RE: ATM The Big Dob Two-Step (-per drive)

[andydtg@phoenix.net]

Here's my combined input for a number of posts concerning
the gear-hobbing thread:

Don Clement: >A factor that I don't believe has been mentioned about erratic
>drive error is the contribution of the quality of the bearings used on each 
>axes. How would one separate the contribution of the bearings from the
>contribution of the gear train? Just a thought but if one had a digitized 
>image of a star (ccd) over a period of time , one could perform an FFT on 
>the data and separate the periodic error from the erratic error. Then one 
>could perhaps relate the period to a particular gear or bearing. Don't know 
>how to relate erratic error to any particular gear or bearing though. Any
thoughts?

I try to test things like shaft straightness, bearing runout, end play
and worm radial runout with a very sensitive dial indicator as the
worm assembly is built up. The gage reads to .00005" & is usually
repeatable to .0002". The total of these type of errors can usually
be held to <.0005", the effect of which depends on the size and 
quality of the worm gear itself.

Carl Brennan: >Is it important that the gear blank rotates as well ??

Yes, the gear blank rotates and is rigidly attached to the rotary
table. For example, if I'm cutting a 240 tooth gear and the table
gear is 180, the tap has to rotate twice for each rev of the table
worm. Since I'm using roller chain and sprockets to connect the
hob to the table worm, the number of teeth I can cut depends on the
available selection of roller chain sprockets, the ratio of sprockets
times 180 has to be an integer also.

Kurt Hillig: >Remember that gears (at least here in the USA) are usually
>spec'd by "diametral pitch" rather than "circular pitch" - a 24-pitch
>gear does *not* have 24 teeth per inch of circumfrence, it's got 24
>teeth per inch of diameter (and I know there's no such thing as a
>1/2"-7.639 thread!). 

I'll defer to the US standard definition you state. By 10 pitch as
on my gears I mean the worm has 10 teeth/inch, thus the 240 tooth
gear has 10 teeth per inch of circumference (at the pitch diameter)
and thus a pitch diameter of 7.639". Obviously my gears are, by this
definition, non standard, the diametrical pitch being 31.416 +/-.
But since they're not intended to mate with anybody else's gears...

Peter Hirtle: >Alan Makintosh went into some detail on his experiences
>making worm wheels in the Maksutov club circulars.  Most of that
>information didn't make it into the ATMT books. <snip>

I tried some of those methods also, the one where you hob a gear
mounted on a lathe crossfeed with the tap in the headstock was
a total disaster.








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