Dennis Brown: Top side bed switches - psmay/mpmd-notes GitHub Wiki

Source

The original version of this came from https://www.facebook.com/groups/mpminideltaowners/permalink/2469383709743767/, written by Dennis Brown, as it was 2021-03-31. Refer to that post for updated information and comments. Some reformatting has been applied (its correctness is not guaranteed).

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How to upgrade your MPMD bed switches to make a major improvement in automatic bed leveling and print accuracy.

**** THIS POST IS IN PROGRESS AND BEING EDITED. THE HARDWARE PORTION IS COMPLETE, BUT THE ALIGNMENT INSTRUCTIONS ARE NOT COMPLETE YET. IF YOU WANT TO FOLLOW THIS POST AND BE NOTIFIED WHEN IT IS COMPLETE, TURN ON NOTIFICATIONS FROM THE MENU AT TOP RIGHT. COMMENTS WILL BE TURNED ON AFTER THIS POST IS COMPLETE****

This post is an overview of the problems and solution with links to more detailed posts with the how to instructions. It is a component to "My printer tune up and alignment process":

Dennis Brown: My printer tune up and alignment process and needed upgrades

Problem #1: The current bed rests on three switches that establish a plane that is theoretically square to the three vertical rails (actually the 6 vertical smooth rods). However, those switches are often not very square to the rails. It is a luck of the assembly draw.

This problem creates bed alignment difficulties and causes the prints to print at an angle instead of straight up. For example, my bed is over 1mm out of square across the bed. A 100mm tall part will have shifted its center by 1mm. This is ok for a game piece, but would be terrible for a functional print.

Problem #2: When actually probing the bed for the automatic bed leveling, the bed is not sensed as soon as it is touched. When being probed directly over a switch (like in a G29 P1), the bed must drop down about 0.3mm before the switch closes . Probe any other location and due to mechanical leverage, it is sensed at a different height.

This creates another bed alignment problem, since the firmware assumes that the bed is at the height a switch triggers (plus a constant offset). This means what is sensed is not where the bed is, and a flat bed will actually seem warped.

Solution: The solution to both of these problems is to reverse the way the bed is mounted and how the switches operate. Three screws adjust the bed square to the three rails.

In this upgrade, new springs will be added under the bed which lift the bed up into a solid stop. The stop position is adjustable to make the bed square to the vertical rails. Normally closed switches will be added to the stops so that pushing on the bed will break the circuit and be sensed as touched. A 120mm borosilicate glass bed is added to make the bed stiffer. This will also add more room so the effector plate will not hit the switches when probing near the edge.

It may sound complicated, but half the work is already done if you have installed the binder clip bed hold downs upgrade, which is a pre-requisite for this. Step C2 of the alignment post linked to at the top of this post:

Dennis Brown: Replacement bed clips

This upgrade is broken down into 14 steps. Steps 1-6 marked with * are already done if you obtain an assembled parts kit from To Be Determined. All the required parts for a kit are shown in the photos.

Most steps are in separate posts to make related photos easier to navigate:

1*) 3D Print the spring bending fixture on a working printer.

2*) Bend new bed springs from jumbo paper clips.

Dennis Brown: Top side bed switches: Bending the springs

3*) 3D Print the circuit board holder.

4*) Assemble the inverter circuit board.

5*) Assemble the bed switch wire connectors.

Dennis Brown: Top side bed switches: Assembling the switches

6*) Make up foam pad springs. I bought a roll of 1/16" thick x 0.5" x 36 yards polyethylene mounting foam from Amazon for $11. This is an incredibly useful foam mounting tape for many projects, and it does not deteriorate over time:

https://www.amazon.com/gp/product/B00TZW05P4

We need 1/8" thick foam, so I folded a short piece in half the long way so the sticky sides went together and the release paper was left on both outside surfaces. I cut it into 0.25" width pieces.

  1. Install the Binder clip bed hold down upgrade if not already done. Step C2 of the alignment post:

Dennis Brown: Replacement bed clips

  1. Install the foam pads under the binder clips. Remove the release paper from one side. Stick the sticky side down back far enough so it does not cover the mounting screw head, as shown in the photos. Do not remove the release paper from the top. It stays on to keep from sticking to the binder clip and attracting bits of junk. The foam is a spring that will make sure the binder clip stays up against the adjustment screw.

  2. Install a 120mm x 3mm glass bed upgrade if not already done. This are available for GigDigit and Amazon.

https://gigdigit.com/borosilicate-glass-bed-120mm-x-3mm-mp-mini-delta/?

https://www.amazon.com/120mm-Borosilicate-Glass-Printer-Surface/dp/B075B1R9YZ/

It should be attached to the bed with a 0.5mm Silicon thermal pad applied to the bottom of the glass (no bubbles). I put a bigger piece on, then cut around the glass with a razor blade. If I pull up the glass from the bed, the thermal pad comes with the glass:

https://www.amazon.com/FBApayipa-400mm-Silicone-Thermal-Heatsink/dp/B01N9HGKR1/

Various top surfaces can be applied to the glass. I use a PEI sheet, some others use glue sticks, hairspray, etc. The choices are many, but it should be a flat surface that can be lightly probed with a hot nozzle without being damaged.

  1. Install the springs onto Binder clip bed hold downs. These just snap in from the front. Make sure to orient them as shown.

  2. Replace the short bed switch jumper with the new circuit board jumpers. You have to take off the bottom plate to do this. Don't turn the printer upside down, just lay it on its side, and don't walk away a leave it on its side for a long time, because the glass plate may come off over time.

This step is a bit tedious if you have fat fingers. A crochet hook could help. The Blue arrow in the photo shows where the wires and connectors come out of the channel. The yellow arrows show which connectors they go into (after removing the original jumper between the spots). The connectors do not come out easy. The new ones do not go in easy and have to be pressed pretty hard to get them to seat the first time. I usually have to wiggle them around a bit to make sure they are seated right before pressing them in hard. It is obvious when the finally go in all the way.

You can put the printer bottom plate back on and put the printer back upright again.

  1. Attach the three new switches to the bed. Refer to the first photo.

Clean the bare aluminum plate where the brass pieces go with alcohol. The brass contacts go on first (remove the release paper). Place them at the edge of the plate and parallel to the edge. They go forward until the connector plastic touches the aluminum plate. Press them down well so they stick securely.

The binder clips drop down after all 3 brass contacts are in place and the adjustment screws are put into the clips. Screw them down until there is just a little bit (~0.5mm) of play before they hit the top of the original bed switches under the bed.

  1. Test that the switches are working. Just power up the printer and make sure that when you push slightly near each switch that the light in the "One Button" changes color.

  2. Align the bed square to the rails. I have done this operation using a machinists precision square. However, I made a custom tool that makes it much easier to do this step. Here is the how to post to make your own tool:

Dennis Brown: Bed alignment square tool

Start with all three adjustment screws adjusted to having a very small gap of the bed floating above the original bed switches (~0.5mm). You can just press on the bed near each switch and you can see how far it moves freely before touching the switch.

The adjustment screws take only the smallest fraction of a turn to make a big difference in alignment, so just tweak it a little once you are close.

I slide the square gently down the rails until it just touches the bed on the close or far side of the bed. I have a light sitting behind the bed, so I can see the gap more easily. You can see this in the picture. Use light thumb pressure to keep the square tool against the rails and against one side of the tower to keep it straight up and down.

One of the screws will not be adjusted. The other two will be adjusted higher to square the bed to all three tower rails.

First check all three rails and take note of how far each is from being square. The rail that touches the closest side of the bed and has the largest gap at the other side is the rail that does not get adjusted. All adjustments will be made to the other two rail locations.

On the other two rails, if the closest side touches first, then tighten the screw (lowers the bed). If the far side touches first, then loosen the screw. There will be some interaction between the two rails adjustments, so you will have to go back and forth between them a couple of times.

Most likely the three towers/rails are not perfectly parallel to each other. This means that after you have the two rails aligned square, the third tower/rail that was not adjusted will not be perfectly square. There is not a lot than can be done to correct this without doing some shimming of the top and bottom frame cross bracing. It is not really worth bothering with, unless there is a significant difference. The best bet is to split the difference so all 3 towers are off just a bit in the same direction by the same amount. This will keep the center of the bottom of a print in line with the center of the top of the print. There will be a slight X,Y scale difference from bottom to top though (and always has been unless the rails are perfectly parallel).

Note: When you take the top or bottom plates off, you affect the tower alignments to each other. Ideally, reassemble the frame with a band clamp around it. That way it will always go back together the same way each time is is assembled after doing some repairs. If you don't have a band clamp, just give the printer a bear hug while tightening the screws on the top or bottom plates. Then use the square tool to make sure the the bed is still square to the rails.

  1. Calibrate the bed M665 & M666 parameters.

Images

  1. 47576411_1934464883333245_4163014016492371968_n.jpg https://www.facebook.com/photo/?fbid=1934464876666579&set=pcb.2469383709743767

The finished bed switch upgrade. The top of the glass has a 0.5mm PEI sticker.

You can see the placement of the foam tape under the binder clips.

The debris shield lower right has a notch cut into it for the wires to snake under.

The brass contacts go on first and are at the edge of the plate and parallel to the edge. They go forward until the connector plastic touches the aluminum plate.

The binder clips drop down after all 3 contacts are in place and the adjustment screws are put into the clips.

  1. 47380576_1934465173333216_2146539481044353024_n.jpg https://www.facebook.com/photo?fbid=1934465166666550&set=pcb.2469383709743767

Installed switch. I carved away a little from the bottom of my print shield for the wires to sneak under.

  1. 47482753_1934465306666536_89267321834569728_n.jpg https://www.facebook.com/photo?fbid=1934465299999870&set=pcb.2469383709743767

Installed switch, other view.

  1. 46460702_1907571126022621_5008448482982232064_n.jpg https://www.facebook.com/photo?fbid=1907571122689288&set=pcb.2469383709743767

Bed Springs made from paperclips.

  1. 47463201_1934465989999801_5667066049783136256_n.jpg https://www.facebook.com/photo?fbid=1934465986666468&set=pcb.2469383709743767

What I imagine an assembled switch kit would look like.

  1. 47493477_1934466189999781_6805395875448225792_n.jpg https://www.facebook.com/photo?fbid=1934466183333115&set=pcb.2469383709743767

The parts needed to add the switches to a bed that already has the Binder clip bed hold downs installed. This would also be all the parts laid out from an assembled switch kit.

  1. 47469078_1934477853331948_6636259706849460224_n.jpg https://www.facebook.com/photo?fbid=1934477849998615&set=pcb.2469383709743767

  2. 47680759_1934659043313829_9090689669468782592_n.jpg https://www.facebook.com/photo?fbid=1934659033313830&set=pcb.2469383709743767

Replace the short bed switch jumper with the new circuit board jumpers. This is a bit tedious. The Blue arrow in the photo shows where the wires and connectors come out of the channel. The yellow arrows show which connectors they go into (after removing the original jumper between the spots). The connectors do not come out easy. The new ones do not go in easy and have to be pressed pretty hard to get them to seat the first time. I usually have to wiggle them around a bit to make sure they are seated right before pressing them in hard. It is obvious when the finally go in all the way.

  1. 48392786_1949242641855469_5253360781059162112_n.jpg https://www.facebook.com/photo?fbid=1949242635188803&set=pcb.2469383709743767

Slide the square gently down the rails until it just touches the bed on the close or far side of the bed. I have a light sitting behind the bed, so I can see the gap more easily. I use light thumb pressure to keep the square tool against the rails and against one side of the tower to keep it straight up and down.