Carbide insert substitutions

[Shotgun]

I've come by a partial set of Borite turning tools. These advertise as using TT-321 inserts.

Trying to piece in what is NOT said, is it safe to assume that any of the available letters can fill in between those T's? TPMT or TNMT (11 degree relief vs no relief) would work equally well?
Why could I not swap in a TNMG (for double sided inserts)?
Would parts specified in metric terms fit equally well? That is, is a 160404 the same as a 321?
Why are they specifying the radius? Does the toolholder always lock you into a radius, so that you can't use a 322 or a 160408?

The most confusing part about buying inserts for a random holder seems to be the several different sets of nomenclature that are used. Carbide Depot seems to make sense of it, but are the inserts really that interchangeable? Or will they be off by 1/4 of a mm?

 

[KyleG]

Someone more knowledgeable may correct me, but I believe you can only alter the third letter (Tolerance). The relief angle is part of the toolholder geometry. You can also vary tip radius without an issue.

 

[Ischgl99]

In a nutshell, you need to use the insert specified by your toolholder for it to work properly, and you can use any radius you want in the toolholder. Read on for more specifics.

The letters in an insert designation have very specific meanings and for the most part, one insert cannot be substituted for another. Your TT toolholders are simply triangle single sided inserts. The TNMT you mention requires a negative rake toolholder to take advantage of both sides and cannot be used in the tools you have. Your tools require inserts that are single sided and have a relief angle built in to them.

A member here, David Best wrote a book about insert tooling that is getting excellent reviews. I haven’t read it myself, but based on his contributions here, I would expect it to be top notch and an excellent reference. https://www.amazon.com/Introduction...refix=david+best+insert+tooling,aps,75&sr=8-2

For the toolholders you have, the four letter designation would be TCMT or TCGT. The M and G are molded or ground edges. Ground is used for things like aluminum and molded is usually better on steel, but you can use a ground edge on steel, but it won’t last as long. TPMT inserts are frequently used in a boring bar and are specific to that insert since the insert is oriented so that the top angle of the insert is horizontal when looking at it from the end and you need the 11 degrees to clear the workpiece, if you use a 7 degree TCMT insert, it might rub on your workpiece, and it might not fit in the pocket properly since the boring bar pocket was designed for an 11 degree side angle on the insert and not 7 degrees. You might be able to use a TPMT insert in a holder designed for a TCMT, but most likely not since the pocket was angled for a 7 degree insert, the 11 degree insert most likely will not sit flat the way it should.

For the size of the inserts, ANSI uses things nomenclature such as 321, 32.51, etc and the metric world uses ISO designations. Many catalogs list both, but if not then you can always use the tables from Carbide Depot to figure it out. The corner radius does not matter for toolholders, but it seems many companies like to specify that. Any radius on the insert should fit in a toolholder.

As a caution, there are metric sized inserts that require metric sized toolholders. I got a set from Grizzly many years ago when I was first starting out and inserts available here to not fit without shimming them, so before buying toolholders from any source, make sure they use inserts you can easily get here.