Negative Rake tooling
At the time this page was originally written, most insert carbide tooling
you saw in the catalogs was NEGATIVE relief tooling meant for heavy cuts
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. In addition these 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
Today, you will see a lot more inserts that wither have a molded chipbreaker
that gives an effective positive rake, or even true positive rake tooling.
The advantage of positive rake tooling is that it requires less force to remove
material from the work. Industry demanded positive rake tooling for a few reasons.
The DISADVANTAGE is you only get half the number of cutting edges per insert.
- They require less cutting force, allowing them to be used on smaller work.
- The lower cutting forces allow them to run carbide out on the end of small boring bars.
- It allows the lathe to be less rigid, and be of lower horsepower, or the metal removal rate to be higher for the same horsepower
These requirements lead to the carbide manufacturers to develop new grades of carbide that not
only were harder than existing grades, but tougher at the same time! This has allowed the creation of
inserts that are extremely sharp, or have agressive chip breakers for fine cuts that are perfect for the
This lead to 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.
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 original tools I used are made by Microdex,
which unfortunately is no longer stocked by MSC, but is sold by
ENCO. Today there are also lots of other choices
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
Update - Part 1 - Holders
Back when I originally wrote this web page, TPG-22x insert and their associated holders
were probably your BEST choice in small carbide inserts. Now, I'm NOT saying they are a bad choice,
and I still use them when I'm roughing really nasty material. Today, I tend to use holders that
hold CCMT 21.5x and other CCMT inserts, along with large holders that hold negative rake holders
that have chip breakers that give an effective positive rake. The larger holders are usually eBay
specials. Good deals can be had
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.
Update - Part 2 - Inserts
As I said above, I've mostly gone to CCMT-21.5x inserts. From what I understand, this shape
was originally developed as a boring bar insert, and to this day, you often find the inserts listed
under boring bars. One of the nice things is you can get uncoated "upsharp" high positive rake inserts
that are designed for Aluminum, and give AMAZING finished (Hint, run them as fast as you can - you should get a mirror finish)
In addition, I use a holder that places the insert at an angle that allows you to both turn and face
with the same holder. YES, if you turn to a shoulder, you WILL have an undercut,
but that can be eleiminated by feeding out across the face. This same undercut limits you to taking
a cut of somewhere in the .060 to .100 depth of cut (aka a diameter reduction of .120-.200). Now I've
never found this to be much of a limitation, as I'll either make 2 passes, or switch to a roughing
insert/tool if I have to go deeper than that!
BTW, those upsharp inserts? Probably the best tool for cutting acetal (aka Delrin) plastic I've found,
BUT you will still get rats nests of swarf. Please be careful
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.
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 .