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Plastic threaded bolt

Plastic threaded bolt

(OP)
I have several plastic threaded bolt which is machine out of stock rod.
Some how when I tight this bolt to a "standard" nut, it is so difficult to turn after few thread engage.
The material is Polypropylene, and now I am thinking to reduce the pitch I.D. in order for me to able to use finger to turn the nut.

Does anybody have this experience, comments?

RE: Plastic threaded bolt

The few symptoms you report suggest that the bolt and nut's thread pitch are somewhat different. This commonly happens when attempting to mate UNF and UNC threads. Given that your bolts were machined from bar, there is a chance of a machining error, or thread distortion from using a dull tool too long, or just a miscommunication about what was ordered.

Check both parts against a thread pitch gage before deciding how to proceed.

Mike Halloran
Pembroke Pines, FL, USA

RE: Plastic threaded bolt

Like Mike says verify you actually have what you think you have/asked for.

Also consider that if you plastic screw is relatively 'soft' and your screw relatively 'hard' it is easy to get some cross threading going on which then causes it to jam.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Plastic threaded bolt

(OP)
It is UNC for sure.
Both Nut and bolt are the same material.
I am thinking of calling out both nut and bolt with class 1A and 1B fit.

RE: Plastic threaded bolt

I've cut threads on plastic rods with a threading die and they almost always cut oversize. Spring-back in the plastic as far as I can tell. My solution was an adjustable die set to cut undersize. Check with the machinist to see if this is how they were cut and ask him to actually measure the threads. They will probably have an oversize pitch diameter. You don't want to tighten any tolerances because then the parts will then be even further out of spec and more expensive.

Timelord

RE: Plastic threaded bolt

(OP)
Timelord,
How do you "QC pass" these part?
What I do is having "finger able to turn", that will QC pass". But it seems that the OD of the threaded rod is a way too small.
It is about 1/2 of the thread engagement :(

RE: Plastic threaded bolt

You QC the parts by measuring the thread pitch diameter. See Machinery's Handbook for the proper method. You ought to check the nuts also to see if they meet their class requirements. The nuts are usually checked with go/no-go gauges. When both threads (nut and rod) actually pass this type of QC, the nut will free run, because all classes of threads have some clearance. Take the problem back to the machinist and make sure he understands his threads are out of tolerance.

Timelord

RE: Plastic threaded bolt

What size?
You would not be trying to thread a 5/16-18 bolt into a M8 nut, would you?
I have seen this happen many times. Mostly by maintenance people. They will go a few turns.
Check bolt and nut with a thread gage. Also try an M8 screw.

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