OK whats with these diamond tool bits?

[SmoggyTurnip]

I have been reading about tangential tool holders like these ones:

http://www.gadgetbuilder.com/ToolHolders.html#Tangent
http://www.eccentricengineering.com.au/


From what I can figure it is neccessary for the tool bit to have a diamond shape to it so that it can have both front relief and side relief. But I have never seen a diamond shaped tool bit. The second link above says "ALL from a standard square HSS toolbit ".



What am I missing here? Do diamond shaped tool bits really exist? Do people make their own?

[ptjw7uk]

I dont think they are diamond shaped just a normal a square hss tool bar held so that only 1 corner is cutting.
Peter

Forgot to say that the tool bit is ground at an angle so the end is diamond shaped.

[TGTool]

Yes, if you slice a square toolbit at an angle to the length, and from corner to corner diagonally, the resulting flat surface will look like a diamond shape. This is the face presented to the cut, or the face you see looking straight down from the top.

[SmoggyTurnip]

Funny, I thought about this off and on over the past year and could not figure it out. Then about a minute after I hit the POST button it all made sense to me.

[dsergison]

well, that will present a diamond, but on an obtuse corner. so it kinda sucks for facing or turning.

[Peter.]

Hi.

Great forum you have here.

I'm thinking of making one of these for external-thread cutting. Does anyone know off-hand the angle that the square hss tool must be cut at to produce a 60-degree cutting angle on the top face for cutting metric threads? If I have this angle I can prduce a jig to make & re-sharpen the cutting tool on my bench grinder.

[Mcgyver]

Welcome Peter,

I'm at a loss as to how a sq tangential tool can be used to cut threads. Remember the cutting edge of a tool must lie on a plane that passes through the lathes axis - in other words the whole length of the cutting edge must be at centre height. Since a threading tool has two cutting edges, it would be difficult to make a threading tool without a flat top, ergo what advantage would there be in grinding the sides and the top of a tangential tool vs just the sides of a horizontal tool bit?

[Peter.]

Hi McGyver, than you for the welcome.

I've read several long threads today and I can't find the reference to it to re-read it, but I'm sure I read that the tangential tool can be used for thread-cutting. I figured, that if a square tool was held so that one corner was touching the work, and the holder held that tool at somewhere like a 45 degree rake with the tool ground so that the cutting face is on a horizontal plane, then the acute angle of the diamond presented to the workpiece would be somewhere around the 60-degrees needed for thread cutting. Easier to draw than explain I guess. Kind of like the photo below but with the tool rotated in the holder 45 degrees.



If this would be possible I could easily make a jig to sharpen my hss bits to cut threads without having the pain of trying to achieve the correct angle by hand.

[lazlo]

I've asked this question before, but I've never gotten a concrete answer: does the diamond toolholder cut easier/nicer, or is the main advantage that it's easier to sharpen the bit?

The diamond toolholders are often shown in the old turn-of-the-century machinery books like Robert Smith's Advanced Machine Work. But those books don't mention any specific advantage to the diamond tool either -- they just show the tool, and how to sharpen them.

In fact, the 5th Ed of Advanced Machine Work shows grinding a diamond tool for a normal Armstrong 1S (straight) toolholder: since the Armstrong is inclined already, you just grind a little more back-rake into the insert, and you have the same tangential tool contact.

[SmoggyTurnip]

Part of the reason I was confused about this is I thought that a tangential tool would have some part of the tool tangent to the work piece - but it really doesn't.