1 1/4"- 4 TPI Acme lead screws

P. Waller

Brass
Former Member
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Mar 10, 2018
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Some may be interested in doing such a job, I was handed one this morning and this is how I do it. It is only 12 parts in 360 brass hex stock and the time estimate from my employer is 21.45 hours for the lathe work. My methods may not be the same as how the guru Youtube "how to" video producers may do it but it works for me and the machines available.

The part is 23 5/8" long but the thread is only 10 3/8 long.
Sample


A sample was supplied by the customer.
Measure the sample Pitch Diameter over wires, the .25" lead is wider then the spindle on many micrometers so a .100" gauge block spans the 2 top wires.


My employer was not going to spend several hundred dollars on ring gauges for a small job so I made one at the lower PD limit that would fit the sample.



Face to length and center drill the threaded end, put the chamfer for the start of the thread on before the threading operation saving a tool change later.



The threading starts Monday, will update then.
Used a Vardex horizontal insert internal threading bar at 375 RPMs, will use a Kennametal Top Notch tool for the external threads, I suspect that it will have to be run somewhat slower.

This is done in a Romi/Bridgeport 15 X 40 lathe which is a sort of CNC Manual hybrid built in the 1990"s, these are very nice machines with VERY easy to learn conversational controls, knowledge of G Code is not required for the most part unless the work requires a feature that the conversational input will not do. They have a lot of safety features in the software that will not run a program if there are obvious tool conflicts or math errors. They are the perfect machine for a hobby shop but a bit to expensive even at 20 years old at $10.000-$20.000 used.
 
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Why are you using hex material instead of round bar?
 
Looking good, keep us posted.
Chuck, if you look at the first pic you can see the hex on the far end.
 
Why are you using hex material instead of round bar?
That is what the customer wants, I suspect that the hex is a drive feature to fit machines that may be well over 70 years old.

This is a guess of course, it is a rare occasion that I know what the part does.
 
Was instructed to finish another job Monday morning and resumed the lead screw job today, most of the finish diameter turning is done. Tomorrow the threading begins using 2 Kennametal Top Notch tools, a full radius 1/8" wide insert for the undercut and a 4 TPI Acme insert. Both of these tools have had a portion of their ends milled away in order to clear the live center on small threads such as these, they are 1" square shanks.



Whilst the 12 parts were running (5 hours) I had the time to assemble a .4"-.7" Mitutoyo dial bore gauge that I received by UPS this morning. The excellent Mits quality that I have experienced over the last 30+ years aside from the case, it is well made but will not fit the assembled tool with its 8 ball anvils. I only use the boxes that tools come in when the tool has many small parts such a a dial bore gauge, anvils, spacers and such.
$370.00 from MSC. Normal Mits quality tool. The acme thread pitch gauge from SPI cost me $96.00 but few people buy them, it has a 1TPI leaf, who makes such threads (-:

 
Finished this afternoon and the threads turned out nicely as one would expect with 360 brass, 265 Rpm's.

The thread lead is .25" which is right around 66 inches per minute feed rate at that speed, the next spindle rpm is 375 which would be 94 IPM, this machine has a max feed rate of 100 when in a rapid move. I chose not to run a 20 year old machine at 95% of its maximun feed (-:

The first pass was .006" DOC, all passes afterwards were .004" DOC, the last 2 finishing were .001" DOC with 2 spring passes.

Did not check the time but it was slightly longer then the estimated 21 hours

 
Very nice looking threading job P.Waller. Wonder why 360 brass was requested and not 954 alum. bronze. That's a big shaft/screw for such a soft material.
 
Very nice looking threading job P.Waller. Wonder why 360 brass was requested and not 954 alum. bronze. That's a big shaft/screw for such a soft material.

I do not know but suspect that they would rather have the screw wear out then the nut, The customer is a 100 year old company and these are replacement parts for machines that may be 70+ years old and the original parts were brass. We just copied the sample.

The Top Notch tools are excellent, one tool will hold many inserts, 30 Deg. metric trapezoidal threads, Whitworth, American buttress threads, unified and metric 60 Deg. V-threads, square and full radius grooving and thin wall parting. A bit pricey for a hobbyist but versatile if a shop does a good deal of small runs.
https://www.kennametal.com/en/products/20478624/62318451/56148232/56148278/56148748.html
 
LOVE this thread!! only thing I can comment on, is the lathe is sitting on 4 x 4's and not level aligned...sighhhhhhhhhhhh.
 
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