# Trouble figuring out outside thread sizing



## Armourer (Feb 1, 2017)

I want to make a spider to put on my spindle on the outboard side of my headstock but am having trouble figuring out exactly what size it is. With my thread pitch gauge it has 16TPI and also a outside diameter of 2.1495". Would it metric? How should I go about doing an internal thread for it? Thanks.


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## Bob Korves (Feb 1, 2017)

You did not tell us what lathe you have or post pictures of the left end of the spindle installation, so it is difficult to visualize what you have or what you are doing.

I have a Chinese 13x40 lathe and am building a hand wheel drawbar arrangement for it, and it will include a spider.  The left end of the spindle, starting from the far left end, first has a smooth inner and outer bore for an inch or so, then has a few external threads extending beyond the spindle end play adjusting nuts.  I have seen people make an internal thread on their spider to fit those spindle threads, and perhaps that is what you have in mind.  I can see no good reason for doing that, it is difficult, it only engages a few threads at most, and I see no reason why it it needed, and good reasons why you do not want it that way.  I made my spider/collet closer adapter so it has a counterbore to clear the threads completely, and just rests against the spindle nuts.  I then added some threaded brass plugs I had laying around to hold the adapter to the O.D. of the left end of the spindle by friction only, screwed through the adapter like the spider bolts, and pressing against the left spindle end O.D.  If you crash something while working with the collet closer, it will stop the collet, but not the spindle.  If the adapter is threaded on to those few threads, it will possibly tear the threads off the left spindle end before all the damage is done.  With my design, the adapter is held only by friction of the brass plugs against the spindle O.D., and is able to slip on the spindle any time things do not want to turn at the same speed, for whatever reason.  The collet closer tube normally holds everything together during use, so the brass plugs just keeps the spider/adapter from falling off the spindle when the collet closer is removed.

It sounds like you are just wanting a spider, so my thought is that the spider should be free to rotate on the spindle if necessary.  There is no good reason to have the spider hard keyed to the spindle.  Think about what happens if the work stops turning but the spindle does not.


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## bfd (Feb 1, 2017)

make a copy of the threads you are talkin about 2.149x 16 and use it as a test bar when you thread the spider. the hole size is the outside dia minus the pitch of the thread. pitch of a thread is 1  over the number of threads per inch in your case 1 over 16 yes 1/16" so 2.149 (don't worry about the .0005 ) minus .062 equals 2.087 that is the minor dia of the inside dia. bill


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## Bob Korves (Feb 1, 2017)

16 TPI is very close to M1.5 threads.  If it is an import lathe it would probably be a metric thread, unless it is a very old import lathe.


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## darkzero (Feb 2, 2017)

The DF-1237G both BusyBee & Grizzly were made in Taiwan as far as I can find. Most Taiwan lathes & mills are built with standard fasteners (yeah, yeah, SAE, inch, UN, imperial, whatever you want to call it) but not always.

Still, could be metric or standard. My Chinese 12x36 has all metric threads except the leadscrews. But my internal spindle threads are 1-5/8"-16TPI. 1.5mm is close to 16TPI but my thread guage told me 16TPI was a better fit than 1.5mm.

I doubt it's a custom size thread. Based on your external thread measurements it doesn't seem likely to be a 2-3/16"-16TPI thread, if it is then the minor is way out of spec. Your measurement is closer to a M55x1.5mm thread.

Regardless I agree with bfd. Stick with whatever pitch measures closest whether 16TPI or 1.5mm, make a matching plug that matches your spindle, & make your internal thread to fit using the plug for test. Luckily for me my spider had to be an external thread so I was able to turn it between centers & could remove it to check fit.


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## 4GSR (Feb 3, 2017)

If you have a set of thread wires, select either 0.040" or 0.0395" wires and take a measurement.  Record this number.  Machine a "Go" gage and thread to this number.  This way, you have a gage to fit the female thread to when cut.  Regardless if it is english or metric, the basic thread bore is about 2.082" going by the OD dimension reported .  Ken


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