The Shop > Tools

Helix angle

(1/3) > >>

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?

Navigation

[0] Message Index

[#] Next page