Jump to content


Photo

Cutting and squaring an aluminium tube

  • Please log in to reply
18 replies to this topic

#1 TG

TG

    Apollo

  • -----
  • Moderators
  • Posts: 1286
  • Joined: 02 Nov 2006
  • Loc: Latitude 47

Posted 04 November 2012 - 10:57 PM

Hello folks,

I am faced with the somewhat scary task of shortening a refractor tube by 1.5" or so. I've been searching CN forums and it seems using a hacksaw with a taped paper guide is the way to make the cut. This part I think I understand but I need help with the next one: how to square the cut. Here is what I'm planning:

Assuming the tube is square right now, I can stand it on a level surface on the other end and then balance a level on it to see if it's square. To get a level surface, I plan to get a flat piece of wood from Home Depot and insert 3 tee nuts and bolts into it. Then I can adjust the bolts till the piece is exactly level. To level it on the top end, I just plan to file it (assuming it's needed) where a level placed across it shows it to be out of square.

Am I overthinking it? Is there an easier way?

Regards,

Tanveer.

#2 glennnnnnn

glennnnnnn

    Mariner 2

  • *****
  • Posts: 257
  • Joined: 20 Oct 2009
  • Loc: San Diego, CA

Posted 05 November 2012 - 03:34 AM

I can't see any flaws in your process, although the easiest way is to just use a block the height of your cut to mark the tube, once you get the tube in the position you know to be plumb or level. You might be surprised at how easily you can determine how un-square a tube is by just turning it and looking. (But) If you draw a circle around your tube on the floor, you have established an accurate location to check with a projection from the wall how far out of true it is.
When you can turn the tube 360 and it just touches the indicator at the top all the way around its square.

#3 RossSackett

RossSackett

    Surveyor 1

  • *****
  • Posts: 1705
  • Joined: 17 Aug 2007
  • Loc: Memphis, TN

Posted 05 November 2012 - 07:28 AM

Tanveer,

You may be overthinking it. I assume you are concerned about squaring the end to maintain collimation (probably of the focuser, since only a maniac would cut away from the objective end). There may be an easier way. I would just flatten the cut end with a piece of sandpaper glued to a flat board, and make the focuser adjustable by widening the mounting screw holes.

Ross

#4 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1972
  • Joined: 10 Dec 2007

Posted 05 November 2012 - 08:27 AM

Am I overthinking it? Is there an easier way?

Regards,

Tanveer.


I believe there is an easier way.

If you stand your tube upright on a flat surface, how much it leans depends on whether the surface is level and whether the tube is square. If the tube is not square the top will wobble as the tube is rotated as it stands. It doesn't matter how level the surface is as long as it is reasonably flat.

Simply make a small vee block on the flat surface to position the tube in. Then watch the top as you rotate the tube in the vee. If it wobbles, the tube is not square. You can do this on a desk or workbench and use a wall to see the wobble. You don't have to actually measure the wobble; just determine if it is more or less as the tube rotates. You can do this with a stiff cardboard wedge or a fancy dial guage. Both will get the same result.

You can get a very square tube working this way since the error at the top of the tube is magnified by the ratio of tube height to diameter.

dan

#5 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5669
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 05 November 2012 - 10:33 AM

Find a machinist and offer him $20 or a case of beer or something like that. Failing that, try a steel/metal or welding supply house. Or perhaps even a vocational school.

Any of those places will have the correct tools and experience to give you a clean and square result. My experience is that the people will gladly help you out and probably refuse compensation, it's a two minute job for them.

#6 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5451
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 05 November 2012 - 10:42 AM

i used a hacksaw and tape on my 100ed.

since the replacement focuser was a VERY tight fit in the tube, it squared itself semi-magically. i just had to make the focuser attachment holes in the tube a bit oblong to accommodate for non-squareness of the end of the tube. but so long as its square within an mm or so (easily achievable with hand tools) it should be ok.

#7 Bill Kocken

Bill Kocken

    Messenger

  • -----
  • Posts: 415
  • Joined: 07 Aug 2004
  • Loc: Coon Rapids, MN

Posted 05 November 2012 - 12:12 PM

Here's another option. Assuming the tube is not tapered, cut it with a power miter saw. Aluminum cuts pretty easily with carbide tipped blades. It's noisy and wear goggles to protect youself from tiny flying chips of aluminum. You could practice by cutting off 1/2 inch to see if you like the result, then cut it to the needed length.

#8 Gene7

Gene7

    Viking 1

  • -----
  • Posts: 877
  • Joined: 10 Jun 2010
  • Loc: Mid Ohio, USA

Posted 05 November 2012 - 12:39 PM

Forget the level, it is too cumbersome. You need to revise your thinking. A square mechanical cut does NOT equal a precise optical aleignment, there are other factors involved. You MUST collimate both ends after assembly and fix it so it does not move. Use a laser and cats eye or whatever.

You can either spend your time fiddeling with the adjustment or fabricating a methodical three point push-pull adjustment, I suggest the latter. Gene

#9 zippeee

zippeee

    Ranger 4

  • -----
  • Posts: 334
  • Joined: 27 Dec 2010
  • Loc: Alberta, Canada

Posted 05 November 2012 - 01:53 PM

Find a machinist and offer him $20 or a case of beer or something like that.


+1

Parting-off in a lathe is without question the most accurate way to do this. However, make sure whoever cuts it uses soft jaws and a die or jig to prevent crushing or scratching the OTA in the chuck.

#10 TG

TG

    Apollo

  • -----
  • Moderators
  • Posts: 1286
  • Joined: 02 Nov 2006
  • Loc: Latitude 47

Posted 06 November 2012 - 11:48 AM

Thanks for the great advice, folks. I'm glad I asked here. The perfect option for me would be an adjustable back-plate but the one I have from D&G has no such feature. In addition, the focuser from A-P just screws in so everything has to be perfectly square. So my modified plan is just this:

1. Cut the tube
2. Put in the backplate + focuser. Insert a laser and check where the spot is showing up on the objective side.
3. Gently hammer in the back-plate (the tube is slightly elliptical rather than exactly circular so the back-plate is a tight fit) till the laser spot is aligned. Mark holes and drill.

I also have a dob-base that I can use to spin the tube around to check for squareness of the cut as one of you suggested.

I do have a machinist who in the past declined payment and just asked for a "case of pop" (he got Izzy). But the tube is painted with a fragile paint and has to be handled gingerly, probably too much to ask for a free job.

Thanks again,

Tanveer.

PS. In case I screw up, I'm cutting the tube to 1.5" leaving the 0.5" to a professional. :-)

#11 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5451
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 08 November 2012 - 10:45 AM

fragile paint = Tak?

cutting one down for a binoviewer?

#12 TG

TG

    Apollo

  • -----
  • Moderators
  • Posts: 1286
  • Joined: 02 Nov 2006
  • Loc: Latitude 47

Posted 14 November 2012 - 02:30 PM

Yippee! I used the paper wraparound method and used a hacksaw to cut 1.5" off. It took about 2 hours (with numerous breaks) and constant cleaning off of the debris. I measured the width of the cut off piece with a digital caliper and its width is within 0.3mm which means its square to +/- 0.15mm. This with a hand-held hacksaw. Amazing! I think I can start attaching the new backplate from D&G.

Tanveer.

#13 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1972
  • Joined: 10 Dec 2007

Posted 14 November 2012 - 02:47 PM

Congratulations! (I always suspected you could do it.)

dan

#14 TG

TG

    Apollo

  • -----
  • Moderators
  • Posts: 1286
  • Joined: 02 Nov 2006
  • Loc: Latitude 47

Posted 14 November 2012 - 03:10 PM

fragile paint = Tak?

cutting one down for a binoviewer?


No, A-P. Cutting it down mainly to upgrade the focuser but having easier focusing for the BV is an added plus.

Tanveer.


#15 Z28500

Z28500

    Ranger 4

  • -----
  • Posts: 331
  • Joined: 27 Feb 2012
  • Loc: N. 30.1 W.-95.6 Texas

Posted 15 November 2012 - 08:11 AM

Sounds like you got a good handle on it and know what your doing.
When I have checked tubes for square on some of the ones I cut, I put the tube straight up on a piece of granite (table, counter top, etc) and put an aluminum framing square next to it, then turned it 90% and checked it again, if it was out a little, I bought a roll of sandpaper 8" wide x at least 2', and sanded putting most of the pressure on the side that needed removing, it ended up square, and flat. Works pretty good. (Wish my chop saw was bigger than 10"!)
Z28500

Attached Files



#16 TG

TG

    Apollo

  • -----
  • Moderators
  • Posts: 1286
  • Joined: 02 Nov 2006
  • Loc: Latitude 47

Posted 15 November 2012 - 07:03 PM

I sanded off paint from the last 0.4" and used a piece of wood to hammer in the backplate till it went in all the way (about 0.5"). Screwed in the A-P focuser, inserted a laser at the focuser end, a circular piece of cardboard cut to fit the tube over the first baffle at the other end, and this is what I saw. It's hard to see but the small circle drawn in pencil is 0.4" in diameter so looks like I'm only 0.15" off from the center. Sweet! The laser was about 53" away so I guess I hit about 0.15/53*57.3*60=9.7 arcmin. I think that's plenty sufficient. E.g. for my f/9 refractor, depth of focus is 0.18mm while this kind of tilt will change focus at the end of a 50mm field by only 0.07mm.

Tanveer.

Attached Files



#17 TG

TG

    Apollo

  • -----
  • Moderators
  • Posts: 1286
  • Joined: 02 Nov 2006
  • Loc: Latitude 47

Posted 15 November 2012 - 07:04 PM

Focuser end after drilling and tapping.

Attached Files



#18 droid

droid

    rocketman

  • *****
  • Moderators
  • Posts: 7193
  • Joined: 29 Aug 2004
  • Loc: Conneaut, Ohio

Posted 16 November 2012 - 01:00 AM

A simple way to make a cut square is to wrap a piece of paper around the tube, make sure the ends are aligned, tape in place and cut.
I learned this building rockets, works like a charm, in fact I had to cut off one inch of the tube in my Widger scope rebuild, used this method.

#19 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1972
  • Joined: 10 Dec 2007

Posted 16 November 2012 - 05:27 AM

I sanded off paint from the last 0.4" and used a piece of wood to hammer in the backplate till it went in all the way (about 0.5"). Screwed in the A-P focuser, inserted a laser at the focuser end, a circular piece of cardboard cut to fit the tube over the first baffle at the other end, and this is what I saw. It's hard to see but the small circle drawn in pencil is 0.4" in diameter so looks like I'm only 0.15" off from the center. Sweet! The laser was about 53" away so I guess I hit about 0.15/53*57.3*60=9.7 arcmin. I think that's plenty sufficient. E.g. for my f/9 refractor, depth of focus is 0.18mm while this kind of tilt will change focus at the end of a 50mm field by only 0.07mm.

Tanveer.


That's the way to do it! If you build it right, it will be aligned without adjustments, and it will stay aligned forever.

dan






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics