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Tightening to 4.5Nm ( or 4Nm... ) strips head of bolts.  

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mderouss
(@mderouss)
New Member
Tightening to 4.5Nm ( or 4Nm... ) strips head of bolts.

Hi there, I'm assembling a Mk 3, and attempting to 'do it right', so I've been trying to tighten the Y axis bolts to the recommended 4.5Nm torque.

This simply doesn't work. Some bolts can be successfully tightened, but in doing so the hex driver sticks in the bolt because it has begun to strip the head. In other cases, the torque setting cannot be reached because the bolt head is stripped first. I've tried this with more than one 3mm hex bit, and also at 4.0Nm torque, with the same problem.

The root cause of this problem is that the hex slot in the bolt head is too large. The measured width of the hex bits is 2.93 - 2.97mm, which is reasonably accurate, but because the hex slot in the bolt head is significantly wider, there is space for the corners of the bit to rotate and jam/strip the bolt head.

So I strongly recommend that Prusa stop recommending a torque setting until they can supply bolts that can take it using commercially available hex bits. And for people assembling their printer - forget about trying to follow the recommended torque values.

Cheers,
Mark

Posted : 13/12/2018 2:13 pm
randolph.l
(@randolph-l)
Honorable Member
Re: Tightening to 4.5Nm ( or 4Nm... ) strips head of bolts.

i found 10 inch pounds work very well for the 3mm fasteners into metal (either into the frame or into nuts) the 5 mm dome head as they are so mucjh larger I don't believe I used the torque driver on them but as I used blue locktite on them they seem secure.... if you have done up enough fasteners in a life as long as mine you do get a feel for when you are turning a bolt too hard for a given material.
I have found the heat treatment on many allen type keys that are included for free with things like Ikea furniture are to put it kindly are suspect and the hardness of the hex bits as supplied in various sets is frequently even worse... I picked up a cheap set of about 60 bits of every imaginable type at a flea market and the were fully soft as if not hardened at all... I used my own heat treating oven and tried bot water hardening and Oil hardening and I could not get them better than about a rockwell in the high 30s... Mild steel indeed so you get what you pay for...there were a few high security drivers for special head designs tat I wanted in the end I machined the ones I needed out of stress proof using the soft cheap ones as models... many high security head designs are not even in my machinery's handbook. I favor the Wiha brand
for full size allen keys Bondhus ball end are also good unless you get stuck with a counterfeit set( they are out there and I doubt a shop will look kindly on you breaking out a set of hardness testing files as you shop.)

Posted : 14/12/2018 6:09 am
thrawn86
(@thrawn86)
Honorable Member
Re: Tightening to 4.5Nm ( or 4Nm... ) strips head of bolts.


So I strongly recommend that Prusa stop recommending a torque setting until they can supply bolts that can take it using commercially available hex bits. And for people assembling their printer - forget about trying to follow the recommended torque values.

Cheers,
Mark

I've broken and/or stripped a number of them as well especially on frame parts, the fasteners are all cheese-grade. Since then I've bought a lot of higher quality hardened steel fasteners from bolt depot.

Posted : 14/12/2018 8:16 pm
Cowasaki
(@cowasaki)
Active Member
Re: Tightening to 4.5Nm ( or 4Nm... ) strips head of bolts.

What are you using to measure 4.5Nm? Its below most torque wrenches and quite a few mechanical wrenchs are inaccurate at such low torque levels.

Posted : 15/12/2018 1:45 am
randolph.l
(@randolph-l)
Honorable Member
Re: Tightening to 4.5Nm ( or 4Nm... ) strips head of bolts.

I used a Wheeler gunsmiths torque driver its scales go from 10 inch pounds (0.8 NM) to 60 inch pounds and came with an actual calibration certificate that was dated the year I bought It for doing the screws on gun scope mounts I have found it to be very accurate.

Posted : 15/12/2018 4:37 am
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