Problem with cylinder and rod

I recently created a design where a 1/8” cylinder pierced a wall where a 1/8” brass rod was to be inserted.

I had a test piece 3D printed by Shapeways and the rod does not fit. What did I do wrong?

If you can share the skp file here, somebody can take a closer look.

How bad is the misfit? If you modeled both parts at exactly 1/8 you may be suffering from shrinkage of the print causing there to be no clearance. Even a tight fit actually needs a bit of clearance or it won’t pass through.

It sounds like you assumed the printing media was dimensionally stable and the hole wouldn’t change size as the material cooled.

Probably best to make sure the hole prints undersized and then drill it out to the desired sized.

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Also, SketchUp does not use true curves and circles. It uses a series if line segments to approximate curves. So if you used an actual true cylindrical brass rod in a SketchUp hole made of a series of flat segments, then there’d be interference at the center of each flat segment.

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An inherent problem in stl also: everything in stl is triangulated.

So the (Dave idea) use it as a pilot hole and drill to fit is the best solution.

Yes. People expect too much from 3D printers. Any foundry worker would tell you that molten materials shrink and warp as they cool, so if you need a precision fit you have to mill or drill after shrinkage is done.

That is what I had decided to do. I was hoping for a design/printing solution, as having to drill using the 3D hole as a pilot hole greatly complicates the manufacturing process. In my case, this hole alone (there are others) requires 16 times 20 manual operations.

Thanks all for your replies.

I you aren’t worried about too precise a fit, you can enlarge the printed hole diameter, typically by about the diameter of the filament print nozzle (if using FDM printing). At least, the printers I use (which have a nozzle diam of a nominal 0.5mm) seem to print along the centreline of the edge of the modelled part, which means that the edges of the print are 0.25mm off the centreline - outside making it larger, or round a hole, making it smaller.

For example, a rectangle drawn 10mm x 10mm will end up around 10.5mm x 10.5mm printed, and a hole drawn 3mm diam will finish up about 2.5mm diam.

To fit a 1/8" (3.2mm) rod, with a nozzle 0.5mm diam, try drawing the hole 3.2 + 0.5 = 3.7mm diam, or 1.85mm radius in SU.

You can probably do the same sort of thing for a laser 3D printer, but I wouldn’t know how much to add to the hole size. Probably, its nominal resolution.

There may be cleverer slicing software than what I use (Lulzbot Cura 3.2) that automatically does the offset for you, but mine doesn’t, and it often takes a few test prints to get mating parts to fit.

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Since my parts are commercially printed, it may be difficult to get the offset measurements directly. So I suggest measuring the printed hole diameter and calculating the actual size to be specified.

A follow up question is how do I increase the hole diameters in SketchUp?

( native tools solution ! )

If both ends of the hole are parallel and perpendicular to the cylinder’s centerline, you could use the ‘Scale’ tool on both end circles separately to scale them to exact diameters by holding down [Shift] and [Ctrl] when using a green side grip. As input you then use the diameter value (not the radius) plus units. Apply this proces on both circles separately so the length of the hole isn’t affected.

Otherwise (no parallel and/or perpendicular situations) you’ll most likely have to “rebuild” the hole from scratch with a new circle (applying Push/Pull) or external long cylindrical body as a new drill.

Thank you for your detailed reply. In my case, the end holes are indeed parallel and perpendicular. I thought something like the process you described would be a solution and once again thank you for your detailed reply.

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Or you can select the end circles in turn and change their radius in the Entity Info window.

If they have become exploded to line segments or a curve instead of a circle, the plugin Lines2arc will recreate them as circles.