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I have a product where we transmit quite a lot of shear force through a small bolted join. On our current design the contact area is about 10mm², the bolt is M2, and the force is 45N, and it works quite well. But now we want more, the requirements are:

  • We want to go up to 300N, in either direction along one axis.
  • No sliding between the parts, even at the micron level. Elastic deformation of a few microns is OK though.
  • Both parts are grade 4 (pure) titanium.
  • The top of the lower part must be flat, nothing sticking up when the second part is removed. Shallow holes are OK.
  • Ideally, the position of the top part in the direction of the force should be adjustable. Preferably continuously, but failing that then in about 0.5mm steps.
  • The join can be made and un-made about 100-1000 times over the life of the product. By a competent and careful operator.

As is, at 300N the joint as is slides around all over the place. I could go up to a M2.5 machine screw, or maybe M3 at a pinch, but some back-of-the-envelope scribbling suggests that won't be enough. I could specify a removable epoxy, but can't find one strong enough. So I thought about mechanically keying the two parts so that they cannot slide.

I could cut both surfaces with a zig-zag profile, like a screw thread. In fact I guess it might be possible for a machine shop to do that with an M3 thread cutting mill (if I pick the right profile) or the corner of an end mill (for a 45 degree profile).

I've not seen such a thing before, at least not at this scale. Does it sound like a good plan? What should I watch out for? Is there an easier option I have missed?

Here's a rough sketch of what I mean: enter image description here Left: as we have it now, for 45N. Right: What I mean by zig-zag profile. I've sketched it as if it were an M3 that's been "unrolled" onto the yellow part.

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    $\begingroup$ Can you post a diagram? Could you have a "lap" joint with a pin and a setscrew as well? $\endgroup$ – Solar Mike Nov 1 '17 at 12:43
  • $\begingroup$ A thousand cycles? There is no such thing as a "competent and careful" operator who will perform this operation even a hundred times "carefully" . You'd better consider this pin/bolt a consumable. $\endgroup$ – Carl Witthoft Nov 1 '17 at 15:14
  • $\begingroup$ @CarlWitthoft The parts are not consumables, but are replaceable. If they only last 50 cycles, we'll throw in a bag of 10 with each product, and the customer can buy more. $\endgroup$ – Jack B Nov 1 '17 at 15:29
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    $\begingroup$ If the angular alignment is not critical, a concentric ring pattern for the gripping surface would be simpler to manufacture at this scale, either by stamping or using a profiled end mill. Also take a look at ARRI Rosette mounts used in camera rigs. $\endgroup$ – Donald Gibson Nov 1 '17 at 18:50
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    $\begingroup$ Put the word "threads" out of your mind for this task. Thread forming is an art in itself and doesn't apply here. You just need to form some kind of grooves or a maybe pocket for the purposes of linear load bearing. $\endgroup$ – Wossname Nov 2 '17 at 9:54
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A "Chamfer End Mill" tool can be used to cut a V groove into a surface.

enter image description here

Make sure to specify a tool that is capable of cutting material at its point as well as along its radius. (Although arguably, the groove doesn't need a sharp point, trapezoidal grooves would work just as well).

Choose your "feeds and speeds" carefully. Titanium isn't good at dissipating heat, so you want to make sure you don't overheat the tool or the workpiece.

I suggest you use a carbide (or better) chamfer end mill, in a very rigid machine with low RPM and feed rate, and plenty of flood cooling. Consult with your machinist and do some experimental cuts on scrap stock first.

Since you're looking for very small feature sizes, you might have good results using an engraving bit, although this would need to be carefully tested to avoid breaking the delicate tool.

You should be able to get good surface finish which will ensure good contact between the metal parts.

Alternatively, you could simply mill a pocket into the yellow part for the blue part to drop down into. That would only require milling to be done on the yellow part and you wouldn't need the grooves at all.

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  • $\begingroup$ My concern with these was that they might not get a sharp enough bottom to the groove. For 0.5mm pitch 60 degree grooves, I think a 0.05mm round is about the limit. While a trapezoidal groove does work, it means a lower contact area. The grooves also need proper tolerance control - if the points touch before the diagonal faces do, then its not going to work. I'll have a look at them though. For milling a pocket, wouldn't that need improbably tight tolerances to work with bidirectional forces? $\endgroup$ – Jack B Nov 2 '17 at 13:22
  • $\begingroup$ @JackB, A pocket with chamfered sides would be self-centering. $\endgroup$ – Wossname Nov 2 '17 at 13:34
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It might be easier to add a part that has teeth machined on to both sides of it from a hard material that under the clamping pressure of the bolt will dig the teeth into the titanium parts.

This might also be cheaper than trying to make properly toleranced grooves on both parts.

The solution I'm proposing is from a type of washer that digs into the material as you tighten the bolt.

enter image description here enter image description here

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  • $\begingroup$ Welcome to Engineering Stack Exchange! I get where you are going with this, but it could use some cleanup to conform to the standards of a good answer on this site. The initial question is a good solution, and should be treated as it's own separate answer, so eliminate that question and form it as a new answer. The second part is good, but perhaps a diagram would be useful, or more description of the type of washer and perhaps reference to other places it has worked before so the poster can go and do more work. Thanks for your answer! $\endgroup$ – Mark Jul 26 '18 at 14:28
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You can also try adding a second bolt next to the first one if possible to increase the clamping force if possible, this will be simpler than what I propose in my other answer. If not possible, changing the grade of the bolt to a higher one to provide higher strength might work.

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