Question re: angle of contact with carbide insert tooling

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GeoNOregon
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Joined: Tue Dec 25, 2018 12:26 am

Question re: angle of contact with carbide insert tooling

Post by GeoNOregon »

I'm just getting started using a lathe, and have a question about carbide insert tooling. I have been unable to find any discussion on the question I have.

For my question, assume I am using a triangular shaped insert to make a simple axial pass on a piece of round stock to reduce its diameter.

Is there a preferred/best/suggested angle of contact between the insert and the material?

Here's what I mean: An equilateral triangle, which the insert is, has three 60° angles. If one side, away from the material, is parallel to the material surface, the angle between the material and either of the two sides would be 60°, (this is what I'm calling the angle of contact). In this position, let's say the insert is in a 'neutral' or 'balanced' position, relative to the material surface, (my terminology, I have found no discussions about this topic.)

Rotating the tool post in one direction would decrease the angle of contact. Rotate the other direction, and the angle of contact would increase.

Is there an advantage, recommendation, suggestion, or situation specific reason to either increase or decrease the angle of contact from a neutral position?

I have enough of a challenge trying to get DoC, RPM and feed rate right to get good surface finish, &/or prevent chatter; adding another factor, and trying to experiment with angle of contact is a bridge too far for my brain.

Thanks for any info, or suggestions of reference sites/sources.

Regards,

GeoD
John Evans
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Joined: Tue Jan 20, 2009 9:33 pm
Location: Phoenix ,AZ

Re: Question re: angle of contact with carbide insert tooling

Post by John Evans »

Well a lot depends on what you have for a lathe ! But over the years I have found the cutting point of the insert to work best at straight in to the work. In other words with your T style insert one 60* face should be parallel to the chuck face. If the "point' of the cutting tool leads into the cut if your lathe has any flex the tool will tend to dig in and cut under size. Carbide or HSS. Learned that lesson on a Atlas lathe with a rocker toolpost !!! You are in for a bit of a learning curve as things like tip radius -chip breakers - positive-negative rake all effect and coating [or lack of] will all effect the way a insert cuts.
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Harold_V
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Re: Question re: angle of contact with carbide insert tooling

Post by Harold_V »

If you own a small, light duty lathe, you would be well served to explore grinding HSS toolbits. Carbide may or may not be satisfactory for your particular application, and, without experience, you have no way of knowing if what you are achieving is in keeping with acceptable results. Carbide on light duty lathes tends to not perform well, although there are circumstances where it will out-perform HSS, such as when machining hard or abrasivie materials. If you are machining soft materials, especially aluminum, you are far better served using HSS, assuming you understand cutter geometry and have the ability to grind tools.

In regards to the angle of your tool, it's common practice for those of us who have machined for years to set a turning tool such that it can serve a dual purpose (facing and turning), as well as right angle shoulders at the ends of cuts. That's not an optimal setting, but it is very functional and proves to be a time saver when one is in production, where every change of a tool takes time, albeit not much.

When a tool is at a right angle to the cut, the least amount of surface area is introduced to the cut. That isn't always the chief consideration, although it is a part of the way to limit chatter, in particular on light duty machines. For the record, I have been in the shop since 1957 (commercially) and can honestly say I NEVER install a cutting tool such that it cuts a 90° angle (generally less, say 85°). I rely on the slides of the machine to establish the 90°.

Assuming you don't have need to face, or to create a 90° shoulder, you'll find that a tool performs best with a slight lead--that being that the tip trails the cuts, but not by a great deal. If the tip leads the cut, as it does when you can face and turn, pressure builds and can often become troublesome, but generally only if the angle is extreme.

H
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