I've been using FSRs for probing for coming up on 3 years. They are 100% reliable, very precise and result in excellent calibration results. I have them on a Taz 4 Cartesian printer, 3 mini Kossels, 2 Kossel 250s, a V1 Rostock MAX, a D300VS delta, my Max Metal delta several other experimental printers and now, my V3. I originally designed this system for the Max Metal but it worked so well that I used it on my V3 and I'm going to retrofit my other deltas with it over the coming year. There are 2 significant aspects to this new mount.
1. Postion the FSRs outside the build plate perimeter.
The "magic" is the red part. The grey part is the Onyx/glass/PEI. As you can see, the FSRs are positioned out away from the print perimeter. This minimizes the "teeter totter" effect when probing 1/2 way between FSRs. You can see it here:
Moving the probing point out significantly decreases this effect. Here is what it looks like on the Max Metal.
And here it is on my V3:
2. Don't overly constrain the contact point on the FSRs. This is one of the two reasons folks have had problems with FSRs (the other is that their bed is not stiff enough - this mostly applies to the Kossel Krowd™ that simply used glass with a Kaptan heater). The plunger system that I originally designed and used works reliably but only if you use high quality prints and prepare and lubricate them properly. Otherwise, they can bind ever so slightly leading to inconsistent triggering. The good news is, this issue is easy to identify and correct.
So, with this new system I simply adhere the FSRs and rubber plungers with sticky tape as you can see here:
The FSR has a sticky back, just peal off the protective film and stick it to the top surface of the printer. The plungers that come with the Ultibots FSR kit also have a sticky back, so peal that and stick it to the top of the FSR sensor itself. Now you put a small dab of silicone seal on the top of the 3 rubber plungers (a SMALL dab) and press the FSR Plate onto them and allow to cure.
I actually don't bother with the silicone, I just have my plate resting on top of the plungers. I've never had an issue with the plate moving. I am working on a simple printable locator that will attach to the top printer plate that will keep the bed from shifting but since this has been working so well, I have not bothered.
Finally, you can see in the last drawing above a recent modification - the FSR Plate now has three ears. In my V3 photo I'm using big binding clips. I needed to add 3 additional rubber plungers under the FSRs to make room for those. But, by tweaking the FSR Plate with the ears I can remove those and use the ears to attach the Onyx/glass with the little blue (or other) holddowns. I do one other thing on my printer, I adhere the Onyx to the top of the FSR Plate with a disk of the 3M 468MP tape we use to attach PEI. This holds the Onyx perfectly flat while allowing it to expand and contract in the X-Y plane without buckling. It works quite well.
