How does thread milling work?

[gfphoto]

I hope it's okay to ask this here because I have no CNC equipment or experience.

I just saw a couple of videos vid1 (http://tinyurl.com/2j4xtg), vid 2 (http://www.vargus.com/popUp/movie.html) of thread milling and would like to understand how the cutter cuts a thread at high speed while traveling relatively slowly around the work?

Also why do the inserts have more than one tooth? I think I can see why - to continue the cut as the cutter moves down in Z - but not sure.

Thanks,

Gary Fuchs

[KiddZimaHater]

It performs a Helical move. Usually it starts at the bottom of the hole, then ramps into the wall while spiraling up and around the walls.
The entire length of the threadmill engages the wall, that's why it has more than one tooth.
They work great in shallow holes, or with tough material like Stainless or Inconel where normal taps tend to break.
http://img149.imageshack.us/img149/3450/threadmill.th.jpg

[gfphoto]

Thanks, it's making more sense, but I watched the first video again and I'm still puzzled by why the cutter teeth don't chew up what they've just cut as the cutter moves around.

I'm a little familiar with threading on a manual lathe and with the mill it looks like what might happen if the carriage movement wasn't synched to the spindle speed on the lathe. What am I missing?

Gary

[bobw53]

Kidd has it right, its really a simple process, if you look in the first video you posted there is a little graphic up in the right hand corner that shows the helical tool path as its happening.

Multiple teeth so you only have to make one full revolution, with the corresponding Z axis move.

You can also get single tooth thread mills that will do any pitch you want, the multiple tooth cutters can only do one pitch.

A few years ago, we were waiting for our CNC lathe to be delivered and had a job for a buttload of little studs, and they paid pretty good. Didn't have any type of threadmill, so ended up using a boring bar with a threading insert in a CNC mill, boring bar spinning in a tool holder and studs sticking up in a vise. 1.25" long 5/16-18, took FOREVER, single flute, single tooth, and lots of deflection and lots of free passes. It was like 18 minutes per part, about 1 minute once the lathe got there.

Advantages, good on big ass threads that aren't feasible to get on the lathe or buy a tap for. Good on tough materials and the coolest thing, you can get a full thread right to the very bottom of a flat bottom hole.

If you need to make a one off with some oddball screwy thread that you can't find a tap for(and if you could you couldn't afford it), you can threadmill it with a single tooth cutter, say a 4-15/16 - 72?

[bobw53]

Thanks, it's making more sense, but I watched the first video again and I'm still puzzled by why the cutter teeth don't chew up what they've just cut as the cutter moves around.

I'm a little familiar with threading on a manual lathe and with the mill it looks like what might happen if the carriage movement wasn't synched to the spindle speed on the lathe. What am I missing?

Gary

Now I see where you are confused, once you get it you're going to smack yourself in the forehead, I didn't get it at first either.

First, the cutters are not like a tap, each tooth is on the same XY plane, they are not in a helix.

Second, the cutter path is a helix, an arc(full circle) in XY and a corresponding linear Z move. The Z move per revolution will be your pitch.

So now lets take our multitooth threadmill, lable the bottom tooth #1 the next one up #2 and so on. Say we are cutting a 1/2-20 internal thread, the thread mill is a 20 pitch, .050" between teeth, and small enough that it is smaller than the minor diameter that has already been drilled. The cutter starts in the center of the hole and moves out to the edge and starts cutting, say a X+ (in reality, you are probably going to helix into it, but for simplicity).

Now we make a simple helical move, going counter clockwise, climb milling. The code for that is really simple and is only one line. Make a circle while at the same time moving .050 in Z. Its a G3 X Y I J Z, but since its a full circle, you can eliminate X,Y, and J.

Once the revolution is complete, tooth #1 will meet up with the thread that tooth #2 started, and so on. Move back to center, pull out and done.

[Jim Shaper]

I'm still puzzled by why the cutter teeth don't chew up what they've just cut as the cutter moves around.

What am I missing?

Gary


The cutter profile acts just like the threads of a bolt as you roll it along the threads of another bolt. If you roll one around the other, but still engaged to the others threads - the moving bolt will follow the helix that a mill's programming is traveling in. The Z move is timed perfectly so that the helix is what it needs to be to keep the cutter in the threads where it needs to be. Rather than having a lead screw timing that move, the computer drives the Z ball screw at the correct rate.

Make sense?

[gfphoto]

once you get it you're going to smack yourself in the forehead

Yes. Ouch.

Thanks guys for the very clear and detailed explanations. I needed that.


if you look in the first video you posted there is a little graphic up in the right hand corner that shows the helical tool path as its happening.

Now that makes sense.

Neat capability.

Thanks again,

Gary