Hacker News Reader

Comment by wcunning | Hacker News Reader

I spent a while learning about sewing machine timing and repair a couple of years ago after buying a very nice vintage Elna at a thrift store for $20. Turns out that the way most of the timing is set is based on the height of the needle when the shuttle mechanism underneath is in a particular place. You loosen the set screw on the collar that connects the upper and lower mechanisms, set the needle to the height of a gauge block from the manufacturer and then rotate the lower mechanism until something lines up, then tighten the set screw. Those blocks are almost all long gone for anything that's still mechanical, and the modern machines are all electronically timed with stepper motors in the top and bottom. Fascinating stuff if you dig into it.

View on HN

That makes me want to go inside my older machine and mark the current positions, say with a sharpy or dye. Mine had the main plastic timing pulley break in half and is un-obtainium. So I epoxied the pieces together, while drilling holes and embedding steel wire reinforcement. Works like a champ, though I go the feeling it was built with planned obsolescence, as the rest of it was built like a tank.

Sometimes such a plastic parts sole function is to break to avoid major damage to the rest of the machine. If you make it stronger and the same conditions occur something else might break or bend that is a lot harder to replace or repair.

You'll see the same in farming equipment, there are usually one or two sacrificial bolts in there that will be happy to snap if the tool gets wedged, rather than that the tool snaps.

I get the sacrificial analogy, but in fact, the larger problem is IF the cam does fail, will it allow interference that will lock up at speed and potentially destroy the highly powered but delicate mechanisms, as you find destroys engine valvetrains, mainframe printers, among others. And this is what it did; when it failed everything locked, but luckily nothing was destroyed because otherwise built like a tank. The alternate explanation is that it is simply a very complicated cam device with many extreme contours, expensive to fabricate from anything but plastic injection, with high force followers that cause it to fail over time, leading to a trip to the sewing machine repairman. edit: I called it a pulley, but it in fact was a complicated cam.

That's a good use for 3D printing, assuming someone knows and can publish the appropriate measurements for the gauge block.

The problem is that no one (myself included) wants to do the work for the measurement. You need to retime the machine in front of you, and once you do, you don't really have any need to make up the gauge... Also, this is probably going to need to be more accurate than I understand 3D printing to generally be. I probably don't need to use my set of Starrett Grade A gauge blocks, but I probably want more accurate than your average ruler.

I have a desktop, sub-$800 printer and can print to 0.1 mm thickness without extraordinary measures and using inexpensive calipers, you can measure down to below 0.01 mm. (Thereby getting potentially more than native precision by repeated attempts.)

Thermal expansion of the plastics being different than the machine is a small issue, which could be solved by measuring, calibrating, and adjusting all at a given temperature.

Your point about limited motivation is spot-on, of course.

Reply on news.ycombinator.com