QCTP progress and problems

TomG said:
Sorry John, I guess that explanation was in another thread. Here it is:

"Back rake is over rated for high speed tooling and generally unnecessary. I grind all of my tools with zero degrees back rake, use them in a quick change tool post and they cut just fine. About the only time I'll ever use back rake is if I'm machining something soft like wood or rubber or some plastics. Back rake when cutting steel is actually a bad idea because it removes material behind the cutting edge of the tool which allows heat build up. Another plus for tools with zero back rake is you'll get a lot more life out of them since the tool geometry doesn't change as the tool is sharpened."


Tom
Click to expand...

Thanks Tom,

That's something new for me to play with. I generally like cutting tools with LOTS of back rake: like with a tangential or the net result of having a chipbreaker. Since grinding the back rake for me is usually a freehand operation (whereas the relief angles are set by my toolrest), it could eliminate a step... Also, since the top of a HSS blank is ground very smooth, I would guess you could, in theory, end up with a better finish with just a light honing of the cutting edge to get rid of the coarse grinding marks.

I'm not sure I understand what you mean by "removes material behind the cutting edge of the tool". As far as I can tell, the cutting edge still does all the work and the back rake just presents a steeper angle to the workpiece for chip control. Can you explain?

Thanks,
John
 
jgedde said:
I'm not sure I understand what you mean by "removes material behind the cutting edge of the tool". As far as I can tell, the cutting edge still does all the work and the back rake just presents a steeper angle to the workpiece for chip control. Can you explain?

Thanks,
John
Click to expand...

John,

Adding back rake makes the angle of the cutting edge more acute. That means there is less metal behind the cutting edge to conduct heat away from it and it's life is reduced. You won't notice any difference cutting non-ferrous metals, but on steels, especially tool steels the effect can be significant.

Tom
 
"i don't know if i had too many beers but im totally lost with the drawing. . . which edge is the cutting edge on your drawing. please ?"

David - Too many beers is an oxymoron. :) The most forward edge is the cutting edge. It slices the work piece. It works very well. I believe the link to the other forum has pictures of it in use.

"does that go in a rear tool post?"

Steve- No, it goes in the front, just like a normal cutting bit.

It's an unusual concept but it really works. Also check out Gadgetbuilder's website for his version: http://www.gadgetbuilder.com/VerticalShearBit.html

Hope this helps.

Steve
 
Frank is exactly correct and I should have pointed that out. My bad.

The tool will actually cut anywhere along the vertical edge, so it can be moved up/down without resharpening. I start out with the center of the bit centered on the material.

Steve
 
Another view while I was cutting a 2MT out of a piece of 1018 CR.

2MTsmall.JPG

It's an easy tool to grind as none of the angles are critical and it works well as long as it is sharp.

Steve

2MTsmall.JPG
 
If you have ever been to a used tool sale you will have come across a box of bits that have some horendous grinds on them. More than likely they all work. Just depends on what you are cutting. A little more side angle, top angle, etc. will sometimes make all the differance in the world.
HSS can be easily ground to any shape you wish. In the picture are some I got this weekend. Form Tools, Boring Bars. Cut Offs, they are all there.

"Billy G" :thinking:

bits 002 (700 x 525).jpg
 
Bill Gruby said:
If you have ever been to a used tool sale you will have come across a box of bits that have some horendous grinds on them.
"Billy G" :thinking:
Click to expand...

I know thats true bill, a few years back when i first started I would scour the flea markets and i bought one bit i remember that cut better than anything I had, it looked really bad. I used it all the time until dull and because i knew nothing i decided to resharpen it the way i was sharpening all the rest. i thought it was a great bit because it was made from better metal but it never cut as well as when i first got it. I should have paid closer attention to it's grind.
steve
 
I thought it was ground by some amateur because everything was so rough, i never thought who made it really knew what they were doing and the rough grinding was just to get the basic shape and the cutting edges were perfect.who ever made it just hogged the unwanted metal out of the way.
steve
 
The right material (12L14,) and the right grind (adding the top rake - duh) and I'm a happy camper. Thanks to all of you who helped.

photo.JPG
 
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