Cad questions (Screw hole size)

[qatmix]

Hi Guys,

I am currently designing my first shocktower on a cad package. I will have some holes for std 1/10 screws (m3) do I make them 3.0 mm or 2.9mm. What do others do?

Thanks
C

[scattyv]

I usually draw them at 3mm but if the shock tower is going to be cut from carbon fibre then I find that they need to be cut to 3.2mm or the screws get stuck.

Hope this helps.

cheers
matt

[qatmix]

fantastic, thanks for this, exactly what I was after.:thumbsup:

[RadioRacers]

It depends on what fit you want and what the manufacturers tolerances are.

This link should help quite a lot http://www.draughtsman.co.uk/information_images/mechanical/clearance_holes.htm.
Also depends what level of detail you want to go into, the more details you use on the drawing the better quality part will come out of it at the end, however this will cost more so there's always a balance.

Is your intent to get them manufactured or just designing them as a training exercise? Which CAD package is it? I work with Inventor and AutoCAD so have quite a bit of understanding with those 2.

[LongRat]

I machine a LOT of carbon fibre plate parts, especially shock towers. I produce my screw holes by plunging with solid carbide 3.0mm flat slot drills. M3 screws will always measure up just under 3mm, a 3.00mm hole will be a clearance fit to the thread. If you produce them with a bigger cutter, you might have enough play to cause noticeable left/right imbalances on some car types, and make the car prone to tweaking on impact.
Some manufacturers interpolate round holes using cutters smaller than the final hole size. This is fine but in those situations you may well find the CAD designs show a larger hole than you actually end up with, as interpolating circles in this way can give more push-off or tool deflection, and some machines just aren't able to produce to size within that sort of tolerance.
The other thing I would add is that making these holes with steel drills is not a good idea, they will wear extremely quickly, giving you rough holes, undersize and not clean before burning up completely in a very short time.

[spennyy2k]

Another problem with interpolation/bore milling is unless you have a good cnc you won't end up with a round hole, you always get tight points at N,E,S AND W ( best way I can describe it). Your best as said above to plunge in with a slot drill as the screw should be around 2.9mm anyway.