What's new
What's new

Grinding a HSS tool to cut acme threads?

Mike W

Stainless
Joined
Jan 12, 2003
Location
Central Kali.
I just did this to cut 10 tpi. My question is how accurate can you get? My mill/drill feedscrew nuts were too loose. New ones were $45 each. So I decided to make my own. The left handed one took 3 tries before I got it right. I made them adjustable to take up backlash. I can make them adjust to fit the feedscrew.
 
I'm interested about your lathe setup (spindle speed, etc.)for cutting ACME threads. I'd like to give it a try. Would also be interested in seeing a couple of pics of your thread cutter.

p.s. My first post on this forum.
 
Welcome to the forum.

I like to use a slow speed for thread cutting. I might get shot down by some of the pros here, but I just feel that even though my feed is controled by how deep I set the compound, I still have a very broad expanse of cutting edge against the work piece. I just think this gives me a little better time to react and feel the cut.
 
RFERG43
You can't cut ACME threads in Huntsville, AL
eek.gif


Drop me an email

Ray
biggrin.gif
 
I usually cut Acme thread by grinding my tool to the exact shape; however i also have 35years experiance; and thread around 120rpm in steel; brass or bronze I'd go up to about 250 if its not a blind hole

Now if your a newbie to threading; here's the best way; make a square cut tool with the cutting edge the same width as the thread tops; cut the thread to full depth; then make your form tool to the right angle to make the Acme but make it narrow by at least 0.020"; cut to full depth again centered in the first cut
Next start to side-cut only go 0.001" each time; make one cut in; then one out (cut one side of the thread then the other)
Once your close to the root width; try your male into the nut; if tight re-cut one side; retry the nut; if still tight cut the other side; repeat till it fits snug; this will give you a tight almost backlash free nut

I had to make a new nut for a 1964 4" boring mill just last week; the guy came back to install the nut and replace some other parts; he told the boss; the new nut was better than the factory supplied items; and he's the Tos repair guy for Canada; it was the cross slide nut; 46mm dia 7mm pitch in Phos/bronze; the shaft was worn but not too bad for that many years of service; a quick lathe filing of the crests and the nut fit great with only 0.005" backlash; I could have got it closer but it was left hand and the shaft was over 8ft long; it's tough to try it up the spindle bore and get a feeling for the fit; I'd have had to remove the tailstock to try from the front of the nut

[This message has been edited by Steve Nuttall (edited 02-19-2004).]
 
Do you have an ACME tool gage ?

If you grind it to the gage you shouldnt have any problems.

Also, due to the stackup of tolerances, ACME thread lenth should never exceed twice that diameter of the threads.This isnt as big a deal with single lead threads, but its readily apparent with mutiple lead threads.

We did a study on this several years ago for the Nuclear Industry on some motor operated valves(MOV's). It was found that on alum.bronze drive bushings, that any more than twice the lenth of the diameter of the threads would cause as much as a 70% increase in horsepower to overcome friction and tolerance stackups. If grease was added into the equation, it usually upped the horsepower requirements even more. Once we stared applying that knowledge our motors lasted 2-3 times longer and our motor burnout rate was signifigantly reduced.Now we use this same testing to determine the wear of the screw, the wear of the bushing and even the lubrication or lack of. This was deemed important for the fact that several MOV's were saftey related, meaning if they didnt work when they were supposed to, it could really screw up the property values around here.
 
That is interesting information. I don't have a ACME gage yet so I used the feedscrew to check the tool blank. I used 1/4" round HSS. I mounted it in a 1/2" boring bar.

You guys will laugh at how I did the grinding. I had some inspiration and tilted my drill press table to 14 and 1/2*, as close as I could measure it with a protractor. I clamped a spin index to the table and chucked up the tool blank.

I used a 5" cup wheel on the drill press to grind one side and then turned it 180* to do the other side. After I got both sides done, I gave the tool a few degrees of relief on both sides.

I will take pictures shortly of how I did the whole job. Once I got the major and minor diameters correct (calculate twice, cut once), the actual threading went pretty smooth.
 
You need to give the advancing (cutting) side of the tool relief greater than the helix angle of the thread. This will vary with the lead and the diameter. This is very important, especially with coarse threaded acme. On a 1/2" 10 tpi acme the relief angle on the tool should be about 8-10 degrees.


[This message has been edited by Evan (edited 02-20-2004).]
 
LeBlond designed a modified square thread back in the late 1930's that was thread milled. In the 1970's I had several made for replacements on the Trepanners we built from the old LeBlond gun barrel drilling machines made for WWII.
 
You 'scrapped' gundrills???

... heart palpitations...
They were not gundrills. They had BIG spade drills that bored the holes in some of the 155mm barrels primarly.
We retrofitted and "suped up" the machines to push more modern tooling for deep hole drilling, primarly for the oil field industry at the time. Instead of running the spindle at 25 RPM, they run them at 700 plus RPM today. Motor HP went from 25 HP to 100 HP variable speed today.
 








 
Back
Top