Making a long threaded screw



Great information. Question though, is that an SAE "whole bunch", or a metric "whole bunch"?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
What I would think of doing is cut the thread from the headstock end toward the tailstock end, using no tailstock but supporting the long unthreaded part with a center rest and other outboard expedients. I would start by cutting a doot of thread normally and flipping the screw over and screwing it into a special 'nut' in the lathe jaws.
I would make a long tight fitting nut and slit one side and semi-permanently (glue?) it to the chuck so it could not move. Then screw in the just threaded part and clamp down and cut threads away from the chuck. When you need to move the screw just loosen the chuck jaws and screw in the section you just threaded.
There might be a small discontinuity where you flipped the screw, but that could be cut off later.
--
Free men own guns, slaves don't
www.geocities.com/CapitolHill/5357/
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

    Hmm ... how about soft jaws on the chuck, bored to the right radius, and single-point threaded? Your initial thread on the workpiece (cut before you mount and prepare the soft jaws) would have to be long enough so you could then spend the time to properly align the tool with the threads (perhaps by disengaging the tumbler gears until you get things in sync), but once that is done, your remaining problem is starting the thread without a lead-in groove or a shoulder. If you have a lever setup to feed the tool into engagement and back out, you could do pretty well, as long as you don't engage too far back after several thread passes have already been made.

    If there is a discontinuity there, there will be one each time you shift the workpiece. It is better to tune things to eliminate the discontinuity from the start.
    But certainly an interesting approach.
    Enjoy,         DoN.
--
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:
| > Seems to me that if you used a die you would likely get good threads, | > but if you pulled or pushed on the stock while it was passing through the | > die you would gain or lose a thread or more over the length of the screw. | > Again, it's a tolerance stackup, but over a long item it adds up to be a | > whole bunch, especially on a lead screw. | | Great information. Question though, is that an SAE "whole bunch", or | a metric "whole bunch"?
Neither. It's an "Oh, s**t!" whole bunch.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Die cutting would not be very accurate for a long thread. The die will want to wander and the concentricity will be bad. I missed the part about cutting a lead screw in the OP. Precision lead screws are usually ground. Although some ball screws and lead screws today are cut out of hardened blanks on thread whirling machines. http://tinyurl.com/5bn5d Non precision lead screws (those not used in a machine tool) are for the most part thread rolled. Long threads like those on all-thread rod are through-feed rolled on a double roll type thread rolling machine. You can easily make a long machine screw to the accuracy required by cutting the threads in sections on a manual lathe. The problem with cutting a lead screw on a lathe is that all of the accumulated error in the lathes drive train, plus deflection, run out, slide wear, and thread form errors in your tool all stack up and prevent you from making a lead screw that's as good as the one in the lathe.

That is what the optical comparator was invented to do, check thread form and thread lead. http://www.qualitydigest.com/may02/html/optcomp.html Modern manufacturers of ball screws and precision lead screws use special electronic gaging machines that check the lead, run out, and accumulated error.
Dan
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

A die won't give a precision lead to the part. Because this is a leadscrew for the lathe, that's not the best way to make the part.
Leadscrews not only have to have the correct thread geometry, but they are effectively length standards in and of themselves.
A die-cut leadscrew would work, but it would impart all the lead error into whatever parts were then fabricated when using it on the lathe.
Jim
--
==================================================
please reply to:
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Ah, I missed the part about it being a lead screw for a lathe.

So...how does one cut a clean leadscrew when one doesn't have a clean leadscrew to start from? Someone made the first one...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Screw threads have been fabricated for a long time, using manual methods like chasing. The real trick to to create the metrology needed to check them, and refine them. Basically you are talking jo blocks and maudsley.
Jim
--
==================================================
please reply to:
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Dave Hinz says...

I would guess (note the word "guess") that the better way would be to turn a couple of threads onto the end of the stock and then stick it into a precision nut which is the work holder for a grinding machine and grind the thread. The grinding wheel is always going to be a constant distance from the nut so just apply constant pressure to continuously turn the stock and work it into the nut and a screw of unlimited length could be done accurately. Maybe I can make an attachment for my lathe!
Steve.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
SteveF wrote:

I have absolutely no idea if this is going to work, but it is just an idea to save on 2nd, 3rd, etc, realignments (which probably saves nothing if you are making it for your own lathe).
1) Make first bit of threaded screw. 2) make threaded tube to match screw, tube extends through head stock, 3) make "lock nut" (or 2?) 4) mount threaded tube into jaws, 5) load lock nut onto thread part of screw 6) load threaded bit of screw into tube 7) lock threaded screw into threaded tube 8) align in jaws and tail stock 9) realign tool. 10) cut next section 11) unlock lock nut(s) 12) screw threaded rod through threaded tube and relock[1] 13) got to 10.
Theorectically you could make it as long as you like, but you need steadies on the long bit,
[1] each turn just advances the rod the size of the pitch.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.