Turny Thing

See also T13689.

The B740 has a turny thing. It looks almost like this:

The outside is perfectly smooth to make it difficult to turn. If you turn it too hard, oof:

This wheel appears to be sturdier and is easier to turn:

String Shelf

When sewing, you need a lot of different string. Bernina sells a Bernina String Shelf.

They do not list the price, but these shelves sell on eBay for about $100.

This is just a shelf for string and costs $100. It seems like it should be possible to build a shelf for string for more like $75 in materials and 40 hours of labor.

String Shelf Shelf

The sewing machine sits inside a Very Nice Table that I don't want to drive any screws into, so I started by making a String Shelf Shelf for the String Shelf to rest on. This is a sort of clamp that can squeeze a ~120mm strip of tabletop to the right of the machine:

The left part is printed twice. The actual shelf is a piece of plywood that goes between the left parts and the middle part, which I drove some M5 threaded inserts into. The final dimension of the plywood is 120mm x 130mm and I should probably just have printed this part instead of making four different sub-assemblies, but I haven't used heat-set inserts yet and I'm not sure if they're trustworthy or not.

Overcome with hubris, I hand-drilled the holes in the far end of the plywood to attach the clamping face and the bit wandered enough to throw the piece significantly out of square. I needed to reprint it anyway since the clamping action wasn't quite right, so I measured where the holes ended up and adjusted them in the model -- the one on the left went down 0.6mm, and the two on the right went up 0.5mm. Then I printed the asymmetrical "as-built" piece with the other adjustments. This felt horrible but worked well and fixed the squareness issue.

I put some of this CatTongue Grip Tape on the inner surfaces of the clamp and it seem to work pretty well: when you wiggle it, the table moves.

String Shelf Stick

I'm using 24" T-Track to create a vertical rail for string. It's bolted to the String Shelf Shelf on the bottom, and capped on the top since the aluminum corners can be a bit sharp/dangerous:

Spool Cap (Small)

Here's a seller offering them for $3.95:

• https://www.heydesewing.com/shop/c/p/Bernina-Thread-Retaining-Spool-CapDisk-Small-x30282370.htm

The shaft is ~5.5mm. I have a 6mm shaft collar on hand, but I suspect the problem is that the part is easy to lose, not that it doesn't function well.

These sewing machine shaft collars are kind of tricky: thread unspools around them, so the inward face must have a completely smooth edge that can't catch or snag. This rules out a simple clamping-style shaft collar with a slit in it.

They also need to be easy to put on and take off by hand (so no set screws) but have enough holding power to resist the pull of the thread.

Two suitable geometries I've used previously are:

These are a bit complex and generally make the part larger (although that's okay for this part). The more "obvious" geometry that commercial parts use (including this one) is to have flexible fingers pointing along the shaft:

...but I suspect that's a difficult geometry for FDM in PLA. That said, I haven't actually tried.

...I suspect that's a difficult geometry for FDM in PLA.

This part appears to work fine, and requires $0.04 of plastic. This is a modest savings on a $4 part, but the customer wanted "like 20 of these" so let's just say that I've saved enough money to offset the cost of investing in another printer.

Bobbin Case

I haven't used heat-set inserts yet and I'm not sure if they're trustworthy or not.

They're very very good.