When is half of 60 not half of 60 ?

[DavidA]

Not wanting to stir up any hornet's nest,  but here goes.

I was reading through my copy of Sparey's 'The Amateur's Lathe' and ,  where he covers screw cutting, he mentions the 'half angle' method of cutting threads. In his example he uses  Whitworth 55 degree thread and says something along the lines of ' Set the topslide over to 27.5 degrees'. 

Which is half of the 55 degrees.  no surprises there.

BUT.

I have noticed that our most excellent Doubleboost,  when setting his topslide to cut 60 degree threads by this method,  makes a point of setting to 29 degrees.  Not the 30 degrees I would expect.

No doubt this is done for a reason as it seems to produce excellent work.

But why ?

Dave.    

[spuddevans]

I may be wrong ( it has happened once before  ) but I think it is so that you are definitely cutting only on one side of the cutting tool. Then for the very last cut you advance the cross-slide instead of the top-slide to give the final true thread shape.

I believe the idea is to reduce the cutting forces on the tool for the majority of the cut until the final cleanup cut.


Tim

[DavidA]

Tim,

I see where you are coming from there.

But won't that leave you with a slightly skewed thread form ? 29 degrees on one face and 32 on the other.

Dave.

[doubleboost]

The last cut is put on with the cross slide
This cuts both sides of the thread at once
Giving the thread the same angle as the tool (well that is how I do it)
John

[philf]

Not wanting to stir up any hornet's nest,  but here goes.

I was reading through my copy of Sparey's 'The Amateur's Lathe' and ,  where he covers screw cutting, he mentions the 'half angle' method of cutting threads. In his example he uses  Whitworth 55 degree thread and says something along the lines of ' Set the topslide over to 27.5 degrees'. 

Which is half of the 55 degrees.  no surprises there.

BUT.

I have noticed that our most excellent Doubleboost,  when setting his topslide to cut 60 degree threads by this method,  makes a point of setting to 29 degrees.  Not the 30 degrees I would expect.

No doubt this is done for a reason as it seems to produce excellent work.

But why ?

Dave.

Dave,

It's always safer to be a degree less than the half angle. If you set it a degree more rather than less you can't generate the correct thread form.

I did a couple of sketches some time ago to show what would happen if someone set the topslide at 60 degrees instead of 30:

The first shows the topslide set to 29 degrees (61 to the lathe axis) and shows that most of the cutting would be on one edge of the tool with just a scraping on the other side.



The second shows the topslide at 60 degrees and you can see that one flank of the thread will be cut correctly whilst the other would be stepped. This would apply to a lesser degree to any angle even very slightly larger than half the thread angle.



As the topslide graduations aren't usually very precise it's safer to set the angle a degree less.

I don't bother - I keep the topslide parallel to the lathe axis and for every 0.1mm I increase the depth of cut with the cross slide I advance the topslide 0.04mm. For the last delicate cuts I just put a cut on with the topslide.

Hope this helps.

Phil.

[DavidA]

Philf,  thanks.

John,

...Giving the thread the same angle as the tool (well that is how I do it)..

Understood.  In fact I retract the question as I am in error regarding the final form of the thread. It has to be correct as the tip is square to the job.



Bo**ox 

Dave.

[doubleboost]

It was a very good question 
The reason for cutting on the leading edge is simply to reduce tool loading
Not so important on a light thread
But it helps on a coarse deep thread
John

[vtsteam]

Hey if you don't ask questions when you don't understand something how can anybody help?

Good for you for asking!! 

[DavidA]

What is annoying me is that I didn't think it through completely. I forgot about the tip being square to the job and that it will clean up the form to the correct 60 degree angle on the last (cross slide) cut.

For your kind words you will each receive a FREE copy of my ritual suicide dvd.
Now,  where did I put my sharp knife.


Dave. 

[vtsteam]

David, this may surprise you, but many of us also don't always think things through!

[Pete49]

Now I'm confused (not that hard really). On the last cut do then move the topslide the same amount as you had been moving the cross slide (ie CROSS SLIDE MOVES IN 5 THOU SO FINAL CUT TOPSLIDE 5 THOU)? oops fat finger. I look at the keyboard not the screen when typing or I should say hunt and peck.
Pete

[philf]

Now I'm confused (not that hard really). On the last cut do then move the topslide the same amount as you had been moving the cross slide (ie CROSS SLIDE MOVES IN 5 THOU SO FINAL CUT TOPSLIDE 5 THOU)? oops fat finger. I look at the keyboard not the screen when typing or I should say hunt and peck.
Pete

Pete,

Don't touch the topslide for the final cut - just the cross slide.

Phil.

[vtsteam]

"Same amount" could be confusing. "To finished depth" might be better.

Top slide and cross slide will travel toward the axis of rotation at different rates per marked division if using graduated collars on screws. Last cut should also be light.

[Pete49]

OK thanks all I think I got it. I'll try it next time I cut a thread    
Pete

[andyf]

I use the angled top slide method until the thread is at full depth. This might be a good point to mention the trick set out in G H Thomas's Model Engineers Workshop Manual (ISBN 1-85761-000- 8) for setting up to so the slides do the trigonometry to get the thread to the correct depth when using an angled top slide to deepen it. You work to a point where the dials read ZERO in the end.

1. Set top slide to almost one half of thread angle, usually to 29.5° (US/metric) or 27° (Whit/BSF). Zero the top slide dial collar. Feed cross slide forward until the tip of the tool just touches the work (nip a cigarette paper or bit of kitchen foil) and zero the cross slide dial collar. Now, the tool is touching the work and both dials read zero.

2. Move carriage to the right so that the tool is in clear air, feed the cross slide forward by the true depth of thread (but see A below) and set its dial back to zero again.

3. Leave the cross slide untouched, and withdraw by means of the top slide until the tool just clears the outside diameter of the work.

4. Move the carriage back to the left, and feed the top slide forward until the tool touches the work again. This determines the starting point.

5. Move the carriage back to the right to clear the work, advance the top slide slightly and make the first cut. Before traversing the tool back between cuts, use the cross slide to withdraw it in the usual way, then return the cross slide to zero and apply further depth of cut with the top slide. Once both dials read zero at the end of a cut, the correct depth of thread has been reached.

GHT doesn't mention the complications which arise:

A. Where the cross slide dial shows the amount by which the diameter will be reduced, rather than the actual depth of cut. Then, in step 2 above, the cross slide must be advanced by double the dial indication . For example, for a thread depth of 0.030”, advance the slide 0.060 as shown on its dial. 

B. When the thread depth is greater than the pitch of the cross slide feedscrew (only 1mm on many small Asian lathes) so the dial has to go round more than once to withdraw when the thread is getting deep. It can get difficult to keep track, particularly if A above also applies. Consider a positive stop on the cross slide, or at least a temporary witness mark on its side at the correct zero point. Likewise, if the depth of thread is more than about 80% of the pitch of the top slide feedscrew, that may have to be turned more than once. But as the top slide will only be advanced, and in small increments, the thread will be obviously incomplete when an “intermediate” zero is reached, and almost fully formed as the desired zero is approached.

Andy

[DavidA]

Andy,

It all seems rather complicated compared with the 'normal' method.  Don't most people just keep on cutting until the nut (or what ever) is a good fit ?
That doesn't apply if you have no nut to try it against.

The problem (B) you referred to can be eliminated by using the dial gauge method illustrated in my picture above.

Dave.

[andyf]

It does sound a bit complicated, David, but it isn't in practice. Anyway, I occasionally make my own silver steel taps for odd sizes and LH threads, where I may not have a nut to try because I need the tap to make the nut...

Anyway, I'm too trigonometrically ignorant to want to calculate depth of thread x cos/the top slide angle!

Andy

[DavidA]

Andy,

I haven't tried making taps yet.  And I also don't fancy calculating the depth of cut allowance for the angle of the topslide.

I'm going to have a go at making a collet chuck to fit my lathe ( inspired by Doubleboost's video; thanks John) and will need to make up the necessary gauges. Lots of thread cutting in my future.

Dave.

[ksor]

 

I'm preparing my FIRST thread cutting on my old 1A616 STANKO - so I'm looking for some GOOD explanations to my frustations 

Let me start with this:

1) "Explanation" has to do with pure theory - so pure and clear that you can understand the underlaying problems

2) "Practice" has to do with what "works for me" and what is "good enough for me"

I think most (if not all) of these postings belongs to category 2) - don't spank me for saying that, but they don't give me the AHAaaa !

  ... what ... something wrong with me ... no, no, of cause not 

Maybe I missed it, but I don't see any place mentioning the cutting tools angle to the Z-axis (the acxial turning axis) - I beleave it should be 90° - right ?

IF THIS IS ADDED some of the postings  - for me I beleave - can be moved from the 2) to 1) category - and then I can UNDERSTAND WHY to use - in principle - any degree (28, 29 or 29.5°) UP TO HALF of the threads degree, but absolutly NOT MORE than half !

As a consequence the FINAL CUT MUST be made by advancing the CROSS SLIDE to clean up the "saw-pattern" on the RIGHT side of the /\  <-- this side !

Did I get it right ?         ... if not then spank me 

[dsquire]

I'm preparing my FIRST thread cutting on my old 1A616 STANKO - so I'm looking for some GOOD explanations to my frustations 
.
.
.
As a consequence the FINAL CUT MUST be made by advancing the CROSS SLIDE to clean up the "saw-pattern" on the RIGHT side of the /\  <-- this side !

Did I get it right ?         ... if not then spank me 

ksor

No spanking today.

I think that you have it figured out now. 

Cheers 

Don

[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]

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.

I could'nt stop thinking about this "explanation" when I went bed  ... 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 

[vtsteam]

Phew! 

[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 

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 
 
DaveH