Heat Compensated Bed Support First Test I have a 305 x 317 x 6.35 mm cast aluminum bed plate in my printer, supported by three screws that are used to level it. The three screws are anchored to a 6.35mm thick aluminum undercarriage which is in turn supported on three standoffs screwed to bearing blocks that ride on two fully-supported linear guide rails that are the Y axis of the machine. When the bed plate heats to ABS print temperature it expands about 0.7 mm. The diagram below shows how the bed expands as temperature rises. This puts lateral pressure on the leveling screws because they are firmly anchored to the unheated undercarriage below the bed plate but the holes in the bed plate are moving apart. The lateral pressure causes the plate to deform slightly, exhibiting the usual hump at the center of the bed. [img]http://mark.rehorst.com/misc/ht_comp_Y/expansion%20diagram.jpg[/img] I thought about how to prevent this problem and keep the bed flat by eliminating the lateral pressure created by the temperature difference between the bed plate and the undercarriage. I decided the simplest way to fix it was to eliminate the undercarriage completely and let the three leveling screws and bearing blocks move independently. It is not an ideal situation from a mechanical engineering point of view, but I thought I'd try it anyway. If it worked I would shed about 500g of moving mass as well as reduce the hump in the bed when it heats up, a win-win situation. Here's what I came up with: [img]http://mark.rehorst.com/misc/ht_comp_Y/heat%20comp%20Y%20axis.png[/img] The bed plate, not shown, is supported by the three vertical screws with springs. Two of the screws are aligned along the Y axis of the machine (reference and B in the first diagram); the one at the left side is the reference screw that is set and never adjusted again. The other Y axis screw, on the right (B ), adjusts the pitch of the bed around the X axis. When the plate expands the two simply move apart along the Y axis, so by disconnecting the bearing block, standoff, and screw from the undercarriage plate, the block and screw at B are free to move away from the reference screw. C is where the more interesting stuff happens. Notice that the expansion line between the reference and point C in the first diagram is not aligned with the Y axis. It has to move diagonally. It took a while to realize that since it was free to move along the Y axis, all I had to add was some freedom of motion in the X axis and it would then be free to move diagonally. I added a short linear guide that I bought via ebay for $9. The bearings and rails in these things are so precisely machined that there is no hand detectable play at all. They move only along the axis of the rail. Today I tore apart the Y axis and set it up to use this new design. Here are some photos of the parts at B and C. [img]http://mark.rehorst.com/misc/ht_comp_Y/02080002.jpg[/img] [img]http://mark.rehorst.com/misc/ht_comp_Y/02080003.jpg[/img] [img]http://mark.rehorst.com/misc/ht_comp_Y/02080007.jpg[/img] I didn't even attempt to print with it or measure the heated bed's flatness. As soon as I put it together I found flaws in the design that allowed too much extraneous movement. In the two pictures below you can see details of the reference point assembly. The movement was occurring where the teflon block was screwed to the side of the aluminum standoff (with a similar problem at the point B assembly. Just pushing and pulling on it caused it to rock. [img]http://mark.rehorst.com/misc/ht_comp_Y/02080001.jpg[/img] [img]http://mark.rehorst.com/misc/ht_comp_Y/02080010.jpg[/img] I decided it wasn't worth testing until I could come up with a more stable design. Back to the drawing board! https://reprap.org/forum/read.php?2,623640,623640#msg-623640 Sun, 14 Jun 2026 21:50:48 -0400 Phorum 5.2.23 https://reprap.org/forum/read.php?2,623640,657592#msg-657592 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,657592#msg-657592
My measurements made with a dial gauge attached to the extruder carriage include any sag that may occur in the X and Y axes and printer frame as well as distortion of the bed surface. It seems that all bed surfaces appear to bow upward. A sagging Y axis would make the bed appear to bow downward. A sagging X axis / frame would make the bed appear to bow upward. So I suspect it's a combo of frame flex, X axis sag, and maybe a little plate distortion due to heating.

It would probably be best to look at the difference between the room temperature and heated measurements to see the effect of heating the bed alone, assuming everything else remains the same.

Whatever the cause, it isn't severe enough to prevent almost edge to edge printing, so for me the matter is closed- I have bigger fish to fry.

Here's a picture of Son of MegaMax heated to 105C (with a few small prints on the bed plate):


]]> the_digital_dentist Developers Wed, 27 Apr 2016 15:31:44 -0400 https://reprap.org/forum/read.php?2,623640,657583#msg-657583 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,657583#msg-657583 sag?

Have seen this in supports for telescope mirrors
Many support points behind a parabolic mirror
in a newtonian configuration much better than just 3
clamps keeping mirror in mount also effect it.
Sag changes with temperature --- some cool mirrors with fan before use
mirror at ambient makes for a good long sharp image session

Perhaps supports somewhat like this:

[attachment 77219 mirror_support.jpg]

:S]]> cozmicray Developers Wed, 27 Apr 2016 15:01:20 -0400 https://reprap.org/forum/read.php?2,623640,657484#msg-657484 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,657484#msg-657484


We also took a picture of Son of MegaMax at 105C, but he hasn't emailed the picture to me yet. I'll post it as soon as I get it.
]]> the_digital_dentist Developers Wed, 27 Apr 2016 11:49:50 -0400 https://reprap.org/forum/read.php?2,623640,635427#msg-635427 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,635427#msg-635427
Quote
JamesK
Wow that's good. A maximum deviation of about .04mm? What's the temperature variation from corner to middle in the high temperature test? It occurred to me that any thermal expansion of the dti probe would add to the measured difference between the hotter centre and presumably cooler corners.

I previously had problems with the measurements while heated- I was getting inconsistent results. I have run the test a few times now and the results are repeatable within +/-10 um which is probably the basic precision of the gauge. Maybe the way it was mounted on the extruder carriage was affecting it the first time. The probe appears to be a pretty high grade stainless steel and the heat doesn't creep very far up during the duration of the test.

I don't know the temperature variation yet. I'll get a picture of the bed with a FLIR camera, possibly later this week. A couple people at the makerspace have them, so I may be able to borrow one.

Quote
Ira
Not a clue if this might help, but if you heated the edges of the plate and let the heat carry to the center would that change the thermal stress enough to matter? If you have more than one heater connected you might try disconnecting one and see if that changes the flatness pattern.

It's just one heater. It may be that just putting a little insulation around the edge of the plate would reduce the heat loos there and provide a more even temperature closer to the edges of the plate.

I'm not sure how much longer I want to pursue this. It's flat enough to print just about edge to edge and I have other things I want to work on, so I'm going to try a couple test prints and see if removing the springs from the leveling screws has returned the print quality to the previous near-perfection. If the quality looks degraded I'll put the undercarriage back into the previous configuration, otherwise I'll leave it as-is.]]> the_digital_dentist Developers Fri, 04 Mar 2016 23:31:43 -0500 https://reprap.org/forum/read.php?2,623640,635407#msg-635407 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,635407#msg-635407 I'm wondering if it could be possible, to run a lead(or ball screw) through dead center and have bed pivot around it for aligning,
building a mechanism, to adjust screw height both ends(including motor, or some other arrangement /flex drive), so it could slope bed,
or just plain 12mm rod throgh center, and have lead screw close to linear block
then if I could support say left of bed with guide block, which maybe needs a pin for bed to rotate,
still not figured out if this rail to would need to slope, not done any cad on it yet, just tossing the idea around.
Might not need bed on springs & screws,
Maybe incorporate these rod ends, or the other male type, a mix of left & right hand threads to help with locking tight(or not).

]]> MechaBits Developers Fri, 04 Mar 2016 21:14:19 -0500 https://reprap.org/forum/read.php?2,623640,635402#msg-635402 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,635402#msg-635402
Ira]]> Ira Developers Fri, 04 Mar 2016 20:31:34 -0500 https://reprap.org/forum/read.php?2,623640,635390#msg-635390 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,635390#msg-635390 JamesK Developers Fri, 04 Mar 2016 18:38:09 -0500 https://reprap.org/forum/read.php?2,623640,635388#msg-635388 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,635388#msg-635388 vimeo.com]]]> the_digital_dentist Developers Fri, 04 Mar 2016 18:26:54 -0500 https://reprap.org/forum/read.php?2,623640,634140#msg-634140 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,634140#msg-634140
I don't have any way to check the bed plate flatness independent of the printer. Measuring the bed plate in the context of the printer mechanism is the best way to see how the plate will perform in a printer.]]> the_digital_dentist Developers Wed, 02 Mar 2016 00:52:56 -0500 https://reprap.org/forum/read.php?2,623640,633845#msg-633845 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,633845#msg-633845
Are you sure, you don't measure flex of the printer as you are measuring from extruder to plate? I would suggest putting the dial fixed to table and then run Y axis and compare it to extruder to plate measurements.
If plate is calibrated for extrusion and it isn't parallel to table, readings would be way off. Also if the plate to table says flat bed, then you know you're having flex in machine.]]> Vuokko Developers Tue, 01 Mar 2016 09:15:43 -0500 https://reprap.org/forum/read.php?2,623640,633808#msg-633808 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,633808#msg-633808 JamesK Developers Tue, 01 Mar 2016 07:48:05 -0500 https://reprap.org/forum/read.php?2,623640,633672#msg-633672 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,633672#msg-633672 The result is about the same as previous results- the corners droop a little, the bed is slightly bowed at the center.

High temperature (105C) flatness test video

I'll make a video of the same test at room temperature and post a link later.]]> the_digital_dentist Developers Mon, 29 Feb 2016 23:18:34 -0500 https://reprap.org/forum/read.php?2,623640,633027#msg-633027 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,633027#msg-633027
Quote
JamesK
It's far from a bad result for a completely new approach - especially one that has removed a whole load of mass from the Y axis.

Thanks, but disappointing just the same. The design is poor from a mechanical engineering point of view- if you think about the stiction of the linear guide bearing blocks, when the reference pushes or pulls the bed, the the other two bearing blocks lag a bit which will probably cause some flex in the bed. Reduced stiction/friction will probably help reduce this problem. The screws and teflon blocks are probably also a little flexible and that stiction/friction in the bearing blocks will cause those other parts to flex too. If the stand-offs weren't so tall the problem would be reduced, but they have to be tall to allow clearance for the drive screw.]]> the_digital_dentist Developers Sun, 28 Feb 2016 13:14:04 -0500 https://reprap.org/forum/read.php?2,623640,633021#msg-633021 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,633021#msg-633021
Quote
amigob
I was wondering why you still use the spring mounts.
If I understand the design correctly, then the system you now have will completely handle the
expansion. So you can make the rest of the mountings 100% rigid.

I like the idea and I think I will also try this on my P3Steel when everything is working.
Directly mount an aluminum heat bed to the linear bearings on one side, little bit bigger distance between the bearings then default. And on the other side use the linear ball bearing guide rail, to handle the X expansion, mounted in the middle of the headbed to 2 linear slide bearings of the Prusa.

I am using the springs because I still need to adjust it. Once everything is working to my satisfaction, I may try it without the springs. If your printer doesn't have a rock solid frame you're going to need the ability to make some adjustments. Springs make that fast and easy.]]> the_digital_dentist Developers Sun, 28 Feb 2016 12:53:39 -0500 https://reprap.org/forum/read.php?2,623640,632994#msg-632994 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,632994#msg-632994 JamesK Developers Sun, 28 Feb 2016 11:45:46 -0500 https://reprap.org/forum/read.php?2,623640,632980#msg-632980 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,632980#msg-632980 If I understand the design correctly, then the system you now have will completely handle the
expansion. So you can make the rest of the mountings 100% rigid.

I like the idea and I think I will also try this on my P3Steel when everything is working.
Directly mount an aluminum heat bed to the linear bearings on one side, little bit bigger distance between the bearings then default. And on the other side use the linear ball bearing guide rail, to handle the X expansion, mounted in the middle of the headbed to 2 linear slide bearings of the Prusa.]]> amigob Developers Sun, 28 Feb 2016 10:57:04 -0500 https://reprap.org/forum/read.php?2,623640,632919#msg-632919 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,632919#msg-632919


It's a little disappointing, but that's what happens when you live on the edge... Note that without a microscope to look at it, the parts look pretty much the same, at least to my getting-old eyes. The part is copy of the limit switch mount I printed when I converted the X axis motor mount to aluminum a couple days ago.

I may try using a larger diameter reference screw and deeper teflon block to screw it into, but I suspect the problem comes from the other end of the Y axis and is mostly a function of the friction/stiction of the preloaded linear guide. More work to come...
]]> the_digital_dentist Developers Sun, 28 Feb 2016 09:08:57 -0500 https://reprap.org/forum/read.php?2,623640,632863#msg-632863 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,632863#msg-632863 the_digital_dentist Developers Sun, 28 Feb 2016 07:07:07 -0500 https://reprap.org/forum/read.php?2,623640,632799#msg-632799 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,632799#msg-632799 Frans@France Developers Sun, 28 Feb 2016 05:05:11 -0500 https://reprap.org/forum/read.php?2,623640,632748#msg-632748 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,632748#msg-632748
Quote
JamesK
The idea of doing away with the carriage once you have a sufficiently stiff bed is brilliant, but, if I understand correctly, all of the bed acceleration now has to be transmitted from the motor to the bed via a single mounting screw - the reference screw in your original description. Have you had to beef up that mounting point to cope with the load?

You are right! The reference screw now moves the entire bed. That has been my greatest concern for doing it this way. I am running the first test print now to see how it goes- so far it looks pretty good. The test part is very small, but tomorrow I'll run something a bit bigger and then make some pictures with a microscope. My biggest concern is Y axis layer registration. If the prints look good, I'll rerun the test with the dial indicator and see if I actually made any improvement. Even if there's no change in bed flatness at temperature, I shaved another 400g of moving mass out of the Y axis.

The linear guides in the Y axis are preloaded which makes friction/stiction pretty high. I have other linear guides that are not preloaded that I may use to replace the current guides so the friction will be much lower.





Quote
MechaBits
the 2 attachment points on the center line, I know the idea is for them to be able to move apart, they are tied with the bed, I wonder if they could be tied to each other, more even distribution of the drive power(just my thoughts, maybe not correct), but put another slide dovetail under the bed between adjuster, like you have on the side.

If you decouple the screw from the undercarriage by adding yet another bearing block, you've got the same situation I have now as far as the single screw pushing/pulling the bed back and forth, only now you're also pushing the mass of the nonfunctional undercarriage.

The test print just finished, I see a slight degradation in print quality that I will examine closer tomorrow. Going to bed. It's late and I can't think straight any more. I'll run more tests and post more pictures tomorrow.
]]> the_digital_dentist Developers Sun, 28 Feb 2016 01:43:43 -0500 https://reprap.org/forum/read.php?2,623640,632712#msg-632712 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,632712#msg-632712 MechaBits Developers Sat, 27 Feb 2016 21:52:33 -0500 https://reprap.org/forum/read.php?2,623640,632698#msg-632698 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,632698#msg-632698 JamesK Developers Sat, 27 Feb 2016 20:25:45 -0500 https://reprap.org/forum/read.php?2,623640,632694#msg-632694 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,632694#msg-632694
I haven't finished the roll screw assembly yet- hopefully later this evening- but I tried moving the bed with it mounted on just the two pitch screws and it seems to work fine. The pitch adjust screw (first picture) is now free to follow expansion of the bed without creating lateral force on the bed plate. When I get the roll screw assembly mounted and can test again we'll see if the bed remains flat at high temperature.




]]> the_digital_dentist Developers Sat, 27 Feb 2016 20:10:30 -0500 https://reprap.org/forum/read.php?2,623640,624814#msg-624814 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,624814#msg-624814
Thanks didn't realize this.
I will put some thin aluminum sheets in between to solve this ( multiple sheets with small gabs in between the sheets ).]]> amigob Developers Thu, 11 Feb 2016 06:00:46 -0500 https://reprap.org/forum/read.php?2,623640,624733#msg-624733 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,624733#msg-624733
I don't know how you're going to anchor those leveling screws, but they need to be rigidly held parallel to the Z axis. If they can tilt or there is any slop in the fit of the screws and their nuts at all, the bed is going to flop around every time it reverses direction. Screws don't normally fit tightly in nuts. You can wiggle them easily. That's why I used the teflon blocks- the screws fit tightly and there's no play (as long as I do a decent job of anchoring the teflon blocks, which I failed to do in this test).

Whatever you do, don't use four supports. It will make leveling the bed without bending it and/or the undercarriage nearly impossible. I know you see that sort of arrangement on a lot of machines, but it is dead wrong.]]> the_digital_dentist Developers Thu, 11 Feb 2016 02:03:02 -0500 https://reprap.org/forum/read.php?2,623640,624717#msg-624717 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,624717#msg-624717
Your first diagram appears to show the support plate is only half supported on rails that are close together,
if the rails where further apart and supported both edges, would that help, or would the expansion cause the same or more issues?
I notice your leveling delta is smaller than using full bed(though your beds bigger),
I suppose if using a larger triangle then the expansion might be harder to compensate for, or maybe just tight things up.

In my first setup I realized I would have to find a way to reduce tilt in Y axis, as direction changed, it was exaggerated in my setup, because I have only a small narrow 25mm bar holding the bed, plus I had one 6mm perspex support attached to a 3mm perspex(while waiting for the heatbed) Plus on top of all that I had an unsecured piece of heavy glass(just for pictures)....the glass was introducing most of the movement/dipping I could see and judder you could feel, so obviously reducing speed & jerks helped a little, but as I will replace soon with cast plate it didn't concern me too much.

Now I have the 3mm heatbed things are much stiffer, I hope when I remove the perspex bed support and replace with cast plate, things will get even stiffer, but still micron movement could still be in there. I will have to decide, do I move from 2 bearings to 3, or 4 One for each corner of cast plate, that should reduce tipping to nothing. I have 500mm rails so have enough space, but if I can get away with 2 great. Ideally the new setup would not have the heatbed supported on springs, as it should be under the cast plate in direct contact(till I find out it's not good enough)

With Mk3 bed under the wider cast plate( about 240mm square), though wonder on the ability of the Mk3 to heat the plate, and it's expansion could cause similar issues, may change to silicone later but then maybe more plate needed to sandwich silicone heater,
but only want pla temps at first so should be fine.

I wanted blind holes in the tooling plate for adjusters to screw into, and adjust from below, but through holes make things easier, you have the through holes on the blocks underneath, I also have that option but with these spigots i'm trying to incorporate, and if I use those cylindrical furniture nut things, the adjustment screw need not be totally vertical.

Waffle Waffle toasty waffle, easier with a picture to show where i'm upto
[attachment 72295 BedClasp.jpg]
Might need another washer or alternate arrangement, suggestions welcome.

If bed is only attached in center(with the leveler i spotted) the expansion wouldn't matter too much, though my bearing arrangement allows them to rotate a little along axis, width expansion might not matter too much, adding more bearings, hopefully they would just move apart, but as you have discovered one attachment point might need to slide or be allowed to rotate a little.
If bed is supported at each corner, it makes it more tricky to add the 3 point leveling without another layer of aluminium, but just using 3 point means the corners of the bed are no longer supported.

[attachment 72297 BedHead.jpg]

I'm hoping to make(piece together) a special stud improve on above, that does away with the spigot, which has to be 1/4" one end, maybe a bit of smooth shaft, but also 6mm thread somewhere for adjustment, other end into the cast plate(none through hole) but stud may need to be telescopic with Knurled Disc Nut but they are a little large, not cheap either.

If somethings on a spring surely there always be some play in the system.
I also wonder how much heat gets carried away through all metal bed link, or melting plastic blocks.]]> MechaBits Developers Thu, 11 Feb 2016 00:30:50 -0500 https://reprap.org/forum/read.php?2,623640,624580#msg-624580 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,624580#msg-624580
The test method is good, the implementation was poor. The plate rocks back and forth on the teflon blocks at the reference and point B because the blocks slide against the aluminum standoffs. The original design with the blocks screwed to the underside of the undercarriage plate prevented such extraneous motion. Once I have redesigned it and fabricated new parts that prevent the undesired motion, I will try again. If it behaves itself when moving the bed back and forth, I'll measure the flatness of the bed again at room temp and hot.

Here is an illustration of the problem that occurs at the reference. The same things happens at point B. I should have been smarter and anticipated it, but I'm getting old and my brain isn't quite what it once was... If the teflon blocks were screwed to the top of the standoffs this motion would not happen.


]]> the_digital_dentist Developers Wed, 10 Feb 2016 14:48:19 -0500 https://reprap.org/forum/read.php?2,623640,624562#msg-624562 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,624562#msg-624562
  • you've performed 2 tests that identify the problem
  • you've established a theory that explains the observed test results
  • what am I missing, where is the part where you modified the experiment to prove/disprove the theory?
]]> plankton Developers Wed, 10 Feb 2016 14:14:28 -0500 https://reprap.org/forum/read.php?2,623640,624549#msg-624549 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,624549#msg-624549 the_digital_dentist Developers Wed, 10 Feb 2016 13:54:30 -0500 https://reprap.org/forum/read.php?2,623640,624546#msg-624546 Re: Heat Compensated Bed Support First Test https://reprap.org/forum/read.php?2,623640,624546#msg-624546 plankton Developers Wed, 10 Feb 2016 13:51:45 -0500