Friction fit delrin disk on a shaft, design question

[Reddinr]

I have a design question. I need to make a 1.5" diameter, 0.25" thick disk that will friction fit concentrically onto a 0.25" diameter shaft. It needs to be tight enough to not move under light force but reusable (taking off, putting on) maybe a few dozen times a year. I tried just "right sizing" the hole in the disk but it got loose over time.

Is there a book or article that shows how to design the plastic spring legs that are sometimes cut into objects like this? Usually, they are sort of "L" shaped legs that are cut into the plastic and arranged every 90 or 120 degrees around the hole diameter. I'm not sure what the technique is called so I can't seem to find anything.

The project is a thread spool holder that the sewing machine manufacturer would like $180 for. I would like to keep that $ and build it from scraps I have (and maybe buy a different tool of some sort). I'll send a picture when I get a minute later today.

Thanks

 

[RJSakowski]

Delrin is pretty lax when it comes to interference fits. I generally used several thousandths. You should have no issue with disassembly. What are the forces that you anticipate?

 

[francist]

An alternate idea if you’re able to cut a small internal groove into the sides of the 0.25” hole a small O-ring might provide enough resistance to hold the disc in place. What make/model of machine?

-frank

 

[extropic]

It sounds like you're asking for a design manual for flexures. If you Google "flexure design" you will see many options.

Alternatively, if you have good access to the circumference of the installed disk, consider drilling and tapping for a small setscrew or spring plunger.

 

[Reddinr]

Flexures! That's it. I couldn't for the life of me remember what they were called. The machine is a large quilting machine, a Bernina Q16 I believe. I think I'm going to try a flexure-based one just out of curiosity. Maybe just glue on a pattern and jigsaw it out for a prototype. The forces are small, just whatever random force a small thread spool would make while spooling out thread. Not much. The key is that the piece needs to be perpendicular, easy on/off in a couple of seconds, but not move.

Here's the assembly so far. The delrin on the right is fine. Permanently mounted friction fit. The left-hand one will be same but slip-on.
The angle block is needed because the mounting surface is at 40 Degrees from vertical. (Glad I measured as I had assumed 45 Degrees...) It mounts with an M5 bolt and T-nut into a grove. Still in rough shape, need to polish the spool shaft and grain the aluminum.

 

[extropic]

From the last picture, it looks like the OEM piece uses a split hub.

You have a lathe?

Turn a cylindrical hub into the disk and split it using a Dremel tool cut off disk. I'm thinking a slightly tapered (or stepped) center hole, with the smaller ID at the end of the fingers and a slip fit at the larger ID.

 

[mickri]

On my sewing machines I just use a wine cork with a tight fitting hole in it to keep the thread spool in place.

 

[Reddinr]

I do have a lathe. I turned and threaded the rod and turned the delrin.

 

[Winegrower]

On my sewing machines I just use a wine cork with a tight fitting hole in it to keep the thread spool in place.

We can think of something way more complicated than that.

 

[Reddinr]

I have seen the cork trick. I think my aunt used that! I do want to over-complicate it a little, you know, to impress the wife.
I was thinking something like this.

Bad drawing: