Helical Threading

This post has been prompted by the reference to helical milling in the ball
nut thread, although it has nothing to do with ball nuts.

I have found helical milling to be a very good and quick way to produce
shortish threads. In fact far quicker than to screw cut on the lathe.
To assist this process I have made some helical milling cutters up.
The bought ones are usually carbide and costly.
What I have done is to machine a top hat piece of steel, the smaller
diameter being 16mm or 5/82 so that it will fit the collet chuck. The
larger diameter, around one inch by one inch long has a slot cut into the
side. This slot isn't central but is offset so that the leading edge, as
the tool rotates, is radial.
Fitted into this slot is chaser from a Coventry Die head. These are
possibly known in the US by another name but they are the self opening die
head that used to be fitted to capstan lathes. There are 4 numbered chaser
in a set. These can be picked up very cheap from auctions, yard sales etc
you only need one of each of the popular sizes as it goes on pitch and not
diameter.
The chaser is retained by a set screw from the front.
To use you need to measure across the widest point, ie. tips of teeth to
the OD. Subtract 1/2 the OD and you have the tool diameter.
Plunge down into the hole, start the spindle and move out so that the
cutter reaches full depth then interpolate one revolution whilst moving
down one pitch.
Some controllers have a G code for this other have to do it by coding.
By tilting the chaser in the holder to an angle it's also possible to do
tapered pipe and Gas threads this way in one pass.

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Regards,
John Stevenson
Nottingham, England