need help machining focuser
Posted 05 January 2013 - 10:22 AM
I want to go real simple for this first one. I was thinking a helical focuser or a Crayford-style and only 1.25" format.
I have a lathe and milling machine.
For the helical it seems that all you need is to internal thread a 1.5+" ID shaft and external thread a 1.5" OD barrel bored open to 1.25" with a set screw?
But the Crayford looks straightfoward as well, no gears needed..?
Posted 05 January 2013 - 10:31 AM
The challenge with a Crayford is to get the flat on the drawtube parallel to the bearing surfaces. With a milling machine you should have no problem doing this, as folks have achieved good results with careful use of a file. Use good bearings.
Posted 05 January 2013 - 10:41 AM
You can find it here
Single pitch threaded helical focusers are maddenly slow. So I turned to multiple pitch threads using old camera lenses for the source of the threads. Saves some time and results in smooth operating helical focusers.
Posted 05 January 2013 - 10:48 AM
Posted 05 January 2013 - 12:59 PM
both slow (when I want to focus) and fast when I put in another eyepiece. I used 10 tpi lead with an acme thread form.
Then on the side of the housing I put a deten, a spring loaded
ball bearing. I did not thread the inside of the housing only
the barrel holding the eyepiece. just pull on the barrel and
it comes out as much as you wish or push in and the barrel goes in. Each time the ball pushes back on the spring and
skips over the thread when you stop the ball again engages the thread and a turning motion screws the barrel in or out. You can see the small barrel on the side which houses
the ball and spring held in by the set screw.
Posted 05 January 2013 - 01:01 PM
Posted 05 January 2013 - 01:24 PM
is made from a piece of aluminum rod 1/2" in diam. I drilled
a hole into the rod about the same size as the ball but didn't go completely through,just enough so the ball was held
back by the edge of the hole that remained.
You can see the ball detent showing in this picture.
Posted 05 January 2013 - 01:38 PM
I works fine in he cold because it's 99% aluminum so all the pieces shrink together (except for the ball and spring of course) you can make the hole,in the rod, with enough clearance so the ball won't seize in the hole when the aluminum shrinks in the cold. I made that mistake on a gem
that I made. I like things to fit so there is no slop. The
mount had CRES shafts that went into aluminun flanges.Beautiful fit at 72 deg but at 25 deg I couldn't
get the mount apart to bring it back into the house when I
was done observing. I've since rectified that.
Posted 05 January 2013 - 02:30 PM
Posted 05 January 2013 - 03:06 PM
also you could buy the eyepiece tube from jmi and build around it. here's a quick sketch but I would reduce the bearings from this initial concept, which is a knock off from crayford john's first focuser?
Posted 05 January 2013 - 06:49 PM
Posted 15 January 2013 - 03:29 PM
Posted 15 January 2013 - 07:51 PM
The reason I chose 11 tpi for the double-start application was that 11tpi is prime not some multiple of the lathe imperial 8tpi leadscrew. If I would have chosen 10tpi or 12tpi then the odd-even thread chaser dial method wouldn’t have worked. Also if I chosen metric pitch then I couldn’t use the half-nut lever.
Posted 15 January 2013 - 10:55 PM
Scopeman, thank you for your contribution. That is awesome and inspiring, I can't wait to post some update.
I'm moving my shop at this time, gonna be complicated ..