When is half of 60 not half of 60 ?

[DaveH]

Just to make another comment, I don't think it matters one iota if the compound is set to 30deg or 29deg. What is really important is the cutting tool is at 60deg and it is equal angles of 30deg, having one 28deg and the other 32deg although they add up to 60deg it can cause you a few problems if it is not set up correctly.
 
Dave

[vtsteam]

Me thinks some of you think too much 

Nah. I like thinking. Some do some don't. 

[75Plus]

Now I don't set my compound slide over to any angle I just go straight in. . 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. 

The right edge is for cutting left hand threads. 

[malbenbut]

When setting at half angle minus one degree the back of the screw cutting tool cleans the back flank of the thread as it is cutting very little metal whilst the front of the screw cutting tool does most of the work. This means you can cut a cleaner thread as the chips don't interfere with each other, the screw deptnh is put on using the compound slide approx one and one eighth the original thread depth.
MBB

[DavidA]

... I don't think it matters one iota if the compound is set to 30deg or 29deg. What is really important is the cutting tool is at 60deg and it is equal angles of 30deg...

On reflection,  I am tempted to think it would matter.

If you did your last cut 'straight in' (to clean up) then you would finish up with a thread that had one face at 31 degree and one face at 29 degree.

Does any one else agree with that ?

Dave.

[vtsteam]

The tool is at 90 degrees David. Any movement of the cross slide inwards from the last cut will cut both flanks.

There are advantages and disadvantages to setting over the compound slide and not setting it over. In the end you pays yer money and takes yer choice.

[DavidA]

Yes,  I just woke up and realised the big mistake.
Please ignore my last response.

Sorry all.

Dave.

Now I can go back to sleep.

[philf]

...
If you did your last cut 'straight in' (to clean up) then you would finish up with a thread that had one face at 31 degree and one face at 29 degree.

Does any one else agree with that ?

Dave.

Dave,

I for one don't agree!

Without any more words I hope this clears everything up!



 

Phil.

[BenH]

I have yet to try screw cutting in the lathe, this is all very interesting and useful thanks

[DavidA]

Funny thing is that ,  in reply #5, I accknowledged that due to the tip being set square to the job and the last cut being 'straight in',  that the correct form would be produced.  So I don't know why I had that lapse of logic.

I was just dozing off when it hit me 'Oh (expletive deleted)  Must change that before they notice.' Too late.

Phil.

Looks as if our posts crossed.  Nice sketch though.  Thanks for that.

Dave

Showing signs of ageing.

[DavidA]

Phil,

Just had another look at your sketch.

I'm not so sure that it does clear things up.  Hate to keep on chewing at this old bone;  but here goes.

If you look at the sketch you will notice that the trailing edge of the cutter tip is inside the line of feed of the topslide. From this I would assume that you will still get some cutting action from this 'back edge' even though most of the work will be being done by the tip's leading edge.

Now.  if you were to rotate the topslide anti-clockwise another degree,  bringing the line of approach to 30 degrees (same as the angle of the tip) then the tip will cut only on the leading edge whilst rubbing the trailing edge against the previously cut thread .

If you take the angle around one more degree anti-clockwise,  to 31 degrees,  then only the leading edge will cut. You will have clearance at the  trailing edge;  which is what we want.  (I think).

The final straight in cut will restore the correct form.

What do you think ?

At the moment I am suffering my way through a nasty bout of some flue like symptoms and am barred from human contact. at least that is what my family say.  So I am trawling the web to keep myself occupied.

[vtsteam]

It won't "rub" on the trailing edge at 30 degrees. It will clear it. If it didn't clear it, it would be cutting it with a light finishing cut.

Also there is no absolute need to take a final straight in cut at the end when cutting at 30 degrees or less. (for a 60 degree thread).

But if you try to cut at 31 degrees or more you will leave a stair step cut on the trailing edge, and then yes, you would definitely want to remove that with a final straight in cut.

When using angles of less than thirty degrees, the trailing edge gets a nice light finishing cut during each pass, while the leading edge does the bulk removal

[DavidA]

VT,

Pretty much as I thought. Maybe the use of 'rub' was not a good choice of words. I should have said parallel with the last cut.

But I think we more or less agree overall.

Dave.

Now I can discard this bone; and perhaps move on to juicier ones.

[philf]

Phil,

Just had another look at your sketch.

I'm not so sure that it does clear things up.  Hate to keep on chewing at this old bone;  but here goes.

If you look at the sketch you will notice that the trailing edge of the cutter tip is inside the line of feed of the topslide. From this I would assume that you will still get some cutting action from this 'back edge' even though most of the work will be being done by the tip's leading edge.

Now.  if you were to rotate the topslide anti-clockwise another degree,  bringing the line of approach to 30 degrees (same as the angle of the tip) then the tip will cut only on the leading edge whilst rubbing the trailing edge against the previously cut thread .

If you take the angle around one more degree anti-clockwise,  to 31 degrees,  then only the leading edge will cut. You will have clearance at the  trailing edge;  which is what we want.  (I think).

The final straight in cut will restore the correct form.

What do you think ?

At the moment I am suffering my way through a nasty bout of some flue like symptoms and am barred from human contact. at least that is what my family say.  So I am trawling the web to keep myself occupied.

Hi David,

As you say at 30 degrees you are right in that the trailing edge won't remove any material - however a fraction of a degree more than 30 degrees will produce a staircase effect. The protractors on compound slides aren't usually that precise. If you think you've set it at 30 degrees you may well have 30.5 degrees and you'll never get a true 60 degree thread. Setting it at 29 degrees should make sure that you don't have this problem.

I've revamped a previous sketch to show external screwcutting with 29 degree and greater than 30 degree compound slide settings. (The angles are exaggerated to show the effects.)In the case of the 31 degree compound slide setting the green dotted line shows the ideal thread flank.) Making the last cut only with the cross slide would clean up the thread correctly but it would have to be a big cut in this case to remove the staircase effect.



I really hope that this finally makes it clear. (Especially as I don't bother with this method!)



Phil.
 

[DavidA]

Phil,

All clear now.

I also don't normaly use this method. So the whole exercise was pretty theoretical.  I would probably use it for,say, eight tpi and less.

So,  let's move on.

Those sketches of yours are pretty good.

Dave