Back rake question

Looking at the web site, it looks like some of the holders have built in side and back rake, and some do not.

Most "friendly" materials can be cut without back or side rake. In some materials it works quite well. In lower horsepower machines, it limits how much you can cut. Other materials, especially on a small lathe, they really want some positive rake.

I'm gonna link you to the middle of a video. It's worth watching if you ever grind your own tools, or maybe just to have some idea what the different angles do. (And just how ugly a tool can be and still work perfectly, provided all the required angles are there). Anyhow, the spot I linked to, (30 seconds or so?) there's some really good (dirt simple) drawings there in the first little bit. Might kind of help with a mental picture of what the chip is doing, and why one might want to select different back and side rake angles.

 
I have the older triangular shaped AR Warner tool set that I think you may also have. Mine have no back or side rake, either. And yes, they cut pretty okay, too. BUT they cannot come even close to the performance of a well ground HSS tool ... not even close.

The typical AR Warner insert has only side and end relief ground into them. Naturally, this allows them to cut. However, they do so with higher cutting forces compared to a well ground tool. The lack of side and back rake forces all the cutting forces to be focused at the cutting edge, which is blunt wall of HSS with a bit of relief angle under it.

In contrast, a good HSS tool will have a shape that allows for the optimal tool lead angle to be used for the given operation. It will have decent relief angles under the side and end cutting edges AND it will have side and back rake angles to reduce cutting temperatures and enhance chip evacuation. The angle of the side cutting edge where all the action takes place is a combination of the side relief angle and the side rake angles, mostly. There is some contribution from back rake, too. This angle forms the cutting edge and is far more acute than that found on your inserts. Accordingly, a good HSS tool will easily cut deeper and more accurately and finish finer than your tool can.

And I'm not guessing at this. I've run cutting test against AR Warner and carbide inserts VS one of my HSS tools and in all cases (except for very hard materials) my HSS tool outperforms these inserts on the smaller lathes that I use. How much better? On my little Sherline lathe, it would be hard for an AR Warner insert to take more than a 0.030" deep cut in mild steel; my HSS tool can take a 0.060" cut in mild steel without chattering, and then it will take a tenth deep cut with a fine finish.

So, the rake angles just happen to be the most influential angles on a tool bit and your inserts do not have them.
 
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