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I've been looking into a Makeblock robotics kit but have found no information on the web that comes from end-users, and one of the main advertised features is not clear to me: The slot threads shown below are straight, while the screw thread that will mate with them is angled. Is there just very little contact between screw thread and rail thread vs. regular screw hole threads? Or would the screw want to rest angled somewhat- and then the head would not be flush with the rim of the rail? Or would the screw deform the aluminum rail if over-torqued?

This is a close up picture of the slot with screws: makeblock closeup

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  • $\begingroup$ These look like quite interesting and adaptable pieces of kit. I particularly like that they are open source (CC-BY-SA 3.0). $\endgroup$
    – Mark Booth
    Feb 12 '13 at 21:54
  • $\begingroup$ Yes I just wish I could find some actual critical reviews of it, there are tons of people talking about the kickstarter for the to-be-made electronics or how interesting it looks, but nobody actually owns a set and is talking about it. $\endgroup$
    – Lucas W
    Feb 13 '13 at 1:46
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It's a clever and attractive design, but it's mechanically weak.

The mating area between the screw and grooves is small, so the pitch angle of the threads isn't as much of a problem. The grooves on opposite sides are offset by half a thread so the screw has less of a tendency to tilt. Mechanical clearance and deformation of the parts is enough to compensate for the remaining geometric mismatch.

The screw will have a strong tendency to strip out the grooves when torqued. This is already a problem with threading into softer materials like aluminum; that's why they invented threaded inserts. It's even worse when only a small fraction of each thread carries any load. The grooves should only be considered suitable for light loading and gentle torquing.

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  • $\begingroup$ Thanks Theran, it's interesting to see this question answered from a general mechanical engineering analysis, since we don't seem to have anyone who has actually used this kit. $\endgroup$
    – Mark Booth
    Apr 11 '13 at 12:36
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It would be interesting if they would have these hard anodized, which would give the surface of the aluminum the same hardness as case-hardened steel. I agree though that the contact area is very small. I do not see the advantage of this over an undercut slot that captures a t-nut underneath, which would still give you the flexibility to tighten anywhere along the length, with full thread engagement, and much higher clamping force.

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