Negative rake carbide design using standard cutters

[More4dan]

I've had this idea for a while and finally had an hour in my shop. I tried mounting a normal carbide cutter on a holder I ground with a 20 degree negative rake. This would allow me to use standard cutters that I can sharpen on a diamond card. This one was made quick and dirty to test the concept. The result was better than I could have hoped for.

I made a quick pass on a resin blank and got the resulting finish. No sanding, as turned in the first blank picture. The second blank picture was with some plastic polish only still no sanding and no MM.

The carbide was a $2 made in China insert. Just lined up the cutting edge to the blank centerline with the tool held level on the tool rest. Comments welcome.

Danny


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[Bob in SF]

Great - thanks!

 

[Talltim]

Sounds like it could be a big savings in the long run. The regular EWT negative rake inserts are a little on the pricey side in my (not always so) humble opinion.

I may give it a try. Thanks for sharing.


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[leehljp]

I could be wrong with my ideas, but I have, for a long time, adjusted the height of the tool post 1/8" to 1/4" below the centerline which put the attack angle of a scraper, (HSS or insert) at the same angle equal to that of a negative rake on centerline level. And by making minute' adjustment (up or down) on the handle of the scraper (or skew laid flat), minuscule adjustments can be made to the angle of attack. It has worked well for me and really makes for smooth cuts.

 

[nebens]

Would you not accomplish the same thing by slightly raising the tool rest and bringing the handle above a perpendicular presentation? Ernie

 

[More4dan]

I do find it a bit easier to hold it flat than at a "fixed" angle as I move across the blank to get consistent results. My goal was to mimic the effect of the negative rake inserts at a lower cost. It also allows beginners to use the same technique for rough turning and making finishing cuts.

For inlace acrylester, I've had poor results when presenting the cutter much below centerline. The blank I tested was an inlace acrylester blank that has given me trouble with a rough turned finish requiring lots of sanding.


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[mbroberg]

I have had two engineers, one who deals in carbide tools and cutters, tell me that turning a standard carbide cutter upside down would have the same effect as a negative rack cutter. I have not tried this yet. Has anybody?

 

[More4dan]

I have had two engineers, one who deals in carbide tools and cutters, tell me that turning a standard carbide cutter upside down would have the same effect as a negative rack cutter. I have not tried this yet. Has anybody?

It would give you a 60 degree negative rake which is pretty steep. The Negative rake carbide cutters use a 20 degree rake. There might not be room on the carbide holder to fit the cutter upside down. If there is, give it a go and let us know how it works.

Danny


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[nebens]

With my limited experience I think I can get almost as smooth a finish with a well sharpened HF roughing goughe as I can get with a fresh round carbide regular or NR.
Ernie

 

[Curly]

If you were to angle the front face of the tool and screw the insert to it the side edge would be up and cutting. The amount of angle would dictate the rake. The disadvantage would be not having anything to index the cutter on and you would be relying completely on the shear/ tension strength of the screw to handle the cutting forces.

 

[More4dan]

If you were to angle the front face of the tool and screw the insert to it the side edge would be up and cutting. The amount of angle would dictate the rake. The disadvantage would be not having anything to index the cutter on and you would be relying completely on the shear/ tension strength of the screw to handle the cutting forces.

Flipping just the insert would allow the down force to transfer to the tool instead of the screw. Most of the tools I've seen have a recess to fit the bottom of the carbide insert and you can't flip the insert.

One could just turn the tool upside down, but all the problems you mentioned would be there. It would work as a quick test I guess.

Danny


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