Helix angle

[DavidA]

Time to dig out your calculators.

I am going to cut some 8 tpi square threads on 0.5" stock.

I have done this before, but never taken the helix angle into consideration. This time I want to do it properly.

I calculate that the tool needs to be rotated 4 degrees anti clockwise (looking down the tool toward the job) to give me the right angle.

Can any one confirm this. Or give me the correct figure and how you arrived at it.

Dave.

[modelman1938]

 Hi. I have never had cause to cut a square thread on the lathe, but I shouldn't think that the helix angle comes into it just the 1/8 lead and the tool would still be fed in at 90 degrees to the lathe axis.

[DMIOM]

hi Dave - my approach would be to unwrap one thread and apply tan rule

Assuming a single-start thread:

If initially assume you're setting the angle at the surface (the crest of the threads):
     0.5" dia stock => circumference 1.57"
     8 tpi => 0.125"
     Angle = tan^-1 (0.125/1.57) = 4.5 degrees

To be honest I'm not sure at what depth you should set the helix angle, if its towards the root diameter then I think the angle would verge towards 5 degrees.

Dave M (IOM)

[DavidA]

Looks as if I have been misunderstanding the exact meaning of helix angle.  It seem it really just pertains to the problem of the side of the tool rubbing on the thread as you cut it.
So the helix angle is really the minimum angle needed to prevent this, and the front cutting edge should still be kept , as pointed out, horizontal to the lathe axis.


Sorry about that one.  And thanks for the quick responses.

I have just been watching a couple of on line videos on this topic and all is now clear.

Dave,

Humble pie again for supper.

[Pete.]

I make it 7.125 degrees

At 8tpi half a turn is 0.0625" lead and tanA=a/b

so at 0.5" diameter  tanA=0.0625/0.5
 
so tanA=0.125

so A=7.125

Why is my angle so much greater?

[seadog]

You're using the diameter rather than the circumference. Remember that the thread is an inclined plane, the length of the hypotenuse being the circumference.

[Pete.]

I get it - pitch over 'distance cut'. Had to think about it for a bit.

[DavidA]

Dave (m),

Out of curiosity I did the calculations and came up with the same as you.

But what was more interesting to me was that I used, for the first time, a Casio fx85GT PLUS calculator.

You need diplomas in computing just to operate this thing. My old fx81 is child's play compared to it.

What a performance.

But I think I have cracked it.

I was wondering if I could down load an application (hate the word 'ap') for my tablet so I could create some programs to do this stuff.

Dave.

[DMIOM]

You're using the diameter rather than the circumference. Remember that the thread is an inclined plane, the length of the hypotenuse being the circumference.

Agree re circumference rather than diameter, but think that "in Tan terms" the circumference is the Adjacent rather than the hypotenuse?

Dave M

[Pete.]

It is, I was using tan because of the (wrong) way I was visualising the problem.

That said, I looked at an online calculator and the figure it produces agrees with using tan=O/A using the circumference of the stock material rather than calculating the length of the hypotenuse of the un-wound thread helix and using sin.

[Jasonb]

The deeper your cutting tool the more clearance you need on the leading edge

[Lew_Merrick_PE]

The convention is to perform your calculations on the Pitch Diameter.  This is not all that important with Square Threads (as opposed to Trapezoidal Threads), but it is worth habituating yourself to using.

(Thinking Radius) pi X Pitch Diameter = Pitch Circumference.  The Lead (.125 inch, in this case) is the advance per revolution (i.e Pitch Circumference).  Thus, the Helix Angle = tan^-1(Lead/(pi*Pitch Diameter)).  [Correction]

[chipenter]

Grind from a round tool bit and turn it to whatever angle you want .

[mklotz]

...
2pi X Pitch Diameter = Pitch Circumference
...

Come again?  C = pi * d  = 2 * pi * radius