Clarification on Threading Infeed
 

I just wanted to clarify the infeed for threading on both sharp v
threads and also UN threads. I think I understand for sharp v threads
the depth of cut is equal to the pitch of the thread. So for a 2 1/4-8
sharp v thread with the compound set at 29 degrees the calcualtion is
..125*cos 29=.1094"

For a UN thread the infeed is 7/8ths of the pitch or
..875*.125/.875=.125
So the infeed at 29 degrees would be .125"

This seems logical since when cutting UN threads you are truncating
the triangle by 7/8 ths. Therefore .125*7/8= .1094

The UN thread is 7/8ths of the sharp v thread.

Is this correct?

Thanks, Steve

Not to worry re. the sharp V at the root. Grind a tiny radius on the V
point of your threading tool (theoretically 1/8 pitch) and that will
eliminate sharpness at the root. Even if you don't do this, you will still
cut very close to a "correct" thread. Best to use a thread gauge or a test
nut to frequently try on the new thread for desired "fit". For casual
threading, frequent testing beats a lot of calculations, and attention,
otherwise required for the "perfect" thread.

Bob Swinney
"Bruno" wrote in message
...
(Steve) wrote:

I just wanted to clarify the infeed for threading on both sharp v
threads and also UN threads. I think I understand for sharp v threads
the depth of cut is equal to the pitch of the thread. So for a 2 1/4-8
sharp v thread with the compound set at 29 degrees the calcualtion is
.125*cos 29=.1094"

For a UN thread the infeed is 7/8ths of the pitch or
.875*.125/.875=.125
So the infeed at 29 degrees would be .125"

This seems logical since when cutting UN threads you are truncating
the triangle by 7/8 ths. Therefore .125*7/8= .1094

The UN thread is 7/8ths of the sharp v thread.

Is this correct?

Thanks, Steve

Steve,

You need to account for a flat on both the crest and the root. If you
use a sharp V tool, you don't get a flat on the root of the thread,
but the flat on the crest is still produced by limiting the infeed.

For now, assume the compound set at 30 degrees
(because the math is easier), external thread, a sharp V tool where
you won't have a flat on the root, and you want a proper 1/8 flat on
the crest, you'd advance the compound by 7/8 of the pitch. So, for
example, a 8 TPI thread should be advanced 1/8 * 7/8 or .1094. But
if the tool is formed for a 1/8 flat at the root, you might advance
3/4 of the pitch (or .0938 for 8 TPI).

To be more precise, with the compound set at X degrees, you'd multiply
the above calculation by Cos(30) / Cos(X). For a 29 degree compound,
that value is about .9902. So, the V formed thread is infeed .1083 and
the UN formed thread infeed is .0928.

Regards,
Bruno

Agreed. But it still helps to correctly calculate the infeed so you
know when you're close. Also, if you're doing threading on
non-standard sizes and/or small pitched threads, you may not have a
test nut or other means to accurately test while cutting.

-Bruno


"Robert Swinney" wrote:

Not to worry re. the sharp V at the root. Grind a tiny radius on the V
point of your threading tool (theoretically 1/8 pitch) and that will
eliminate sharpness at the root. Even if you don't do this, you will still
cut very close to a "correct" thread. Best to use a thread gauge or a test
nut to frequently try on the new thread for desired "fit". For casual
threading, frequent testing beats a lot of calculations, and attention,
otherwise required for the "perfect" thread.

Bob Swinney
"Bruno" wrote in message
.. .
(Steve) wrote:

I just wanted to clarify the infeed for threading on both sharp v
threads and also UN threads. I think I understand for sharp v threads
the depth of cut is equal to the pitch of the thread. So for a 2 1/4-8
sharp v thread with the compound set at 29 degrees the calcualtion is
.125*cos 29=.1094"

For a UN thread the infeed is 7/8ths of the pitch or
.875*.125/.875=.125
So the infeed at 29 degrees would be .125"

This seems logical since when cutting UN threads you are truncating
the triangle by 7/8 ths. Therefore .125*7/8= .1094

The UN thread is 7/8ths of the sharp v thread.

Is this correct?

Thanks, Steve

Steve,

You need to account for a flat on both the crest and the root. If you
use a sharp V tool, you don't get a flat on the root of the thread,
but the flat on the crest is still produced by limiting the infeed.

For now, assume the compound set at 30 degrees
(because the math is easier), external thread, a sharp V tool where
you won't have a flat on the root, and you want a proper 1/8 flat on
the crest, you'd advance the compound by 7/8 of the pitch. So, for
example, a 8 TPI thread should be advanced 1/8 * 7/8 or .1094. But
if the tool is formed for a 1/8 flat at the root, you might advance
3/4 of the pitch (or .0938 for 8 TPI).

To be more precise, with the compound set at X degrees, you'd multiply
the above calculation by Cos(30) / Cos(X). For a 29 degree compound,
that value is about .9902. So, the V formed thread is infeed .1083 and
the UN formed thread infeed is .0928.

Regards,
Bruno

Also agreed! Having a thread gauge close by is good for verifying infeed.
Technically, you will need 2 of these "fishtails" - 1 for Sharp V and 1 for
UN. As Bruno says, for non-standard sizes (when you are nutless) there is
no substitute for doing it right.

Bob Swinney

"Bruno" wrote in message
...
Agreed. But it still helps to correctly calculate the infeed so you
know when you're close. Also, if you're doing threading on
non-standard sizes and/or small pitched threads, you may not have a
test nut or other means to accurately test while cutting.

-Bruno


"Robert Swinney" wrote:

Not to worry re. the sharp V at the root. Grind a tiny radius on the V
point of your threading tool (theoretically 1/8 pitch) and that will
eliminate sharpness at the root. Even if you don't do this, you will
still
cut very close to a "correct" thread. Best to use a thread gauge or a
test
nut to frequently try on the new thread for desired "fit". For casual
threading, frequent testing beats a lot of calculations, and attention,
otherwise required for the "perfect" thread.

Bob Swinney
"Bruno" wrote in message
.. .
(Steve) wrote:

I just wanted to clarify the infeed for threading on both sharp v
threads and also UN threads. I think I understand for sharp v threads
the depth of cut is equal to the pitch of the thread. So for a 2 1/4-8
sharp v thread with the compound set at 29 degrees the calcualtion is
.125*cos 29=.1094"

For a UN thread the infeed is 7/8ths of the pitch or
.875*.125/.875=.125
So the infeed at 29 degrees would be .125"

This seems logical since when cutting UN threads you are truncating
the triangle by 7/8 ths. Therefore .125*7/8= .1094

The UN thread is 7/8ths of the sharp v thread.

Is this correct?

Thanks, Steve

Steve,

You need to account for a flat on both the crest and the root. If you
use a sharp V tool, you don't get a flat on the root of the thread,
but the flat on the crest is still produced by limiting the infeed.

For now, assume the compound set at 30 degrees
(because the math is easier), external thread, a sharp V tool where
you won't have a flat on the root, and you want a proper 1/8 flat on
the crest, you'd advance the compound by 7/8 of the pitch. So, for
example, a 8 TPI thread should be advanced 1/8 * 7/8 or .1094. But
if the tool is formed for a 1/8 flat at the root, you might advance
3/4 of the pitch (or .0938 for 8 TPI).

To be more precise, with the compound set at X degrees, you'd multiply
the above calculation by Cos(30) / Cos(X). For a 29 degree compound,
that value is about .9902. So, the V formed thread is infeed .1083 and
the UN formed thread infeed is .0928.

Regards,
Bruno

Robert Swinney writes:

Not to worry re. the sharp V at the root. Grind a tiny radius on the V
point of your threading tool (theoretically 1/8 pitch) and that will
eliminate sharpness at the root.

1/8 pitch? Shouldn't the valleys be truncated by 1/4 P? 1/8 P is the
truncation of the peaks? That's how I understand _Machinery's Handbook_.

Richard J Kinch wrote:
Robert Swinney writes:


Not to worry re. the sharp V at the root. Grind a tiny radius on the V
point of your threading tool (theoretically 1/8 pitch) and that will
eliminate sharpness at the root.


1/8 pitch? Shouldn't the valleys be truncated by 1/4 P? 1/8 P is the
truncation of the peaks? That's how I understand _Machinery's Handbook_.
That's nuts.