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Equipment Discussions >> Classic Telescopes

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AllanDystrup
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How to straighten a bent shaft (Unitron)
      #6148124 - 10/20/13 10:06 AM Attachment (24 downloads)

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 ) :

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

Edited by AllanDystrup (10/21/13 06:03 AM)


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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6148139 - 10/20/13 10:15 AM Attachment (23 downloads)

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

Allan


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terraclarke
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6148203 - 10/20/13 10:59 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


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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: terraclarke]
      #6148258 - 10/20/13 11:33 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


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dgreyson
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6148296 - 10/20/13 11:58 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.

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hottr6
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6148317 - 10/20/13 12:14 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.


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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: hottr6]
      #6148342 - 10/20/13 12:30 PM

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

Allan


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tag1260
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6148362 - 10/20/13 12:44 PM

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.


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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: tag1260]
      #6148382 - 10/20/13 12:57 PM

Quote:

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


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roscoe
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6148424 - 10/20/13 01:17 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


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actionhac
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6148425 - 10/20/13 01:20 PM Attachment (11 downloads)

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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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: actionhac]
      #6148452 - 10/20/13 01:36 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

Edited by AllanDystrup (10/20/13 01:43 PM)


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Ron500E
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Re: How to straighten a bended shaft (Unitron) new [Re: actionhac]
      #6148458 - 10/20/13 01:38 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


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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: Ron500E]
      #6148469 - 10/20/13 01:47 PM

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

Allan

Edited by AllanDystrup (10/20/13 01:47 PM)


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fjs
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6148475 - 10/20/13 01:50 PM

Quote:

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)


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grendel
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Re: How to straighten a bended shaft (Unitron) new [Re: fjs]
      #6148535 - 10/20/13 02:48 PM

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


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fjs
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Re: How to straighten a bended shaft (Unitron) new [Re: grendel]
      #6148548 - 10/20/13 02:58 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.

Edited by fjs (10/20/13 03:17 PM)


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greju
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Re: How to straighten a bended shaft (Unitron) new [Re: hottr6]
      #6148559 - 10/20/13 03:03 PM

Quote:

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.


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roscoe
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Re: How to straighten a bended shaft (Unitron) new [Re: fjs]
      #6148560 - 10/20/13 03:05 PM

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

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orion61

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Re: How to straighten a bended shaft (Unitron) new [Re: greju]
      #6148563 - 10/20/13 03:08 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.

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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: fjs]
      #6148577 - 10/20/13 03:18 PM

vise vis-à-vis vice
versa


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dgreyson
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Re: How to straighten a bended shaft (Unitron) new [Re: orion61]
      #6148614 - 10/20/13 03:43 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!!


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fjs
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6148645 - 10/20/13 04:07 PM

Quote:

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.


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terraclarke
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Re: How to straighten a bended shaft (Unitron) new [Re: fjs]
      #6148707 - 10/20/13 04:48 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.

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dgreyson
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Re: How to straighten a bended shaft (Unitron) new [Re: terraclarke]
      #6148967 - 10/20/13 07:31 PM

That sounds like sensible advice Terra

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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: terraclarke]
      #6149202 - 10/21/13 01:25 AM

Right Terra, this will be my 1. approach.

Thank you all, for all the good advice.
Truly a treasure chest of knowledge in this forum !

Allan

Edited by AllanDystrup (10/21/13 01:26 AM)


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Jon Isaacs
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Re: How to straighten a bended shaft (Unitron) new [Re: terraclarke]
      #6149314 - 10/21/13 06:08 AM

Quote:

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.




Terra:

Looking at it, it is steel and it is unlikely that it is very hard steel, it should be sufficiently ductile if one wanted to use a hammer. "Fatigue" seems unlikely to me unless it were already cracked/fractured and then it would probably grow the crack no matter what one did.

Heat could be good but would likely damage the finish. Heat sinking it would save the plastic, not sure about the finish.

I would use a vise initially, aluminum jaw covers are nice but there will likely be some residual deformation which would will need a final straightening.

Jon


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bremms
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Re: How to straighten a bended shaft (Unitron) new [Re: Jon Isaacs]
      #6149335 - 10/21/13 06:47 AM

Yes you will need to do some final tweaking after the vise. If you are good at delicate tasks, once it is close you can use a few nuts to hold it in position just shy of the bent area. cllamp those in the vise and use gentle pressure. Depending on the type of steel. The fix can be easy or it will crack and break. It's most likely a mild steel . The vise method can cause extra deformation if you are not careful. If I the unbent length is too long in the vise you will cause a bend in the opposite direction.
I would bend it back by inserting into a threaded fixture slowly and gently bending it back. Thread it in far as possible... Gently apply pressure... Little more maybe thread it in more .. You are bending it back the same way it was bent. It will work better than the vise method. If it fractures.. Well, it went well past the elastic limit and it wouldn't matter how it was fixed.


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terraclarke
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Re: How to straighten a bended shaft (Unitron) new [Re: bremms]
      #6149342 - 10/21/13 07:10 AM

Hookes Law always applies.

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terraclarke
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Re: How to straighten a bended shaft (Unitron) new [Re: terraclarke]
      #6149420 - 10/21/13 08:41 AM

Be very careful. You can see in the following photo, posted by one of our CN Classics members, the point of weakness where the threaded portion meets the unthreaded portion. You can see in the photo that the shaft was bent prior to fracture. Don't know if the fracture occurred at the time of bending or during an attempt to straighten it.

http://www.cloudynights.com/classifieds/data/28/44420-1368137649DSC06295-larg...


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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: terraclarke]
      #6149530 - 10/21/13 09:53 AM

Dang, you've got me worried now Terra...

Well, I can't use the shaft as is, so I'll clamp softly
(and carry a [strike]big[/strike] tiny hammer...)

Allan

Edited by AllanDystrup (10/21/13 09:56 AM)


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terraclarke
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6149595 - 10/21/13 10:29 AM

Allan

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actionhac
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Re: How to straighten a bended shaft (Unitron) new [Re: terraclarke]
      #6149623 - 10/21/13 10:45 AM

Wow Terra thats a good piece of detective work and may have saved Allan from a heart failure.
The item you found looks like it could be brass and if Allan's is the same material fracture is likely.
I would be very careful and possibly not even try straightening it if in fact it is brass. If its steel I'd say proceed with caution.

Robert


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Jon Isaacs
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Re: How to straighten a bended shaft (Unitron) new [Re: terraclarke]
      #6149641 - 10/21/13 10:58 AM

Quote:

Be very careful. You can see in the following photo, posted by one of our CN Classics members, the point of weakness where the threaded portion meets the unthreaded portion. You can see in the photo that the shaft was bent prior to fracture. Don't know if the fracture occurred at the time of bending or during an attempt to straighten it.






When there are threads present, a stress riser exists and fracture is more far likely. Whether it occurred during the initial bending or the straightening, when it breaks, it breaks. Without actually seeing the threads on the photo, it does look to be in the threaded region.

If it were to break, good machinist could bore out the plastic and machine a new shaft but one would need a friend who is a machinist to make it worthwhile.

Jon


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terraclarke
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Re: How to straighten a bended shaft (Unitron) new [Re: actionhac]
      #6149739 - 10/21/13 12:01 PM

Thanks Robert. I knew I had seen a picture of a similar bent and broken Unitron mount shaft a few months ago so this morning I just set out to ferret it out by looking at posts and adds of several of our regular Unitron folks here and voila. It was Steve's.

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sgorton99
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Re: How to straighten a bended shaft (Unitron) new [Re: terraclarke]
      #6149785 - 10/21/13 12:22 PM

Yes, that is my broken shaft from my 128. Good detective work Terra! I have been watching this thread with great interest, as my long shafts on my 155 need some minor straightening as well.

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grendel
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Re: How to straighten a bended shaft (Unitron) new [Re: sgorton99]
      #6150169 - 10/21/13 04:25 PM

you can hold the handle end of the shaft in the vice (using the vice as a heat sink) then heat the shaft, you can then slacken the vice, move the shaft into the jaws to straighten.
all my metalworking vices have soft jaws used to cover the serrated jaws, usually made from sheet metal, folded over the jaws of the vice.
and we call stuff bent too (or kinked)
Grendel


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bremms
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Re: How to straighten a bended shaft (Unitron) new [Re: grendel]
      #6150294 - 10/21/13 05:41 PM

My feeling is, if its bent like that. It's useless try and bend it back very carefully. It break it breaks... You can trim it off and re thread a little more of the shaft. My Zeiss had a bent azimuth adjuster ( happened in packing) the seller sent me back some money to compensate. I knew it would not bend back without breaking.I was able to shorten it a little and it's hard to tell. It is about 8-9mm shorter.

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dgreyson
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Re: How to straighten a bended shaft (Unitron) new [Re: bremms]
      #6150311 - 10/21/13 05:51 PM

I wonder if that shaft is also one of those odd ball non standard Japanese threads?

A magnet will show if its Steel or chrome plated brass or other non ferrous metal.


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actionhac
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Re: How to straighten a bended shaft (Unitron) new [Re: dgreyson]
      #6150493 - 10/21/13 07:56 PM

Another idea is to not disturb that bend and add another bend in the opposite direction to bring the shaft back to straight?



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roscoe
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Re: How to straighten a bended shaft (Unitron) new [Re: actionhac]
      #6150687 - 10/21/13 09:57 PM

Well, here's my general theory of repairs: What you have here is something that's 'broken'. it's bent and therefore not useable. If your repair is a failure, what you will have in the end is a part that is broken. Same as now. If your repair is successful, you will have a working part, a vast improvement, and it makes sense to research the best way to do so, but if it fails, you are in the same place (broken) physically, but a step ahead mentally - you've learned something from the experiment.
And in the worst case - it snaps right off - you'll be in the same place I am ---- with a vintage focuser with the shaft bent a bit right beside the drive gear one one side, and broken clear off on the other.
I'm going to bring mine to my neighborhood machinist and have him center-drill both broken shaft ends, and install a steel pin to connect them. perfect? no. good enough? yep!


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bremms
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Re: How to straighten a bended shaft (Unitron) new [Re: actionhac]
      #6150690 - 10/21/13 09:58 PM

They did use some odd threads. The metric threads on the Tasco 11TR spider are not any standard metric thread.

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strdst
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Re: How to straighten a bended shaft (Unitron) new [Re: dgreyson]
      #6150699 - 10/21/13 10:02 PM Attachment (8 downloads)

Quote:

I wonder if that shaft is also one of those odd ball non standard Japanese threads?




My first/only Unitron 114 came with a bent shaft as well. I was able to remove it from the knob and replace it with a carriage bolt. I filed a section of the threads away and cut the head off to look just like the original. Looks and works just like it is OEM.

Nothing uniquely Japanese about the thread pitch on mine. I did a thread about it in 2008 but can't find it and I could only find one picture in my library tonight. Oh well you maybe get the idea.


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starman876
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Re: How to straighten a bended shaft (Unitron) new [Re: strdst]
      #6150807 - 10/21/13 11:00 PM

The reason these shafts break right where the threads begin is because the threads are a very sharp cut into the metal. Of they had rounded edges they would not break so easily. This is a classical mistake when machining anything and then cutting into it without rounding out the cut. Everything I have ever seen that is machined like that will always break at that point when stressed. I think I saw advice on heating it before it is bent back straight. You must remember the bend has already been made and metal is now displaced on both sides of the rod. most likely the best way to get it straight would be to have a threaded device you can insert the bent rod into that would be long enough that it would straighten the rod when inserted all the way.

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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: starman876]
      #6151138 - 10/22/13 06:33 AM Attachment (10 downloads)

As I’ve said previously: thank you for all the good advice! I think there’s no ”best approach” for straightening out a bent shaft – it will depend on the material, dimensions, hardness plus the degree and location of... ”bent’ness”.

My Unitron 128 slo-mo shaft is small, only moderately bent (ca. 10 degrees), and the shaft is rather soft and thus reasonably malleable. I decided to start with the approach suggested by Marc Bremmer (bremms) , ie. inserting the shaft into a threaded fixture and slowly, gently try bending it back...

As I’d already taken the EQ mount apart for cleaning & lub’ing, I decided to use the solid and propely threaded slo-mo nut on the declination part of the mount as the fixture : threading in the bent shaft as far as possible, orient and then very gently leaning on the shaft, feeling it bending back, ever so lightly..., then threading the shaft further in and repeating the process.

All along, I felt I had a had a very good ”feeling” for the matrial, and thus an instant feedback of how much preassure to apply, and when to thread further in and repeat the process. As Mark has described, there’s probably not the same precision in working with a vice (versa) -- plus the risk of degrading the threads. The shaft ended up close to straight, and it was now easily threaded all the way into the nut on the mount.

Now I just need to glue the head on the dec locking shaft, where one of the 3 ”wings” has been broken off. The seller told me the mount had recently taken a head on crash straight on the dec controls, so that explains the problems with the shafts. Very solid and nicely engineered stuff those Unitron mounts, but of cource you can’t expect delicate parts as control shafts to take that kind of beating!

Allan


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Rich (RLTYS)Moderator
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6151143 - 10/22/13 06:35 AM

Looks good.

Rich (RLTYS)


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terraclarke
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Re: How to straighten a bended shaft (Unitron) new [Re: Rich (RLTYS)]
      #6151171 - 10/22/13 07:25 AM

Good job Allan. Glad it worked out. You can't even tell really that it was ever bent in the first place.

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dgreyson
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Re: How to straighten a bended shaft (Unitron) new [Re: terraclarke]
      #6151517 - 10/22/13 11:33 AM

Now we have another tool in our arsenal for fixing stuff.

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bremms
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Re: How to straighten a bended shaft (Unitron) new [Re: dgreyson]
      #6152881 - 10/23/13 02:21 AM

Glad that worked out Allan. I have a good bit of machine shop experience.
That method seems to work very well. Sometimes, there is nothing you can do but make a new part.


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Jon Isaacs
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Re: How to straighten a bended shaft (Unitron) new [Re: AllanDystrup]
      #6152957 - 10/23/13 04:49 AM

Quote:

As Mark has described, there’s probably not the same precision in working with a vice (versa) -- plus the risk of degrading the threads.




For what it's worth:

Actually, I think there is more control with a vice. If you are bending it by hand, you are controlling the force applied. If it begins to fracture (or probably even soften), you cannot possible react fast enough to keep that fracture from growing and eventually fail because unstable cracks grow at very high speeds, hundreds of meters a second, even the fastest servo-hydraulic load frames cannot stop a crack from growing.

With the vise, you are prescribing a displacement, you are only bending it as much as the vise allows, if the material starts to weaken, the force is relaxed.

In a similar way, small hammer taps can be effective, you are only adding a small amount of energy, sufficient to slightly deform the material the way you desire but not sufficient to cause large deformations.

Jon


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AllanDystrup
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Re: How to straighten a bended shaft (Unitron) new [Re: Jon Isaacs]
      #6154752 - 10/24/13 02:20 AM

Quote:

Actually, I think there is more control with a vice...




Without having specific competence in metallurgy, I still think - as I wrote previously - that :

Quote:

there’s no ”best approach” for straightening out a bent shaft – it will depend on the material, dimensions, hardness plus the degree and location of... ”bent’ness”



I think you may be right Jon, that at least for a hard (steel), relatively long and thick shaft, a continous pressure (preferably combined with heating of the material to make it more malleable) may be the best way to apply and dissipate the force along the length of the material; And as illustrated by Robert (actionhac) in a previous post, the pressure point on the shaft can be precicely adjusted along the way, by using appropriate wood supporting in the vice.

I'm pretty sure this approach would have worked too for my tiny, only slightly bent soft shaft, but in this case I would still recommend the "fixture method" as the easiest way to apply pressure/corrections in small and well controlled increments, while slowly rotating and moving the pressure point up the shaft, and at the same time continously checking that the shaft threads easily into the nut. At least this did work out well for me

Allan


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