Thursday, February 11, 2016

Printable Mounts for FSRs

I've installed FSR (force sensitive resistors) on all of my printers now. I developed a universal mounting system that I've used on Rostock Max, Mini Kossel, Kossel 250 and Taz 4 or 5 Cartesian printers. I print these in PLA but if you use a heated bed - and especially if you heat the bed to more than 60°C - you need to make sure the mounts are well insulated from the heater. The Rostock Max and Orion printers have a nice solution - a 1/4" thick melamine "snowflake" that the heater and glass mount to. This provides excellent insulation AND stiffness to get reliable triggering of the FSRs. I hypothesize that some of the issues I've read on the Web about unreliability of FSRs are due to two issues:

  1. The mounting system is over constrained or not assembled correctly and binds
  2. The glass build plate and flexible (silicone, Kapton) heater attached at three spots around the perimeter is too flexible so probes over the FSRs give different results than probe points between them (where the bed is suspended between the mounting points)
Make sure to address these, along with insulating the mounts, and you should expect good results.

The printable mounts and instructions can be downloaded from Thingiverse: http://www.thingiverse.com/thing:1335095


4 comments:

  1. Hey Mike,
    Thanks for uploading these. I was wondering if you had any pictures of the actual mounting in one of your deltas. Things like do you position them in the 3 axes, or do you place them opposite the towers? Is the bed only attached to the rest of the printer via the 3 silicon pads?

    Thanks!

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  2. That's a great question!

    On my Mini Kossel and Kossel 250 printers I position these on the base horizontals in the middle - so they are on the opposite side of a tower. Here is a photo of an earlier version of the mounts on a Mini Kossel so you can get the idea: http://mhackney.zenfolio.com/p696832041/hfde2809#hfde2809

    On my Rostock Max I used existing holes in the melamine snowflake insulator. These are actually 30° from the towers, here's a photo: http://mhackney.zenfolio.com/p694116148/h23399ca8#h23399ca8

    If things are setup well, it shouldn't make any difference where the FSRs are located. Although I do prefer to have a probe point right on the FSRs. On the Rostock Max, I do not but it seems to calibrate better than the other machines, so go figure!

    The bed must be constrained so it can not move in the X-Y plane. It isn't sufficient to just sit it on the silicone pads. On the Rostock, the plunger part is screwed to the bottom of the melamine snowflake so the bed can not move.

    On the Kossel 250 I use a little clip to position the glass. It also keeps the bed from lifting (note that the little tongue int he photo does not touch the top of the bed): http://mhackney.zenfolio.com/img/s6/v142/p1644964468-4.jpg

    I have not published the version with the little clip yet, that's coming soon.

    On a new K250 I'm experimenting with a dab of silicone to hold the bed on! I think it will work great as I did that on my TAZ 4 printer and it has a moving Y. Basically, adhere the FSR to the printed mount. Adhere the silicone pad to the FSR (there is adhesive on the UltiBot pads) and then put a spot of silicone on top of the pads and lower your bed assembly on them. Press a bit to seat and let cure. The only drawback is that it means re-siliconing if you need to remove the bed for some reason. I've thought about using a locator pin arrangement so the bed could separate easily but so far this has not been an issue.

    ReplyDelete
    Replies
    1. Thanks a bunch MHackney! Mounting my bed on the FSR's now!

      Delete
  3. Ok, I've uploaded the alternative mount and hold down for the Kossel 250 and the photo I linked above.

    ReplyDelete