I'm looking at R8 carbide indexable end mills from CDCO to use for
cleaning up rusty stock and squaring up stock. One says it uses
tpg-32 inserts. Are those the same as tpg-322?
Another alternative is a similar 2.5" indexable end mill, which uses
a different insert. SPG422 carbide Positive rake, 15 Degree lead angle
I have a Bridgeport with the 2J head. Which of the two is more likely
to give a good finish on hot and cold rolled steel?
What difference does the positive rake and 15º lead angle make?
Here's a link to the 2" one
I would suggest you look at the price of the inserts - if you tool can use
inserts without holes, the inserts are cheaper - look for a common size
(like TPG-32 or the SPG-42) - look at a catalog on how to decode the
number - T=triangle, P=positive, first number =diameter of inscribed circle
in 1/8ths of an inch (e.g. 3/8), and so on - all of the on line insert
company catalogs have a page or two explaining this - that will help a lot.
See what inserts you can get cheaply, surplus
Get a copy of the Enco [or another] catalog that has instructions
on how to decode carbide specifications.
for sale prices see
TPG stands for "Triangular Positive Ground"
TPU stands for "Triangular Positive Unground" [on the top], and
is about the cheapest carbide insert available. For home shop
turning and milling use these are generally entirely adequate.
If possible buy a package [on sale] as the price per insert is
generally much less.
Generally TPU/TPG inserts will have 11 degree side relief angles.
To decode the numbers
x is the diameter of the largest circle that will just fit onto
the insert in eights of an inch, thus a TPG3yz is a triangular
insert that will just enclose a 3/8 diameter circle.
y is the thickness in 1/16s of an inch. thus TPG32z is nominally
2/16s or 1/8 thick.
z is the corner radius in 64ths of an inch. Thus as TPG322 would
have a 2/64ths or 1/32 corner radius
SPG stands for Square Positive Ground" and the numbers are coded
the same way. Note that you get an additional corner with the S
inserts, but you may find out when you do a per corner cost that
the TPUs are cheaper.
Long involved discussion here. You have to have positive rake
inserts for the back of the tool to not drag on the work. While
there are mill and lathe holders for negative tools, this
generally requires more rigidity and power than the typical
home/hobby machine. Moltrech among several others discusses the
effect of lead angle on milling.
Be reminded that you will [theoretically] need two grades if you
machine both aluminum and steel. C2 for Aluminum and C5 or C6
for steel. However for the typical low volume low stress home
shop machining more than likely either one will be perfectly
satisfactory with both materials.
Given that you will be tending to chip and break the inserts
milling rusty material, possibly with hard spots, I urge you to
consider a fly cutter using HSS lathe tools. The cost is low and
as you can resharpen the HSS tool bit on most any grinder or belt
sander, this is one of the most economical solutions,
particularly for the home shop where production and speed are not
Making a fly cutter was a traditional apprentice project,
frequently with two HSS bits, one roughing and one finishing,
however as you are just starting machining I would buy a single
bit fly cutter set to start and then after you gain some
experience and insight into the terminology such as axial and
radial rake, you can build your own, sizing to fit your
For an import fly cutter set see
you want a fly cutter with an R8 shank see
a more elaborate fly cutter see
Let the group know how you make out.
Unka George (George McDuffee)
The past is a foreign country;
they do things differently there.
L. P. Hartley (1895-1972), British author.
The Go-Between, Prologue (1953).
I am just adding to what the previous posters have said. The advice they
have given is sound. I would only like to add the pros and cons to the
two styles you are looking at. I am probably missing some, but:
pro: can face mill up to a square corner.
con: slightly weaker and more prone to breakage. Lighter cuts will need
to be taken. only 3 edges per insert.
pro: stronger, more heavy cuts can be taken.
con: can't mill up to a shoulder(when used with the lead angle shown)
Of the particular holders shown, some things to consider:
a four insert tool will be faster that a 3 insert tool. Probably not a
big deal for a hobbyist.
The square insert tool lead angle will slightly improve insert life and
can be helpful on brittle materials to prevent edge breakage (of the
workpiece). It also looks to have more axial rake than the triangle
holder. This will improve surface finish, and is slightly easier on the
machine. The triangle inserts seem to begin the chip with a maximum edge
force, and the result is a noticable difference in the noise, in my
experiences with the two types. It's probably harder on the machine.
Of the two, I personally would probably use the square insert holder 90
percent of the time.
Brent Muller wrote in
From what little research I have done, triangular inserts are a lot more
common, and therefore likely to be cheaper due to availability surplus.
I looked into this a number of years ago, so square inserts may have
become a lot more common place by now.
eBay has 63 hits for "TPG", versus 39 for "SPG" under carbide inserts, so
the square ones are certainly available these days.
Thanks for all the info. It was nice to have some pointers to
references as well as the voice of experience in what to look out for.
I do have a fly cutter which I built way back when I bought my
Hardinge Horiz / vertical mill 20 years ago. As a single point tool,
it's slow to clean up stock and the RPM is slow as well since it's
using HSS, that's why I was interested in the carbide indexable mills
now that I have a Bridgeport. Fly cutters are indeed a low cost
solution, with the tradeoff being time.