Minor diameter

[jdsackett]

Hi guys. I just got a 5c collet adaptor and need to make a drawbar. Thread is 1.240x20. For a minor i come up with 1.179. Right?? Wrong?? Close?? Not even in the same zip code?? Appreciate any help here fellas, not the greatest math whiz. Regards, j.d.

 

[mklotz]

The 77% depth-of-thread tap drill size would be:

1.24 - 1/20 = 1.19

 

[Ken I]

Is that really a 1.24 or is that what you measured ?

I would have thought 1.25 (1¼") would be more likely.

Ken

 

[jdsackett]

Thanks guys, was pretty close ;D Ken, it's supposed to be 1.240. At least that's what the specs are for a 5c collet outside thread. Thanks again, j.d.

 

[Bill Gruby]

This will help. The thread on a 5C is 1.238-20 RH.

"Bill Gruby"

http://www.loganact.com/tips/collet.htm

 

[Jasonb]

I think it depends a bit on who you get your collets from. I've seen anything from 1.230 to 1.245.

As you have got the collet just make the thread to fit in much the same way you would adjust a split die to give the required fit.

Jason

 

[jdsackett]

Marv, that's a much simpler formula than i have. The one i have is 1/tpi*.61343 *2 and then subtracted from the major. I like yours better. Regards, j.d.

 

[MachineTom]

For a collet drawbar you can be loose, a 75% is 1.191 a 77% 1.189 this is the tap drill size.

 

[shred]

I made a 5C drawbar a while back (first bit of internal-single-point-threading I'd done on the lathe). The thread is pretty loose, which is good since my collet threads are all over the map. There's plenty of threads engaged so it's not a problem like that.

 

[mklotz]

Marv, that's a much simpler formula than i have. The one i have is 1/tpi*.61343 *2 and then subtracted from the major. I like yours better. Regards, j.d.

The more general formula, which includes the desired depth-of-thread, is:

TD = MD - 0.013*DOT/TPI

where:

TD = tap drill diameter
MD = major diameter
DOT = desired depth-of-thread expressed as percentage

When DOT = 77, this formula reduces to the one I gave above...

TD = MD - 1/TPI