how to cut DOUBLE LEAD left hand acme threads

[bmuss51]

I need advise on how to cut double lead left hand acme threads on a manual lath. and the lathe has electronic gear shift , so it will be hard to just index the spindle 180 degrees to start the second cut. yes' i have done right hand double lead threads but it was on an old geared lathe. but that was 35yrs ago. and the real kicker is the piece is 5 foot long with the same size shaft at one end with no under cuts at threads end and no fallow rest avail. because of readout attachment.
could anyone give step by step intructions?

 

[steamer]

What size ACME?

Diameter and pitch?

Dave

 

[Maryak]

1. How long is the threaded portion of the 5' ?

2. Once you know the pitch of the thread, (as opposed to the lead), then second thread can be cut by moving the compound slide a distance equal to the pitch having 1st made sure it's tracking parallel to the bed. This assumes you have no threading dial. If you have a threading dial and the thread to be cut is of the same genre as your leadscrew then the second thread is cut using opposite numbers on the dial. Otherwise back to method 1.

Hope this helps

Best Regards
Bob

 

[Ken I]

Following from Maryak's comments - what is the lead of your lathe's leadscrew ? (relative to the lead on your L.H. thread)

A 5' long thread (assuming its 1-2" diameter) is not going to be possible without some form of travelling steady.

No undercut ?? ie you want the thread to wash out - over how many pitches ? I've only ever seen specialist screw cutting lathes (Such as a Herbert Cri-Dan which was cam operated) capable of doing that - but maybe someone on this forum has a trick rig for doing it.

Those sharp thread terminations on some Acme threads are because the thread was milled rather than turned.

Ken

 

[MachineTom]

While not Acme threads I've made a few double start threaded pieces. With a double cut thread what you really do is cut half the depth, of pitch depth that would be cut normal for that thread, so for a double start 10TPI, you will only cut the thread the depth of a 20TPI thread. To cut that thread, first do the thread as normal, noting the setting of the compound dial at the start of the first thread, that the depth will be 1/2 the normal 10TPI. After cutting the first thread, reset the compound to the setting recorded for the first thread. Now you want to take the amount of the pitch of the cut thread which is .050*4=.200" and crank the compound out that amount. This is now the point to start the second thread, you will use only crossfeed until you touch off, then infeed with the compound as normal.

The reason for *4 the cut pitch is that the sine of 14.5° is .250 this being an Acme thread. Just remember to touch off with the crossfeed, then feed with the compound. The same way for the nut, and note to flop the compound for the internal threads.

It takes longer to type this than do it.

 

[bmuss51]

i found out that the thread is 1 1/4" 5 left hand and it can also have a under cut at the end of the threads which goes to the full dia. for a length of 6". but the job is in brass that is 4' long on one shaft and stainless on the second piece that is a total of 6'with the 6" section on that shaft also. i can use a steady rest but not a fallow rest because of the y readout. and for the nut it is case iron and might have to be cut also if a tap can not be found.
and thanks to all that replyed,but sense i am giving more info maybe you guys can give me some more insight as to a setep by step with things to watch out for which will give problems.

 

[steamer]

You could do this two ways that I can think of ....I attached pictures of a job I did below.

I'm assuming you have a 8 pitch leadscrew. If you have a 5 pitch it may be a good deal easier.

The offset method.

Set your compound parallel to the work piece. Your going to need a stout lathe as your going to plunge cutting this thread if you do it this way.

You need to set your gearing to 2 1/2 pitch.
Then cut a single thread for the length required In brass or aluminum this should not be a problem
That attached photo shows a 2 3/4 - 4 double start

Cut to full depth. DON"T DISENGAGE THE HALF NUTS! Back the lathe up to take successive cuts.
Now return to the starting position and using the compound, advance the tool by 0.400" with out changing the half nut location. You should be in between the threads of the previous cut. Cut this thread to full depth.

You may need to rough the thread with a roughing tool that is smaller than the finishing tool if only to take the bulk of the material out of the thread and improve tool life. Tedious!

The other way to do this would be the index method.

Set the lathe for 2 1/2 threads. Set the compound for 14.5 degrees. and grind and align your tool accordingly.
This method allows you to cut on only two sides of the tool as opposed to 3 with the previous method. If your lathe isn't that stout...do it this way. Again DON"T DISENGAGE THE HALF NUTS!

As to the cut, I would consider cutting it between centers here as the lathe doesn't really allow for offsetting the gearing. To do that I would mount a 4 jaw chuck and chuck up a small piece of round stock. Turn a 60 taper on this piece. It will run dead true provided you don't move it after machining the taper.

Now mount the work between centers and mount a drive dog on the piece. Mark two opposite jaws on the chuck 1 and 2 with a magic marker.

No drive the part with jaw number 1 and cut the thread as usual. Once to full depth, back out and rotate the part/dog to be driven by the opposite #2 jaw. DO NOT REMOVE THE DRIVER DOG! Again cut to full depth.

While your cutting these threads, you will need to lace the part against the driving chuck jaw so that it stays in contact at all times.... a piece of leather shoe lace works great here.

Either way will work. or perhaps you will take this advice and your situation and come up with a better way for you.

If your cutting stainless, I would do the index method which would reduce the cutting forces considerably.


Good Luck!

Dave

 

[bmuss51]

sorry for the time delay, but the job had got pushed back till next week.
i will be using a lebond servo shift lathe and i have avail carbide inserts available, but for the 5 thread double cut what insert should i use like a 5 pitch acme or a ten pitch insert?
could anybody tell me the depth of cut using the compound dial set to 14.5 degrees (like .06" on the compound dial) and again this is for a 1 1/4" dia shaft that will have a double lead 5 pitch acme thread.
could i get a good thread sense it is odd numbered thread do the index of the two leads by doing even numbers for the one lead on the threading dial and for the other lead use the half numbers?

 

[kvom]

I am assuming that from your description that each of the two threads is 5 tpi and that there are 10 threads/inch total. If in fact it's half that then you need a 2.5 pitch insert, which I doubt exists as the helix angle would be very steep.

You need a 5 pitch insert, since since that's the pitch of the helix. Depth of an acme thread is half the pitch, but in this case it's a 10 pitch to use, so the depth is 1/20 or .05. I suspect with brass going straight in is just as good.

Using the thread dial will only help you if the leadscrew is 5 tpi or a multiple. You need to cut the threads without disengaging the leadscrew, reversing the feed or the spindle to return the tool for each pass. See Dave's description above. Then use the compound to advance the tool by one pitch and cut the second thread the same way.

I would make the nut first. A tap will be expensive, but these guys might me able to help: http://www.westerntap.com/

Or you could save a lot of grief by outsourcing to a CNC shop if this is a 1-off part.

 

[bmuss51]

the thread is 5' long and i do not know of cnc equipment local that can turn 5' plus at one time of that size.

so from the answers i got so far: if i set the shaft up on a manual lathe i would use a 5 pitch insert and cut to a depth of .05" with the compound turned to 14.5 degrees and once the feed is engaged you let it engaged and go for the repeat cuts by backing off the cross slide and reversing the spindle to backup for the next cut. and then to get the second lead i would advance the cross slide .400" and repeat the above steps.
does this sound correct or am i missing something?

 

[kvom]

You would advance the tool by .100 to get to the second lead. Since you can't disengage the leadscrew you need to advance using the compound. Or else, turn between centers using a faceplate and lathe dog so that you can turn the work 180 degrees.

I suggest you try your technique on a short piece first.

 

[MachineTom]

The profile you will cut is a 10 TPI profile..060 depth, the TPI of the gearbox is set at 5 TPI, The helix angle will likely cause rubbing of the cutting bit, and will need to be relieved befor you start. On acme threads unlike 60° threads each cutter has to be made for the exact profile being cut, you can't use a 5 tpi cutter to cut a 10 tpi profile.

After cutting the first thread you offset the compound .100 and cut the second thread.

You must have a follow rest made up or you will be unsuccessful in any threading of a 5' work piece.

If this is a double start 5 tpi, needing to cut 2.5 TPI for each, it would work the same, but you need a different lathe.

The print should tell the story on the real pitch

 

[f350ca]

Just throwing ideas in the air, but wouldn't you use a cutter ground for 10 tpi, cutting to the depth for 10 tpi, but it advances at 5 tpi. Then the second thread falls between the first helix.
Ignore this Tom just said it while I was typing.

 

[kvom]

Since you have a steady rest you might be able to cut the threads a foot or so at a time moving the rest along the work.