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How to straighten a bent shaft (Unitron)

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#1 AllanDystrup

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Posted 20 October 2013 - 09:06 AM

Today I bought a heap of Unitron/Polarex stuff from the estate of a Danish amateur astronomer.

Apparenly this Fellow had been very active in visual astronomy, had a pair of Celestron Cat's plus some Polarex scopes, and had liked to mix/match and ATM with his gear.

After having sold off the Cat's, the family allowed me to fill my rear seat with what was left of Unitron/Polarex stuff (for the very fair price they asked), among other things a D60/f700 guide scope and a 128C mount (the guide will fit nicely on my Zeiss Telemator, and I can use the mount for my 114 :jump:) :

Now, the stuff obviously has beeen out of use for a long time, it's dirty and needs cleaning & restoring. among other issuses, the slow motion shaft of the declination on the 128C mount is bended :


Any suggestions for straightening out a bended shaft (without risking to break it) ?

Thanks,
Allan

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#2 AllanDystrup

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Posted 20 October 2013 - 09:15 AM

Here's a pic of the bended Unitron 128C slo-mo shaft...

Allan

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#3 terraclarke

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Posted 20 October 2013 - 09:59 AM

Wow Allan, what a great haul. You are going to love that eq. mount! They are wonderful. Looks like you got a duatron too. How cool.

Terra

#4 AllanDystrup

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Posted 20 October 2013 - 10:33 AM

Yep, a lot of good stuff there...
The 128 EQ came with the drive for 60mm too :),
working ,-- but all needs a good cleaning & lubrication.

Allan

#5 dgreyson

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Posted 20 October 2013 - 10:58 AM

Spot heat it with a propane torch until its close to red hot and then it will be mallible enough to straighten out. Could be hard on the knob. Someone made a jig here in a different thread that would pull it straight but dunno how universal that is aa a fix. Cold bending wont work.

#6 hottr6

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Posted 20 October 2013 - 11:14 AM

Put it on an anvil and tap the bend with an engineer's hammer. As the bend straightens out, roll the shaft on the anvil, tapping the shaft to get the bend (presumably straight) you want. Don't use heat as it will melt the plastic handle.


#7 AllanDystrup

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Posted 20 October 2013 - 11:30 AM

Hmmmm, to heat or not ...
seems to be the :confused:

Allan

#8 tag1260

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Posted 20 October 2013 - 11:44 AM

Not being familiar with it but will the knob come off?
You may also be able to get some of it out by clamping it in a large vise and using that as sort of a press. It may be less stress on it to get some out that way rather than beating it all out with a hammer.

#9 AllanDystrup

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Posted 20 October 2013 - 11:57 AM

clamping it in a large vise


No, the knob doesn't come off.
But clamping the shaft in a vise sounds like an idea worth trying.

Allan

#10 roscoe

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Posted 20 October 2013 - 12:17 PM

You'll get most of it out in a vise, (you may want to pad the jaws with scraps of hard wood) but the last part will have to be done with a hammer - find one with a flat, smooth face, and also a flat, smooth piece if sturdy steel for an anvil. Many light taps, rather than a big boom, is the way to go, rotating it regularly as you tap on it so the high point is up.
good luck!
R

#11 actionhac

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Posted 20 October 2013 - 12:20 PM

I like using a vise also. With wood supporting the shaft the knob will not interfere. Squeeze out the bend and roll it on a flat surface to check progress.

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#12 AllanDystrup

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Posted 20 October 2013 - 12:36 PM

Thank you Guys !
I've decided to go ahead with the vice as a first approach.
Seems like the most gentle, incremental and yet controlled way of straightening out the bend.

ps @Shane :
The problem with tapping the bend with a hammer is, that the end of the shaft is threaded (doesn't show well on my photo), so the tapping risks destroying the threads.

Allan

#13 Ron500E

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Posted 20 October 2013 - 12:38 PM

I'd bring it to a small machine shop and have them use a hydraulic press to slowly get it to its original position.
Not sure how much it would cost in Denmark but in the States, if they even bother to charge you, it would around $20 USD.
Kind Regards,

Ron

#14 AllanDystrup

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Posted 20 October 2013 - 12:47 PM

Good idea Ron -- i'll check what's available in the neighbourhood.

Allan

#15 fjs

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Posted 20 October 2013 - 12:50 PM

I've decided to go ahead with the vice as a first approach.
Seems like the most gentle, incremental and yet controlled way of straightening out the bend.


NO! definitely not a vice! Drinking while performing delicate tasks is NOT recommended! (use a vise)

#16 grendel

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Posted 20 October 2013 - 01:48 PM

its a vice in the uk, but dont ask the Americans what colour!
Grendel :-)

#17 fjs

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Posted 20 October 2013 - 01:58 PM

Oops! I knew about 'colour', et al, but not 'vice'; sorry. Thank you for setting me straight!

P.S. Why can't you spell 'defense' correctly? We are not removing fences here.

#18 greju

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Posted 20 October 2013 - 02:03 PM

Put it on an anvil and tap the bend with an engineer's hammer. As the bend straightens out, roll the shaft on the anvil, tapping the shaft to get the bend (presumably straight) you want. Don't use heat as it will melt the plastic handle.


This works well with a piece of soft wood on two sides.

#19 roscoe

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Posted 20 October 2013 - 02:05 PM

To preserve the threads, you could use a block of hard wood instead of metal for your anvil.

#20 orion61

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Posted 20 October 2013 - 02:08 PM

If you heat it, wrap the knob and part of the shaft in Modeling Clay or Play Doh it will absorb the heat and not allow the heat to transfer up the shaft and melt the plastic knob.

#21 AllanDystrup

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Posted 20 October 2013 - 02:18 PM

vise vis-à-vis vice
versa

#22 dgreyson

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Posted 20 October 2013 - 02:43 PM

Another trick is to wrap aluminum foil and then lightly clamp vise grips on the shaft as a heat sink, I'd forgotten about that.

Good luck!!

#23 fjs

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Posted 20 October 2013 - 03:07 PM

vise vis-à-vis vice
versa


truly hilarious, Allan.

I don't think you will go wrong with the 'vice' as long as the shaft is not bent too severely. If it is bent more than about 20 degrees or so; yes, I would want to heat it. If you choose to heat it(and I wouldn't blame you for doing that), there are many ways to deflect the heat away from the knobs. Any type of clamp affixed to the shaft between the heated area and the knobs SHOULD do the trick (WILL do the trick if adequate). What is required is the understanding that you are providing a large lump of metal that will soak up the excess heat before it reaches the plastic knobs. Aluminum(Aluminium) is well known for its heat transfer abilities.

As you can see from my post; we use the word 'bent' here. I don't know if Grendel would agree.

#24 terraclarke

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Posted 20 October 2013 - 03:48 PM

I would go with a vice and hard wood like oak in the jaws to not mar the threads. The vice will allow you to slowly and evenly apply compressive stress to the metal without as much chance of metal fatigue. Repeated blows with a hammer will possibly cause fatigue, metal crystallization and fracturing. Also, fast heating and uncontrolled cooling can result in metal crystallization and fracturing, aside from damage to the plastic knob. A heat sink is problematic. Use the vise and periodically, slowly release the pressure and examine then rotate the bolt as need be. That would be my first coarse of action. Heat or hammer as a last resort.

#25 dgreyson

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Posted 20 October 2013 - 06:31 PM

That sounds like sensible advice Terra






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