View Full Version : Can someone explain...
02-22-2005, 01:37 PM
the logic behind the price of carbide inserts? For example, a TCMM with one side useable runs $5 - $6, while a TNMG with 2 useable sides and no relief to manufacture is only a buck or two?
I'm thinking a holder can be made (as I've seen suggested) so the surface on which the insert sits would slope down toward the tip to create an effective 3* positive relief at the tip of the TNMG insert. That would give you 6 cutting tips on one insert, as opposed to the 3 you'd get with a TCMM.
Would the TNMG chipbreaker groove allow this insert to cut when presented to the work at this angle?
02-22-2005, 02:52 PM
I think the surface should slope from the tip back? for a pos relief angle?
Correct me if wrong.
02-22-2005, 03:40 PM
The TNMG designation implies "negative" rake, but there is actually no relief at all, i.e. the vertical surface of the insert is at 90* to the horizontal surface. I may be confusing the terms rake and relief.
02-22-2005, 03:43 PM
Seems like supply & demand, or what the market will bear. I sometimes use a tiny TMPT, which is awesome when you have limited tooling workspace. Nobody sells these cheap.
CNMG is what is run 75% of the time. I've found these on eBay for 51 cents each, USA mfg, cermets. Also Polish inserts (TMX/Bison) from Penn Tool Co. cost about $3 each. WNMG's are another readily available (read cheap) insert. CNMG's give you 4 edges (8 if you buy the "other" toolholder), WNMG's give you 6.
02-23-2005, 01:57 AM
<font face="Verdana, Arial" size="2">Originally posted by Lynn Standish:
...so the surface on which the insert sits would slope down toward the tip to create an effective 3* positive relief at the tip of the TNMG insert. </font>
This does NOT give you a positive insert angle - this is a highly NEGATIVE angle. It is not possible to use a negative geometry insert to replace a positive geometry insert - the insert geometry does not allow for it.- when you try to exceed the specs you cause excessive flank wear and destroy the insert. False economy. Get the right tool the first time!
02-23-2005, 01:00 PM
Thanks for the reply. Guess I'll forget it and go with the more expensive inserts. I still don't understand why they are priced the way they are, but I can't do anything about it anyway.
02-24-2005, 12:59 AM
The pricing has more to do with the volume of inserts sold and the chemical make up of the insert than anything else. Some of the alloys and coating used today are extremely expensive and cutting research is not free either - so you have to expect the manufacturer ot regain their cash flow some how, and when you consider that inserts only account for 3% of the production cost in machining materials they really do not cost that much (82% is capital equipment costs, 15% materials). Inserts are cheaper in that you save time by not having to grind and hone bits and switching to a fresh corner of the insert only takes a minute.
Once you understand the nomneclature or designation of the inserts you will be able to select and bid on inserts at a greatly reduced price on eBay. I recently bought Perfect Circle inserts (o.165"ic) that sell for $19 /each (cdn$) at Kennametal here in Edmonton for $5/30 +$6 shipping(US$)! It is rare that I ever have to buy inserts from Kennametal or Sandvik - patience is a virtue here. http://bbs.homeshopmachinist.net//wink.gif
02-24-2005, 01:23 AM
Look at the insert.
How much is 'working" volume, and how much is just there to hold the rest of it?
Now look at a brazed tool.
How much total volume of carbide is on that?
Inserts really stink from a material efficiency standpoint. You are paying for a lot of totally wasted material which can never do any cutting.
I don't know the logic of the different shapes, costing such diffrent amounts, but it likely comes down to volume.
In industry, as pointed out, they pay off big time.
For us, they can really suck from a $$ per cutting point basis, even though you get three points per insert.
Or you may find them perfect.
I have been working some serious material removal with 4140 pre-hardened, which machines nicely as long as the tool is good.
Its an arbor for the mill, raw stock 1 1/2". I needed to take one end down to 1/2" for the journal, the other to an MT3, leave a collar at 1 1/2", and the rest to 1.250. Big pile of bright blue chips. The old Logan probably never has worked that hard before.
I got as much work out of a single small radius triangle insert point as I got out of any two of the 5 brazed carbide tools, plus one HSS tool that I have gone thru so far.
I sharpened the HSS 3 times for wear, and the brazed carbide all got micro-chips on them that I don't have (yet) the machinery to regrind to usable form effectively. I still honed them each at least once before having to give up and set them aside.
The insert point that I have gone thru wore, it didn't chip. I could detect that, and gnage it out. The brazed chipped, and started giving that mirror polish that indicates they are burnishing and not cutting.
A large (1/16) radius carbide I tried I did not have the ooomph to use. The smallest nose ones have been working. They are, I think, 1/64 nose.
Totally worth it, but a more industrial task than most. I hadn't pulled out the insert or brazed tools in a year, but the inserts did it this time.
[This message has been edited by J Tiers (edited 02-24-2005).]
02-24-2005, 09:55 AM
As Thrud suggested, eBay is my lowest cost source of inserts. Try running an automatic search each day for the inserts you need, or post on the WANT IT NOW section of eBay. I still have to pay retail (ouch!!! hate that word) for one or two oddball shapes that don't get auctioned often.
One of the best general reference sources is Valenite's CD-ROM (no catalog available at this time):
Informative sections on chipbreaker designations, feed, DOC, SFM, in non-technical language.
JT, Valenite & Iscar both state that you should get between 15 to 30 minutes cutting/edge. I've observed that running the spindle in reverse & crashing the tool really shorten the life <LOL>. Even then, at 25 cents/cutting edge, it doesn't hurt too much.
It's O.K. to call me a cheap bastard...I consider that a high compliment!
02-24-2005, 12:22 PM
Might be a silly question, but why aren't HSS inserts commonly made in all the shapes. Seems economical, re-sharpenable, etc...
02-24-2005, 01:05 PM
JD, I think because carbide is not a metal, per se. It's a powder that's pressed into molds, baked, coated, etc. Some tooling is now made of powdered metal, like endmills, but the price is equal to cobalt(...manufacturers say you can run higher SFM than cobalt, haven't tried them yet).
By the time the insert maker invests in R&D, moldmaking cost, etc, they have no financial incentive to make a low-cost insert.
02-24-2005, 02:00 PM
<font face="Verdana, Arial" size="2">Originally posted by precisionworks:
JT, Valenite & Iscar both state that you should get between 15 to 30 minutes cutting/edge. I've observed that running the spindle in reverse & crashing the tool really shorten the life <LOL>. </font>
Didn't do that, but I did stall the spindle a couple times. That generally did knock out the brazed carbide, it had welded on and took out a chunk when it broke loose.
At smaller diameters I could take 0.050 and even 0.125 DOC, but at full diameter the belt was "quacking" loudly at 0.010, and I was on the edge of stalling. I backed off to 0.005 and a more agressive feed for roughing down. About 250-300 RPM in the backed-off mode, or something over 100 SFM.
I think I did get at least 30 mins. The first point was not fresh.
My operation is NOT like those industrial ones....power not there to work it hard, so I'd expect longer life.
02-24-2005, 04:04 PM
JT, sound like you're in the sweet spot for your setup. If you don't have Valenite's book or CD, email me & I'll fax the pages on chipbreaker selection - that really has helped me. My toolroom instructor always said "Take the most DOC with the fastest feed that the lathe will stand". As the part diameter lessens, it's like having a bigger motor. You can only push the small machines until they stall or the belt slips, which happens pretty often here!