Carbide for the Home Shop Machinist (HSM)

I find it a bit ironic that many, if not most home shop machinists (HSMs) DON'T use carbide tooling. Today, in the average hobby woodworking shop, most people wouldn't be caught using a steel blade on their tablesaw, but most machinists use steel tools to cut metal! The complaints you hear are very similar to what you heard with tablesaws 20 years ago - Takes more horsepower, you can't sharpen them, more expensive etc.

Back when I got my first lathe (an Atlas 6" - not a large high HP lathe by anyone's standards), I decided to try carbide tooling. I went out and bought a few cheap brazed tip carbide tools, and I got HORRID results. One day, not long after that I was looking through the MSC catalog, and I found a line of small shank , indexable insert carbide tools from Microdex. I called MSC tech support and asked about them. They said they were great for small lathes because they were positive relief. I bought one holder (about $35) and a few inserts (it takes TPG-22x or TPMR-22x inserts - I'll explain this number in a minute). I put the tool in the lathe and got GREAT results. Over the next year and a half, I converted entirely to carbide tooling. This is what I have found out.

Holders

Negative Rake tooling

Most insert carbide tooling you will see in the catalogs today is NEGATIVE relief tooling meant for modern high speed, high horsepower lathes. The reason professional shops LOVE this kind of tooling is that it gives you twice as many edges per insert, therefore halving their insert costs and the inserts are more robust, therefore they tend not to break if abused, again lowering their cost in both inserts and the cost to change them.

Positive Rake tooling

The advantage of positive rake tooling is that it requires less force to remove material from the work. This allows the lathe to be less rigid, and be of lower horsepower. It also is much less likely to distort the work. The DISADVANTAGE is you only get half the number of cutting edges per insert.
Recently, there have been a bunch of "ISO" tool holders that have come on the market. These tools are positive rake. Most of these tools are available in the smaller sizes that are most useful to the HSM, like 3/8" and 1/2" shank, where most standard tooling STARTS at 1/2" shank, which is fine for people with 12" lathes, but won't even fit on a 6" lathe.

Holder Terminology

At this time, I still use the same 3/8" shank positive relief tools I started with even though I've bought a bigger lathe (12"). The tools I use are made by Microdex, which unfortunately is no longer stocked by MSC, but is sold by ENCO as their "Made in USA" Small shank Insert holders, as well as by Campbell tool and Blue Ridge tool.

The standard right hand, turn to shoulder 3/8" tool holder from Microdex has the industry standard code of MPAR-6. I believe the M stands for Miniature, the P stands for Positive Relief, the A is the holder type (A is 0 deg Lead - a.k.a. Turn to shoulder, B is 30 deg, C is 60 deg - read threading, F is facing) and R is right hand, AKA turn TOWARDS the headstock (would be an L for left hand), and the 6 is the number of 16ths of an inch the shank of the tool is in height - a.k.a. 3/8". A 1/2" shank tool would be a MPAR-8.

This is a picture of a MPAR-6 insert holder with a TPMR-221 insert (In quick change tool post at top), an MPAL-6 (Left Hand) Insert holder (Bottom), and a TPG-222 Insert. The Dime is for size comparison.

I recently added a few NEW tool holders to my tool box. The First, a type CTAPR-8 from Valenite is a standard right hand turning tool that still takes TPG-22x inserts. It seems to work just as well, if not better than, the Microdex holders (Holds inserts that are on the small side of the tolerance levels better) but it is NOT available with a 3/8" shank, only 1/2" and above . I have also added a threading tool, but have only cut one or two threads with it so far (I need to get some more shop time), and it seems to work fine so far, but no final opinion yet

Inserts

Most of the tools I use take what is known as a TPG-22x or TPU-22x inserts. These are about the cheapest inserts you can buy, with import inserts costing about $1. The T stands for Triangular, which with a positive rake tool means you will get 3 cutting edges from each insert. The G or U stands for Ground or Utility. Utility inserts are supplied "as Cast", and typically are a little cheaper, the G inserts are ground to a finer finish. The 22x part is the SIZE of the insert. Your holder will only take one size (the first 3 digits) - In this case, 2/8" IC, a.k.a. it's a 1/4" insert, 2/16" thick (a.k.a. 1/8"). The x is the TIP radius in 1/64". Common sizes are 1 and 2, leading to a full designation of TPG-221 or TPG-222, although other sizes are available. TPMR inserts are basically the same, except that the have a chip breaker molded into the insert.

Carbide Grades, or "Boy, am I confused!"

Next comes the topic of what KIND of carbide. Having talked to the various carbide insert Mfgs (See letter Below), what we came to is this. For the home shop that doesn't want to stock lots of kinds of carbide (which gets expensive, as inserts are usually sold in boxes of 10, and ONE insert will last months in the home shop), the best bet is to buy one of the TiN coated or Multi Coated grades. These grades are meant for high speed cutting of steel, but will work of Al and CI, but with "Degraded" performance. In this case, by "Degraded", I mean you have to cut at a lower speed, which, in the home shop is actually a blessing.

03/11/1998

Yesterday, I sent an E-Mail off to many of the carbide Manufactures, asking what they recommend insert wise. To see the letter, and what they say, click here

Carbide Grade Cross Reference

Here is a link to Carboloy's Carbide Grade Cross Reference

Turning Speeds, or "You want me to run my lathe HOW fast?!"

One of the Good/Bad things about carbide tooling is that to get a GREAT finish, they like to run FAST. How fast? Well a good grade of Tri-Coated insert, turning say, 12L14 steel, would like to run in the speed range of 500 - 1000 sfm. What does this translate into? If you are turning a piece of 1" diameter stock, we are talking 2000 - 4000 RPM! The average home lathe wont GO that fast. They will however work OK at lower speeds.
Remember I said that Tri-Coated inserts need to run with degraded performance turning Aluminum and Cast Iron? It turns out that the Tri-Coated inserts have to run slower than the "proper" inserts. By slower, the manufacturers are talking in the range of 1000 sfm, or ONLY at the speeds we are cutting steel. As the average home shop lathe is going to have problems getting use to 1000 sfm anyway, this isn't a problem, but a blessing in disguise, as with the "Proper" grades of insert, you can run Cast Iron at up to 1500 sfm, and Aluminum at up to 3500 sfm or 7500 sfm with exotic (read diamond) tooling .

Carbide Manufacturers

Overall, I LOVE carbide tooling, and I only use steel tools for special purposes, like form tools.

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