Delring press fit tolerancing

[XaeroR35]

I have plastic (Delrin) pins that need to be press fit into holes on a fiberglass plate. How do I determine the tolerance for this? I assume the tolerance will be different than for metal press fit. Epoxy could be used as well, but would prefer to stick with a permanent press fit tolerance.

 

[drawoh]

XaeroR35,

I am fairly certain epoxy will not work on Delrin. Delrin has a low coefficient of friction. I would not consider any press fit with it, to be reliable. Fibreglass is not a homogenous material, making any hoop stress analysis way more interesting than you need it to be.

I would look into some other design. Could your pins snap into the holes?

--
JHG

 

[KENAT]

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[KENAT]

Epoxy can be made to work on Delrin - however will require some attention to detail and may still not be as strong as with other materials.

Look into surface preparation of the Delrin, either chemically or mechanically to give the epoxy something to bond to.

I once got creative with gluing magnets into Delrin where I made it a stepped through hole countersunk on the far side. When bonding made sure the csk's were filled with the epoxy - so kind of cast a riven in place.

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[Tmoose]

What do the pins have to do?

 

[XaeroR35]

This is basically a revision of a current design where they did not call out tolerances at all. They stated to ensure .002 interferences of the pin to the hole. Our drafting/checker department wants tolerances put on the drawing (understandably). I don't trust the previous design but it works as intended so I am just trying to come up with correct tolerance scheme. ANSI B4.1 give interference fits but these seem to be for metal only. I would think the delrin would deform and need more of an interference to stay put.

 

[KENAT]

Actually, specifying the required interference (with a tol on that interference) may functionally be a better way to go & is my preference (as a former checker too;-)).

Will depend if the pins are installed by the machine shop or similar creating an inseparable assembly or if they are installed in a completely separate step.

This way the machine shop can optimize the fit - in extreme cases even measuring individual pins and reaming the hole to suit (may not be an issue with Delrin which as you point out will probably deform more).

Bad thing is it makes final inspection more of a challenge.

Look at the FAQ link I gave above - for non standard combinations of materials or other non standard situations look up tables may not be suitable/applicable and some math may need to be done. Even on the ANSI standard fits you need to consider the insertion forces/pull out forces/resultant stress etc. unless they explicitly list certain material combinations that I don't think from memory they do.

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[MintJulep]

I don't trust the previous design but it works as intended

Take the time to find out how it was really assembled then. I wouldn't be surprised if it was not made to drawing.

 

[gruntguru]

Drawoh. Difficult analysis shouldn't be a reason to change the design. If analysis gets too difficult you simply perform some test fits.

je suis charlie

 

[drawoh]

gruntguru,

Tapping the holes and screwing the pins in probably would be easier, especially if he uses a lock nut from the opposite side. There are a whole lot of problems with press-fit delrin pins into FRP.

--
JHG

 

[boo1]

remember the thermal effect

 

[Pud]

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