When is half of 60 not half of 60 ?

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vtsteam:
Yes to 90 degrees to the axis of rotation.

Sticking my own neck out here:

I'm not sure whether you have to clean up a saw pattern, technically, because it seems like there shoudn't be any AS LONG AS the angle of set over is equal or less than the half angle of the thread.

Why?

If the set over is equal to the half angle of the thread then the trailing tool edge follows its cut edge, but doesn't cut anything.

If the set over is less than the half angle, the trailing edge is always doing some cutting as it gets deeper -- similar to a straight plunge cut, although the leading edge is doing MOST of the cutting.

if the set over is more than the half angle, then you would get stair stepping because it is being pulled away from its cut edge with every wind of the compound slide.

Hope that's right. Please don't hesitate to send a dunce cap if I have that wrong.

ksor:

--- Quote from: vtsteam on September 18, 2013, 05:23:04 PM ---Yes to 90 degrees to the axis of rotation.

Sticking my own neck out here:

I'm not sure whether you have to clean up a saw pattern, technically, because it seems like there shoudn't be any AS LONG AS the angle of set over is equal or less than the half angle of the thread.

Why?

If the set over is equal to the half angle of the thread then the trailing tool edge follows its cut edge, but doesn't cut anything.

If the set over is less than the half angle, the trailing edge is always doing some cutting as it gets deeper -- similar to a straight plunge cut, although the leading edge is doing MOST of the cutting.

if the set over is more than the half angle, then you would get stair stepping because it is being pulled away from its cut edge with every wind of the compound slide.

Hope that's right. Please don't hesitate to send a dunce cap if I have that wrong.

--- End quote ---

I could'nt stop thinking about this "explanation" when I went bed :loco: ... and sometimes "it" comes to you while sleeping ... or trying to ... and it did !

So here I am again and you'r right:

if the compound angle is LESS THAN half the thread angle there will NOT be any saw pattern - the cut will have the correct shape (if your cutting tool is grinded in the right shape !) of the thread but not yet deep enough.

If the compound angle is GREATER THAN half the thread angle there will be a saw pattern on this side --> /\ of the cutting.

To be sure allways to have the right shape of the thread it is "best practice" to set the compound angle a Little LESS THAN half the thread angle.

There is NO need for for doing any cleaning cut or do the last cut with the cross slide.

Now the explanation is clear to me too :med: :med:

vtsteam:
Phew! :ddb:

vtsteam:
One more thing I have been thinking about:

As the angle of set over reduces down to zero (infeed is now 90 degrees to lathe axis), both the leading and trailing edges are cutting the same amount.

If you cross over to the other side, the trailing edge starts to cut more than the leading edge. And the cut will be smooth until you again reach the half angle on that side. After that it will be stair stepped.

Now why would you want to cut on the trailing edge?

Well, I was thinking that you have to grind a whole lot of relief into the leading edge of a threading tool. You need to add the thread's helix angle PLUS whatever cutting edge relief you want.

But the trailing edge has the opposite situation. It would only require the normal cutting relief MINUS the helix angle.

If for arguments sake you wanted say 10 degrees of cutting relief for your lathe tool, and the helix angle happened to be 10 degrees, and you decided to try to cut on the trailing edge, your cutting tool wouldn't need any relief ground into it at all.

Now, I don't know if a lathe tool will actually cut a thread well on the trailing edge -- I have to try it first. So theory may have to give way to reality. But I AM curious about this.....

ps. One reason why cutting on the trailing edge might not go well is that it is analogous to climb milling. The thread tends to push the lathe tool, rather than the lathe tool pushing into the work. So backlash may have a similar effect. If so, It may be that reducing the cutting clearance angle would be helpful, and also it might be helpful to give the leading edge some work to do ie. reduce the set over angle.

DaveH:
Me thinks some of you think too much :lol:

Now I don't set my compound slide over to any angle I just go straight in. I have a very good reason for doing this I buy single point carbide insert tips. These tips whether 60deg or 55deg come with both edges ground. I have paid for the two sharp edges I am damn sure I am going to use them :lol:
:beer:
DaveH

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