Cherry MX mount.

User avatar
Eszett

20 Jul 2015, 17:48

Hi guys! Can someone help me with information please. I need the exact measurements of the Cherry MX mount. Here is what I need:
stem (male) cross, arm width: ... (1mm?)
slot (female) cross, arm width: ... (1.1mm?)
stem (male) cross, width: ... (4mm?)
slot (female) cross, width: ... (4.1mm?)
stem (male) cross height: ... (~5mm?)

What I read in the Wiki is not entirely clear, for example:
Cherry's MX keycap specifications give the keycap slot dimensions as a cross 4.1+0.05 mm wide, with arms 1.17±0.02 mm wide[1] (this would make them 1.465 mm long). There are no known specifications for the switch stem. Signature Plastics gives the dimensions (presumably of the keycap) as a cross 0.159ʺ (4.04 mm) wide, with arms 0.047ʺ (1.19 mm) wide.
What is correct? I need exact specification, so that the male cross fits the female cross perfectly, and doesn't sit too tight or too loose.
Last edited by Eszett on 20 Jul 2015, 19:30, edited 1 time in total.

User avatar
Muirium
µ

20 Jul 2015, 18:15

What are you doing?

Here's a relevant thread. Mr Interface designed mount adapters that work… with some issues. Rsbseb has the measurements and is working on a batch.

http://deskthority.net/vendors-f52/alps ... rs#p243299

User avatar
Eszett

20 Jul 2015, 18:25

Hi Muirium! Im trying to sketch a keycap prototype. Thanks for the hits, I PMed rsbseb in that matter.
Last edited by Eszett on 20 Jul 2015, 18:38, edited 2 times in total.

User avatar
Muirium
µ

20 Jul 2015, 18:29

Ah, he's the right man to ask. He's also working on keycaps of his own.

User avatar
Eszett

20 Jul 2015, 18:38

Alright, Muirium. Update: My neighbour lent me his digital calipers. What I measured is abit uncertain:
about 1.03 mm to 1.09 mm (?) for the stem arm width
about 3.93 mm to 3.98 mm (?) for the stem cross width
The problem is, I can only measure the stem. I can't measure the keycap slot with the calipers I use, they won't dip into this tiny slot. Especially important is the offset between stem and slot. Maybe 0.05mm for each contact?

Since pictures tell more than words:
Zwischenablage01.jpg
Zwischenablage01.jpg (71.63 KiB) Viewed 7465 times

User avatar
vvp

21 Jul 2015, 11:58

I would say that it depends on the process you want to use to make the stems. Ask your stem producer (who know the process he is using) what the difference between switch stem and keycap stem should be for just the right fit.

If you would want to make the stems with a 3dPrinter then you need to make more versions and find out which is the best. Different 3dPrinters will have different tolerances. Here is an example of a stem for an FFF 3dPrinter: FreeCAD Document.

andrewjoy

21 Jul 2015, 13:12

I always wanted to 3d print an IBM cap to fit over and existing slider had a google but could not find one

User avatar
vvp

21 Jul 2015, 13:52

I do not know. I do not have the an IBM cap nor the slider to try it.
But it is not hard. The cap I posted is probably about 4. iteration. I guess it took me about 4 hours to get it right on my printer. If you have a 3d printer and the IBM cap does not have too thin wall or too thin pillars then you can do it ... in the worst case in about a day. If you do not have a 3d printer but have a lot of money then you can get it right through Shapeways after a few iterations too.

Here is the picture of the cap in the FreeCAD document:
http://deskthority.net/workshop-f7/katy ... ml#p189679

User avatar
Eszett

21 Jul 2015, 14:04

@vvp Hi! What are your measurements? I can't open FreeCAD files, but I may install FreeCAD later just for the sake ... Yes, the resulting measurements also depend on production process, material. However, I'd like to avoid going trial and error, with 3dprinting alot of keycaps, just to realize, that they don't work and throw them into garbage. I prefer to design on exact specifications to avoid as much mistakes to reduce prototyping to the minimum. BTW, would you like to help me with prototyping, ie. 3dprinting some caps?

User avatar
vvp

21 Jul 2015, 18:07

Sorry, no time, nor interest in keycaps. You can get them for 10-20 ¢ a piece from 7bit (that is if you do not want something fancy but the most simple ones). I did it only as a test whether it can be done at all. FFF printers are not very reliable for small features. Especially the ones which use bowden extruder (like the one next to me :) ). If you need reliable small features then you need an SLS or an SLA 3d Printer. These printers will need the dimensions tweaked differently.

The dimensions are attached. These are suitable for RepRap 3d Printer (i.e. simple FFF/FDM) with Ø 0.5 mm nozzle.
stem.png
stem.png (25.49 KiB) Viewed 7396 times

User avatar
rsbseb
-Horned Rabbit-

21 Jul 2015, 18:27

Eszett wrote: Hi Muirium! Im trying to sketch a keycap prototype. Thanks for the hits, I PMed rsbseb in that matter.
Sorry I didn't see the pm on geekhack. I've had some health issues lately and have not spent as much time on line the last couple of weeks. I'm not sure what the factory design specs are because there is a variation from one finished component to another. The way you need to design your parts is going to vary from one material or manufacturing method to another. Environmental conditions such as temp and humidity and length of processing time may also impact the finished product. If you haven't done one before you will probably need to experiment a little before getting it right. This is the issue I am dealing with on the adapters right right now. Hopefully you have a handful of samples to work with for measuring and testing against.

If I can be of more specific help let me know and I'll try to do my best to be of assistance

User avatar
Eszett

22 Jul 2015, 18:47

@vvp I will order printing from a 3d printing service, so I don't know much about different printer models. The dimensions you give differ abit from the other ones I've heard so far. Maybe due to the 3dprinter you use, which adds a little more plastic, than designed?

Signature Plastics says: 4mm * 4mm * 1mm (for the female "cross")
BlueNalgene says: 4.1mm * 4.1mm * 1.35mm (for the female "cross")
You say: 4.3mm * 4.3mm * 1.3mm (for the female "cross")



@rsbseb Yes, I PMed you.

User avatar
vvp

22 Jul 2015, 21:04

Yes, FFF 3d printers add about 0.1 mm to each wall. It is because the wall is rough. It is not a perfect plane. But the plastic is extruded precisely as if the wall would be perfect. When measuring you will measure on the top of the bumps. So a part looks (and even behaves) a tiny bit bigger than the model. Most 3dPrinting services use SLS 3d printers. Those should not not produce parts as much bigger as FFF 3d printers. I do not have experience with SLS or SLA 3d printers so I cannot give you estimates how part sizes must be altered so that the result is right.

Edit: Also FFF printed holes tend to close a bit as cooling plastic contracts. The smaller they are the more they are closing.

User avatar
Eszett

23 Jul 2015, 00:04

@vvp Alright, I will take this into account. When printing via FFF, cutouts must be slightly (~0.1mm) scaled up, while solids should be slightly scaled down. Thanks for the advice!

User avatar
Eszett

27 Jul 2015, 23:59

Little update. I still don't know what measures of the keycap will work for me, but what I got to know so far is the measures of the Cherry MX switch stems (the male cross): 4mm x 4mm, and the "arms" from left to right are 1.2mm thick, and the armes from front to back are 1mm thick. Note: you can't mount a keycap which fits precisely to this cross in any other direction than designated, because the arms from left to right are thicker than those from front to back. That's probably the reason why "the barrel" was invented, which is a stem counterpart (female cross) with uniform measures in all directions. My Filco ABS keycaps have this round barrel which holds the female cross. DSA keycaps from SP have it too. Here a illustration of what I want to say:
Zwischenablage.jpg
Zwischenablage.jpg (227.02 KiB) Viewed 7255 times

User avatar
vvp

28 Jul 2015, 02:30

My guess would be that the "barrel" is used more to save the plastics than to allow mounting of the cap rotated by 90°.
Some keycaps do not even have a barrel, only a thin shell around the cross. This way they save even more plastics.
My model uses as much material as will fit there because it is hard to 3D print too thin structures using FFF/FDM method. If you want to design for FFF/FDM then use as much material as you can fit around the cross.

User avatar
Eszett

28 Jul 2015, 04:43

vvp wrote: If you want to design for FFF/FDM then use as much material as you can fit around the cross.
Good point. I will follow that.

User avatar
Eszett

29 Jul 2015, 02:21

My first FDM-3dprinting test run turned out to be total crap. Unuseable. The female stem became much too small (~3mm) and inaccurate like a amorphic hole. I guess I have to scale the 3d models female stem up by 1mm (e.g. 5.3mm wide), just that it turns out right (4.3mm). Could be still unuseable because the FDM printing technology seems to be too inaccurate for such tiny items. (?)

User avatar
vvp

29 Jul 2015, 11:01

Yes, FFF/FDM is not good for too small objects like the stem. But it is doable as my green FFF printed keycap shows. The keycap was done by printing the provided CAD file. The rough setup description:
  • slicer: Cura
  • material: ABS
  • Ø 0.5 mm nozzle
  • 0.2 mm layer height
  • minimum layer time was probably about 15 - 20 seconds
  • no heat chamber
  • fan was on
  • brim was on
  • the keycap was printed up-side up
For FFF/FDM printers, the problem are areas of the object where one isle of a horizontal cut of the object has area smaller than 12 mm² (e.g. thin pillars). There are two reasons for this:
  • if the pillar is tall then it will bent under the sideways friction force of print head printing the top of the pillar
  • when entering/exiting the pillar, the print head must extrude/retract the filament and this cannot be done very precisely (especially true for printers with bowden extruder) => there will be errors in the amount of extruded plastic => these errors are more damaging when they cannot even out over bigger area
Shrinkage of 1 mm on 4 mm stem is way too much. As I said, the errors are in the range of 0.1 mm maybe 0.2 mm at most. The exceptions (i.e. locations where errors may be bigger) are:
  • outer corners - e.g. if your nozzle has Ø 0.5 mm then the filet radius of corners will be at least 0.25 mm (it will be slightly bigger due to surface tension, shrinkage, over-extrusion)
  • small holes - due to surface tension, shrinking and over-extrusion small holes are closing more than by 0.1 mm; this starts to be really noticeable when holes are smaller than 3 mm; the smaller they are the worse it is
  • bridges and stuff printed on support - due to gravity the filament hangs down a bit; you can expect about 0.3 - 0.5 mm here; try to design your part so that anything printed on support is not visible in the final part and does not need to be precise
There must be problem with the printer settings or it is a very crappy printer when you are getting 1 mm error on 4 mm ojbect. My guess is that the printer did run too hot (maybe in a heated chamber) or too quick and probably without a fan activated. The picture may (or may not :) ) help to identify what is wrong.

Edit: SLS and especially SLA 3d Printers will do much better for small stuff. But they are not common among people. But 3d Printing services would probably use an SLS printer by default (e.g. Shapeways uses SLS as the default method).

User avatar
Eszett

29 Jul 2015, 19:47

IMG_0129.JPG
IMG_0129.JPG (208.58 KiB) Viewed 7176 times
Hi vvp! Here is a photo. "1mm" shrinkage was just my guess, but the keycap stem (female cross) turned out way too small. In my 3d model the female cross has the measure 4.15mm wide and each arm 1.25mm thick. See on the photo how small it is! The other issue is that the female cross is filled with supporting substance, and I can't remove it. The keycaps were printed upside up. Maybe I should try to print them upside down to resolve this issue?

Would you mind to show me the 3d model of your "green keycap"?

User avatar
vvp

30 Jul 2015, 09:42

The picture of green keycap is in this post:
http://deskthority.net/workshop-f7/katy ... ml#p189679

The printer looks like it produces a good result for such a thin stem. Looks like the extruder is not using bowden (or it is very well calibrated).

Check with the producer whether the support material can be dissolved in something which would not dissolve the keycap material itself. E.g. there are materials for FFF/FDM printers which can be dissolved in water; these are often selected for support. It is possible that the bottom layer is over-extruded so that it sticks to the bed better. If so then this would close the bottom of the cross more, but the rest (higher up the stem) may be fine.

If possible then dissolve the support material from the stem and try to cut out the proper cross from the bottom layer. See if the rest of them are OK.

Ask them to print the keycap without support material. It should go fine. The bridges will be short. The printer should not have a problem making them. From the picture, it looks like even the outer stem diameter is smaller than it should be. That is not good. I do not know why it is so. Such a thing never happened to me. You may be forced to make the stem shape as I did it in my model (i.e. you would not be able to rotate the keycap by 90°).

You can try to print it upside down but I doubt it helps much.

User avatar
suka
frobiac

30 Jul 2015, 11:25

Just for reference here are some crappy shots of several MX mount designs I employed in the past, printed with SLS:
Various MX caps
Various MX caps
caps_various.jpg (169.28 KiB) Viewed 7141 times
Maybe one of the simpler or open geometries that could be more easily reworked afterwards would allow to also print them on less exact machines?

But even with SLS one does benefit from some scaling experiments prior to finalizing production :D :
Empirical size for the stems
Empirical size for the stems
stem_various.jpg (137.48 KiB) Viewed 7141 times

User avatar
Eszett

30 Jul 2015, 19:58

@vvp Yes, I will ask them for printing without supporting material. And print upside down. I would like to change over to SLS/SLA, but it is more expensive, and I don't like 50+€ costs just for prototyping :-(

@suka Good idea to print a series of differently sized stems. And I like this "open geometry cross",
Zwischenablage01.jpg
Zwischenablage01.jpg (62.06 KiB) Viewed 7098 times
since I see some advantages: 1) it makes it easy to scratch supporting material out 2) it gets rid of the thin walls (more stable) 3) and it is easier to rework. I doubt that I can rework ABS though(?)

User avatar
vvp

31 Jul 2015, 12:19

Note that open geometry means smaller islands (separated 2D areas) in the horizontal cut of the keycap. Something which was one island with a hole will become two separate islands. This means there will be a retraction when moving form one island to the other one. Though one can suppress the retract in the slicer and clean up strings later on - but this will lead to errors in the filament amount deposition.

Retractions mean errors in the amount of the material deposited especially with the bowden extruders. This problem is about 8x smaller with direct extruders. From your picture, I would tell that your service uses a direct extruder.

Thinner pillars (smaller islands) also mean that they will bend more easily due to friction with the moving print head. So if the pillars will have wavely surface then they are too thin for the printer used.

Try it out and you will see whether the printer can handle it. This is just to get you prepared if the result from an FDM/FFF printer will be bad.

What do you mean by "reworking ABS"?

User avatar
Eszett

31 Jul 2015, 18:23

vvp: the FDM Printer my provider uses is a HP Designjet 3D.

With "reworking ABS" I mean scratch and cut it with a knife.
Last edited by Eszett on 31 Jul 2015, 21:09, edited 2 times in total.

User avatar
vvp

31 Jul 2015, 20:57

Ok, that printer does not use bowden extruder so it should be able to print also quite small areas. Probably only few mm^2 is possible. Moreover it is dual extruder so even tall thin objects can be printed by surrounding them in a support material which will provide the needed rigidity.

The support material should be dissoluble in soapy water. Try it out. Just put the cap into warm soapy water. If it does not go well and if you have a ultrasound cleaner then you may try to do it inside it. No clue what kind of soap to use. Try more of them if your 3D support provider does not specify it.

Yes, you can carve ABS. They claim the support material is sharp. I guess they mean brittle. If so use glasses when removing it manually.

User avatar
Eszett

01 Aug 2015, 01:55

Hi vvp! I've tried to break the stems (which has thin walls) of the keycaps, but they are quite firm, so up to now I don't have any issues with stability.

Yes, the engineer of my provider (fabberhouse.de) said to dissolve their supporting material, an alkaline solution is needed with a PH value of at least 10, and they use one with a value of 13. Additionally the alkaline solution has to be heated up to 70° celsius, and stirred constantly. Soap has an PH value of 9-10, so I will try to follow your idea, and heat the keycaps in soapy water up to 70°.

User avatar
Eszett

08 Aug 2015, 06:10

My next batch of mounts were all too wide. I can try again and again, my guts say, it makes no sense to find the perfect dimensions, since this 3d printer (this 3d print service) is too inaccurate for mounts. Even when I find the perfect dimensions, the printer will print deviating results, some caps will fit, some won't. So my idea is apply a different strategy: to shrink my a little bit too loose mounts. Some recommended cling film, but is there a more proper way to do it? Maybe nail polish?

User avatar
vvp

08 Aug 2015, 11:48

Interesting. I would tell the HP 3dPrinter should have better repeat-ability.

I have a simple home made RepRap (a Rostock variant) and the repeat-ability is not bad. I printed as a test about 12 keycaps (3x4 grid) and two of them needed to be discarded right away and one of them did not come out as well as the rest.

Sorry. No good idea how to add material to the inside of the keycap stem. Acetone (i.e. nail polish) will melt ABS very well. I use it to paint some of my models (or only parts of them when I'm lazy) to improve adhesion and smooth the surface. I'm not sure whether it will lead to a slight size increase or decrease. (Smoothing would indicate smaller bumps and therefore smalor size on the top of the bumps but maybe acetone leads to ABS swelling a bit.) You could paint with a thin ABS melted in acetone. That would add material but inside the stem the surface tension will force most of the added material to the concave corners. So it will not work well.

SLS and especially SLA should work much better for this but they are typically more expensive. That is a a pity.

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