Tuesday, September 29, 2015

Not Everyone Uses CNC

I am building a part that replaces the bottom of the tripod on two Vixen telescope tripods: the HAL-110 and HAL-130.  They are the same shape internally, but the screw holes are slightly differently spaced.

I have historically drilled and tapped the holes in my part by using the legs as patterns.  For the CNC project, best to get the locations exactly and use the mill to drill pilot holes.  But it the spacing is different, and there is no symmetry at least at  the .001" scale.  My guess is that some badly paid Chinese worker has a pattern for the holes for each leg.  I just hope that he doesn't damage it, because then I will have to revise my program.


Will said...

Are you saying that the hole locations are not equidistant to each other, and/or to the leg opening? In other words, consistent from leg to leg, just not patterned in a recognizable measurement system? You did check in metric, yes?

Those look like cast end pieces. Does it look like the screw holes in those tips are part of the casting? Are they using self-tapping screws into blank holes, or does it look like the holes are cast, but then tapped after?

If those tips are cast with the holes, then you can be fairly confidant that the leg holes will not change in the future, as a mold for casting parts is very expensive, and not changed without good reason.

If this becomes a problem, you might consider redesigning your part with internal slots for floating nuts, that would accommodate offset screw holes within a moderate range of travel. This would require some additional machining steps, possibly T-slots, and the purchase of over-sized or flanged nuts. Maybe just counterbored holes from the backside would work.

Anyway, you have options, if it becomes necessary. Shouldn't become a big or expensive problem, though.

Clayton Cramer said...

I mean there are no straight lines that pass through holes in rough alignment with each other. These may be casting, but they go into an aluiminum casting which may be drillede and tapped after insertion into the leg.

Will said...

Those appear to be flathead screws, so it is doubtful they are drilling the casting with the actual leg as a drill guide. That leads to random accuracy, which translates to screw heads sitting high from not fitting into the taper, or broken screws from trying to draw them down to flush.
If it's not done with a cnc setup for repeatability, it would have to be a substantial drill guide to maintain accuracy over time, for production.

It's remotely possible that they are drilling the leg and the foot together, but this is unlikely, as the leg and casting requires anodizing afterward. If you are seeing bare aluminum there, maybe.