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machinistmarty

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#1
i have to cut a stainless steel, 3", 1 1/2 stub acme on a manual tool mex lathe . it has no follow rest, the thread is about 4' long. Any suggesstions? i've cut many acmes including double lead but never a pitch this steep or a thread this wide. the crest of the thread is .333 the lead is .666.
 

machinistmarty

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#3
I'd think about building a follow rest...
yea, that was my first thought. I also thought about cutting a v thread to the depth, then cutting a no 4 acme , with my compound at 180 and then advancing the width with the compound, kinda like doing a double lead but just making the thread wider.
 

brino

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#5
What's the total length / how much is sticking out of the chuck?

Can you add drill a centre hole and use a live centre in the tail stock?

-brino
 

4gsr

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#6
If this is for a paying customer, And he's willing to pay the additional cost, I would find someone with a thread milling machine. Be a heck of a lot easier than trying to singlepoint such a very coarse thread as this. If you're in the Dallas/ Ft Worth area, I know of a place up there I used to use many years ago for thread milling lead screws. They are still in business, I don't know if they still do thread milling, only one way to find out.

But if this is for yourself, Follow the advise already given, a follower rest is a must. The jaws may/will have to be modified with some kind of wide jaw to straddle at least two threads at each position of the jaw. Think this out a bit or it will eat your lunch in a heart beat as you get into it!
My thread calculator says the thread has a nominal depth of .210" and a root width of .280". And of course a pitch if .666". Will your lathe cut this pitch?

I would cut this thread in steps using a say roughing tool ground to a 4-pitch Acme threading tool. Since you are cutting stainless steel, that's nightmare no. 1. I would work on getting the thread depth down to about .125-.150 deep first. Sidestep the threading tool, both directions to get the crest width of the thread close to .300". I forgot to tell you to set your compound at 90 degrees to the cross slide or parallel with the work piece. Once you get it about .125-.150 deep, work on getting down to around .200-.210 deep. Once to depth, start side stepping the threading tool to get the .280 wide root width. As you get to the proper root width and depth, use a freshly ground threading tool with a nose width of about .150" wide. Any wider width is probably going to induce chatter into the thread. You don't want to do that. What ever you do, don't use a full width threading tool. It takes a pretty stout lathe to handle a threading tool to full width. Especially cutting stainless steel. Be sure to use a good cutting oil such as Mobil Mobilmet 766.

Ken
 

machinistmarty

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#8
th
If this is for a paying customer, And he's willing to pay the additional cost, I would find someone with a thread milling machine. Be a heck of a lot easier than trying to singlepoint such a very coarse thread as this. If you're in the Dallas/ Ft Worth area, I know of a place up there I used to use many years ago for thread milling lead screws. They are still in business, I don't know if they still do thread milling, only one way to find out.

But if this is for yourself, Follow the advise already given, a follower rest is a must. The jaws may/will have to be modified with some kind of wide jaw to straddle at least two threads at each position of the jaw. Think this out a bit or it will eat your lunch in a heart beat as you get into it!
My thread calculator says the thread has a nominal depth of .210" and a root width of .280". And of course a pitch if .666". Will your lathe cut this pitch?

I would cut this thread in steps using a say roughing tool ground to a 4-pitch Acme threading tool. Since you are cutting stainless steel, that's nightmare no. 1. I would work on getting the thread depth down to about .125-.150 deep first. Sidestep the threading tool, both directions to get the crest width of the thread close to .300". I forgot to tell you to set your compound at 90 degrees to the cross slide or parallel with the work piece. Once you get it about .125-.150 deep, work on getting down to around .200-.210 deep. Once to depth, start side stepping the threading tool to get the .280 wide root width. As you get to the proper root width and depth, use a freshly ground threading tool with a nose width of about .150" wide. Any wider width is probably going to induce chatter into the thread. You don't want to do that. What ever you do, don't use a full width threading tool. It takes a pretty stout lathe to handle a threading tool to full width. Especially cutting stainless steel. Be sure to use a good cutting oil such as Mobil Mobilmet 766.

Ken
this is a job at work, an obsolete part. Lathe is a 28 inch swing Toolmex. I do have no 4 acme carbide inserts. And that was exactly the way i was going to attack this job. i've done a lot of acmes. lots of double lead acmes. just never one with a pitch this steep or this wide of a thread. It's 316 stainless and i know its going to be a pain in the butt. i do wish i had a follow rest but no such luck.Jaws are good its a 14" four jaw cushman chuck. Lathe is quite stout and can handle it. chatter is my biggest concern. not sure if i can measure over wires or not, simply because it's a stub acme, but i do have the lock nut that goes on it. (its actually a stop, this is a jack.) it actually will get a 4 pitch v thread on the other end so i can clamp on that end. i'll probably turn me a short recess on the end the double depth of the thread to make it easier to get to the right depth. I'll do this job next week, I'll try to post a few pics
 

4gsr

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#9
Ok, no follower rest. Chuck on the material where there is about 24" sticking out of the chuck. Cut 22" of thread. Pull the other 22" plus inches out of the chuck, support in steady rest with a sleeve or cat head to run in steady rest and cut the rest of the thread. I really think your only going to be able to cut 12" of at a time. Which means having to un chuck and re chuck at least four times to complete the thread.

EDIT: If it's of any help, the M.O.W. is = 3.1353/3.1004" using .328" dia. wires. Yeah, I know, there is no way to hold three wires to check this.
So a single wire measurement is 3.0676/3.0502 with exactly 3.000" OD material. The is based on a 2G fit thread.
 
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machinistmarty

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#10
Ok, no follower rest. Chuck on the material where there is about 24" sticking out of the chuck. Cut 22" of thread. Pull the other 22" plus inches out of the chuck, support in steady rest with a sleeve or cat head to run in steady rest and cut the rest of the thread. I really think your only going to be able to cut 12" of at a time. Which means having to un chuck and re chuck at least four times to complete the thread.
Ok, no follower rest. Chuck on the material where there is about 24" sticking out of the chuck. Cut 22" of thread. Pull the other 22" plus inches out of the chuck, support in steady rest with a sleeve or cat head to run in steady rest and cut the rest of the thread. I really think your only going to be able to cut 12" of at a time. Which means having to un chuck and re chuck at least four times to complete the thread.
there is a relief at the end of the acme threads so i could thread half then turn it around and cut the other half, picking up the thread would be the same either way.Good idea, i may give it a try. also i could thread the other thread where it attaches while chucked that way. Will wires bottom out being a stub? havent figured the size yet but i could probably get away with some 3/8" drill rod and bridging the gap on the bottom with a piece of tool steel. thats how i do double leads. Thanks alot!
 

machinistmarty

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#11
there is a relief at the end of the acme threads so i could thread half then turn it around and cut the other half, picking up the thread would be the same either way.Good idea, i may give it a try. also i could thread the other thread where it attaches while chucked that way. Will wires bottom out being a stub? havent figured the size yet but i could probably get away with some 3/8" drill rod and bridging the gap on the bottom with a piece of tool steel. thats how i do double leads. Thanks alot!
Also as a side note, I've been doing manual machining for 33 years.(same company,papermill) Never be afraid to ask questions!
 

Asm109

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#12
Why 316 SS? Nothing about a lathe lead screw cries out for that. I'd buy a chunk of Stress proof long before I used 316.
 

machinistmarty

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#13
Why 316 SS? Nothing about a lathe lead screw cries out for that. I'd buy a chunk of Stress proof long before I used 316.
this is a jack on the fourdrainer of a paper machine. its wet all the time, has to be stainless
 

4gsr

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#14
Not familiar with paper mill operations that much. What I do know, the water used in processing can be very corrosive in some of the processes. That's the reason for using 316 SS. It's either this one or go to a duplex stainless steel. The 400 series is probably out of the question. The PH series may work, be a little bit easier to cut in the 2H1150 condition for 17-4PH. I'm sure the paper mill has gone thru all of the options of materials over the years and settled on 316 ss.
On the wire size to use, you may have to cut a flat on the wire so it will clear the root of the thread. 3/8" drill rod will be too big. Need something in the range of .3125 to .3440". Ken
 

machinistmarty

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#15
Not familiar with paper mill operations that much. What I do know, the water used in processing can be very corrosive in some of the processes. That's the reason for using 316 SS. It's either this one or go to a duplex stainless steel. The 400 series is probably out of the question. The PH series may work, be a little bit easier to cut in the 2H1150 condition for 17-4PH. I'm sure the paper mill has gone thru all of the options of materials over the years and settled on 316 ss.
On the wire size to use, you may have to cut a flat on the wire so it will clear the root of the thread. 3/8" drill rod will be too big. Need something in the range of .3125 to .3440". Ken
i tried .3125 today and it bottomed out, i know i can grind a flat. I looked up the stub acme chart in the machinist handbook, and it gave a range of .318-.428 for wires on a 1 1/2 lead stub acme. I will just be doing a comparison check with the old shaft so i don,t think it would matter much. I do have a stop nut to check the threads. And yes the water is very corrosive. the fourdrainer is where they spread the slurry on the wire and start sucking the water out of it. Were a coated paper mill and everything is bleached and enhanced for brightness etc.
 

4gsr

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#16
Another good selection for steel for high chloride and acidic environments is Inconel 925. You think 316SS is tough to cut, this grade is right there with it, too!
 
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