Internal threaded tube issue?

Hello!

At work we have been doing some internal threads that keep coming out slightly different per part. One will be very loose, the next will be very tight.

We are using a ½-13 long reach tap cutting into polypropylene, and need the thread to be about 4.5 inches long. It creates a threaded tube that is used in some assembly’s we are doing, but when putting stuff together, some can be hand tightened, some are too loose, and others are hard to turn at all. Everything gets done with the same tool on our turning center and we started to buy undersized rod in order to bore out the center to make sure it was the right size. I am not using coolant as the material should self-lubricate, and the ones we have available may damage it.

Any idea what might be happening there? We are hoping to iron it out so we can automate the processes a bit more.

Thanks in advance!

(Also, first post. Sorry if it’s in the wrong place.)

I would be willing to bet that heat (& chip packing) is causing your threads to be different.
Plastic does not like to get hot.

Could you direct chilled air down the bore, as it taps?
Possibly even a peck tap cycle, to allow for chip removal (even with a sp flute tap) and cool things off at the same time.

Just my $.02.

Doug.

I have done a little work with polypropylene and two things come to mind. First of all, that stuff will quickly dull a tool making subsequent passes "tighter" as it's cutting less and more pushing the material out of the way until it springs back. The other factor that could be a problem is a consistent temperature. When the threads are cut at one temperature and assembly done at another, the expansion or shrinkage can become an issue. Along those same lines, a tool that is hot from friction will cut differently than one that is stone cold.

Maybe find a coolant that doesn't have an adverse effect on the polypropylene?

Hope that is of some use,

-Ron

I make a nylon part with a 1/4" tapped hole in a 3/8" OD rod. I make lots of them. It took me a little time to sort out the tap. I don't have much experience with polypro, so take this with a grain of salt.

What works best for me is to use the least amount of the tap as possible. I ground away all the lead-in pitches except one, and I chamfered that thread to help the tap start. I'm basically trying to make the first pitch of the tap cut full size. I run it very slow to keep the heat down, and I run coolant. I rinse the part with tap water as soon as it comes off the machine to get rid of the coolant.

It took a couple tries to get the tap to cut the way I wanted, but it was worth the trouble. That tap has made many thousands of parts, and the hole still gages.

For a deep hole like the OP, I would try grinding away everything except about 3-4 pitches of full size threads. OP is likely using a pulley tap, so there probably isn't a spiral flute version available, but if I could find one, that's what I would want to start with.

Edit to add: I was wondering- why don't the tap manufacturers make taps for plastic? I looked, all I could find were hand taps up to 1/4".

TAPS FOR PLASTIC - OSG

Don't they know we drill and tap plastics all the time?

Thank you for all of your replys
I had a felling that heat might be contributing to the issues but I was not sure if its was going to effect it overall. I have noticed chip changes when boaring out the hole as well, going from consistent strands to folding over itself. I will try to look for a coolant to taping wax to mitigate that.

I dont think the tap has dulled (at lest I hope not, its new with less then 8 parts made), but I will also keep that in mind. although its a bit expensive to grind down we might be bale to make our own for later use.

Iv been doing a hybrid of power/hand taping the thing, where we run it thought in one go, but I might have to cycle in/out it or the chip brake you mentioned. Another thing is that we are moving the part and pulling the tap along in an anchored center chuck, so might have to change the setup slightly.

As you have found, polyprop is a true thermoplastic, it goes very soft with heat and has quite high thermal expansion, we used to assemble interference fit collars over mating parts by boiling the collars in water and fitting them hot.
PP needs to be cool so why not cut your assumed 4.5" bits from the long length, cool them in a fridge and then tap with the minimum contact tap proposed above, I assume you are not having to enlarge the hole to tapping size, if you are use a drill reduced in size behind the tip to reduce friction.

Polypropylene is also really soft so make sure your tap holder or whatever is floating freely and not binding.

MetalCarnage said:

I have done a little work with polypropylene and two things come to mind. First of all, that stuff will quickly dull a tool making subsequent passes "tighter" as it's cutting less and more pushing the material out of the way until it springs back. n

Click to expand...

Plain polypropylene? I'd expect taps to last million holes on plain polypropylene, maybe you got glass or carbon fiber filled version?
Like PP+GF30

Other thing we don't know is the wall thickness.

If this is thin wall PP, it will be very easy to expand under the tool pressure. A close-fitting sleeve that fits over the OD might help control the expansion.