[dgoddard]

I'm about to do my first internal threading operation. The thread is a 1-5/8" - 16 tpi. From way steel machined on turning and boring figure it is probably in the Brineel hardness range of 175-225. My lathe is a converted Grizzly 0602 (10x22). Using Mach3 wizzard for G-code. On smaller dia stock I used 500 rpm for external threading. With the larger dia and my first attempt at internal, thought would ask for some recommendations from those who have been there, done that.

Merry Christmas

Opps, left out that will be using laydown indexable A60 carbide insert on 3/8" bar.

[PixMan]

Merry Christmas to you too.

Use the fastest speed that you feel comfortable with for this. The materials is fairly soft for carbide tooling, and with a threading operation the feed rate is high. Still, you would be hard-pressed to exceed the limitations of the carbide insert. The bigger danger would be to go too slow, as you may chip the carbide from too much force at low speed.

Your real limitation may be the control itself and it's limited maximum processing speed for all that happens in a threading cycle. That 500 rpm you used for an external thread is fine, but if the machine cycles faster you'd be OK at twice that speed.

[Willyum]

I think the thing to consider is how fast the tool will be traversing. For every rotation of the Spindle the tool will move 1/16"(16TPi). So, your feed per rev. is .0625! At 500rpm your Feed in Inches per Min. is 31.25. At 1000rpm your machine will be threading at 62.5 IPM!

The Dia. of the part to be threaded, the hardness of the material and the specific insert should have a recommended SFPM (surface feet per. min.) to consider as well. The insert manufacturer should have tables telling you what the recommended SFPM is. (I'm not familiar with the Brinell Hardness scale and couldn't find a decent conversion to Rockwell to give a more proper answer. So, if the last paragraph was redundant... please, disregard)

I would recommend not to exceed 200rpm, but if you have had success threading a similar TPI thread at 500rpm start there and adjust speed as you go.

My 2 cents,
Will

[PixMan]

At 200 rpm he'll have more problems with chipping carbide inserts than he would with exceeding a maximum feed rate. Carbide threading inserts crave speed. If the machine has a fast-processing control that can keep up, run it.

I have successfully cut 16P threads at up to 2500rpm on Fanuc 18iT and 16iT controlled machines many times. At up to 2000rpm, no problems. Above that, you are best to have a little extra room at the beginning and end of the Z axis travel to avoid having the X axis start retracting before it's reached the final Z position.

[Willyum]

I see what your saying Pixman and your point about leaving a little engaging/disengaging room on the threading pass is a good idea.

However, I've threaded 300-400 series SST with carbide inserts as low as 80 rpm on a manual lathe with no "chipping" problems. My recommendation isn't for someone who can cut threads at 2500 rpm in a turning center, it's for someone about to cut their first ID thread. Also, turning threads on a Hardinge CNC turning lathe is much different then turning threads on a CNC converted Grizzly lathe.

That's my take on it anyway,
Will

Not all carbide inserts are created equal. See recommended sfpm per material for each specific insert.