Re: [CAD_CAM_EDM_DRO] Helix Angle

Hi,

Thank you for the replies but they don't help as everyone came up with a
different answer.

What I'm trying to do is cut a 3 start helical slot around an aluminium bar
to make couplings for stepper motors. The way I would machine them is to
hold them in the lathe chuck and use a slitting saw in a milling spindle set
at the helix angle of the slot, which happens to be 8 TPI. This is easy to
do, once I have the helix angle, but not very productive. As I wanted to
manufacture a lot of these couplings, of different sizes, I have designed a
jig which consists of a screwed threaded bar which the part to be machined
is locked to. The threaded bar is used to advance the part to a slitting saw
held in a horizontal miller but the jig must be set at the helix angle.

All the machining is easy once I have the helix angle but drawing the slot
in ACAD is a different matter. I tried to do it all day yesterday with
results that didn't please me. I draw it the same way I machine it, not
forgetting I got a different angle to all of the posts :-)

As to the question why 18mm with an 8 TPI thread. I'm English and come from
that generation that had to learn measurements ambidextrously and as such I
think in imperial and draw in metric. I machine in either depending on the
machines scales. Metal here can come as imperial or metric and one is often
sold for the other. If I order 3/4" I'll sometimes get 19mm or visa versa.
The 18mm measurement was after machining a 19mm or 3/4" bar to 18mm.

Anyway my question is still the same, which of us is right. The thread I'll
be using will be square or acme form.

Regards,

Terry

> You'll be soooo pleased to know that the helix angle of the thread tip
> and root are not the same!
> To see how to calculate the angle, imagine a piece of paper wrapped
> around the appropriate diameter (root or tip or anywhere in between). If
> you trace the helix to the paper, along with a circumference line
> (straight around) and one lengthwise, then unwrap the paper and flatten
> in. The three lines will (if you connected them right) form a triangle.
> The line that was the lengthwise one will be the thread lead distance.
> The line that was the circumference will have a length equal to the
> circumference (duh!).
> These two lines form a right angle.
> The hypoteneuse is the helix line. The tangent of its angle will be the
> thread lead divided by the circumference.
> Thus for the thread tip, the angle will be arctan(0.125/(pi*0.7087)),
> which is 3.2134 degrees.
> For the thread root the angle will be
> arctan(0.125/(pi*(0.7087-0.125*tan(60)))), which is 4.6217 degrees.
>
> Although I'm not sure why you need this to generate a CAD model for the
> helix...
> -Kevin Martin
> -----Original Message-----
> From: Terry Owens [mailto:terry@...]
> [...]
> This may at first sight appaer to be off-topic but it isn't. I nned to
> know the answer so I can draw it in CAD to make a part for a CNC
> machine.