Tuesday, April 30, 2019

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/ustep 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/ustep (at 16:1 ustepping).  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.

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, 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.  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.  
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.

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.



16 comments:

  1. "I flexed the heater in the toasted area and it cracked, so I decided it would be safe to reuse it and ordered a new one without any adhesive."

    That would read more naturally as "unsafe" than as "safe", is it a typo?

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  2. Thanks for enough of a hint in the image to find that particular supplier of PTFE. I had been wondering whether I should bother with PTFE for the kinematic mount I intend to put on the printer I'm rebuilding, but that price made it clear to me.

    The idea to bond the TCO to the heater with high temp silicone seems like a great one and another idea I plan to steal.

    I learn something new every time you blog. Thanks! ☺

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  3. I think the recommendation from Kenovo to put RTV on the edges of the pad are to act as strain relief to keep the edges from lifting away. That's my theory anyway.

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    1. In SoM the PEI was/is attached to the bed plate with the same 468MP adhesive that the Keenovo heaters use. I found that it started letting go after about 2 years and had to reattach it twice so far. When it lifts, the adhesive remains on the bed plate and let's go of the PEI starting in the corners and spreading around the edges, so you might be right.

      Or maybe plasticizers that keep the adhesive sticky are burning off and at the corners/edges they are free to escape, or maybe oxygen gets to the adhesive and ruins it and a bead of silicone stops that process.

      If there are any experts on adhesives here we'd like to hear from you...

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  4. I was looking at your UMMD design in the fusion 360 files, and I have a question for you. The z-plate is lifted by a belt on each side. The top mounting point, that holds the belt via an idler wheel, is attached to a vertical member, just below the horizontal member that the x-y stage rests on.

    Is there a reason you can't attach it to that horizontal member instead? I think you'd probably get like 50-60mm out of that movement alone?

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    1. Thanks for your comment! You're right, and if I spent more time thinking about how to do it I could probably come up with a way to tension the Z axis belts when the pulleys are mounted on the horizontal XY stage frame members, but what I have is working, the change will require yet another extruder carriage redesign, I have more than enough Z motion, and I have other projects to work on. When my brain starts to itch I may eventually make the changes and if I do I'll post them here.

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    2. Yeah after I posted I realized that you would have trouble tensioning the belts. I can think of some ways to do it, but none as simple as what you have, and possibly not as effective either.

      I'm thinking of building one of these this summer. I will crib quite heavily from your models, and I've been reading your blog entries. One of your entries (the one on why certain segments of the belts need to be parallel to the axes) just saved me from a mistake on a small machine I'm building right now. I had redesigned some motor mounts to take bigger motors, and moved the center point of the motor axis quite a bit to accomodate them, since I didn't think it would matter. I'm re-printing it the right way now.

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    3. A lot of first-time mechanism designers make the mistake of not routing belts parallel to the guide rails. I might have made the same mistake in UMMD but for an article I saw on the web that mentioned it. I'm glad the information was able to save you some trouble.

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  5. I've been lovingly copying and ripping off your designs here for a while, and am currently framing up my printer. I wonder about your thoughts on headspace above the XY frame - how you calculated the height from the top of the carriage to the "ceiling" of the workspace. I'm trying to keep things compact, but still allow for enough space to allow for proper filament runout

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    1. I didn't really calculate it. I just gave it what I thought would be enough and it has proven to be that. I feed the filament down through the electronics enclosure right to the center of the bed. Slicers drop prints on the center of the bed so it keeps the filament path short and straight. If I print something really big, or a bunch of small parts covering the bed plate, the filament feeds OK and doesn't create much drag on the extruder carriage.

      Some people prefer to route the filament through a tube in the drag chain that connects to the extruder carriage. That allows minimal height above the XY stage, but I generally don't find height of the printer to be nearly as much of a problem as the footprint. Feeding filament through a long tube increases the drag on it that must be overcome entirely by the extruder motor.

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  6. I made a variant on the thermal isolating PTFE mounting blocks that are adjusted through the bottom instead of requiring through-slots to adjust from the top. This let me use inexpensive and common acorn nuts instead of the relatively more expensive round-headed screws, and let me bury the adjustment features under the printable part of the bed instead of requiring tabs off the side. Here's my writeup, with credit because credit is due... ☺

    https://forum.makerforums.info/t/making-thermal-isolating-blocks-for-kinematic-mount/77841

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    1. It looks pretty good! The main reasons I used PTFE for the leveling screw blocks is that it holds the screws tightly so they can't wobble. When you tap a piece of metal to take a screw it normally has a lot of clearance between the screw threads and the hole threads allowing the screw to wobble in the hole. If the leveling screws wobble, the bed will wobble. PTFE grips the screws, allows them to be turned easily, and doesn't melt or soften when the screws get hot (that's why I didn't use printed plastic). I did an experiment with nylon, but it gripped the screws so tightly I couldn't adjust them.

      I didn't tap the holes for the leveling screws. The steel screws will roll their own threads in the soft PTFE and the result will be a tighter grip on the screws than if the holes are tapped. You won't need a grub screw to hold the adjustment.

      Acorn nuts should be fine. It is not necessary to mill a slot in the plate for the pitch screw head- you can mount a piece of metal with two parallel rails for the screw head to rest against, too.

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  7. Do you still have those CAD files? Links seem to be broken :( Thanks!

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    1. I just checked all of the links and they look fine in my browser (Chrome)...

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