Re: ATM Design advice for tracking Dobs

[andydtg@phoenix.net]

John D. Upton wrote:

>    For those of us into DIY activities to the extreme, would you be 
>willing to talk about your hobbing techniques?  I bought a small 
>combination machine last year to work on telescope parts and have been 
>intrigued by some of the hobbing techniques described in the ATM books. 
>I'm still learning, but am always looking for a new challenge.
>
>    Do you have a full (semi?-)pro NC shop set up to do your work or do
>you work with more modest machine set-ups? I'm sure I'm not the only one
>on the list who would like to know more of your methods (and products). 

I have a small collection of metal working machines, a 10" South bend 
bench lathe, an Atlas manual vertical mill, grinders, a cutoff bandsaw, 
a 12" vertical bandsaw & the usual assortment of micrometers, gages,
& tooling. The only piece of NC equipment is myself, I use AutoCAD
to design the stuff I make. Someday I'll be able to get bigger/better
stuff.

I've tried some of the gearmaking techniques, like using ordinary
taps in a lathe to free-cut gears - didn't work well for me.
Melting gears with threaded rod or using pipe clamp slotted
strip can make a functional gear but ususally get lot of
lead an/or periodic error. 

The gearhob technique I use now uses Acme threaded rod (1/2-10)
which has a tooth profile almost exactly like that of a 20 deg 
pressure angle actual worm gear. I mount the gear blank
on an extremely precise rotary milling table (+/- 5 arc sec)
and motor drive the table, with the acme tap rotating at
an integer fraction multiple of the reduction of the mill
table. The tap is fed radially inwards into the plastic gear
blank and gradually cuts the exact pitch and depth teeth
into it, as the same motor drives the table and the tap
in synch via a chain sprocket drive. 

I use these gears in my smaller alt-az computer driven
mounts, as well as to make focal plane rotators.







Andy Saulietis / DTG Alt-Az-Fp Drive Systems
12617 Harriet Ln
Santa Fe, TX 77510 USA
409-925-8854 Voice/Fax