More Changes to UMMD's Z Axis

More Z Axis Updates

I "finished" UMMD about 1.5 years ago, but there have been quite a few changes to the machine over that time.  In particular, I have made a lot of changes to the Z axis and related parts that I will summarize in this post.

Pulleys and Belts

The original Z axis used 3 mm pitch steel core belts and 40 tooth pulleys.  I can't recall how I ended up using those parts- maybe I had them on-hand- but that combo led to an unfortunate 18 um/full step in the Z axis.  After a few changes and some careful calculations, I ended up with 60 tooth 2mm pitch drive pulleys and belts, and now have glass core belts on the machine.  That gives a nice, round 20 um/full step.  The glass belts stretch about 3x as much as the steel core belts, but still not enough to matter.

One of the 60 tooth 2mm pitch drive pulleys.  The larger diameter of the pulley necessitated a redesign and fabrication of the Z axis top pulleys to keep the belts parallel to the linear guides.

The Z axis top pulley mounts had to be remade when I changed the drive pulley diameter to keep the belts parallel to the guide rails.  The original mounts had two carriage bolts to hold them in place and prevent the plate from rotating.  The new design has an antirotation tang that fits into the t-slot and uses a single carriage bolt to hold it in place.

Update 1/20/20:  A year or so ago, before I changed from steel core to glass core belts in Z, one of the Z axis drive pulleys came loose and rotated on the drive shaft.  I was recently doing some work on the XY mechanism and decided that it would be a good time to fix that problem.  I pulled the Z axis shaft out of the machine and milled two flats at each end so the drive pulley set screws would prevent rotation on the shaft.

The original pulley mounting bracket at the top of the Z axis used two carriage bolts to hold it in place and prevent it from rotating.  

This is one of the final top-of-the-Z-axis pulley mounts.  It was milled from a piece of 8mm thick tooling plate left over from the bed plate.  There's an anti rotation tang on the back side that fits into the t-slot.

Extruder Carriage

The extruder carriage has undergone more changes than any other part of the printer.  I used different extruders, different hot-ends, and different carriage designs.  The original carriage was made from a single piece of aluminum tubing with the extruder, motor, and hot-end all hanging below the X axis bearing block.  I thought that it looked too much like a pendulum, so I moved the extruder and motor above the bearing block leaving just the hot-end below.  I eventually settled on a two piece design that has the extruder and hot-end mounted on a metal plate with the belt clamps mounted on a smaller piece of tubing.  That allows the extruder and hot end to be removed without taking the belts out of the clamps or even relaxing the tension on the belts.  One thing about the design that has been a constant was the extraordinary length of the carriage.  This was necessary because of the way the bed was lifted on the Z axis.

Eventually, the very long extruder carriage started 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, almost final 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 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.

Z Axis Belt Clamp Redux

By now you've probably seen that I had a problem with the original belt clamp design that led to a failure of the steel core belts.  I redesigned the belt clamps based on a design I have used in SoM for about 6 years without any problems.

The original clamp design worked like this.

And it failed like this!

New Z axis belt clamp design folds the belt back on itself to lock it in place.  The open side of the clamp (facing the camera in the photo) is closed with a rectangular aluminum nut plate that's held in place with 4 screws.

Extruder Carriage Modifications

Now that the bed lifted higher, I was able to cut the long, 5mm thick aluminum plate that mounts the extruder and hot-end on the carriage about 60mm shorter, allowing the hot-end to mount closer to the extruder.  The PTFE tube that connects the extruder to the hot end is also 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.

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.  The black hot-end clamp has a flag (to the right of the cooling fan) that bumps the X=0 switch.

This is the final extruder carriage design.  The extruder and hot-end mounting plate is 5 mm thick aluminum, and the belt clamp mounting tube is 1.5" x 2"x 1/8" aluminum tubing.  The belt clamps and hot-end clamp are printed ABS parts.  The plate holding the hot-end and extruder can be removed without taking off the belt clamps or releasing the belt tension.

Some of you may be thinking that my extruder carriage is ugly as sin, with visible wires, no "professional" looking covers, etc.  There's a reason for that.  The extruder and hot-end are the most unreliable parts of the printer.  Problems with either often require some disassembly.  I prefer to keep everything right where I can see it and easy to get to without having to take off a bunch of covers.

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.

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 mounted the new, adhesive-free heater on the bed plate using Permatex Red high temperature silicone purchased at a local auto parts store.

The TCO, previously mounted on the edge of the bed plate was moved to the heater and mounted using the same high temperature silicone that was used to mount the heater on the plate.  This was done so that if the heater comes off the plate, the TCO will stay with the heater and hopefully shut down the power before it starts a fire.

The new bed heater mounted on the plate using high temperature silicone.  The TCO is also attached using the same high temperature silicone inside the blob near the center of 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 (and cheaper than filament for the printer).  

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.  So much neater than the original!

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.

If you just want the CAD model of the sphere-head screws that are used for pitch and reference adjusters, here you go.  You don't have to use the same spherical head screws I used.  In fact, if you'd prefer to make all the leveling adjustments below the bed, you can just drill through the support as I did at the roll screw, use long screws with thumbwheels, and then put acorn nuts on the ends of the reference and pitch screws.  Use appropriate diameter/width of the hole and slot for the acorn nuts on the reference and pitch adjusters.

Update 1/11/22: very important! When you are preparing the PTFE blocks for the ball head screws, do not tap the holes in the PTFE and do not use threaded inserts. Threaded inserts are best used for screws that you're going to drive in and remove frequently. This isn't that. When leveling the bed you're going to be turning these screws maybe 1/4 turn, maybe a few times during the life of your printer. You don't need an insert. Also, threaded holes in inserts and nuts always allow for clearance between the nut and screw threads to ensure that it will be easy to turn the nut/screw. That clearance allows the nut/screw to wobble in the threaded hole. That's the exact opposite of what you want here. If you tap the holes or use threaded inserts, the screws will wobble, and if the screws wobble, the printer's bed will wobble. You should drill tap-size holes (in this case, 4.25 mm for the M5x0.75 threads on the ball head screws) into the PTFE blocks and then just turn the screws into those untapped holes. Steel screws are much harder than PTFE and will happily roll threads into the plastic. Don't worry, the PTFE won't grip the screws so tightly that you can't adjust them (but nylon will, so don't substitute nylon for PTFE! I know this because I tried it). The screws won't wobble in the PTFE so the bed won't wobble on the screws. PTFE is self-lubricating, so you don't need to use any thread cutting oil when you drive the screws in. 

Finally, once in a while I see people suggesting that PTFE is not good for this application because of "creep". Don't worry about it. I've been using PTFE blocks for this purpose in my printers for >5 years and never had any problems.

Electrical Connections

I had great results using Wago 221 lever nuts when I wired the Duet controller, so I decided to use them to make the bed connections.  I designed and printed an ABS housing that is screwed to the support tee.  Another printed ABS part that fits tightly into the t-slot provides strain relief for the cable.  A Fusion360 file for this and other Wago mounts is here.

I used the Wago mount on the left to make connections to the bed heater and thermistor.  It has a tang that fits into the 8mm wide slot on the bed support tee.

I mounted the Wago bracket on the back side of the bed support tee, where the screw terminals had been. That was a mistake. It's hard to see it back there, hard to install and remove it. I tried to move it to the front side where I could inspect it and release wires easily but, alas, I had cut the cables from the bed heater too short to reach the front side of the support tee. I may turn the whole bed support assembly around so the electrical connections will be at the front side of the bed. This is a mistake I won't repeat in my next printer.

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 (you can see one of them in the first photo at the top of this post).  Finally, I had to shorten some of the cables that run from the hot-end up to the extruder carriage cable.

Son of MegaMax Gets a New Y-Axis

My second printer, Son of MegaMax (SoM) has been a workhorse at the Milwaukee Makerspace for over 3 years, but there have been a few things I didn't like about it, so I decided it was time to make some changes.

The PEI has been reglued onto the bed surface a couple times and the edges were starting to lift up again.  I also wanted to change from the 450W, 24V heater to a line powered heater that would get the bed up to temperature faster, and eliminate the giant industrial power supply that sounds like a vacuum cleaner.  I gave up on the ball screw drive that has been limiting speed due to a severe mechanical resonance and went back to belt drive.  I also wanted to convert to a kinematic mount for the bed plate.

SoM's bed heater, shortly after it was put into service.

Bed plate just removed from SoM.  Dark spots are scorched kapton where air bubbles got between the heater and the bed plate.  This is how heaters eventually fail...

The New Design

I spent some time modeling the changes I wanted to make and got busy in the machine shop at the makerspace.  My machining skills and time are very limited, so whenever possible, I try to make use of existing parts and materials that will require a minimal amount of machining to make them work.  If you've read any of my other blog posts, you'll know that one of the materials I use a lot is square aluminum tubing.  In UMMD, the pulley supports, motor mounts, and extruder carriage are all made from square aluminum tubing.  I used the same tubing to make the bed supports and motor mount for the new Y axis.

The Kelvin-type kinematic mount uses three leveling screws- one each for reference, pitch, and roll, just like the mount in UMMD.  The reference screw sits in a chamfered hole in the bed plate, while the pitch screw sits in a chamfered slot that allows the bed to expand when heated.  The expanding bed plate is free to slide against the roll screw that simply supports the bottom of the bed plate (no holes or slots).  Springs at each leveler hold the bed plate down on the leveling screws.

Occasionally people ask me why I didn't use a Maxwell-type kinematic mount instead of the Kelvin-type that I used.  The answer is simple: the Kelvin type mount only requires slots/holes to be milled/drilled in the Y direction, just like the motion of the milling machine table.  The Maxwell type mount would require milling three slots, 120 degrees apart.  That would require a rotary table which we have at the makerspace, but it's a real PITA to set it up on the machine.

This is the new carriage plate and bed support designed for kinematic mounting of the new bed plate.  The sphere head screws are the reference and pitch adjusters, and the screw on the left is the roll adjuster.

The tubes used to make the mounts were first cut to a few mm longer than needed, then I drilled the holes, and finally milled the edges to final dimensions.  The angled edges dimensions weren't critical, so I marked the angles on the tubes with a marking pen, then clamped the tube in a vice on the mill table, tilting the tube so that the line I drew was parallel to the top of the vice.  Pieces of wood stacked inside the tubes kept them from collapsing when they were squeezed in the vice.

The springs attach to bolts screwed into the plate on one end and the bed support tubes on the other end.

Springs and the screws that will be used to anchor the springs to the bed plate.  I drilled holes then cut the heads down.

The bed hold-down springs hook in the screw heads.

UMMD has a kinematic bed plate mount and it is extremely stable.  Of course, it moves the bed in the Z axis, not Y, like SoM, so we'll see if the concept holds up in a bed-flinger type printer.

Here's the carriage assembly.  The reference adjuster is on the right, pitch on the left, and roll adjuster at the top of the photo.  The plate that links the three bearing blocks is 2.5 mm thick aluminum that was cut on a band saw (no milling on this piece, though the milling machine was used to accurately drill the holes for the bearing blocks).

Heater mounted on the bottom of the bed plate. The left side tab will sit on the reference screw, the right side tab will sit on the pitch screw, and the tab at the top of the picture will sit on the roll screw. 

This is the reference ear of the bed.  The dark circle in the chamfered hole is where the spherical head of the reference screw contacts the plate.

This is the pitch ear of the bed plate.  The dark lines in the slot are where the spherical pitch adjuster screw head sits.

This is the reference adjuster screw assembly.  The pitch adjuster is identical.  The screw on the side is there to anchor the spring that will hold the bed plate down on the adjuster screw head.

This is the roll adjuster assembly.  The end of the screw supports the bed plate from below.

This is the reference end of the assembly.  Putting the carriage plate on the bearing blocks instead of on the leveler tubes keeps it far from the bed heater.  All the leveling screws are threaded into teflon blocks that won't melt or soften when the screws get hot.

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This is the pitch end of the assembly

This is the assembled roll adjuster.  The knob has 16 ridges, each of which represents 50 um of vertical displacement.

Motor Mount

The motor mount was made from a piece of 2 1/2" square aluminum tubing.  I had to raise it a bit by putting a piece of 1/4" thick aluminum under it so that the belt could easily be clamped to the carriage plate.  The motor mount is held down by two 5/16" carriage bolts that fit into the t-slots in the base plate.  Belt tension can be set by pulling on the motor mount, then tightening the bolts.  I used a 20 tooth pulley.

End Pulley

I had a piece of junk from something I took apart years ago that looked like just what I needed- a milled aluminum bracket with two bearings pressed into it.  The bearings have 1/4" bore, like the motor shaft, so I simply mounted a pulley on a 1/4" shaft that I also had from a junk tear-down.  The only problem was that the shaft height of the motor and the end pulley differed by about 1 mm.  That meant that the belt clamp would have to accommodate that difference.

Y axis end pulley

Belt Clamps

I wanted to mount the belt clamp on the carriage plate, so I calculated the necessary thicknesses of the clamp to keep the belt parallel to the Y axis guide rails.  On the motor side of the clamp, the belt would be 5.6 mm above the carriage plate, and on the end-pulley side, it would be 6.6 mm above the carriage plate.  So I designed a printable clamp with those distances in mind.

At first I designed a one-piece clamp, but thought about it and decided it would be less likely to become a source of backlash if I split it into two pieces.  That way when the bed reverses direction, the belt tension will always keep the clamps in position without introducing any backlash.

New Y axis belt clamps.  Splitting the clamp into two pieces reduced the possibility of backlash.  Steel pins secure the ends of the belt in the clamps and the belt teeth interlock in the slots in the clamps.

Electronics

The original heater was a 24V 450W unit, so it needed a big power supply- 24V at 31A.  That power supply had a fan that sounded like a vacuum cleaner.  Since I have switched to a line powered bed heater, I didn't need the high DC power so I replaced the main power supply with an LRS-220-24, a 24V 8A supply (still overkill) with no fan at all.  Complete silence!

The new, 750 W, line-powered bed heater is capable of getting much too hot, so I added a thermal cut-out for safety.  In UMMD I bolted the TCO to the bed plate, but decided it would be safer to have it attached to the heater.  That way, if the adhesive on the heater lets go (as the adhesive on UMMD's heater is doing now, after about 2 years of use), the TCO will stay with the heater and be able to do its job.  I used the same TCO that I used in UMMD, but in SoM I attached it to the heater using high temperature silicone.  UMMD will be getting modified as soon as I get around to reattaching the bed heater.  When the adhesive eventually lets go on this bed plate I'll reattach it with high temperature silicone.

Here's the heater with the TCO added- it's in the blob of blue goop next to the thermistor at the center of the bed.

Power to the bed is switched using a Crydom D1225 SSR.  It's wired through the 10A circuit breaker that serves as the power switch for the printer, and then goes through the TCO on bottom of the heater.  I used Anderson Power Pole connectors for the heater and thermistor connections to the controller.

Performance

I've been able to crank the acceleration up to 3000 mm/sec^2 and print at 100 mm/sec, and I'm not done tuning it yet.  That's a big improvement over the 40 mm/sec limit that was imposed by the resonance in the ball screw setup that used to drive the Y axis.  Print quality is excellent, as always.

A Few More Changes

I connected the power supply ground to the line input ground and also to the frame of the printer - that should have been in the original build.

SoM's lighting has always looked a little dim, so I added some of the same 24V white LED strips that I used on the top of UMMD.  Much better!

I replaced the Titan extruder with a BondTech BMG.  That means I need a new print cooling fan duct design, so I'll be working on that over the next few weeks.

The BMG mounted on SoM's extruder carriage- the hot end offset from center is different than the Titan, so the print cooling fan would no longer fit.  I'm redesigning that now...