Tuesday, April 30, 2019

More Changes to UMMD's Z Axis

More Z Axis Updates

UMMD's very long extruder carriage was starting to bother me. I can't really say that it was creating any problems in the prints, but it just didn't seem right.  Any minor wiggle in the X axis guide rail would be amplified by the long lever arm that the hot-end was mounted on, so I finally decided to do something about it.


Here's the extra long extruder mounting system that I wanted to shorten.  The extruder and motor are mounted just above the X axis bearing block and the hot-end is connected by a PTFE tube down below.  The length was needed so the hot end could reach the bed surface.

Bed Lifting Brackets and Z Axis Belt Clamps


If I was going to shorten the extruder carriage, the bed had to go up higher.  The easiest way to make that happen was to swap and flip over the bed lifting brackets that hold the bed assembly on the Z axis.  That raised the bed by about 50 mm, and moved the lever arm from the extruder carriage that whips around at high speed and acceleration, to the bed that only goes up and down a little.  Probably a good trade off.

The new positions of the bed lifting brackets.

While I was doing that, I changed the way that the Z axis belt clamps attach to the bed lifting brackets.  When I first built the machine, I didn't realize how hard it was going to be to release the Z axis belt clamps because of the dual layer PC panels that fit into the printer's frame (I'd have to remove a frame member to move a panel out of the way).  I also didn't anticipate the amount of experimenting I'd be doing with the Z axis.  Releasing the belt clamps from the brackets required use of a right angle screwdriver to get at the screws that were on the outside of the brackets, with very little room for my fingers to fit in the space.  I needed to flip the screws so that the heads were on the inside of the brackets instead of the outside.

The old way... my knuckles are up against the PC panel on the left.  There are four screws that I have to take out on each side of the Z axis.  The screws goes through a metal plate that holds the yellow belt clamp against the Z lifting bracket.

Much easier access to the Z axis belt clamp screws.  The tapped holes in the bracket were drilled  out to allow the screws to pass through the bracket and belt clamp and thread into a nut-plate on the opposite side of the belt clamp.

I drilled out the threaded holes in the brackets so that I could just push the screws through from the inside, and made two aluminum nut-plates with four tapped holes that the screws now thread into.  The belt clamps get trapped between the brackets and the metal plates just like before, only the screws are now easier to access.  It was so easy- I should have done it years ago!  Now if I want to remove the belt clamps I can just use a screwdriver from the inside of the brackets, under the bed support, where there is plenty of room to work and I can see exactly what I'm doing. Nice!  That will make future changes to the Z axis a lot easier.

Compare the two pictures above to see the differences in the bed lifting brackets.

This is one of two new nut-plates that clamp the Z axis belt clamps to the lifting brackets.  The material is 3 mm thick aluminum and the holes are threaded for 6-32 screws.


Extruder Carriage Modifications


I cut the long, 5mm thick aluminum plate that mounts the extruder and hot-end on the carriage about 60mm shorter and remounted the hot-end closer to the extruder.  The PTFE tube that connects the extruder to the hot end is a lot shorter than it was.  I feel better about it now.

The metal plate on the extruder carriage used to bump the X axis endstop, but that part of the plate was cut off (maybe I should have left part of it there to bump the switch).  I printed a new hot-end clamp that includes an extension that bumps the switch.

The old extruder carriage- the metal extension plate used to bump the X axis endstop, circled.


And here's the newly shortened extruder carriage.  There's not much room for bolting on a print cooling fan, but I rarely print PLA anyway.

Bed Heater


The 468MP adhesive holding the heater on the bottom of the bed plate started letting go several months ago, so I decided to peel the heater free and reattach it using high temperature silicone.  I made an attempt to remove the heater using the scraper I use to release prints from the bed, but it didn't work- the parts of the heater that were still stuck to the plate were really stuck to the plate.

I contacted Keenovo about it and they pointed me at this site for instructions on how to remove a heater from a plate and this site for instructions of preparing a plate to receive a heater that has 468MP adhesive.  Here's their manual on the heaters (which I had never seen before).

They recommend a few things I was previously unaware of, including sealing the edges of the heater with a bead of high temperature silicone, maybe to keep the adhesive from "drying out" and letting go?  Maybe I should seal the edges of the PEI sheet for the same reason...  They also recommend using a mechanical "sandwich" construction to ensure that the heater stays attached to the bed.

Per Keenovo's instructions, I heated the bed plate (to 100C) and used a scraper to release if from the bed.  I gouged the silicone in a couple spots, but fortunately didn't expose any of the heating wires.  Once I had the heater loose I looked at the underside.  The area that had come off the bed plate had been running very hot and singed the silicone on the underside of the heater.  I flexed the heater in the toasted area and it cracked, so I decided it wouldn't be safe to reuse it and ordered a new one without any adhesive.  I'll stick the new heater to the plate with high temperature silicone.


The burnt bed heater.  The dark section cracked when I flexed the heater in that area, so I have ordered a new one without adhesive and I will cement it to the plate using high temperature silicone.
I'm going to mount the TCO on the heater using the high temperature silicone so that if the heater peels off the plate again it might prevent damage or fire by staying with the heater.

Leveling Screw Block Redesign


Once I had the extruder remounted on the shorter plate and went to relevel the bed, I noticed that when I turned the roll screw, it was causing the bed to shift laterally.  That's shouldn't happen!  I found that the PTFE block holding the pitch screw was tilting/shifting in the t-slot.  The narrow PTFE block was held inside the t-slot by two small screws and they weren't holding fast so the block was wobbling in the slot.  I tried to tighten the screws and they stripped the holes in the PTFE.

Here's the original roll adjuster- the other two are about the same.  The PTFE block fits into the slot and is held in place by two small screws whose heads you can barely see in the bottom t-slot, behind the long roll adjustment screw.  It wasn't a very solid or reliable way to mount the PTFE blocks.


It was time to redesign the leveling screw blocks for more secure attachment to the support frame.  I was out of PTFE and the "local" plastics shop is about 40 miles away, and I just need a relatively small amount to use for this and future projects, so I did some shopping on ebay.  The first thing that struck me was how expensive PTFE is, or looks at first glance.

PTFE is a commodity, and you buy commodities by the price per weight.  The ebay listings usually have dimensions listed in inches, and PTFE has a density of 0.08 lbs/in^3, so I calculated the price/lb including the shipping cost when I compared the different listings.  It didn't really matter what the exact dimensions of the block were because I'm going to cut it up and mill it anyway.  I mostly use small blocks of the stuff, not large sheets, so I looked at bar/block listings at least 3/4" thick.

Here's a typical offering:

This one is a total of 13.125 in^3, which will weigh 1.05 lbs.  At a total cost of $23, that works out to about $22/lb. Ouch!


Here's an example of a pretty good deal:

These blocks of PTFE are 71.25 in^3 and have good dimensions to allow a lot of small parts to be made by cutting it up and milling.  71.25 in^3 will weigh 5.7 lbs.  I've probably used 1/10 that much PTFE in the last 10 years.  Total price is $36.80, which works out to $6.45/lb.  That seems like a pretty good price for PTFE.  
I ordered the block in the second photo.


The PTFE arrived in the mail- a literal brick!  I went to the makerspace and went to work on it.  In a couple hours I had three new PTFE blocks finished and ready to go.


The new PTFE leveling screw blocks.  You're looking at the bottom of the block on the left.  The tang just fits into the 8mm wide t-slot to prevent the block from rotating.

The bed support tee with new PTFE leveling screw blocks installed.  Each block is held in place with an M4 screw and t-nut.  The thickness of the blocks matches the length of the threaded part of the leveling screws- 13 mm.

One of the new leveling screw blocks.  The blocks are 30 x 24 x 13 mm.



Here's the reference leveling screw with the new PTFE block in place.  I deliberately set the end of the PTFE block 5mm back from the edge of the t-slot so there would be more room for the spring.

The CAD file for the new design including the bed support and the bed plate itself is located here.

Electrical Connections

I had great results using Wago 221 wire blocks when I wired the controller, so I decided to use them to make the bed connections.  I found that the Wago's are a size that matches the t-slot, so I decided to use some high strength tape to hold them to the bed support tee.  I added one for a ground wire connection to the bed plate that I had neglected to install when I built the machine.  I used a printed part to provide strain relief for the cable at the bed support.

Miscellaneous


I had to make a couple other small changes to accommodate the new configuration.  I printed new bottom-of-the-Z-axis bumpers to keep the bed assembly from going too far down.  Finally, I had to shorten some of the cables that run from the hot-end up to the extruder carriage cable.



Sunday, April 14, 2019

Floor Jack Pads: Pushing The Limits of 3D Printed Parts

My 12 year old Audi TT needs new shocks and I am preparing to do the work myself.  I've changed struts on two other cars, so I have most of the tools and a pretty good idea of what to expect.  Now I'm in the process of researching all the correct part numbers to order.

One tool that's been missing from my ever-growing collection is a floor jack.  I fixed that deficiency yesterday with a trip to Harbor Freight Tools where I bought a 3 ton, low profile, steel jack for $89.  I have no illusions about the quality, but it seems sturdily built (it weighs about 80 lbs) and should be fine for my infrequent uses like replacing the struts in my car and rotating the tires once in a while.

The jack did not come with any sort of pad on the saddle, and I don't want to try using it without one, so I did a little research.  Volkswagen Audi Group vehicles use a common lifting point "socket" that is best used with a jack pad that is made to fit.  I looked up commercial offerings and found some for about $8-10, made of polyurethane.  Polyurethane?  I can print that!

One of the jack pad makers was kind enough to provide dimensions:

Audi jack pad dimensions.

I modeled one of the pads in Fusion360 in about 30 seconds.  The commercial pad was only 69 mm in diameter but the saddle on my jack is 93 mm in diameter, so my model has a 90 mm diameter base.

My print used fluorescent green TPU- I won't have any trouble seeing it in the bottom of a drawer or toolbox.


UMMD has a 0.4 mm nozzle, so I used TPU filament in 0.24 mm layers, 0.5 mm line width, 6 perimeters, 8 top and bottom solid layers, and 40% triangular infill.  It used about 101g of filament and took about 5 hours to print at 40 mm/sec.  The print came out beautiful, and like all TPU prints, it's super tough.

Here's the jack with the naked saddle.  You need some sort of pad to protect the car!

Here's the jack with my custom 3D printed pad in the saddle.  The bump on top of the pad fits into the jacking receptacle on the car's frame.
Let's see if it's tough enough:


Well there you go!  TPU is one of the most amazing filaments you can get for a 3D printer!  It's easy to print (220C extruder, 45C bed, 30-40 mm/sec) and produces incredibly tough prints.

A view of the 40% triangular infill looking through the bottom of the Audi floor jack pad.

I used concentric infill for the bottom and top layers.  Other solid fill layers were set to rectilinear.  This nice Moire pattern appears in the bottom of the pad. The black smudge was acquired when I jacked up the car for the video.  Next time I'll take photos before I test a print under load.



Now I'll have to print a jack pad to fit my wife's car...

BMW jack pad dimensions.

BMW jack pad printing with 50% infill in fluorescent green TPU.


Done.



The Fusion360 models for Audi and BMW jack pads are here.

Saturday, April 6, 2019

Repairing M-Audio BX5a Studio Monitors




I've had a pair of M-Audio BX5a speakers for use with my computer for a few years and over the last year or so, the gain in one channel, and then the other, has steadily decreased.  They're pretty decent computer speakers and I didn't want to throw them away and get new ones, so I searched the web to find service information but came up with nothing.

These speakers are biamplified and the specs claim 40W for the bass driver and 30 W for the tweeter.  The transformer that powers the speakers is rated at 2 x 16V 1.5A (marked on the transformer).  There is a pretty big heatsink on the two, probably class-AB, amplifier chips, but it's mounted inside the enclosure and the only thermal communication with the outside environment is via the steel back panel of the speakers (that is warm all the time when the speakers are powered) that the heatsink is mounted on and a little air that might make it to into the box via the bass vent. The result is that the speakers run warm.

Heat and electronics is never a good combo and leads to failure of electronic components.  The parts that are most affected are semiconductors that tend to fail catastrophically, and electrolytic capacitors that tend to degrade over time.  The speakers still worked, but gain was dropping, suggesting that the semiconductors were still functioning, and that signal coupling capacitors or power supply electrolytic and bypass capacitors may be failing.

I decided to repair the speakers using a brute-force approach that I used to use when repairing old vacuum tube radios- start by replacing all the electrolytic capacitors.  It's a reasonable approach given the low cost of capacitors and their relatively high probability of failure.  Also, if one has failed, others operating in the same environment may fail soon, too, so replacing all of them is the best way to ensure that the speakers will work for a few more years before they need any more attention.

Disassembly


Whenever I'm going to do something like this, I take lots of photos as I go so that if there's any doubt about what goes where, I can review the pictures and get things back together the right way.  I suggest you do the same.  As I take screws out, I put them in a tray so they don't get lost.  When there are different types/lengths of screws, I will sometimes turn them into their holes so that I will know which ones go where.

The first thing I had to do was get the speakers apart.  I have some M-Audio AV-40 computer speakers which suffered from failed capacitors a while ago.  They were a nightmare to work on because of the way the speakers were built, so I expected the same in the BX5a speakers.  I was pleasantly surprised to find them pretty easy to take apart.

There were only a couple tricky things to deal with.  Once the back panel was open, I had to reach into the speaker and clip a zip tie off the wires that connect to the drivers so that I could get my hand in far enough to disconnect the wires from the drivers.  There's an LED on the front panel and the wires from it go to a connector on the amplifier circuit board.  For some reason they put glue on the connector which made it a PITA to separate.  The wires from the power transformer went to a connector that was also glued.


Clip the zip-tie circled in green to release the LED and speaker wires, then reach in and pull the speaker leads off the drivers.  The LED attaches to the amplifier board with a connector so it gets released there.  There's a drop of glue holding the connector on the PCB for some reason.

Unscrew the two ground wires circled in green, then cut the zip-tie circled in pink to remove the ground plate to gain access to the amplifier PCB.  Unplug the transformer from the amplifier board - the connector is glued.  The final step is to unscrew the input connectors and volume control, then the three screws that hold the heatsink on the rear panel of the speaker.

This is the amplifier PCB and heatsink.  The nonpolar electrolytic caps are all green. There are two on the small PCB behind the volume control pot and one on the main board. The other electrolytic caps are all black.  All are rated for 105C.


After getting those items sorted out it was pretty easy to extract the amplifier assembly from the box and identify all the capacitors.  I drew a picture of the amplifier board and added major landmarks, then marked the values of the capacitors and their approximate locations.  The PCB is marked with component numbers so I made a list of all the numbers and values.  I also measured the diameter of all the caps so that when I ordered replacements I could be sure they'd fit in the same space.  The board is actually pretty generous with space around most of the parts, so matching the component sizes wasn't entirely necessary.

Capacitors and their locations on the PCB.  I just started using the Pilot white board markers and they are fantastic.  The colors are very bright and they erase cleanly and easily.  Highly recommended!


The 1 uF and 0.47 uF non polarized electrolytics were hard to replace.  Non polarized electrolytics are often used for coupling audio which means they may be exactly the caps that need to be replaced to get my speakers working like new again.  I was unable to find electrolytic replacements, so I used film capacitors instead.  Film caps will probably perform better and last longer, so it isn't a problem.  I found suitable parts in my junkbox, and picked out replacements from the DigiKey catalog.  The film caps have wider lead spacing than the original parts and are physically larger, but they fit into the board just fine.

I made a spreadsheet that has all the original capacitor values and designations and a list of DigiKey part number replacements for them.  All the replacements are rated for at least 105C operation, and in a few cases I was able to select parts that were rated for >1000 hrs operation at that temperature. It costs about $11 per speaker for all the capacitors.

Replacing the Caps


The parts are all through-hole type which makes replacing them very easy.  Be sure to pay attention to the polarity of the caps as you remove and install them.  I removed and replaced the old caps one by one.  I grabbed the cap that was coming out with a pair of pliers and heated its leads on the underside of the PCB until the part was loose enough to pull out.  Then I cleaned up the PCB as needed with some desoldering braid, and installed the new cap, matching the polarity of the old cap.  I kept a printout of the spreadsheet on my work table and checked off each capacitor as it was replaced to ensure I wouldn't miss any of them.   It took me about an hour per speaker to replace the old caps.


Reassembly


Reassembling the speakers is the exact opposite sequence of disassembly.  When you put the speakers back together, you have to be careful to reconnect the driver leads the right way.  In the first photo, above, the black lead for the bass driver connects to the terminal nearest the bottom of the photo, and the red lead goes to the other terminal.  The tweeter's black lead goes to the connection nearest the top of the photo and the white lead goe to the other terminal.  Be sure to reattach the ground wires and the metal ground plate that goes under the main PCB.

Results


The speakers sound like new again.  I used to leave them powered up all the time which probably led to the short lifespan of the original capacitors.  In the future I'll power the speakers off when I'm not using them so they don't sit and cook.  Maybe they'll last longer this time...


Wednesday, March 13, 2019

Argon Laser!

The boss was cleaning out a closet at work and was about to toss an old dental curing light into the trash until I spotted it and rescued it.  It was a LaserMed Accucure 3000.  When I got it home I opened it up to see why a dental curing light would use a fiberoptic wand and would weigh about 20 lbs.  I was expecting a high pressure arc lamp and some filters, but to my pleasant surprise, it contained an Argon laser tube!  Woohoo!

I plugged it in and flipped the power switch.  It lit right up!  Double Woohoo!

Here's a peak inside the curing light.  I was expecting a high pressure arc lamp and filter, but nooooo, it uses a LaserPhysics Reliany 300b argon laser tube!

The output is a pretty blue green color.  I blasted it through a diffraction grating and found 6 lines- 3 distinctly blue and 3 more green than blue.

When I first turned it on and checked the output power using the built in power meter it was reading about 80 mW out.  I did some reading and found that argon lasers like to be operated occasionally to keep the power level up, so I've let the thing run for 10 minutes at a time a few times in the last couple weeks and now the power output is up to about 120 mW.  This thing is incredibly inefficient- it takes about 1kW from the power line to produce about 0.0001 kW out!

I'm looking for info on the tube- it's a LaserPhysics Reliant 300b- so far web searches come up empty.  If you have info I'd love to see it.

Now I have to decide what to do with it.  Someone at the makerspace has some mirrors mounted on galvanometers...

Sunday, February 24, 2019

Update on the Tangle-Free Filament Spool Holder

A year or so ago I posted this design for a filament spool holder that prevents tangles caused by the filament springing over the flanges of the spool:

The original design didn't use rubber bands- I just twisted the nut tight once the roller was pushed down on the spool flanges.  Unfortunately, if you leave something finger tight at a makerspace, it will get taken apart... daily!  I replaced the regular nut with a nylock nut and added the rubber bands.  Then the rubber bands started disappearing, of course!

A couple weeks ago one of my friends at the Milwaukee Makerspace, Tom Klein, who has forgotten more about machining than I will ever know, did a modification to the design.  He saw that I was using rubber bands to pull the top roller down on the spool's flanges and thought it would be better without the rubber bands.  I couldn't have agreed more.

I provided a CAD drawing of the original roller, he dug through the bar stock at the makerspace, and then he got busy on the lathe.  The top roller is now made of steel, and is heavy enough that no springs are needed:

Top roller made of steel, includes F608 bearings like the original printed plastic roller.  The weight of the roller eliminates the need for rubber bands to pull it down against the filament spool flanges.
The bolt is secured with a nylock nut and is left just loose enough that the roller can slide up and down in the slots in the frame.

If you have a lathe, I can wholeheartedly recommend this modification.

Thanks Tom!

Saturday, February 16, 2019

Designing 3D Printed Parts for Assemblies

Here are some of the techniques I have used to design assemblies of 3D printed part and non printed parts.

The first thing I do when designing 3D prints to fit with non printed parts is make CAD models of the nonprinted parts with enough detail to cover mounting the part to the print or other non printed parts.  I have built up a library of nonprinted parts such as stepper motors, bearings, linear guides, etc. that I can reuse as needed, and Fusion360 allows direct import of 3D models of hardware from the McMaster-Carr catalog which can make life really easy.

Once I have modeled the non printed parts, I put the CAD models into their final positions relative to each other, then design the printed parts around them.

Get yourself a good caliper.  It is the handiest tool there is for modeling existing objects.  You can get cheap one on sale for about $10 at Harbor Freight Tools, but if you spend about $40 you can get one that doesn't have to be zeroed before every measurement, won't lose the battery cover, and will work for over a year on a single battery.

Copying/Modeling Existing Objects


In the printed blower photo below, I made an impeller (green) that was based on the design of an impeller from a CPAP blower.  It looks pretty complicated by it's actually pretty easy to make such a thing.

First I took a picture of the CPAP impeller and measured its diameter and vertical dimensions.

Here's the CPAP blower impeller, photographed from above.  The goal is to copy the complex curves of the vanes on the rotor.

Start by drawing the base disc that the vanes will grow out of and import the photo of the impeller being copied.  Align the photo with the disc.

Model the base disc, and import the photo of the impeller and align it with the disc.
Then use spline curves to trace the outline of one of the vanes:

Trace the outline of one vane with spline curves and pull it up in Z.

Create a sketch in the XZ plane to taper the vane to match the original

Sketch that will be used to taper the vane toward the edge of the impeller.

Revolve the sketch and use it as a cutting tool to taper the vane.

The vane is tapered by revolving the sketch.

Separate the vane from the disc by cutting the part with the top plane of the disc.
The vane is separated from the disc by cutting the vane component with the top plane of the disc.
 Copy the vane using a circular pattern.


The vane is replicated by creating a circular pattern.

Finally, add mounting holes to match the motor and fillet edges, etc., and combine all the pieces into a single component.  Export the STL file and you're good to go.


Here's the end product printed.

The same technique can be used to copy all sorts of things.

I used a similar technique to copy a finial from an antique table that a friend was refinishing.  
In that project, I traced the outline of 1/2 of the profile of the object then revolved it into a solid.

If you buy parts from China, you'll often find that if there is a drawing showing dimensions on the web page where you order the parts but it may or may not match the parts they actually ship to you.  I normally try to buy or scrounge gears, pulleys, belts, motors, switches, bearings, etc., first, and then design around them.  I have learned the hard way to wait until I have the parts in hand so I can measure them and make sure things will fit.

Belts


In machines that use belts, you have to clamp the belts to the moving parts.  I like to use self locking belt clamps in which the belt folds back on itself with the teeth engaged.  The problem is that every manufacturer's belts are different thickness, so you have to customize the clamp slots to match the belt.  I printed this gauge to test the belts.  It has a series of slots that are 1.0 mm to 3.0 mm wide in 0.1 mm steps.  I use it to check the width for a single pass of the belt, and then fold the belt and see which slot will hold it with the teeth engaged.

GT2 belt gauge for designing self locking belt clamps.
Self locking belt clamp.  The entry and exit slot widths are determined using the belt gauge and the belt.

Bearings


Hole gauge used to check fit with hard disk drive bearings.  So far this gauge was used for a filament spool holder and Van de Graaff generator that used HDD bearings for the rollers.
Cutaway view of the top of the Van de Graaff generator.  The green parts are HDD bearings that let the roller (blue) spin very smoothly.

Filament spool holder that uses HDD bearings (12 of them!) for the rollers and pivots.


Rectangular gauge used to fit the Y axis bearing blocks on the ends of a "square" aluminum tube used for the X axis in the sand table project.

The black rectangular tube fits tightly into the Y axis bearing block in the sand table (early version).

Bosses and Alignment


Bosses and mating holes can be used to ensure accurate alignment of printed parts.

Worm gear project that illustrates the wrong way to use bosses. The bosses and mating holes ensure accurate alignment of the two halves of the shell, but in this design, the screws go through the bosses the wrong way.  The problem is that the boss is relatively small and weak.  When the screw starts turning threads into it, the boss is liable to break off.


Test print made to check spacing between the shafts.  1/2" drill bits were used for the gear shafts.
Bosses (done correctly this time) used to align the two halves of the box.  The large screw hole goes in the boss, and the smaller hole that the screw will thread, goes in the boss's mating hole.

Cutaway view of a boss with the screw in place.  I usually make the boss 1 or 2 layer thicknesses shorter than the mating hole.  The screw hole in the boss (the blue part) is larger than the screw diameter, so the screw just drops in.  It will thread into the smaller hole in the mating part (orange).
Here's a project I did a few years ago- it involved bearings, gears, a motor and three printed parts that all fit together:

Snakebite extruder printed parts and bearings.  The base fits on a NEMA-17 motor.

Bearings in place... rectangular bosses align the two top cover pieces to each other and the bearings.
Top cover halves in position...  Bosses in the top cover align it to the base.
Gears installed...
Final assembly.

The other way to ensure alignment is to use steel pins and tightly fitting holes.  You can buy 3-5 mm diameter stainless steel rods cheaply and cut them to the lengths needed, and have plenty left over for other projects.

Motor Mounts


One thing I see a lot of is motor mounts that look like L brackets, printed in plastic.  That may work OK if the L is made of steel or thick aluminum, but plastic is a lot more flexible than steel and aluminum.  Under belt tension or forces applied by gears, motor mounts like that will flex and that will cause belts to ride hard on pulley flanges, or gears to track poorly. 

When I'm designing a motor mount, I start with a solid block and carve away just enough of it to allow belts, pulleys, and gears to be installed, then add whatever I need to mount it on the machine.  You end up with a much more solid structure with minimal flex that way. 

Looking at these two mounts, which do you think will flex more if it's printed?

Here's a printed motor mount from the sand table project. 
Here's one half of a motor and gearbox assembly I designed and printed.  It was very solid.

Motor and printed gearbox.  No thin, flexible parts allowed!
Here's another motor mount that was in the sand table very briefly.  It was quite solid and performed well.


In many situations, the best motor mount will not be printed, it will be made of metal.  In UMMD, my coreXY printer, I made motor mounts from square aluminum tubing which is far more rigid than using a L bracket.  You can drill mounting holes on the end opposite the motor, or on two sides, making placement very easy.  Aluminum tubing is available in many sizes to work for different motors, and is pretty easy to work with.  The hardest thing to do is cut the large hole for the motor's pilot (that's the name of the cylindrical bump on the shaft end of the motor).  In NEMA_23 motors it has to be at least 38.1 mm diameter.  There are a lot of ways to cut that type of hole- a hole saw on a drill press, or a milling machine with a boring bar, etc.  Aluminum is easy to cut with a hack saw and drills easily, too.

Aluminum motor mounts I made for UMMD's XY stage.  I cut the large holes using a boring bar on a milling machine, but there are ways to do it with less sophisticated tools.
Here's one of those mounts installed in UMMD.


One advantage of using aluminum tubing to make motor mounts is that it's thermally conductive and will act as a heatsink to help the motor run cooler.

Strength

I frequently use modifier meshes in Slic3r to increase fill density around screw and bolt holes.  Meshes can be created in Slic3r, but I prefer to make them in CAD and export them as a separate STL file(s) that import into Slic3r.

If the modifier mesh touches the surface of the part, it will be visible in your print.  You can make it invisible by putting it entirely within the print surface.  If I am going to reinforce a vertical hole, I will put the bottom of the mesh about 0.4 mm above the bed plate and make it about 0.4 mm shorter than the print.

Thumbwheel with internal modifier mesh (green) in CAD (Fusion360).  Note that the modifier mesh is 0.4 mm below the top surface and 0.4 mm above the bed plate.
First bring in the thumb wheel...

Then import the modifier mesh...

Then set the fill density of the modifier to 100%...
Then check the preview to see that the modifier is there and looking as it is supposed to- the pink solid infill surrounding the hole.
Using a solid modifier around the screw hole lets me crank the nut down on the thumb wheel screw without worrying about breaking the printed wheel.

Some people like to use threaded brass inserts especially for parts that will be screwed and unscrewed frequently.  Keep in mind that the inserts are larger and their threads will never strip out, but they are still installed in printed plastic.  If you apply too much force to the screw, you'll break the insert free of the plastic.  We have a Taz printer at the makerspace that uses threaded brass inserts to hold screws that lock the Y axis guide rails to their mounts.  Turning the screws too tight has jacked the brass inserts right out of the guide rail mounts.

I often use thread rolling screws for plastic.  They look a lot like wood screws without the sharp point and have widely spaced threads that don't strip out the holes very easily.  You can use wood screws and sheet metal screws too, as long as their sharp points are inside the print.

Holes


Holes always print a little smaller than the design size, so it can be useful to print a gauge of different sized holes to allow testing for fit.  If you need really accurate holes, it is best to print slightly undersized pilot holes and then run a drill bit through them.

Hole templates like this one can be a quick reference to check printed holes against other hardware like bearings, screws, and bolts.

A printed hole template like this can be used to check printed hole size against design size.  Numbers indicate design size, not actual hole size.  The printed hole size will be a little smaller. than the design size.  Make sure your extruder is calibrated before you bother printing something like this.
If you're going to be mounting a bearing or other round object in a printed hole and you want it located accurately, when you slice you should use "random" seam placement so that the layer start/stop zits get scattered around the hole instead of all lining up to form a seam that will displace the bearing a little.

Thumb-wheels

Thumb-wheels have all sorts of uses including leveling and zeroing beds, hold down clamps, etc.  There are different ways to make them, but I have found a pretty easy way that makes reliable thumb-wheels every time.

The trickiest part about thumb-wheels is preventing the screw from turning inside the printed plastic wheel.  You can't do it with a round screw head.  You need to either use a hex head bolt (great for larger stuff, but not so good for small screws) or put flats on the head of the screw using a grinding wheel or belt sander.

I use a lot of 6-32 screws because I have a bunch of them and they are a reasonable size for most things I need to adjust with thumb-wheels.  You can't usually find them with hexagonal heads, so I grind flats on the sides of the heads.  When grinding, you need to hold the screw tightly without damaging the threads and without burning your fingers as it heats up.  I drill a hole into a wood block and drive the screw part way in.  Then I put one side of the block on the belt sander table and start grinding until the the flat on the head meets the screw threads.  Then I turn the block over and grind the other side of the screw head.  The flats come out parallel and I don't burn my fingers.

6-32 screw head ground flat.  Careful positioning of the screw in a wood block allows you to preserve most of the tool socket.

All the hardware you need to make a reliable thumbscrew.  You can leave out the lock washer if you use a nylock nut or a drop of Loc-tite.




Use a center hole that just fits the screw, and put in a depression for the flattened screw head.  Flutes are made by drawing a circle at the edge of the wheel, using it to cut away the side of the wheel, then using a circular pattern to duplicate the cut around the wheel.



The 32 TPI thread is about 794 um pitch which is very close to 800 um.  Using 16 flutes means that each flute represents about 50 um of vertical displacement when you turn the screw.

Clearance between moving parts

One of the harder things to do is design moving parts for mechanical clearance.  You can use a sketch and a rectangular or circular pattern to check simple clearance issues:

A sketch (blue) was done on an offset plane and uses a circular pattern to check for movement and clearance at the optical limit switch on the right side of the model.

You can use "joints" in Fusion360 to show part motion in 3 dimensions.  Place the joints and then grab one of them and move it with your mouse- the model will move just like the actual object will when it is assembled.

Rotary joints (the flags) were added at the bearings

Select one of the rotary joints as a handle.

Move the joint and the assembly moves.

You can use this type of thing to model moving assemblies and check for interferences.

Infill Patterns

Slicers have different infill patterns available.  Some of the patterns consist of straight lines that run back and forth across the interior of the print (rectilinear, grid, triangles, stars, etc.), others are interesting looking 3D structures (honeycomb, 3D honeycomb, gyroid, etc.).  Here's something to think about: patterns that print as a series of straight lines will print faster than patterns that use short lines and curves going in different directions.

All the motion in the printer is subject to acceleration and deceleration (and jerk or junction deviation).  If an infill pattern consists of a series of short, straight lines going in different directions, such as the hexagon infill pattern, it will print slowly because the extruder carriage never gets to the target speed because it is limited by the acceleration.  The printer is going to shake a lot while printing that type of infill, too, because the direction of motion changes every few mm.

Short segments and many rapid direction changes means hexagonal infill will print slowly and shake your printer silly.
I've never been convinced that there's any advantage to using the pretty looking honeycomb or gyroid patterns compared to the much faster to print grid, triangles, or star patterns, so I avoid complex 3D infill patterns for mechanical part type prints.  The complex patterns do have their uses, especially if you print objects in which the infill pattern is visible.

The rectilinear grid pattern lays down lines of plastic at 45 degrees and 135 degrees on alternating layers.  That makes it very quick to print, but the infill lines on any layer only touch the lines on the adjacent layers where they cross which results in a weak infill that is most suitable for holding up the "roof" of a part that won't be subjected to a lot of force.

Rectilinear infill bridges previous layer with layers of infill touching only at the crossing points.


If you want strong infill that prints quickly, try grid, triangles, or stars.  All three of those infill patterns place all the pattern lines on every layer (unlike rectilinear grid).  That makes the patterns strong and because the lines are all straight and go across the width of the print, they print quickly.



Your Turn


I hope you'll find some of this stuff useful in your designs.  If you have some better ideas, I'd love to hear them...