I've had a couple extruders that the M3x50 is not catching the threads in the back plate of the extruder. Swapping out the toolhead will work. We've also tried swapping the Interface board cover in case there was a thick or warped one, @MikeR tried re-tapping the threads, but these things did nothing. He also took some quick measurements and did not see anything out of the ordinary. So I'm wondering if this could be either a supplier mistake with the threads or just the fact that the screw has such little to grab onto, that maybe a longer screw would be a better fix? This problem is causing to slow things down a bit on the line.
So from looking at a problem plate. the hole locations for the screw and for the tool head alignment feature look to be in the correct locations. And From looking at a plate that does mount vs the one that doesn't, nothing looks to be any different. @tutley any other ideas for me to check?
I think a look at the size of the holes/threads on some of these is worth a look. I also noticed what looks like a slight chamfer on that hole. If the chamfer is made large at that spot it could reduce the number of threads available to grab as well.
This fastener hole is utilized to mount the tool head to the test stand, it is also possible that this could be causing damage to the threads if not done properly. Checking that these aren't being over-torqued during that process would be a good idea as well.
According to the drawing, the hole diameter is 3mm with a .75mm +/-. This means the hole could be anywhere from 2.25mm to 3.75mm and would technically be in spec. After measuring the hole diameter of a back plate that does mount which was ~2.5mm and one that did not mount which was ~2.85mm. I think its possible if the hole is too big, the screw we use will not be long enough to grab the usable threads. I feel we could use a slightly longer screw to prevent this from being a problem. We currently use a M3x50, if we moved to a M3x55 i feel this could resolve the problem.
The ideal drill size for an M3x0.5 thread is 2.5mm. I think we would be in better shape specifying a 2.5mm diameter hole with a 0.25mm tolerance. This would allow the holes to be drilled from 2.25 to 2.75 prior to being tapped.
I just spoke with @MikeR over here at Rockies, and he showed me the problematic part.
In this particular instance, the threads have been stripped out on the side that the screw engages, so the part is damaged beyond usability.
He was going to countersink the locating holes a bit more to see if that would help, but i dont think that will make any difference.
There are just no threads left to engage.
We discussed switching to a longer screw, however the m3x50mm is the longest screw offered in the blue dyed option.
Counter sinking the holes larger did not help with this issue. If we could move to stainless steel screws we would be able to get a longer screw that would engage with the threads more so if the first few threads are stipped we will still be able to screw the back plate onto the carriage. @eBeardslee what are your thoughts on this? I believe we went with blue screws for a marketing/tech support purpose.