Parting off blade shape.

[Larry$]

Looking at an insert they almost all have some radius on the corners. The radius reduces the area that is in essence torn off. The point where the metal fails. Is that torn area then burnished to create the beautiful finish we see? I use HSS parting blades that I either eyeball sharpen, lazy method, or more precisely grind on the tool grinder. Even though right off the grinder these have "perfectly square corners", so the sides would not be cutting and only a torn chip would be produced the thickness of the feed, almost instantly a corner radius is worn on to the tool. This is because an absolute sharp corner has little material supporting it. So we now have a form tool with a radius corner with progressive shear cutting until the metal fails and some tearing occurs. The wiping action of the progressively thinner cut is therefore going to burnish the surface a little bit in the torn area. This produces a flowing of the metal much like using a sharpening steel on a knife. SWAG System!

All this is merely a bit of mental gymnastics. If I manage to get an acceptable cut I'm happy and go on to try and limit the next mistake learning opportunity.

 

[Parlo]

If parting inserts are relieved and they cut on the sides why doesn’t that make the surface finish worse? How can they cut a square groove without square sides?

Why would cutting on the sides ( around the corner radii ) make the finish worse? It's common to pop a radius on a corner to improve surface finish. It only cuts the sides one feed/rev deep. Most cutting tools are relieved i.e. have clearance following the cutting face to prevent rubbing.

If you see the earlier video and this image https://drive.google.com/file/d/1QyfOBTufVaYmN_7zdSYRsrYRQBbSRcW5/view?usp=sharing it will explain how a groove is cut with square sides. The quality of the finish on the sides, as always, depends on the feedrate. This determines the overlap between the side grooves and ultimately the surface finish which is measured by the height of the peaks.

" how can they cut a square groove without square sides " takes the thread back to the original posts.

 

[Parlo]

Looking at an insert they almost all have some radius on the corners. The radius reduces the area that is in essence torn off. The point where the metal fails. Is that torn area then burnished to create the beautiful finish we see? I use HSS parting blades that I either eyeball sharpen, lazy method, or more precisely grind on the tool grinder. Even though right off the grinder these have "perfectly square corners", so the sides would not be cutting and only a torn chip would be produced the thickness of the feed, almost instantly a corner radius is worn on to the tool. This is because an absolute sharp corner has little material supporting it. So we now have a form tool with a radius corner with progressive shear cutting until the metal fails and some tearing occurs. The wiping action of the progressively thinner cut is therefore going to burnish the surface a little bit in the torn area. This produces a flowing of the metal much like using a sharpening steel on a knife. SWAG System!

All this is merely a bit of mental gymnastics. If I manage to get an acceptable cut I'm happy and go on to try and limit the next mistake learning opportunity.

'The radius reduces the area that is in essence torn off.'
The material is sheared around the radius, producing the wall finish. There is no burnishing after the shearing, as the tool width becomes less than the slot width as the tool progresses deeper.

'so the sides would not be cutting and only a torn chip would be produced the thickness of the feed, almost instantly a corner radius is worn on to the tool.'
The corners wear because the material is shearing the feed depth around the corners. This feed / rev depth is the length of shear on the sides of the tool ( approximately equal to the wear radius ).

 

[Shotgun]

I realize that times are rough, but are we expecting this horse to become edible if we beat it enough?

 

[Parlo]

If a post has errors, should I leave it or attempt to further explain the concept?
Perhaps this video will convince the doubters that a parting tool needs to cut on the sides and can't just cut at the front.

 

[Jim F]

I realize that times are rough, but are we expecting this horse to become edible if we beat it enough?

I was just thinking the same thing..........

 

[Susquatch]

Several YouTube comments from retired engineer of 55 years, Bill Shiff were deleted because he proved that a trapezoidal ( triangular ) parting tool cut around its perimeter and not, as stated just on the primary edge. With respect for Bill, this is the best evidence I could supply to dispel the myth and to respect his wisdom.

Jezz, and here I thought this thread was looking for opinions on what edges of a parting tool do the cutting. It seems obvious to me that it is three edges or as a wise user pointed out earlier, the front top surface.

I didn't know that we were trying to validate the opinions of a wise old engineer. But as a slightly less old and much less wise old man I agree with his view completely.

 

[Parlo]

Jezz, and here I thought this thread was looking for opinions on what edges of a parting tool do the cutting. It seems obvious to me that it is three edges or as a wise user pointed out earlier, the front top surface.

I didn't know that we were trying to validate the opinions of a wise old engineer. But as a slightly less old and much less wise old man I agree with his view completely.

Thanks mate, you were a tough crowd but I got there in the end lol. Now onto YouTube to debunk the video.

A surprising result arose after shooting the coarse feed video, the part stayed the same length! I was measuring over the remaining peaks.