Carbide Threading Inserts

[terry_g]

I have recently purchased some inexpensive insert tooling for my lathe and now am wondering why I waited so long. I know when you buy a 10 pack of inserts for less than $20 they aren't going to be high quality carbide.
I bought a 12mm internal threading bar that take 11IR A60 inserts that will cut 48 to 16 threads per inch. I have been using high speed steel and brazed carbide for years. One of the thing I learned a long time ago was the need for a radius on the tip of the cutting edge. A sharp point is very easy to chip. A .010" radius on a high speed steel bit is usually adequate, a little more .015" to .020" on a brazed carbide bit really helps. The threading inserts I have a sharp point and not a radius. Turning inserts have a few options for different radius's but the threading inserts do not.

Last night I was turning a 20 TPI internal thread and about three cuts from finished the tip of the insert chipped. I ended up chipping all three cutting edges by the time I was finished. I took a diamond file and put a slight radius on the least damaged cutting edge and did the finish cut with it. When cutting a 20 TPI thread in a deep bore with a small threading bar the bar the bar is very close to the thread and I suspect a cutting got between the bar and the thread and made it flex away and then dig in chipping the bit.

 

[Proxule]

Aye, same experience as you, And when I looked into these years ago, I couldn't make heads or tails of the nomenclature.
A60 covers 16-48 TPI
AG60 covers 8-48 TPI
G60 covers 8-14 TPI

Thanks for posting!

 

[PeterT]

I use offshore threading inserts & cheapo toolholder from Ali. Who knows if they were same manufacturer but they were similarly inexpensive. I have had no such chipping issues. Pictures show 303 stainless at low(est) lathe speed which is not how they are supposed to be run. But I've also done 12L14 & 1018 steel. Brass & such is a no brainer, finish is beautiful. Anything that cuts crappy with a regualrr insert will be a bit worse with threading insert generally speaking.

Assuming your setup & feed was ok, one thing I am suspicious of is some inserts look to be clones of Carmex. Carmex sells anvils corresponding to different thread sizes. But the offshore sellers rarely offer them so what angle did they mill the pocket plane? I'd have to refer to my notes but that was a concern of mine. You have to pay attention to internal vs external & the other designations which soeme sellers do a brutal job of listing. But anyways just passing on my experience, they certainly work once the planets align.

 

[StevSmar]

I just got this book on insert tooling:
Introduction to Indexable Tooling for the Metal Lathe: A User Guide

Seems very comprehensive and will be useful once I start dabbling in inserts.

 

[gerritv]

For any of those inserts they have a tip radius. Of much higher importance is tip width. that determines the quality/fit of resulting thread for a given pitch. A typical A60 insert is ok at 16-20tpi but as you get finer pitch the tip width starts to create problems. Your pitch might be ok but the thread shape will be wrong and result in a sloppy fit at best.

Left to right, 16, 20 and 48 pitch screws with A60 insert at 20X magnification.

 

[gerritv]

I have posted links to this site before https://www.machiningdoctor.com/charts/unified-inch-threads-charts/ , if you scroll down to Basic Dimensions charts you can see that at 16 tpi the root is 0.0156", at 48tpi it is 0.0052"