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1-800miner
12-01-2010, 02:50 PM
I have to cut an internal buttress about 2 or 2 1/2 dia.
Never cut one before and could use any tips.
First question:Do I set the compound at the angle and work the tool in like v-threads ?Or do I bring the tool in at a 90 angle and cut the flat?
Secondly: How do I know when to stop? The mating piece is over eight feet long,no way to bring it to the lathe to mate it to the part being cut .
Maybe I should cut a male part first in order to sharpen my skills and to have a test part for the internal threads.

Carld
12-01-2010, 03:08 PM
I will say your going to have to make a short male test thread the same dimensions as the long shaft.

I have cut two buttress threads and I plunge cut both of them. I made a cutter that fit the thread I was copying.

It was not an easy job, there had to be a better way.

dalee100
12-01-2010, 04:50 PM
Hi,

I've done external buttress threads for punch press balls screws. I always wanted to choke the machine operator for locking the press down and the maintenance guys for torching it off. They do suck to make.

I had to grind a what was pretty much a form tool cut them. So, I had the compound feeding straight in. The tool cut on both sides simultaneously. Though it got to be quite a load on the tool as the depth of cut got deeper. I used a 1/2"x1" piece of M42 HSS to hand grind the tool.

Oh, and don't forget to make sure you make the tool for threading the correct direction, buttress threads can be run both ways.

Good Luck!

dalee

Boucher
12-01-2010, 05:17 PM
These folks make inserts for American Buttress and API Buttress threading.
http://www.toolflo.com/cats/On%20Edge.pdf they are on page 136 of their catalog.

Ron of Va
12-01-2010, 06:27 PM
Just a couple of days ago cut a left handed buttress thread (.750-7 tpi) to save an old Starrett vise I got second hand. I found the two most difficult problems were grinding the tool to match the profile of the old vise screw. Secondly, the presentation of the cutting tool to the work is also critical, a fishtail doesn’t cut it. I set the compound to 90 and fed the tool straight in to the work the proper amount for a 7 tpi. (.071) I took .002-.003 per pass.

I was fortunate enough to be able to mount the old screw in the lathe and adjust the tool post so the ground tool fit the old threads perfectly.
Good Luck.
http://www.hunt101.com/data/500/medium/ButtressThrreads.jpg

1-800miner
12-01-2010, 08:00 PM
I measured the thing today,it's 2 3/4 shaft threads look like they are 3/4" deep.
This is going to be a tough job for my wore out 12" atlas.
The form tool needs to be over an inch wide.
I wonder if I can make two tools,cut the flat and half the angle to depth,then come back with the second tool and finish the angle.
I can see it,just don't know if I explained it very well.
I am repairing a Lincoln # 7015 four post car lift.One column has the buttress screw driven by a gearmotor and the other three post have cables tensioned to the screw so it all lifts evenly.
The lower limit switch died and the screw ripped out the threads on the bronze lift nut.
I can't get a reply from Lincoln so I have to do it the hard way.

dalee: I promise I will measure and draw everything a dozen times before I turn on the lathe,thanks for the headsup,that could easily be overlooked

Carld
12-01-2010, 09:29 PM
I suggest you rough out all you can before you start using the form cutter to finish the thread. I also suggest you try it on a piece of scrap brass or aluminum before you do the real nut.

That is going to be a very hard load on your lathe. Don't get in a hurry and take very light cuts.

How are you going to hold a 1" square cutter in a boring bar? Are you going to weld the form tool to the end of a bar to make the threading tool?

Man, that can get real intense real fast.:eek:

whitis
12-01-2010, 09:48 PM
That seems like a pretty wide chip to take on your lathe, even with taking multiple shallow passes.

Consider making a tool which is only 1/4" wide with the requisite angle 1/8" wide and a 1/8" wide flat. Or go 3/8". Set your compound parallel to the axis of rotation so you can use it as a fine offset relative to the lead screw position. Then take lots of passes to rough it out:

4321
765
98

Advancing less than half the tool width for each one, longitudinally.

You can also do this, reversing left to right, with a form tool.

You haven't given the pitch and angle of the thread, that I can see.

If your threads are 0.75" deep on a 2.75" shaft, then the bore of your nut would only be 1.25 which only gives you 1/2" shaft to support a 3/4" deep cutter, less a margin for clearance.

Last time you were lucky in that the nut apparently stripped out while the lift was on the floor. If it strips out or has some other failure while someone is under a car ...
Make sure you round off the inside corners so you don't create stress risers.

dalee100
12-01-2010, 09:53 PM
I measured the thing today,it's 2 3/4 shaft threads look like they are 3/4" deep.
This is going to be a tough job for my wore out 12" atlas.
The form tool needs to be over an inch wide.
I wonder if I can make two tools,cut the flat and half the angle to depth,then come back with the second tool and finish the angle.
I can see it,just don't know if I explained it very well.
I am repairing a Lincoln # 7015 four post car lift.One column has the buttress screw driven by a gearmotor and the other three post have cables tensioned to the screw so it all lifts evenly.
The lower limit switch died and the screw ripped out the threads on the bronze lift nut.
I can't get a reply from Lincoln so I have to do it the hard way.

dalee: I promise I will measure and draw everything a dozen times before I turn on the lathe,thanks for the headsup,that could easily be overlooked

Hi,

Honestly, and with no disrespect meant, your 12" atlas isn't capable to do the job. It's not going to have enough power or rigidity to make those cuts. I was using an 18" Colchester with a 10hp motor and it had all it could do. A 15" LeBlonde with a 5hp wasn't up to it. Though I was making ball screws from 3 1/4" 4140.

Unlike vee type threads, the tool has to cut both sides at the same time. You maybe able to rough out a small part of the thread, but at some point you will need form the whole thread. And it will be at the bottom half of the thread where the load is the greatest anyway.

I would honestly pass on this job or find a lot bigger lathe to use.

dalee

Don Young
12-01-2010, 09:58 PM
While you are making the male test piece, you might want to make a tap or chaser. Then you could rough out the thread on the lathe and finish it with the tap.

It seems to me to be a real challenge for a 12" Atlas. I have one and mine is certainly not very rigid. I would probably try to cut it in several sections and would probably not even use a tool that cut the entire form. That would take some careful planning and measurement but I think it is feasible. Practice a lot on scrap before you start the real thing.

A good sharp tool with the correct angles will certainly make things easier.

camdigger
12-01-2010, 10:05 PM
A 1" wide by 3/4" deep thread sounds like an application for thread milling to me.

Toolguy
12-01-2010, 11:14 PM
Might be able to thread mill it with a dovetail cutter. Or get a couple of carbide inserts for that thread and make a thread mill. I've done that a few times for specials.

dp
12-01-2010, 11:36 PM
How about using a buttress thread insert?

http://www.usshoptools.com/current_year/threading_grooving/carmex/pdf_laydown/laydown_american_buttress.pdf

macona
12-02-2010, 01:45 AM
3/4" deep threads on a 12" atlas? Not going to happen. Your gearbox wont even be able to match the pitch of the thread. This is going to take a big lathe, or a cnc mill and thread milling.

And personally I would never feel good about a part of mine going on a car lift. eek..

oldtiffie
12-02-2010, 04:29 AM
Good advise Macona.

Its usually difficult to cut a thread with a lead 2+ x the lead of the lead-screw - often considerably less.

Despite what some say, it is not all that difficult to cut that form and size thread on a small(er) lathe at all. I will cover that later if needs be. Just about everybody here will have all that is needed in terms of the tools required to cut the thread.

But before some start throwing thread milling - particularly internal thread milling - about, I suggest that they take a chill pill and read this extract from Machinery's Handbook.

http://i200.photobucket.com/albums/aa294/oldtiffie/Machinery_HB27/Mach_HB_gear_milling1.jpg

I bought this machine from the UK - no NOT China but the UK - for such stuff amongst lots of others for high-helix (small lead) spiral gears as well as thread milling - also for small leads.

http://www.hemingwaykits.com/acatalog/The_Quick_Step_Mill__.html

http://i200.photobucket.com/albums/aa294/oldtiffie/Lathe_misc/Lathetail-stock12.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Lathe_misc/Lathetail-stock7.jpg

My limitation is my 3mm lead (~ 8tpi) lead-screw.

Ian B
12-02-2010, 04:32 AM
As others have said, this is really too big a job for the lathe that you have. However, if there's one thing HSM'ers excel at, it's stuffing a quart into a pint pot and doing jobs waay past their machine's normal capabilities.

Cutting the whole sloping flank with a full width form tool will just cause chatter, especially as it'll be on the end of a boring bar. What you could do, however, is work along the sloping and vertical faces cut by cut.

Rough the thread out, and then clean up the sloping and vertical faces separately. Feed down the vertical flank with the cross slide, and then set the compound slide to 45 degrees and work along it. Both flanks might take 5 or 6 passes each. This way, you can use a form tool that's only 1/5th of the sloping flank's total width.

Good luck, post pics!

Ian

oldtiffie
12-02-2010, 05:15 AM
You've got it Ian.

Eating an elephant can seem a daunting task - if you want to do it in one go - but its a lot easier with lots/more chewable and digestible bites.

I'd have done the roughing using what are essentially "parting tools" and the cross-slide for the straight plunge in (as for square threads) and then the top-slide set at 45* for the 45* flank.

The rest is just a clean-up - taken slowly.

The cuts are making a spiral not a groove and with that size helix angle I'd be tilting the screwing tool/s.

I don't know how the Law applies in the OP's state or country, but here in OZ car hoists (as in commercial garages etc.) come under the "Cranes, Hoists and Lifting Gear Code" and all components parts or assemblies must be designed, made to and Inspected in accordance with the Code.

The Code also covers and requires testing for replacements if required.

The hoist is required to be checked by an Inspector and load-tested at set intervals.

If the hoist fails a test or is overdue it is regarded as being "out of test" and is not to be used until it is "passed".

There is no way I'd make that nut here.

The hoist has clearly had a substantial failure and must only be repaired and probably tested and Inspected in accordance with the Code.

I don't think I'd like to be served with a writ or a warrant if anyone got hurt or worse.

JoeLee
12-02-2010, 07:59 AM
I would go with an insert for the thread you need to cut.
Look at the link below and you can download thier program for selecting the right insert for the job. I use it all the time, it takes the guess work out of everything and they threads come out perfect.
http://www.vargus.com/vardex/template/default.aspx?pCatId=7

JL..............

1-800miner
12-02-2010, 10:23 AM
Well I better tell "the rest of the story".
It's my own car lift.I bought it at a military auction years ago.Bought four,sold three. In hind sight I should have sold two.
They had three phase three horse motors.I have single phase.
So I replaced the motor with a 3/4 horse with a hundred to one gear reducer in between(I know, but it is all I had)
It was slow but so am I.It worked great for twenty years,until the limit switch quit. At x100 torque against the weak side of the threads it failed.
If I can repair the lift I will change out the motor and gear reducer to somthing more modest.

Carld
12-02-2010, 11:04 AM
Ok, from the start I didn't think your lathe is up to the job. I suggested roughing most the metal out and then finishing with the form tool and that may work. Others have suggested using a smaller form tool and stepping it out to clean up the finished thread but that is difficult to do.

I guess you will be using some grade of brass for the nut. First you need to make a male test stub of the thread probably in aluminum. Then try to make a nut using aluminum to see if you can even do it. Roughing out the internal thread will be tricky and I suggest you do the cutting on the back side so you can watch the progress. It will be very tricky to get a good finish using a smaller cutter because of all the overlapping passes required. You will probably have to use a full size cutter to get an acceptable finish and doing that with a boring bar is going to be even harder.

In the end I think it may be better to have it done on a CNC lathe and they would not have to use a full size form tool to do it.

1-800miner
12-02-2010, 12:19 PM
Don't know if I can get it cut,but I sure started an interesting thread!
I sure like the looks of that tool post milling machine,I can see lots of possibilities.
Prolly buy a new car lift for the repair price ,but I gotta try, just because.

macona
12-02-2010, 12:45 PM
Here is another option. Cast a nut. Maybe something like silicon bronze. Make a mold over the screw and then cast a core from that. I am betting you can get good enough surface finish from the cast for a car lift. Not talking breach block lugs here.

Ian B
12-03-2010, 01:28 AM
If you do try it by cutting the profile in steps, don't worry too much about getting the sloping flank perfect - it doesn't do anything, it's only the perpendicular flank that's taking the load. It needs to be somewhere close to give the loaded flank the required support, but it most likely won't even be touching the threaded shaft.

Ian

1-800miner
12-03-2010, 09:49 AM
Never thought about casting! I have a lot of high density styrofoam. I can chuck that stuff up and cut threads until I get a good one. Then cast it.
I will have to play with cutter shapes so I cut rather than tear the foam.
Any thoughts on that?
Maybe I can fit a quart into a pint jar?

macona
12-03-2010, 02:34 PM
What I would do is take the screw out and make a box around it about 2x the length of the nut. Then coat the screw in mold release and use silicone mold material to pour half around the screw, let it harden, put release on the new rubber and pour the top half. Now you have a mold you can make a core for. Use silica sand and sodium silicate to make a no bake core. Or a baked plaster typed core might give a better surface finish. Use that to cast around for the new nut.

I have never seen really great results with the lost styro methods.

1-800miner
12-03-2010, 02:52 PM
What a great idea. I should have titled this thread "how not to cut a buttress thread"

macona
12-03-2010, 03:27 PM
Or "Saving your buttress one thread at a time."

ajfreeze
12-31-2011, 12:47 PM
I want to know how this came out....anyone else?

JoeLee
12-31-2011, 04:09 PM
I had to cut an American butress thread inside a collar to match a screw shaft. I bought the full profile insert for my Valenite internal boring bar and it was a snap to cut. All I had to do was make sure the bar was parallel to the work and the height was set right and in about 20 passes I was done.

JL.....................

Lew Hartswick
12-31-2011, 07:06 PM
I want to know how this came out....anyone else?
Me too. A year should be enough to have a final answer. :-)
...Lew...

mike4
01-01-2012, 12:23 AM
You've got it Ian.

Eating an elephant can seem a daunting task - if you want to do it in one go - but its a lot easier with lots/more chewable and digestible bites.

I'd have done the roughing using what are essentially "parting tools" and the cross-slide for the straight plunge in (as for square threads) and then the top-slide set at 45* for the 45* flank.

The rest is just a clean-up - taken slowly.

The cuts are making a spiral not a groove and with that size helix angle I'd be tilting the screwing tool/s.

I don't know how the Law applies in the OP's state or country, but here in OZ car hoists (as in commercial garages etc.) come under the "Cranes, Hoists and Lifting Gear Code" and all components parts or assemblies must be designed, made to and Inspected in accordance with the Code.

The Code also covers and requires testing for replacements if required.

The hoist is required to be checked by an Inspector and load-tested at set intervals.

If the hoist fails a test or is overdue it is regarded as being "out of test" and is not to be used until it is "passed".

There is no way I'd make that nut here.

The hoist has clearly had a substantial failure and must only be repaired and probably tested and Inspected in accordance with the Code.

I don't think I'd like to be served with a writ or a warrant if anyone got hurt or worse.
It is possible to make parts as long as you make them using material which is the same or higher quality as original .
The testing can be completed by a "competent Person"
A lot of us exist in the maintenance field , if you work in these areas there is no scrimping on material or tolerances .
I also make replacement parts for final drives in machinery at times , it is not an area to be taken lightly ,however even original manufacturer supplied componemts can and do fail with often serious consequences.
If you always follow the original specs and use the correct material , you will find that there isnt a problem, unless you create it .
Michael