Jump to content


Photo

need help machining focuser

  • Please log in to reply
16 replies to this topic

#1 psi_chemie

psi_chemie

    Mariner 2

  • *****
  • Posts: 204
  • Joined: 01 May 2011
  • Loc: Leawood, KS, USA

Posted 05 January 2013 - 10:22 AM

Any astro-machinists out there that can point me towards a focuser project for a Newtonian scope? Step by Step?

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..?

#2 dpwoos

dpwoos

    Apollo

  • *****
  • Posts: 1387
  • Joined: 18 Oct 2006
  • Loc: United States

Posted 05 January 2013 - 10:31 AM

The main problems with a helical is that the "speed" if fixed by the thread pitch, and they wobble. I am sure the wobble is correctable with some sort of tensioned/split-nut arrangement.

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.

#3 John Jarosz

John Jarosz

    Astro Gearhead

  • *****
  • Posts: 3209
  • Joined: 25 Apr 2004
  • Loc: Fairfax, Iowa

Posted 05 January 2013 - 10:41 AM

I wrote a CN article on this subject quite a while ago.

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.

John

#4 psi_chemie

psi_chemie

    Mariner 2

  • *****
  • Posts: 204
  • Joined: 01 May 2011
  • Loc: Leawood, KS, USA

Posted 05 January 2013 - 10:48 AM

THANK you John. Once I find some plans and begin on a design I place to post pictures of the process.

#5 Norm Meyer

Norm Meyer

    Mariner 2

  • *****
  • Posts: 252
  • Joined: 08 Feb 2009
  • Loc: Warren, ME 04864

Posted 05 January 2013 - 12:59 PM

I made a helical focuser some years ago and I like it. Mine is
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.
Norm

Attached Files



#6 Norm Meyer

Norm Meyer

    Mariner 2

  • *****
  • Posts: 252
  • Joined: 08 Feb 2009
  • Loc: Warren, ME 04864

Posted 05 January 2013 - 01:01 PM

here is another view with the threaded barrel extended.

Attached Files



#7 dpwoos

dpwoos

    Apollo

  • *****
  • Posts: 1387
  • Joined: 18 Oct 2006
  • Loc: United States

Posted 05 January 2013 - 01:07 PM

Nice! How does it work in the cold?

#8 Al8236

Al8236

    Apollo

  • *****
  • Posts: 1129
  • Joined: 18 Aug 2010
  • Loc: 48.9173N 122.1390W

Posted 05 January 2013 - 01:11 PM

Crayfordjon has posted a few different designs, here is one
http://www.cloudynig...&Board=atm&N...
There are others if you want to search.

#9 Norm Meyer

Norm Meyer

    Mariner 2

  • *****
  • Posts: 252
  • Joined: 08 Feb 2009
  • Loc: Warren, ME 04864

Posted 05 January 2013 - 01:24 PM

The small barrel or rod that holds the ball and spring
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.

Attached Files



#10 Norm Meyer

Norm Meyer

    Mariner 2

  • *****
  • Posts: 252
  • Joined: 08 Feb 2009
  • Loc: Warren, ME 04864

Posted 05 January 2013 - 01:38 PM

dpwoos,
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.

Norm

#11 m. allan noah

m. allan noah

    Viking 1

  • -----
  • Posts: 752
  • Joined: 14 Aug 2009
  • Loc: Virginia, USA

Posted 05 January 2013 - 02:30 PM

I built one along the lines of John's idea, using camera lens parts. I took apart about a dozen lenses, and found one that had a large ID, which i bored out to 2". Then i used some epoxy to attach an F-stop ring from a different lens to use as a hand grip for focusing. Then I glue the whole thing into a thin outer shell from a third lens. Very minimal machining in the end.

allan

#12 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7726
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 05 January 2013 - 03:06 PM

you can make a kineoptic type helical focuser without using threads and you'd be able to push/pull as well as turn the tube.

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?

Attached Files



#13 psi_chemie

psi_chemie

    Mariner 2

  • *****
  • Posts: 204
  • Joined: 01 May 2011
  • Loc: Leawood, KS, USA

Posted 05 January 2013 - 06:49 PM

OMG this is awesome, thank you all! I have some information to process, and perhaps a cutting tool to grind or two, but I will hopefully be able to show some progress soon! I have some ball nuts to repack.

#14 scopemankit

scopemankit

    Ranger 4

  • -----
  • Posts: 390
  • Joined: 27 Jan 2009
  • Loc: south africa

Posted 15 January 2013 - 03:29 PM

I have made a few crayfords, here is a rough plan and a pic. hope it helps. (Three of them are crayford helical focusers).

Attached Files



#15 don clement

don clement

    Vendor (Clement Focuser)

  • -----
  • Vendors
  • Posts: 802
  • Joined: 02 Feb 2011
  • Loc: Running Springs, California

Posted 15 January 2013 - 07:51 PM

A method I have used with a standard UN thread form for fast focusing that would work on thin walled tubing is the multiple-start thread. For example I machined a double start 11 tpi thread using a 20UN full topping insert (thread profile height of 0.0307”) on my manual lathe that has an imperial leadscrew. The trick was to use the odd and even markings on the thread chasing dial. Even numbers (2,4) of the thread chasing dial for the first thread start and odd numbers (1,3) for the second start. Here is a picture of a double-start 5/16-11UN tap I made using this method from oil hardened drill rod.

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 Image

#16 psi_chemie

psi_chemie

    Mariner 2

  • *****
  • Posts: 204
  • Joined: 01 May 2011
  • Loc: Leawood, KS, USA

Posted 15 January 2013 - 10:55 PM

Cool Don- I love your work. I want to try many methods but I have been thinking about trying a double start on my lathe- I've been playing with the thread dial and think I can do this.. I also want to try and make a tool for something like Norm's helical I love the threads.

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 ..

#17 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7726
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 17 January 2013 - 06:36 PM

don't know if you ever used sketchup but...

here's a 3d model you can move around pull dims...

2in crayford focuser






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics