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Installing Focuser

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#1 DavidD

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Posted 11 November 2012 - 09:52 PM

I installed a different focuser on my tube this weekend, and ran into some issues that someone here can likely help with.

The OD of the focuser was REALLY close to the ID of the tube. I could get it started, but go no further than that. So....

I put the focuser in the freezer for an hour. That was enough to get the focuser into the tube, but rocking it back and forth, mm by mm. I tried my best to keep the holes in line, but when I got it in, they were visible, but off. I figured the screws would pull it in, so I started the screws in, one at a time, working slowly. I think I did manage to pull it in just a bit, but in the process, I twisted two of the screws out.

Now, what should I do? How should I have trued up the screw holes? Will I ever be able to remove this focuser if I want to put the original back in?

Also, what should I have done in the first place? Should I have used some sort of lubricant? Should I have heated the tube (I did think of this later, as the tube was getting colder as I worked the focuser in, which probably didn't help my efforts)?

Any other advise?

David

#2 Pinbout

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Posted 11 November 2012 - 10:45 PM

Its a refractor?

Isn't there an adapter that goes between the focuser and the tube?

#3 RossSackett

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Posted 12 November 2012 - 07:45 AM

Well, it looks like you now have a pretty good press fit that is unlikely to release itself, so perhaps there isn't anything else you need to do.

I presume that twisting out the screws stripped the threads? You could re-drill and thread the holes, which would take care of any misalignment and save the appearance of the job.

Or you could try two strap wrenches to see if you can twist it off, perhaps with a little penetrating oil (take the objective off first, if it is a refractor).

Ross

#4 glennnnnnn

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Posted 12 November 2012 - 12:01 PM

That was a good effort, and a smart trick with the freezer, but its in there now and at least you don't have to worry that its not straight.
(I'm assuming that you used a caliper and measured ID & OD and just thought you might get it to work.)
There's probably not much you could do to enlarge the receiving-end of the tube, but sometimes there's a little sharp ridge on the edge that can hang things up a bit if they're close and tight. Lightly sanding the inside can take off a tiny thickness of paint, which might help.
The focuser would be easier to deal with by lightly filing the insertion area, or even using some emery-cloth until you have taken off enough for a fit that's still snug, but not super-tight. If you're worried about the finish on your focuser you can wrap it with masking tape to keep it blemish-free and clean.
As for those holes, if they're close you could just drill and tap them for the next-larger sized thread, and nobody will ever know.
-Glenn

#5 DavidD

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Posted 13 November 2012 - 12:24 AM

Yes, it's a refractor.....no adapter in between.

I did worse than strip the screws....they are still in the holes. I twisted the heads off. I'll need to get a screw extractor (been meaning to pick one up anyway) to remove them.

The screw I can remove has a stripped area, but doesn't look like it was cross-threaded. I'm tracking down replacement screws but didn't make it to ACE before they closed. Also, I think the original are aluminum screws. Will SS work, or will they cause more issues?

I thought about strap wrenches, but don't want to scratch the tube up. Can I safely use them? The only one I saw at Lowes fit up to 3.5", and my tube is 4".

I did measure the ID/OD and they would indicate it would fit. The inside of the tube is blackened all the way to the edge, which I think caused a lot of the friction. I didn't think to sand down the focuser a bit before trying to fit it.

I'll get the screws out and access the state of the threads. If they are stripped, I'll go up a size and make it at least secure where it is. I think I'll likely not get it back off, so I hope I'm happy with it.

David

#6 glennnnnnn

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Posted 13 November 2012 - 06:34 AM

Think about it like this: The inside and outside are essentially parallel, so you're making each of them into a (very) slight taper.
Stainless will take a whole lot more torque and beats any other material. Stainless is the best!
If the broken screws are Aluminum you can drill them out with a small (1/16") bit.

#7 DavidD

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Posted 13 November 2012 - 11:13 PM

Thanks glen(nnnnnn) -

I was a bit worried at putting an SS screw into Aluminum. I've had that arrangement seize up on me before.

On idea has crossed my head. I've bought a longer screw that will fit into the one open hole. Can I put that in, and gently tap it with a hammer to nudge it closer to where it needs to be??

David

#8 glennnnnnn

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Posted 14 November 2012 - 08:00 AM

I wouldn't do that!
Either get a shorter screw or get the threads working for the one you have and cut it shorter.
If you mean the head of the screw isn't flush, you might have some excess material around the hole that you can remove with a larger drill-bit, in this case known as a "de-burring tool" held in your hand and twisted to make a more flush area for the screw-head.
EDIT: I think I misunderstood your question about moving the screw-head. You must mean to try to re-align the holes by twisting the focuser slightly in the desired direction.
I Still Wouldn't Do That! Better to just enlarge the holes and rethread them for the larger screw. If you try to move the focuser by tapping on the screw, you'll probably just mess up that screw. I really doubt if you're going to move that focuser. Its stuck. If you look, you can find the correct-sized drill bit packaged with the tap to fix the new holes. First get the new screws, then the drill bit and tap, and hopefully you haven't already made the new holes too large, or you'll have to get larger screws, etc.
Honestly, I'm having to scramble a bit here because I built airplanes and the things that I automatically take for granted are probably not the way you do an operation like this. After you've drilled a million holes and stuffed them with every imaginable sort of rivet or screw, you do things without hesitation, and I know there are those here on CN who are freaking out because they might have to drill a hole. I've been there too.
-Glenn

#9 rboe

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Posted 14 November 2012 - 08:45 AM

Ideally it would nice to be able to pull the focuser off, if only to get at the darn screws a bit better. On the other hand, if the screws are soft that will make removing them a whole lot easier.

Wrap the tube in maskig tape to minimize scratches on it. I have a spring loaded center punch, good for creating the dimple the drill bit is supposed to follow once you try to drill out the screws.

Once you have a hole through the screw it should be easy to remove but it sounds like the tube body is providing some interferance. I think you'll have to elongate the holes in the tube a bit to compensate.

#10 Gene7

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Posted 15 November 2012 - 02:14 PM

David,
You need a lot of help. You need to start thinking before you act. My first suggestion is

to get the objective lens off the tube before you mess that up by pounding, sandpaper grit,

or metal chips. Of course never remove the glass from the mount, only the mount and glass

intact from the tube.

I say a refractor needs to have the objectve optics aligned and the focuser collimated.

It is not good enough to mechanically align the focuser and objective mount and expect the

glass to be aligned, or at least checked, since the glass is of greater precision than the

metal.

To do that alignment you need a 1 1/4 inch mount laser and Cheshire alignment unit. First

you tilt adjust the focuser until the beam is in the center of the objective, then you use

the Cheshire in the eyepiece and tilt the objective lens so the images line up in the

Cheshire. Does your scope have means of tilting these two objects? If not typically new

mounting holes are drilled with some allowable play in the fit. Thus, there cannot be a

tight fit of the focuser to the tube.

After the objective is removed from the tube I suggest you take it to a machinest and let

him remove the focuser and remove a bit of metal where it fits the tube. He cannot drill

and tap new mounting holes, that is your job after collimation. Remove all metal chips so

they do not cause you trouble later. Better luck. Gene

#11 roscoe

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Posted 16 November 2012 - 07:47 AM

Perhaps to get the focuser back out, you could take the objective/cell off, and reach down the tube with a long enough stick or dowel, and working evenly all around it, tap the focuser body back out.

If the broken screws are sticking out into the holes, if you have a dremel, you could perhaps grind the protruding part off.

#12 DavidD

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Posted 16 November 2012 - 11:08 AM

First to address what I meant be putting in the longer screw.....I meant to tap it on the side, to nudge the placement.

Of the two screws that twisted off, only one protrudes slightly, and yes, I can Dremel that down if I can easily extract it.

Ron - yes, I find a spring loaded punch to be invaluable!

I think with all this advice, the best plan is to work the focuser back off, and make it a looser fit.

I'll try to keep this thread updated on progress.

David

#13 DavidD

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Posted 17 November 2012 - 04:38 PM

The good news is, I have the focuser off. I was able to get enough of the protruding screw bodies off to get them under the tube. I took the objective off and wrapped it in a clean cloth. I took as much of the focuser apart that I could. I was able to put a long board through the front and tap several places on the inside of the focuser. I was also able to tap on the finder shoe, using cloth to protect everything. Once I got it started, I was able to tap it the rest of the way off with a screwdriver inside the focuser rim.

Now.....the two screws did not extract cleanly. I think I'll leave the one hole that a screw did not break off in as a 4mm. I cleaned it up with a tap, and it looks ok. The other two holes need to be cleaned up more, and I'll likely need to enlarge them to 5mm. I have already done that on one of the holes, but the other one is being much more stubborn, and I've broken off an extractor and a tap in it already. I'll have to go buy another 5mm tap, along with 5mm screws to match the two enlarged holes, before I can finish up.

So, now what to do?? I think what I will do is put the original focuser back on for now. I'll figure out a way to sand down the new one, but in the mean time I'll still be able to use the scope. I've already spent half a day on this, and have run out of time for now.

Any other advice?

David

#14 roscoe

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Posted 17 November 2012 - 04:53 PM

David,
Lubricate the tap as you work it in. I'm sure there's the perfect lube for aluminum, but any light oil will work. also, wind it in perhaps 1/4 turn, back it out till it breaks the chip off (maybe 1/8 turn) then advance a bit more. An extractor won't take a broken tap out. if there is some way you can grab it with pliers or tap it with a punch or small screwdriver in one of the grooves and start it unscrewing, it'll come out.
Good luck!
Russ

#15 Pinbout

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Posted 17 November 2012 - 05:27 PM

I would make sure the broke screws were ground down even with the casting and drill and tap 3 new holes. it would be a lot easier. :grin:

#16 Sean Cunneen

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Posted 17 November 2012 - 06:20 PM

I like rubbing a candle around tight-fitting parts. Especially helps with Aluminum. I've been in your spot before, I have an 8" classical Cass with a backplate machined by demons. I have to go through the same thing everytime I reassemble(which isn't often) I have found that sometimes I can use an awl to maneuver screw holes in line, but I doubt that would work in your case. A little hand sanding with fine sandpaper will get things in order. A tap should clean up your holes just fine. I am a fan of SS screws as well. A little 3-1 oil does the trick for me. I am SO GLAD you didn't whack anything with a hammer, I once put a huge clamshell in an RFT trying to get the focuser off with a rubber mallot. Flints are VERY delicate.

#17 glennnnnnn

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Posted 18 November 2012 - 08:56 AM

Solve the big problem first: get the focuser to slip easily in (and out of) the tube. Then see how the holes line up.
I would just drill the old screws out.
The use of an awl is good advice. As is lubrication.

#18 DavidD

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Posted 18 November 2012 - 01:24 PM

Ron, the tap broke of with about 1/8th sticking out. I was able to back it out with a pair of needle-nose pliers. The tip of the screw extractor broke off, too, but I was able to tap it through from the back side, as it was not firmly engaged.

#19 Z28500

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Posted 19 November 2012 - 01:27 AM

Hey, that candle part sounds like a good idea!
After drilling and tapping several hundred holes (including mistakes) on that scope/tripod I've almost finished, I started using baby oil on the taps, but yeah, anything slippery will work.
Z






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