rmac wrote: ↑Sat Sep 10, 2022 9:37 pm
Harold_V wrote: It is important to understand that negative rake is NOT intended to have a sharp cutting edge.
... and ...
Harold_V wrote: The edges on both sides should not be sharp.
Harold, are you saying here that I was off base earlier when I was wondering if thoraxe's tool might be worn out from prior use?
Absolutely not! But knowing what's wrong with a cutting edge becomes important. In my experience, when a negative rake insert gets used as intended, it breaks down where the outer portion of the cut makes contact with the tool (the edge degrades at that interface]. Once that starts happening, there's a rapid degradation of the tool's performance, and if one persists, the end result is often a broken insert. This condition is common when multiple parts are being roughed with the same depth of cut per pass. The tip of the tool may be in acceptable condition, and the cutting edge may, otherwise, still look good. If a shallower depth of cut is taken, the tool may perform acceptably.
I sure don't want to confuse anyone with bad information.
That's noble, as some don't care, so long as they are heard.
When I try to evaluate a negative rake insert, what I look for is chipping, wear, and/or cratering of the insert. Those features are quite obvious if compared to a pristine insert. If there's any indication the cutting edge has been chipped, I make a decision on taking a risk of getting a few more miles from that particular tip, or to index to a different one, known to be in acceptable condition. If I find that all corners on both sides have minimal damage and the insert is of little value, I often will pick the best tip and use it for heavy roughing. I'll risk breaking the insert because it's no good anyway.
The typical negative rake insert is NOT ground. They are generally dispensed as they are made, and it shows if you know what you're looking for. That usually results in dull (not sharp, but smooth and very regular in appearance) edges, which are far less important in negative rake as compared to positive rake inserts. Remember---a negative rake tool is NOT intended to cut at the tip.
Aside from that, can you explain what you mean above by "both sides"? Both sides of what? That's not clear to me.
Sorry. I should have been a little more explicit. By both sides, I mean both top and bottom faces of the insert.
The typical (negative rake) triangular insert offers six cutting tips, three on each side. That's not typically true of positive rake inserts, where rake is created by tilting the insert, which, in doing so, creates negative clearances. That necessitates front and side clearance must be ground on the insert, so the use of both top and bottom is not possible. By sharp contrast, if a positive rake insert is ground from a negative rake insert (commonly done), the tool holder itself creates the required clearances, so both sides of the insert can be used. Such inserts are far better suited for finish cuts and perform poorly for roughing.
If folks are hell bent on using carbide inserts, they should become familiar with the differences in inserts and holder so the choices they make are in keeping with their needs. Grabbing a random holder isn't in anyone's best interest, assuming they are trying to solve a given problem.
I often get on my soapbox and try to encourage folks to learn how to grind their own tools. The knowledge gained is priceless, as once one understands the geometry involved, it's dead easy to see what's wrong with a cutting edge---and offers one the ability to correct less than acceptable geometry. I like to say that a lathe man is held captive as long as he's unable to fashion his own cutting tools. It's true!
Our GlennW is well versed on this subject. His comments should override mine, as my distance from the production shop for so many years has left me lacking a great deal of knowledge. Much of what I speak was acceptable 40 years ago, but much has changed in that time, including the introduction of micro-grain carbide, along with improved bonding techniques. The carbide of today is a far cry from that which was in common use in the 70's.
Glenn also has some experience with Cermet inserts, which are very good for machining hardened materials. Perhaps he could offer some guidance.
H