WSF: fitted parts

I'm modeling an object in pieces that I want to fit together after printing. I don't need the parts to "snap and lock", but I do require a decent fit to keep the parts from sliding. Building inverse shapes is simple enough: three long rectangle solids next to one another with the middle one part of one object and the outside two as part of the other will form a simple anti-slide 'lock'. But how much space between would be required? I assume if there is no space I may have to force the parts together or pull out an exacto blade. I would prefer to not cut the parts after printing.

TIA

I'd say to leave a 0.1 or 0.2mm gap each side of the central block. I'm doing things like this to make tabs that hold robot parts together in one form or the other (for example, the tabs on Vantage's shoulders), and I've found 0.1mm either side allows the parts to slide in and grip together quite nicely, while still being able to pull apart when you want them to.

Hope that helps.

Andy

Thanks! That is exactly what I needed to know.

I asked Shapeways the same question and got the answer that the parts should just be the same size (ie. no gap).

This worried me slightly - given the slight surface roughness I could see this resulting in a tight interference fit.

On the motor pod that I had printed recently I used 'no gap' just to see what happened.

There were two areas that needed to fit together:

- the first was a rectangular (radiused corners) plug into an appropriate socket (approx 10mm x 30mm in cross section) and this was a nice smooth fit with no apparent slop;

- the other was a more complicated snap together join with small feature sizes (the snap ridge was 1mm x 1mm) and this didn't work quite so well the parts don't exactly snap together - or perhaps more accurately I suspect that once I snap them together it will be difficult to 'unsnap' them.

So I think that perhaps the gap allowance required may vary based on the size of the parts being fitted.

It is interesting to see Andy's figures - I will give them a go next time I need this sort of thing.

Tim

It does vary a little, it's true.

As a rule of thumb, I've found ball joints don't need any clearance to fit, as long as there's plenty of airflow in the channel (if the joint is designed to go through a range of motion, say, 90 degrees, then the room needed to account for the shaft's movement allows air in the socket to escape when the ball is pushed in). If the ball is the same shape as the opening (most of my neck joints, for instance, are only designed to rotate and rock a little, so the opening has a 4mm diameter like most of the balls), then a little clearance needs to be allowed for.

I've used tabs, like on Vantage's shoulders above, to hold parts together temporarily (in my case, as I said, it's to hold them together when the figure is in one form or the other). Most of these tabs are about 1mm x 2mm, and 1mm high; in these cases I've allowed the 0.1mm gap all round.

Then there are the 'snap' hinges (like Salvo's gun barrel), which consist of a cylindrical pivot (generally around 3-5mm diameter) which snaps into a socket via a pair of nubs on either end. I've found these need a little more clearance to allow movement, and I've stretched it to 0.2mm either end, except at the tips of the nubs themselves, which I give no clearance for friction.

Hope that's helpful.

Andy