cant get angle of thread cut correct

Thanks Hopper, I will try that method once ive practiced a bit more now I know the correct topside angle set-up, will put some rake on as suggested JasonB.

If that is a Myford you are using you should have a set of fiducial marks to either side of the setting scale. If you use these to set the top slide the angle read directly and you don't have to do the sums.

Martin

Hopper, a 30°, 60°, 90° triangle has sides of 1, 2 and square root of 3 (1.73), the hypotenuse is the side that is twice the base . Going across 5 thou for every 10 thou in gives an infeed angle of 26.56°. Since this is less than 29° it is probably fine but will cause more cutting on the right hand flank of the tool than a 29° angle. If you want to get nearer to 29° use 5.5 thou across for every 10 thou infeed. The 2:1 ratio works well for 55° Whitworth threads where you want an infeed angle just less than 27.5°.

Martin

Doh, i knew when I posted that that it sounded odd. (the 1, 2, 3 triangle dimensions.) I really should avoid trying to use words of more than three syllables like hypotenuse. Thanks for pointing out my mathematical mayhem.

Nonetheless, the 2:1 ratio seems to work well for me for the past 40+ years since I was taught to do it that way. As I said, it cuts mostly on the lead edge of the tool. The other edge takes a very light cut at the same time, cleaning up any roughness or steps left from the the previous cut. So final form of the thread is down to the exact shape of the tool rather than the exact angle of infeed. For Whitworth 55deg forms I usually knock a few thou off the topslide movement.

3, 4, 5 is what you want for a triangle though not 30/60, 1,2,3 is a block

A 1:2:3 triangle is a very special triangle with internal angles of 180°, 0° and 0°

Martin

It's so special it doesn't exist! In Euclidian geometry if three points are colinear then by definition they do not define a triangle.

Andrew

Seems irony was lost here!

Martin

I'd challenge that as:

"Three points property: Three points either lie on a line or lie on a circle."

The example given is merely a degenerate triangle, if two of its points were co-incident (e.g. a 2, 2 ,0 triangle) it would be a right degenerate triangle.

Neil

Yes, but a degenerate object is not the same as the other objects in a class, and is generally considered to belong to a seperate class. So a degenerate triangle isn't considered to belong to the class 'triangles'.

Getting back to the OP I don't understand how the screwcutting created a buttress thread with the toolbit perpendicular to the work, even if the topslide was set at the wrong angle?

Andrew

1,2, and the sq root of 5 should work ( I think )

Neil    Don't the three points of a triangle always lie on a circle ?

Edited By roy entwistle on 12/05/2016 16:44:29

3,4,5 triangle is interesting square these and add and you get this ,. 9+16=25 may be others but the maths of 75 years ago is a bit misty now .Enjoying the conversation.

Andrew, the sketch was probably not a good rendition of what was produced. The thread form was asymmetric with one side correct at 30 degrees and the other at 60 degrees due to the incorrect setting of the top slide angle. This would produce a 90 degree thread similar to a buttress thread.

Martin