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They slip off, or they will not go on, or the one you force cracks. Off-the-shelf leg caps come in jumps, so the one that fits your chair may simply not exist.
You buy caps for your chair legs and they slip straight off. Or they will not go on at all, and the one you force cracks. Most people have been there.
Off-the-shelf leg caps come in jumps. 16mm, then 19mm, then 22mm.
So what do you do when your chair leg measures 17.5mm? It will not fit the 16mm. It rattles in the 19mm. Neither purchase is the right one.
This is not the manufacturer being unhelpful — it is what mass production costs. Every size needs its own mould, so a finer set of sizes means more moulds. Jumps are inevitable, which means a certain share of the world's chairs will never have a cap that fits.
We are preparing to sell 3D-printed parts that customers specify by dimension, and leg caps are one of them.
You enter what your calipers read. 17.5mm means 17.5mm. Round legs, square legs, rectangular legs. With no mould involved, a finer resolution costs nothing extra.
None of that is more than a change of method. What became interesting came next.
Building to the measured size is a statement about the design.
A 3D printer melts material, stacks it, and lets it cool. Material shrinks as it cools. How much depends on the material, and varies with the machine and the temperature settings even for the same material. Hand the same file to two different setups and the finished dimensions differ.
That is why the clearance is specified separately from the leg size — how much room to leave between the leg and the inside of the cap. Too tight and it will not go on; too loose and it falls off. The right value cannot be known except on the machine that will print it.
So we give away a test piece for measuring clearance. It carries a row of different clearances; you print it on your own machine and find which one slides on and stays put. Enter that figure with the order, and what you get is matched to your setup.
Saying "we build to your measurement" and stopping there would be the easier sales line. In practice the dimension is settled by the printing environment as much as by the design. Hiding that just moves the problem to the moment the part arrives.
Once dimensions are free to specify, useless specifications become possible too.
Take the height. If the leg does not sit deep enough, the cap falls off in use. We originally allowed as little as 2mm of engagement; testing showed 2mm slips off. Any specification leaving less than 5mm of depth is now declined, with the reason given.
Building what was asked for and shipping it is easy. The person on the other end receives something they cannot use. Possible to make and fit to use are different questions.
There is one more thing we decided against: a recess for a stick-on felt pad.
Carving a shallow recess into the floor-facing surface so a felt pad can sit in it looks like a thoughtful touch. Printing it is another matter. Put the recessed face down and the ceiling of the recess hangs in mid-air. Put it face up and the closed surface spans a cavity instead. Either orientation needs support, and the finish suffers.
So the base is flat, and an ordinary stick-on felt pad goes straight onto it. Better something that prints cleanly and achieves the same thing than something that can be fitted but prints badly.
Square tube legs have rounded corners. If the inside of the cap stays square, those rounded corners jam and the leg never seats fully.
That dimension appears on no drawing. So we ask people to look at the leg in front of them and specify the corner radius as well. The outside radius then follows automatically — inside radius plus wall thickness, which keeps the wall an even thickness all the way round.
This is still in progress. Printing one and checking it in the hand comes next, and I expect that will teach us something more.