I'm probably not the only one to be confused by the indexible tooling, so I thought I'd share information that took an afternoon to gather.
My objective was to buy some inserts to match the insert holders I had from Harbor Freight. I have the 3/8 inch shank set of holders, part number 39932. That set came with a set of inserts of unknown sopecifications. The holders are identical to the second set I bought from ENCO last week for $19.99. That was part number 250-1400, a "3/8" SHANK INDEXABLE HOLDER 5 PC SET". The ENCO set did not include inserts.
I eventually found that I wanted TCMT21.51 inserts. I got them for $2 to $3 each from carbidedepot.com. Below is the explanation of how I determined what I needed. n You can follow along at
http://www.carbidedepot.com/formulas-insert-d.htm.
My first challenge was to find out what size my 3/8 inch tools used. I finally learned that they are not measured by the length of the side nor from tip to tip. It's measured by the size of the largest circle that fits totally within the outline of the insert. This is known as the Incribed Circle or IC. See
http://www.mathwords.com/i/inscribed_circle.htm for a picture and discussion.
I suspect that the IC is used instead of length because the other dimensions change with the shape and the treatment of the cutting tips.
Once I learned that I use 1/4 inch IC, I next needed to find out the rest of that silly code. It's all described well in Machinery's Handbook. Page 757 of edition 27, under "Cutting Tools". Some parts of the code are pretty clear and some take some thinking.
It was easy to determine that I was using
Triangular inserts, so the first letter was a T. The holder is ground to use only triangles.
I measured the angle on the tip of one of my existing inserts and found it to be about 7 degrees. The holder is straight across, no rake. That made the rake 7 degrees, which makes the second letter a "
C". The web site said that 7 degrees was good for aluminum as well as light cuts in steel.
I wanted a nice, sharp, ground finish, "tolerance" class G but I could not find any that were cheap. I settled for "M" tolerance, knowing that I would have to check the alignment if I swapped inserts.
The last leter was the geometry of the insert. I had screws with a bevel on them, so I need a hole and the hole needed to be countersunk. My old inserts had chip breakers, since it made deeper cuts cleaner. I had specified a rake, so only one side could be used. Add all those up and you get a 'T'
So now we have TCMT. What about the 21.51? That's three fields. 2, 1.5 and 1.
2 = IC = number of 8ths in the IC. In this case 2*1/8 = 1/4 inch IC
1.5 = Thickness = number of 16ths. In this case 1.5*1/16 = 3/32
1 = the radius of the cutting point in 64ths. In this case 1/64th
That't it. TCMT21.51. You can play with the thickness to change the cutting height if your tool post does not have a height adjustment
I had to decide if I wanted 0, 1, or 2 64ths radius. I found that I can see the difference. A 0 is great for threading and for cutting hard materials. It will dull and chip easier. A 1 is a good general purpose edge. A 2 gives a very smooth finish. It should last longer.
Using the right insert makes a big difference. A C2 TCMT221 had a hard time with a piece of steel I was trying to turn. A C6 TiN coated TCMT21.51 cut it like it was butter.
Let me know if pictures are desired.
Daniel
