LOL, tetchy eh Norm?
Full faith in the "man of dividing plates", John
Stevenson, had the pleasure of meeting him recently
and a waffler he ain't. I see by latest posts my faith
was not in vain. Gidday John.
Good day Tom,
Wondered if you had got back OK, bit of a long walk downhill to where
To the group, Tom was in England a while ago and popped in one Friday
afternoon and we had a very good chat only spoilt by having to cut it
short as we had prior arrangements that night, collections, deliveries
Always nice to put a face to well respected posts.
Visit the new Model Engineering adverts page at:-
It seems to be the case that most watchmakers lathes include a 60 hole
dividing plate on the back of the drive pulley since it is conveniently
divisible by 2; 3; 4 etc. . I'm in the process of making a replacement
and hit upon this dividing by 60 problem too.
My 4" lathe has divisions on the front gear of the back gear; but being
somewhat old, the 360 circle (the only one divisible by 60) has worn
away in places. The procedure recommended to me was to get some
35mm film and use the sprocket hole spacing to make a band with 60
holes which fits round a woodend disc and put this on the back of
the mandrel. I've not tried this yet and must confess that
I'm tempted to try and borrow a 60t change gear to save the trouble
(my lathe is plain so doesn't have changewheels itself).
I am not sold on this 35mm film idea - I am constantly trying to improve my
precision, and this does not appear to be a step in the right direction.
On the lathe that I use currently I have been thinking of using a gear
fitted on an expanding mandrel in the end of the hollow tailstock, 60 would
be favourite, but I would have a choice. Its a shame the bull wheel has 75
teeth, if it was 60 then I could use that instead.
I think I have seen indexing holes on a chuck backplate too. That could be
I'm not too sure of it either (I keep hoping someone else will try it
first and tell me if it works or not!); the problem seems to be the
space between the first and last holes; it seems that the inter sprocket
hole spacing on 35mm film is very accurate; but there is bound to be an
error in joining the ends; it would probably be fine if this home made
"plate" were to be put onto a worm reduction gear as this would reduce
the error by the worm reduction factor; but if I had a 60 to one worm
reduction dividing head, then I wouldn't need a 60 hole division plate!
If you are really *that* desperate, a length of old metal tape
measure tacked around the circumference of a plywood disc got me out of
trouble on one occasion about 20 years ago!
Could you not get a 'friend' to drill appropriate holes around the
circumference of either a chuck body or the rim of a chuck backplate which
you can then index with a simple detente?
Alternatively, Hemingway - www.hemingwaykits.com -sell a kit for
the George Thomas dividing head, this includes a 60t gear wheel of about
three inches in diameter which I'm sure Kirk Burwell would sell you as a
stand alone item. He's on 01746 767739.
Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"
On 27 May 2006 19:52:23 +0100 (BST), Alan Bain
[Apropos using 35mm film to form dividing plate]
With the greatest respect gents, this method will improve your
The whole point of this method is that it is transforming an accurate,
pre-made linear scale into an angular one, by wrapping it around the
circumference of a disk.
So all you have to do is to be able to turn a disk to a set
circumference (i.e. diameter). You need no measuring equipment
whatsoever, just your piece of film and a rough pi.
Cut your film to the number of holes you want (say 120). Cut another
piece with one third the number of holes - this gives the approximate
diameter of the disk you will require. Make the disk a bit bigger as
your pi is size three-and-a-bit.
Mount disk in lathe and turn down until when you wrap the film around
it, the ends meet. If they overlap, change to putting-on tool.
How will this improve your precision? Well, because whatever you do on
the cross-slide is magnified just over six times on the circumference,
you will very quickly get a very good idea of how much you can
reliably and repeatably take off. You will also learn the trick of
setting-over the top slide so you can improve the resolution of your
cut by maybe a factor of 10. You will also learn when to stop cutting
and start using other metal removal techniques (polishing/lapping).
And what if the ends don't meet? If the gap is, as some of our
trans-atlantic friends like to say, a RCH, what difference does this
make to the angular accuracy? Googling shows me that film-hole spacing
is 0.187" [-ish], so 120 of these makes 22.440" circumference. Let's
assume our RCH is 0.004", giving a disk circumference of 22.444" (disk
radius 3.572". Your final division, instead of 6 degrees (0.10472
rad), is 0.10582 rad, between a twentieth and a tenth of a degree
big; the other divisions being about one sixtieth of one tenth of a
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