I am going to pick up a set of indexable turning tools from Enco. I
think these are made by Microdex (USA). They have 1/2" shanks and take
TCM-321 Inserts. There is also a set of 5/8 shank holders that I
believe take the same type of inserts. I have a Sheldon 13x36 Lathe w/
variable speed and can reach in the neighborhood of 1800rpm. My
question is are these uncoated inserts and do the inserts set at an
angle in the holder to achieve the proper rake angle or is the rake
built into the insert? Also, should I get the 1/2" or 5/8" shank? I am
also going to order a BXA Wedge Toolpost when I order the holders.
Any suggestions are appreciated. Thanks, Steve
I like the 1/2" shanks best on BXA toolholders- the 1/2" shank tools sit
flush with the side of the holder.
A 5/8" tool will fit in a BXA holder but the side of the shank hangs out by
I'm not a insert expert, but for that size lathe make sure the tools you buy
have positive relief, see
I have a 13x36 Jet with a BXA post and I recommend 5/8" tools because they
are stiffer. There will be a time when you need to turn with a lot of
overhang and the 5/8 will be less prone to chatter. Also, I have not ever
seen a situation where 1/2" tools would do a job that could not be done with
The toolholders hold the inserts flat (zero back rake). The only effective
back rake is a result of the chip breaker groove(if your inserts have one)
that runs parallel with the edge.. Front and side clearance is ground into
the insert and typically runs around 5 - 11° depending on the insert type.
There are several different inserts that fit the tools with various tip
radii, clearance angles, chip breakers, coated, uncoated, carbide, cermet,
etc. The ones that come with the tools are generic and better ones are
available from the tool houses.
Steve wrote in message ...
I am no expert, but I sure would consider making or buying holders that take
tnmg inserts, reason is they are dirt cheap from Enco and Travers, and you
get six cutting edges. With a holder that is set to give you a positive rake
you can do very nice work with these inserts. Plas Tools makes some nice
reasonably priced holders
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Before you do, consider making your own. See
If you decide to go that route, e-mail (without the antispam) me for
some additional material I wrote since '98.
I'm still using those same holders with good results.
Huh? First, you are talking about negative rake inserts - the tNmg
I know you can tilt the insert to get any rake you want from a positive
but I don't see how you can get positive rake from a negative rake
insert. If it has
6 cutting edges on a triangle, it must have a 90 degree angle on the front.
I do a lot of aluminum, so I have pretty much stayed with positive rake
This is a good point, if you plan to do heavy cuts or lots of overhang, you
should at least have one 5/8" tool, stiffer is better.
In my case I already had a bunch of 1/2" tools when I bought my BXA holder,
but I also inherited one 5/8" tool with the holders that I use for heavy
cuts. I mostly use my 1/2" tools though, I don't do heavy cuts very much
(mostly prototype and one off work) and the inserts are cheaper for the 1/2"
No! These (if they are what I think they are) are cheap ones,
with inserts too thin, and not properly supported by a sold carbide
anvil. As a result, they break/crumble too easily. Also, a full 5/8"
shank will avoid (or minimize) the problem of chatter from too thin a
shank. I've even had problems using them in something as small as a 5"
Well ... yes, but some of them result in too little clearance,
and some with too little support under the edge of the carbide.
But when you include a properly shaped chipbreaker groove, you
get a positive rake, thanks to the groove. The TNMP is one example,
IIRC. I've got a lot, but I sometimes forget the designation.
I got some good Valenite 5/8" shank holders for them, after
getting a lot of the inserts in an eBay auction. The Valenite holders
have a ground-flat carbide anvil to support the inserts, so the insert
is less likely to break under heavy cuts.
I use these a lot on my Clausing 12x24", and the Sheldon
mentioned in the first article in this thread should be similar in
I even use them with an Aloris 16N holder on my Phase-II
wedge-style toolpost. The 16N takes the triangular inserts and holds
them at the right angle for negative rake on a plain one, or positive
rake with the proper chipbreaker groove. It holds two of them -- one
for turning, and one for facing -- just move the holder to the other
dovetail on the toolpost.
They are not as strong when you start cutting serious steels.
I've used the ones mentioned above on 30-taper toolholders, to convert a
CAT-30 to the right thickness for the quick-change spindle on my
Bridgeport. I've also used it to turn down the case-hardened pilot
bearing on a 40-taper 1" milling arbor. That cut was hard enough so the
swarf was bright red, and the inserts had no problems.
Not mine. They tip the positive/negative (TNMP-321) inserts forward to
provide 5 degrees of clearance. This still leaves 5 degrees of positive
rake on the cutting edge.
There is a big difference between a chip breaker groove and a groove to
provide rake. Compare an insert with a chip breaker groove to one such
as the TNMPs. The chip breaker leaves a narrow flat near the cutting
edge. A negative/positive insert has a groove that goes right to the
Micro dex does not make to the best of my knowledge a holder that uses
inserts with a hole in the middle. I have a heap of Microdex holders
of all sizes and styles and have yet to see one that did not use
clamping style method of moounting with the chip breaker built on the
clamp itself. Are you sure your not buying those el cheapo types for
39 or 49 or even less $$. I dunno, maybe microdex decided to get into
that line now as well, but I have never seen em......which really
don;t amount to a hill of beans anyhow, but those type of holders as
pointed out the inserts cost a lot more money to buy and are not as
substantial and durable.
I for one do not get as nice a finish using a negative rake insert in
a toolholder with the rake built into the thing to give me that
induced 5 deg droop. They work fine for the most part in heavy
roughing cuts, lots of times my chip breaker grooves will erode away
or break and have yet to get a finish what I would call nice with
them. I still prefer the inserts that are real postive rake insert,
even if you only get three cutting edges.
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Yup, especially with aluminum, that is my experience. I have done
some aluminum work that people thought was diamond-turned or
something, truly mirror finish, with the positive-rake molded
chipbreaker inserts. They last forever, sometimes months of light
use on a single edge.
Now that I have a "real" lathe (Sheldon R15-6) I may have to experiment
with some negative rake cutters and see what they can do. But, up to
now, the positive rake inserts have done very well for me.