It would be relatively easy to single point a 4-start thread on a CNC lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam

SJ wrote:
It would be relatively easy to single point a 4-start thread on a CNC lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam




With a dog and a four slot faceplate or chuck. Set the part on centers
and move the dog 90 deg in the chuck or faceplate for each thread.

John

On Sat, 01 Apr 2006 23:02:43 GMT, "SJ" wrote:

It would be relatively easy to single point a 4-start thread on a CNC lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam

Lotsa ways to do this. Two ways involve the compound. Either set
parallel with the Z axis or set at an angle. With the compound set
parallel you cut the first pass to depth, advance the compound 1/4 of
the lead, thread to depth, repeat until done. Example: Say you want to
cut your thread so that every rev of the screw moves the screw 1 inch.
That's your lead. But you want 4 starts so the thread form can be
smaller. So your pitch would be 4 threads per inch. And 4 TPI= .250"
Look in the book for the thread depth of a 4 TPI thread. Touch the
threading tool to the part and zero the dial for the X axis. Make
several passes until you have moved in on the X axis until the tool
cuts the full depth. Then advance or retard the compound .250". Repeat
the cutting operation. Move the compound. Cut. Etc. You may want the
compound set at 45 degrees to clear the tailstock because you need to
use a center to support the part. Or maybe you want to set the
compound at 29.5 degrees if you are cutting a 60 degree profile
thread. If the compound is set at any angle other than parallel to the
Z axis use trig to figure out how much to advance the compound. E.G.
the multiplier for 45 degrees is 1.4142 so the compound would need to
be advanced (or retarded) .3536. In the above example the Z axis is
the longitudinal axis and the X axis is the cross slide.
ERS

Eric R Snow wrote:

On Sat, 01 Apr 2006 23:02:43 GMT, "SJ" wrote:

It would be relatively easy to single point a 4-start thread on a CNC lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam

Lotsa ways to do this. Two ways involve the compound. Either set
parallel with the Z axis or set at an angle. With the compound set
parallel you cut the first pass to depth, advance the compound 1/4 of
the lead, thread to depth, repeat until done. Example: Say you want to
cut your thread so that every rev of the screw moves the screw 1 inch.
That's your lead. But you want 4 starts so the thread form can be
smaller. So your pitch would be 4 threads per inch. And 4 TPI= .250"
Look in the book for the thread depth of a 4 TPI thread. Touch the
threading tool to the part and zero the dial for the X axis. Make
several passes until you have moved in on the X axis until the tool
cuts the full depth. Then advance or retard the compound .250". Repeat
the cutting operation. Move the compound. Cut. Etc. You may want the
compound set at 45 degrees to clear the tailstock because you need to
use a center to support the part. Or maybe you want to set the
compound at 29.5 degrees if you are cutting a 60 degree profile
thread. If the compound is set at any angle other than parallel to the
Z axis use trig to figure out how much to advance the compound. E.G.
the multiplier for 45 degrees is 1.4142 so the compound would need to
be advanced (or retarded) .3536. In the above example the Z axis is
the longitudinal axis and the X axis is the cross slide.
ERS

if you have play in the compound nut and cannot lock the compound you
will get an drunken screw or worse.

John

John wrote:
Eric R Snow wrote:

On Sat, 01 Apr 2006 23:02:43 GMT, "SJ"
wrote:

Got a faceplate with 4 slots?
Can you run the job between centers?
Thread one lead.
'Index' the workpiece one slot (90 degrees).
Run another thread - etc.

Ken.
--
Volunteer your idle computer time for cancer research
http//www.grid.org/download/gold/download.htm
Return address courtesy of Spammotel http://www.spammotel.com/

....
if you have play in the compound nut and cannot lock the compound you
will get an drunken screw or worse.

If you're just talking about backlash, I disagree. My lathe has significant
backlash in the compound, not really a problem as the force is always in the
same direction.

FWIW, my favorite method for this job is to leave the compound set at 30.
Cut one lead. Do the math to advance the start point by backing out the
cross slide and advancing the compound to the next lead and cut again.

karl

"SJ" wrote:

It would be relatively easy to single point a 4-start thread on a CNC lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam


I cut 4 start 36-TPI all the time on an Atlas 618. While I use the
proper depth of cut for 36 TPI, the lathe gearing is set to 9 TPI. My
thread dial has four marks and each mark works a different start (this
may vary on other lathes). I make a pass for all four starts, then
advance the compound for the next one.

Once I needed to do a 4 start 32 TPI and I just couldn't work the
thread dial the same way. I had to cut two threads to depth, and then
I slipped the gearing from the tumbler to the spindle 1/4 revolution
and did the other two threads.

For internal threads, extra clearance was required on the tool.

-Bruno

On Sat, 01 Apr 2006 21:50:55 -0500, John
wrote:

Eric R Snow wrote:

On Sat, 01 Apr 2006 23:02:43 GMT, "SJ" wrote:

It would be relatively easy to single point a 4-start thread on a CNC lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam

Lotsa ways to do this. Two ways involve the compound. Either set
parallel with the Z axis or set at an angle. With the compound set
parallel you cut the first pass to depth, advance the compound 1/4 of
the lead, thread to depth, repeat until done. Example: Say you want to
cut your thread so that every rev of the screw moves the screw 1 inch.
That's your lead. But you want 4 starts so the thread form can be
smaller. So your pitch would be 4 threads per inch. And 4 TPI= .250"
Look in the book for the thread depth of a 4 TPI thread. Touch the
threading tool to the part and zero the dial for the X axis. Make
several passes until you have moved in on the X axis until the tool
cuts the full depth. Then advance or retard the compound .250". Repeat
the cutting operation. Move the compound. Cut. Etc. You may want the
compound set at 45 degrees to clear the tailstock because you need to
use a center to support the part. Or maybe you want to set the
compound at 29.5 degrees if you are cutting a 60 degree profile
thread. If the compound is set at any angle other than parallel to the
Z axis use trig to figure out how much to advance the compound. E.G.
the multiplier for 45 degrees is 1.4142 so the compound would need to
be advanced (or retarded) .3536. In the above example the Z axis is
the longitudinal axis and the X axis is the cross slide.
ERS

if you have play in the compound nut and cannot lock the compound you
will get an drunken screw or worse.

John
John,
It's true, a loose compound will make a bad thread. No matter how you
cut it if you are single pointing a thread.
ERS

On Sun, 02 Apr 2006 15:42:44 GMT, Bruno wrote:

"SJ" wrote:

It would be relatively easy to single point a 4-start thread on a CNC lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam


I cut 4 start 36-TPI all the time on an Atlas 618. While I use the
proper depth of cut for 36 TPI, the lathe gearing is set to 9 TPI. My
thread dial has four marks and each mark works a different start (this
may vary on other lathes). I make a pass for all four starts, then
advance the compound for the next one.

Once I needed to do a 4 start 32 TPI and I just couldn't work the
thread dial the same way. I had to cut two threads to depth, and then
I slipped the gearing from the tumbler to the spindle 1/4 revolution
and did the other two threads.

For internal threads, extra clearance was required on the tool.

-Bruno
Greetings Bruno,
The methods you just described are just two more ways to make
multi-start threads. The "slipping" of the gear train is a method not
mentioned very often, and it's good you brought it up. Also, the
clearance on the tool is important for external threads too. The tool
needs to have the clearance for the lead. This is not always obvious
and it's an important point you brought up.
Cheers,
Eric

Karl Townsend wrote:
...

if you have play in the compound nut and cannot lock the compound you
will get an drunken screw or worse.


If you're just talking about backlash, I disagree. My lathe has significant
backlash in the compound, not really a problem as the force is always in the
same direction.

FWIW, my favorite method for this job is to leave the compound set at 30.
Cut one lead. Do the math to advance the start point by backing out the
cross slide and advancing the compound to the next lead and cut again.

karl




I just did a double lead acme thread last week, with the nut. The pitch
was .250 and the lead was .500 inch. I set the compound at 0
and plunge in, since the lathe is rigid enough and it eliminates the
possibility of pushing sidewards and moving the tool. Also the lathe is
equipped with a infeed adjustable stop for threading which makes
threading a pleasure rather than squinting at the little numbers to
reset the cross slide each time.

I screwed up in that I didn't make a thread gauge for the nut before I
made it. I had to pick up the thread two times after removing the nut
from the lathe to check for fit. I left out the extra tolerance for a
long nut ( 8 inches) on a multi lead thread and that was probably why
the nut didnt quite fit on the first two trys. The shaft was 86 inches
long with the thread length about 58 inches. The nut was cast iron for
added strength I guess. It was a rush repair job for a big machine.



John

It allows the nut (4-start as well, of course) to travel 4 times as fast but
engage the same number of threads. So your lead is quadrupled while your
pitch stays the same.

Take a look at the cap on a standard gallon milk container. You'll see it
most probably has multiple threads (sometimes as much as 8). This is so that
you don't stand there twisting too many turns before you get to your milk,
but enough threads also engage the cap to hold it on when its screwed back
on.

My application calls for only 1 revolution of the nut (as opposed to 4) for
a 0.2 inch travel on a 1/4"-20 screw.

Sam


"Brent Philion" wrote in message
...
As much as it sounds like a nice peice of machining what is a 4 start
thread used for?

On Sat, 01 Apr 2006 23:02:43 GMT, "SJ" wrote:

It would be relatively easy to single point a 4-start thread on a CNC
lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam

Brent Philion wrote in
:

As much as it sounds like a nice peice of machining what is a 4 start
thread used for?

Fast lead. In the example here a four start twenty pitch thread will move
the nut 0.200" per revolution versus 0.050" per rev for a standard 20
pitch.

So, why not just cut a five pitch thread and be done with it? Well in this
case the major is 0.250", a five pitch thread would be so deep that there
would be next to nothing left at the minor diameter. The 20 pitch four
start is shallower and leaves a lot more material at the minor diameter. So
the multi start thread gives you a fast lead and more strength.


On Sat, 01 Apr 2006 23:02:43 GMT, "SJ" wrote:

It would be relatively easy to single point a 4-start thread on a CNC
lathe with a C-axis, but not having access to one, how would you cut a
4-start 1/4"-20 thread on a manual small lathe (like the mini-lathe)?


BTW,

You don't need a C-axis to cut a multi start thread on a CNC. You simply
need one pulse signal from the spindle. You don't even really need an
encoder on the spindle.


--

Dan

Quid Aere Perennius

Brent Philion wrote:

As much as it sounds like a nice peice of machining what is a 4 start
thread used for?


The cap on a fountain pen is a common application (and why I cut 4
start threads). The cap will engage within a quarter turn, and it
doesn't take much turning to fully close the pen... something that is
done often enough to want the operation to be quick and reliable.

As much as it sounds like a nice peice of machining what is a 4 start
thread used for?

On Sat, 01 Apr 2006 23:02:43 GMT, "SJ" wrote:

It would be relatively easy to single point a 4-start thread on a CNC lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam

in one word

Gotcha

for the added complexity it threadds 4x as fast but has the same pitch
stenght and torque as a nut with 4x the thread

On Sun, 02 Apr 2006 21:01:36 GMT, "SJ" wrote:


It allows the nut (4-start as well, of course) to travel 4 times as fast but
engage the same number of threads. So your lead is quadrupled while your
pitch stays the same.

Take a look at the cap on a standard gallon milk container. You'll see it
most probably has multiple threads (sometimes as much as 8). This is so that
you don't stand there twisting too many turns before you get to your milk,
but enough threads also engage the cap to hold it on when its screwed back
on.

My application calls for only 1 revolution of the nut (as opposed to 4) for
a 0.2 inch travel on a 1/4"-20 screw.

Sam


"Brent Philion" wrote in message
.. .
As much as it sounds like a nice peice of machining what is a 4 start
thread used for?

On Sat, 01 Apr 2006 23:02:43 GMT, "SJ" wrote:

It would be relatively easy to single point a 4-start thread on a CNC
lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam