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Recommended Interference fit for Black Acetal

Skillet Maker

Plastic
Joined
Dec 1, 2022
I'm in the process of making new wheel tools for my metal spinning setup. I'm interested in trying Black Acetal as a medium to see how it will hold up to spinning non ferrous metals such as copper, pewter and silver. I've already made a spin form from this material and really like it so far. I'm not finding the correct interference fit recommendations for my application. I am running timken a2126/a2037 tapered roller bearings. They are about 1.25" OD 0.375 width and 0.375 shaft. I need to friction fit the race into the Acetal and am concerned about heat expansion. Any suggestions and insight would be great!
Thanks!
Peter
 
For my work I do trial and error to dial things like this in. But I do it on a sub spindle lathe and "walk" the ID to what I need and toss the rejects. The ID that works will run thousands of parts so not a big deal. I would look to get a boring tool that can under cut the ID opening so the bearing gets trapped once pressed in. You would want the acetal minimum body temp warm or even soaked in hot water before pressing.
 
I would go at least .05mm undersize on the bore of the acetyl and if possible use a circlip to stop it backing out. Forget about gluing it acetyl doesn't glue. It would help if you posted a sketch or drawing of the acetyl part.
 
The issue is going to be creep. You can get a pretty large interference fit to work with Delrin but each time the material heat cycles it will "stretch" a bit and will loosen eventually. It might be worth a try to see if the benefits you're thinking you will get will outweigh the lack of longevity, For the materials you are forming you might try Delrin AF (anti-friction) for the forming roller in a plain bearing configuration (without the roller bearing). I'd stick with polished steel forming rollers and using Delrin for the forms. Don't think Delrin would hold up as a roller. But my experience is with rolling fairly heavy steel so may be skewed by the high forces required.
 
I second the .002" press and circlip idea, also agree, glue is not an option. Also, depending on how much force is being applied, a cross pin or set screw into the axle/ shaft may be appropriate.
 
Thanks everyone for all the helpful comments! I'm going to take a picture of what I'm going to replicate and share it here.
As far as heat build up, I don't really know... I would suspect around 200 deg.f
 
The originator of Delrin, DuPont, used to have a tech note that answered such questions as press-fit dimensions and the like.

DuPont sold the Delrin product line to another company. The design data is now ahttps://www.delrin.com/general-design-principles-for-dupont-engineering-polymers/

Look in section 10 of "General Design Principles for Delrin® Engineering Polymers" for the press-fit data.
Thanks for the link but I don't seem to be able to read it.... It goes to a form page of sorts...
 
If you do decide to do this with acetal, use a quality product. Quadrant's Acetron is good stuff, and very consistent. There is plenty of junk acetal - with porosity, weird residual stress, etc.

Your 200F application is a bit warm for this. With time and pressure that is enough to make small changes to the memory of the material.
 
Here is a picture of the wheel I'm making out of the Acetal. The wheel is about 3" dia. and 1" thick. I picked up the Acetal several years ago from Professional Plastics so I don't know the brand or quality. I will say it machines beautifully!
 

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Thanks for the link but I don't seem to be able to read it.... It goes to a form page of sorts...
I see an interactive viewer with a tiny folder icon to download the file. But here's a direct link to the guide in PDF form:


Looks like DuPont divested the Delrin division in November 2023 to a private equity firm ("TJC L.P."), which now operates it as Delrin USA, LLC. I guess it's possible that the division wasn't profitable, and/or DuPont wanted to raise some cash. Unsure whether that will affect the manufacturing and sales of Delrin plastic.
 
That general design parameter link specifies .02-.04mm (.0008-.0015in) for a steel shaft.

However, as stated earlier in the thread, it does note that creep is an issue as the parts go through heat cycles. Have you considered adding a part like a bushing that is made of delrin before the interference fit? That way you could design a more secure method to affix the bushing to the shaft, then interfere between the two delrin elements.
 
Looks like DuPont divested the Delrin division in November 2023 to a private equity firm ("TJC L.P."), which now operates it as Delrin USA, LLC. I guess it's possible that the division wasn't profitable, and/or DuPont wanted to raise some cash. Unsure whether that will affect the manufacturing and sales of Delrin plastic.

I think that more likely was that delrin had become a commodity, the patents having long since expired.
 








 
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