How to cut buttress threads?
I have to cut an internal buttress about 2 or 2 1/2 dia.
Never cut one before and could use any tips.
First questiono 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.
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.
It's only ink and paper
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.
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.
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 90º 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.
Last edited by Ron of Va; 12-01-2010 at 07:30 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
Last edited by 1-800miner; 12-01-2010 at 09:03 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.
It's only ink and paper
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:
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.
Originally Posted by 1-800miner
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.
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.