Another 4-start thread question - 1/4" internal thread
 

Hi and thanks for all your responses to my question yesterday about how to
cut a 4-start thread on a small lathe. I'm going to try the dog approach
since it seems the easiest to set up and probably gives good results.

My second question is about the other half of the equation: the nut that the
4-start 1/4"-20 screw uses needs to obviously have the same threaded type to
work properly. The nut is about 1/2" thick. Because the hole is small (for a
1/4"-20 thread), other than having a custom 4-start tap made (expensive?),
are there any tricks to cut this 1/2" long 4-start 1/4"-20 internal thread?
I know that my current internal threading tool for the lathe can't do a hole
this small.

Thanks again,
Sam

Another 4-start thread question - 1/4" internal thread
 

SJ wrote:

Hi and thanks for all your responses to my question yesterday about how to
cut a 4-start thread on a small lathe. I'm going to try the dog approach
since it seems the easiest to set up and probably gives good results.

My second question is about the other half of the equation: the nut that the
4-start 1/4"-20 screw uses needs to obviously have the same threaded type to
work properly. The nut is about 1/2" thick. Because the hole is small (for a
1/4"-20 thread), other than having a custom 4-start tap made (expensive?),
are there any tricks to cut this 1/2" long 4-start 1/4"-20 internal thread?
I know that my current internal threading tool for the lathe can't do a hole
this small.

Thanks again,
Sam




I have never tried it but I would try a tap standard 1/4 20 and grind
off all but one row of threads for clearance. Leave four full teeth on
the existing row and grind the rest off. Run the feed at 5 tpi. You
may need more relief because of the greater lead so you may have to
grind off the back side of the four teeth left to get the added
clearance needed.


John

Another 4-start thread question - 1/4" internal thread
 

"SJ" wrote in
news:tcVXf.213875$H%4.187993@pd7tw2no:

Hi and thanks for all your responses to my question yesterday about
how to cut a 4-start thread on a small lathe. I'm going to try the dog
approach since it seems the easiest to set up and probably gives good
results.

My second question is about the other half of the equation: the nut
that the 4-start 1/4"-20 screw uses needs to obviously have the same
threaded type to work properly. The nut is about 1/2" thick. Because
the hole is small (for a 1/4"-20 thread), other than having a custom
4-start tap made (expensive?), are there any tricks to cut this 1/2"
long 4-start 1/4"-20 internal thread? I know that my current internal
threading tool for the lathe can't do a hole this small.

Well your choices are to buy a tap, make a tool to single point the thread,
or make your own tap.

Remember if you make your own tool to single point, you need to calculate
the helix angle of your thread and have that many degrees plus a couple
more ground into your tool for clearance. I'm coming up with about 16.4
degrees, do you would need about twenty degrees ground into the leading
edge of your threading tool.

You don't need any clearance at all on the trailing edge. In fact your tool
can look like this [//] just so long as the trailing edge is less than 16.4
degrees. For the ID thread you'll probably need to grind this way for
strength. Just don't stop the feed in the cut, as the tool has no clearance
when the tool is not moving at the thread pitch.

--

Dan

Quid Aere Perennius

Another 4-start thread question - 1/4" internal thread
 

"john" wrote in message
...
snip---
I have never tried it but I would try a tap standard 1/4 20 and grind
off all but one row of threads for clearance. Leave four full teeth on
the existing row and grind the rest off. Run the feed at 5 tpi. You
may need more relief because of the greater lead so you may have to
grind off the back side of the four teeth left to get the added
clearance needed.


John

That won't work-----not even close. The helix angle is wrong, so you won't
achieve the lead you require, and even if you could, the teeth wouldn't have
proper clearance.

Harold

Another 4-start thread question - 1/4" internal thread
 

Harold and Susan Vordos wrote:
"john" wrote in message
...
snip---

I have never tried it but I would try a tap standard 1/4 20 and grind
off all but one row of threads for clearance. Leave four full teeth on
the existing row and grind the rest off. Run the feed at 5 tpi. You
may need more relief because of the greater lead so you may have to
grind off the back side of the four teeth left to get the added
clearance needed.


John


That won't work-----not even close. The helix angle is wrong, so you won't
achieve the lead you require,

The lead and the helix angle are two separate items. The lead is .25 on
a four start 1/4 20 screw. The lead is determined by what you set your
lathe to cut at.

and even if you could, the teeth wouldn't have
proper clearance.

If you have any skill in tool grinding you can increase the relief
without touching the front profile of the four teeth to accomodate the
increased helix angle caused by the larger lead. The front profile of
the four teeth doesnt change for a four start thread.



John

Another 4-start thread question - 1/4" internal thread
 

"john" wrote in message
...
snip-




That won't work-----not even close. The helix angle is wrong, so you
won't
achieve the lead you require,

The lead and the helix angle are two separate items. The lead is .25 on
a four start 1/4 20 screw. The lead is determined by what you set your
lathe to cut at.

and even if you could, the teeth wouldn't have
proper clearance.

If you have any skill in tool grinding you can increase the relief
without touching the front profile of the four teeth to accomodate the
increased helix angle caused by the larger lead. The front profile of
the four teeth doesnt change for a four start thread.

I'd have to see that to believe it, and I've ground one hell of a lot of
tools by hand, including complex form tools for gear teeth and other
critical applications. I've been hand grinding toolbits since the early
50's, many of those year in commercial shops, including running my own for
16 years. As you might surmise, I'm no stranger to the cutter grinder nor
to threads, which I mastered long ago, and to mil specs.

Yes, I agree----in principle, the helix angle could be altered, but doing it
without a cutter grinder, or even better, a thread grinder, your chance of
success is small. Backing off each tooth exactly to the cutting line
without altering it is a tall order. I'm calling BS on your concept
unless you include specific grinding equipment.

Harold