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I'm a non-engineer doing a little design project.

I'm going to be running some M5-ish (have some flexibility) setscrews into a shaft of about 6mm. There's no reason why the shaft can't be flatted off a bit so I'm presuming I'll do that.

There's no motors or electronics involved and the ends of the shafts are getting a pretty minor (I don't have numbers, sorry, I know I'll need them at some point and will need to do testing to get there) amount of force applied from a distance 20-30mm away. The setscrew connection must be highly resistant to slipping or loosening over extended (could be effectively permanent) use but also must be re-settable occasionally. When it does get reset, it could be right in the area of prior settings on the shaft, and it will have to be done with high accuracy (say down to a resolution of .02mm ideally), so I want to avoid marring as much as possible. (If this isn't possible then the whole thing will have to use a pinch bolt instead of a setscrew.) The shaft can be any material, within reason.

The part will have some exposure to vibration but I don't intuit that it will be more than blue loctite will protect against.

How do I do the math to determine what exact materials to use and the torque on the setscrew to avoid the screw loosening, slipping, or significantly marring the surface of the shaft?

Any recommendations for reading on the topic would be great.

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  • $\begingroup$ Use a nylon tipped set screw(McMaster-Carr) with some Loctite threadlocker and just tighten the set screw by hand with your allen key as tight as you can. Done, it doesn't have to be any more complicated than this. Just select the Loctite for the degree of permanence you want. $\endgroup$ – William Hird Jun 5 '17 at 22:30
  • $\begingroup$ @WilliamHird I was thinking about this, and the simplicity is appealing, but I kept reading in various places that the nylon tipped screws weren't a good choice for a permanent/semi-permanent installation. $\endgroup$ – Nathan Knutson Jun 6 '17 at 0:07
  • $\begingroup$ the tip can be any material softer than the shaft , the key is to use Loctite so the screw doesn't come loose. $\endgroup$ – William Hird Jun 6 '17 at 3:07
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While 6mm is at the small end of shaft sizes, why not use a standard keyed shaft and hub? The standard key is 2mmx2mm for a 6mm shaft. For a normal fit, the shaft tolerance should be (-0.004/-0029) and the hub (+/-0.012). The setscrew tightens into the key so damaging the shaft becomes a non-issue. A keyed shaft/hub isn't going to slip unless you shear the entire key and if that happens, you've got much bigger problems. The alignment is repeatable to within the tolerance between the width of the key and the keyslots on the shaft and hub.

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  • $\begingroup$ While not exactly the question I was asking, this is a really good idea. $\endgroup$ – Nathan Knutson Jun 5 '17 at 21:02
  • $\begingroup$ But the question you asked was about how to do something that would make achieving your actual goal a pain in the rear. $\endgroup$ – DLS3141 Jun 5 '17 at 22:33
  • $\begingroup$ That is true enough :) $\endgroup$ – Nathan Knutson Jun 6 '17 at 0:04
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A few thoughts :

  • In terms of protecting the surface finish of the shaft, hardness is the most important property so ideally you want a relatively hard shaft and a relatively soft screw (at least at the point). Of reasonably easily available materials something like silver steel may be a good bet as it has decent hardness even without heat treatment and also comes precision ground and so is good for shafts. With heat treatment you can significantly improve this at the expense of toughness. Possibly the best solution might be to surface harden it eg by case hardening.

  • For the screw itself you really want some thing with a dog point as this should prevent the thread getting damaged and making it difficult to remove. Having the screw in a relatively oft material will obviously help prevent damage to the shaft but conversely will reduce the torque and thus the clamping force which can be applied. One way around this is to use a brass insert between the screw and the shaft, as long as there is enough space. If this is impossible then at least makes sure that the tip of the screw has a good surface finish and is free from burrs etc.

There is no obvious way of actually calculating what you need as these sorts of wear situations really tend to be empirical and there is no simple way of calculating exactly what you need beyond the fact that having the shaft surface as hard as possible will certainty improve matters.

In terms of torque you will probably want to use the maximum torque that the screw will take as the greater the preload in the screw the less the chance of the interface flexing or slipping which is what is most likely to cause damage. This needs to be balanced against the fact that higher strength screws will also be harder, although if you can put something like a brass insert between the screw tip and the shaft this will help a lot.

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  • $\begingroup$ Alright, the brass insert is a good idea that seems like it would obliterate a lot of the challenges here, so I'll look at that. Can you elaborate on dog tip versus flat for this application? $\endgroup$ – Nathan Knutson May 6 '17 at 21:49
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If you have the space why not use a cylindrical clamp which slides over the ends and has its own clamping screws so that the clamping force is on the whole shaft not just provided by one or two setscrews.

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  • $\begingroup$ Yeah, that's what I was trying to get it with the pinch bolt comment. I could do that, but space and complexity are both concerns too. $\endgroup$ – Nathan Knutson May 6 '17 at 19:29

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