Thread tapping question

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Displaced Canadian

Displaced Canadian

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Millersburg, OR
I have noticed when I tap the inside of a blank all works well. However when I tap the inside of a piece of derlin to hold a threaded piece to polish there is quite a wobble. I'm using 3/4" derlin and I have a line drawn so it does into the collet the same way each time. I'm guessing when I'm threading it holding the tap tool by hand I'm not getting it straight. My question is would getting a tap guide help hold the tap straighter or do I just need to practice more? Run out isn't an issue any piece of materiel in the collet doesn't wobble any when turning. Any thoughts?
 
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If you don't support the tap parallel to the piece, it will never go in straight. If you have a drill chuck for the tail stock, put the tap in the chuck and keep the tailstock loose enough so that it will move as the tap advances.
 
You will have better results if you use a tap guide or the chuck in the tail stock trick. IF you do the chuck in the tail stock you will have to turn the spindle by hand. The tap guide make s it a little easier. I have done both methods. Also if your delrin walls are thin when you tap them they will stretch with the tap as it turns. Plenty of wall thickness helps there.
 
On my metal lathe the tailstock is so light that it would be easy to get out of alignment if it wasn't locked down. A tapping guide was worth the $15 to me.

A couple other things you might do (if you don't already). Face off the material before drilling. Use a spotting drill or center drill to start (and make sure you are on center).

As a check, after drilling your hole (before using the tap), take your drill bit and insert the shank end (assuming it's the same size and not reduced) in the hole. Turn on the lathe slowly and see if you are running true.

If the drilled hole isn't straight and on center, it doesn't much matter how you tap. :smile:

Ed
 
There are several things to look at:

Are you using the Delrin rod as is? If so, I would turn down the OD slightly to make sure it is perfectly round and parallel, most Delrin rod isn't as it is extruded.

Face off the end, as already mentioned, and have the minimum length protruding from the collet to avoid flexing the Delrin when drilling/tapping the end.

Make sure the thread is tight, if necessary use a slightly undersize drill for the size thread you are using.

Using a piloted tap holder is, as already mentioned, a must.

You don't mention how big the thread is, but make sure you have plenty of wall thickness, Delrin is quite flexible.
 
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Definitely going to have to get a tap guide. LMS has one that fits directly into a MT2. I make the holders out of 3/4" delrin and so far the biggest hole it it is 9mm so still a fair bit of meat on them. I do think shaving them down a little would be a good idea because the factory size is a tight fit in the collet. Thanks for the help.
 
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