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Crayford Focuser Cracked

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#1 ChrisMax

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Posted 05 April 2014 - 02:48 PM

I just discovered a crack in the Crayford Dual Speed Focuser on my Orion Astrograph 8 f/4. As you can see it's in the plastic cover. I'm wondering if it should be replaced. Any opinions?

Thanks

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#2 Kim K

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Posted 05 April 2014 - 04:23 PM

Assuming the plastic cannot be glued and that Orion will not sell a replacement then I would make a plate out of 1/16" aluminum that covers the bolt holes. Get longer bolts from your hardware store. I would not glue the plastic since the crack is separating which indicates stress.

Hope this helps...

#3 obin robinson

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Posted 05 April 2014 - 05:59 PM

I would make a plate out of 1/16" aluminum that covers the bolt holes.


Ditto. I would get some aluminum and use the plastic part as a template for the shape and the hole placement. You don't need super thick aluminum. Even 1/8" thick would be much stronger than the plastic.

obin :)

#4 ChrisMax

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Posted 05 April 2014 - 07:19 PM

Thanks for the helpful suggestions! I'm wondering though if the plastic is just a cover.

#5 Geo.

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Posted 06 April 2014 - 10:06 AM

No, the part is bearing the thrust of the set screw and not very well. It was either a bad molding or bad design. I laugh every time I read of plastic as an engineering material.

#6 ChrisMax

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Posted 06 April 2014 - 11:41 AM

I've taken the focuser apart and this is the part I need. Anyone know where to buy one?

Thanks George yeah now that I see the part it is indeed a really bad design. Maybe I should just upgrade to a moonlite focuser and be done.

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#7 gdd

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Posted 06 April 2014 - 12:15 PM

Thanks for the helpful suggestions! I'm wondering though if the plastic is just a cover.



My brand new AT8IN does not have a plastic cover over that part, I think it is probably not needed. It does not look like it would even keep dust out.

Gale

#8 hottr6

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Posted 06 April 2014 - 12:19 PM

I *think* that focuser is made by Kunming Optical. Try calling Lunt as they are distributing a lot of Kunming products and parts.

#9 ChrisMax

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Posted 06 April 2014 - 01:41 PM

Hi Gale

Unfortunately George is right about the plastic cover. It does bear the thrust of the set screw which is why it cracked. I will post a photo of the repair when it is done.

Thanks Shane I'll check it out.

#10 SteveG

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Posted 06 April 2014 - 02:59 PM

Have you called Orion? They've sent me replacement parts on many occasions.

#11 tedbnh

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Posted 06 April 2014 - 06:21 PM

With nothing to lose, I'd try clamping it tight, milling out a hole the size of a double-hole washer (the kind for hanging pictures) that looks like a figure-8. Then drop in the washer with some JB-weld and let it dry. The washer will take up the stress.

#12 ChrisMax

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Posted 06 April 2014 - 06:59 PM

Hi Ted

It's pretty thin plastic to mill out. Orion must be saving a lot of money using such thin plastic in such an important part of the focuser.

#13 gdd

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Posted 06 April 2014 - 10:07 PM

Are you sure that plastic was supporting anything? Maybe the holes were not aligned correctly causing the pinion to cut into the plastic. What held the plastic in place? On my AT8IN focuser that that part looks like yours with the plastic removed. The pinion with the knob just sticks out with nothing supporting it.

Is that second picture of a plastic part, or a metal part with the plastic cover removed?

Gale

#14 ChrisMax

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Posted 07 April 2014 - 04:52 AM

Hi Gale

The part is a piece of molded plastic and the tension screw nut is embedded in it. When the screw is tightened the plastic bears the tension. Over tighten the screw and the plastic cracks which is not really all that hard to do as the plastic is thin. It should have been made of metal. If you look at the focuser on the latest Orion Astrograph it has been redesigned and has eight screws in this part not four as shown in my photo. So they must have had many of these focusers fail in the same way.

#15 gene 4181

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Posted 07 April 2014 - 11:18 AM

I've taken the focuser apart and this is the part I need. Anyone know where to buy one?

Thanks George yeah now that I see the part it is indeed a really bad design. Maybe I should just upgrade to a moonlite focuser and be done.

+ 1 on that thought

#16 Jon Isaacs

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Posted 07 April 2014 - 01:14 PM

I've taken the focuser apart and this is the part I need. Anyone know where to buy one?

Thanks George yeah now that I see the part it is indeed a really bad design. Maybe I should just upgrade to a moonlite focuser and be done.


Is it really plastic? I would think that particular part would be aluminum. I can only see a tiny glimpse of the bore but it appears to be aluminum.

Jon

#17 hottr6

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Posted 07 April 2014 - 02:25 PM

Is it really plastic? I would think that particular part would be aluminum. I can only see a tiny glimpse of the bore but it appears to be aluminum.

If it were aluminium, there would be no need for the threaded insert.

#18 ChrisMax

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Posted 07 April 2014 - 02:40 PM

Hi Jon

Yes it is really plastic and thin. I'm almost finished with the repair I've done and will post a picture. Hope it works!

#19 ChrisMax

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Posted 07 April 2014 - 03:04 PM

I've taken the suggestions above but used 26 gauge (0.016) sheet metal. It seems to be working fine in fact feels stronger. Well at least it may save me $200+ on a new focuser!

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#20 T1R2

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Posted 07 April 2014 - 03:14 PM

my aluminum lens cell has threaded inserts

#21 ChrisMax

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Posted 07 April 2014 - 04:23 PM

For those of you interested I'll post a few photos of the piece and repair. The focuser is back on the scope and fully functional. Now I can get the auto focuser for it instead of spending $200+ on a new focuser. I epoxied the two metal washers on the inside for additional strength.

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#22 ChrisMax

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Posted 07 April 2014 - 04:27 PM

Closeup of where the rubber meets the road. It's really a simple mechanism.

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#23 ChrisMax

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Posted 07 April 2014 - 04:29 PM

The final shot on the scope. Thanks for all the very helpful suggestions!

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#24 Jon Isaacs

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Posted 07 April 2014 - 09:08 PM

Hi Jon

Yes it is really plastic and thin. I'm almost finished with the repair I've done and will post a picture. Hope it works!


Chris:

I sure hope your repair holds for you. If it doesn't, a thicker aluminum plate might do the trick..

As a mechanical engineer, I find it difficult to imagine that part is plastic.. I would expect at least cast aluminum, the loads and stresses can be quite high with that screw in the center. You ought to send that photo to Orion.

Jon

#25 T1R2

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Posted 07 April 2014 - 09:33 PM

as Jon has pointed out, I don't think its plastic either, but we could be wrong, one thing I've noticed is anodized aluminum can have plastic sound when tapped with a finger nail, and even feel plastic, but if scratched with a finger nail on an exposed edge, it will sound metallic.






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