On Sunday, May 9, 1999 at 12:00:00 AM UTC-7, alain labbe wrote:
Hi, i'm fairly new to metalworking but I'm having a lot of fun. I do have a
small problem thought. I cant seem to find what size a shaft or bar should
be to thread it with a die. For example, i had to make a plug out of brass
(to fix an eccentricly drilled flywheel) out of 3/8 round. I tried to
thread it 3/8-16 and just couldn't get the thread started with a small taper
on the end. Ended up removing an undefined amount of metal, just to get a
long taper before I finaly succeded in starting the thread. I have really
cheap dies which certainly doesnt help but what is the rule. Should a
3/8-16 thread be done on a smaller than 3/8 rod? The one I did turned out
ok for the purpose but I'd like to know what I really should have done.
Thanks
Alain Labbe

use a tap and die chart to help you find the initial inside and outside diameters needed for the tap and die you will be using.

On 2018-10-22, wrote:
On Sunday, May 9, 1999 at 12:00:00 AM UTC-7, alain labbe wrote:
Hi, i'm fairly new to metalworking but I'm having a lot of fun. I do have a
small problem thought. I cant seem to find what size a shaft or bar should
be to thread it with a die. For example, i had to make a plug out of brass
(to fix an eccentricly drilled flywheel) out of 3/8 round. I tried to
thread it 3/8-16 and just couldn't get the thread started with a small taper
on the end. Ended up removing an undefined amount of metal, just to get a
long taper before I finaly succeded in starting the thread.

One way to do it is to turn the diameter down at the end to just
fit inside the die (Minor diameter of the thread) -- and if you get a
copy of _Machinery's Handbook_ there are tables to give all of this.
But you can simply test-fit while it is in the lathe.

I have really
cheap dies which certainly doesnt help but what is the rule.

"Really cheap dies" may be ones made for cleaning up damaged
threads. They really are not good at starting threads. Often, you can
recognize those designed for cleaning up damaged threads by their having
a hex outside, instead of round.

Good quality dies and taps are worth buying. For most purposes
I find the "Spiral Point taps" or "Gun Taps" to be better for any
through holes. The spiral point chases the chips ahead of the tap, so
you don't have to frequently reverse the the tap to break chips. For
blind holes, you want a set of three taps -- a "starting tap" (long
taper), a "plug tap" (fairly short taper), and a "bottoming tap" (almost
no taper, to finish the bottom-most threads in the hole.)

Also -- another nice form of tap with no chip problems is a
"roll tap" or "thread-forming tap". It requires a different diameter
tap hole, and works better on mild steel aluminum or brass and softer
plastics. It causes material to flow from the bottom of the thread to
form the upper part of the thread. (You can find these all in the MSC
catalog (http://www.mscdirect.com)).

However, a reasonable die will likely have one side with a long
taper, and the other without. Start with the long taper side, then (if
you need the thread to go close to a shoulder) reverse the die to finish
the thread with little taper.

Should a
3/8-16 thread be done on a smaller than 3/8 rod?

You want the end turned down to the the minor diameter for a
short distance to help start the die onto the shaft. And, you can use a
lathe center in the tailstock to push the die holding the die square to
the shaft.

Start out a bit over-length, and expect to turn of the reduced
diameter portion once the thread is finished.

The one I did turned out
ok for the purpose but I'd like to know what I really should have done.
Thanks
Alain Labbe

use a tap and die chart to help you find the initial inside and
outside diameters needed for the tap and die you will be using.

And normally, the part which you are threading with a die will
be the nominal diameter of the thread (3/8" in this case), exclusive of
the turned down cylindrical end to help start the die.

Enjoy,
DoN.

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