Fine Focus Problem

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bcjolly
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Fine Focus Problem

#1 Post by bcjolly » Mon Jul 15, 2019 3:14 am

I recently purchased a American Optical Series 10 microscope for my daughter. The fine focus knob on the right side works as intended. The fine focus knob on the left side appears to slip. There must be a clutch or some type of mechanism connecting the two knobs. I think I can fix it but would like to have some idea of what I am up against. All help appreciated.
Barry C Jolly

apochronaut
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Re: Fine Focus Problem

#2 Post by apochronaut » Mon Jul 15, 2019 12:39 pm

The center flat panhead screw that fastens the knob to the shaft keeps tension against a bowed spring washer, which acts as a kind of clutch. When the lubrication in the system is adequate, the knob works with very little tightness on that screw. It can become loose if the knobs are reversed against each other because the screw by design has a high surface area of contact with the aluminum knob.
The fine focus should require a nice delicate touch. If there is any tightness or backlash at all, then the knob could be slipping due to undue resistance in the system. In that case, it is most likely just a lubricant issue. The mechanism in those is very sturdy.

bcjolly
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Joined: Sat Jul 06, 2019 2:50 am

Re: Fine Focus Problem

#3 Post by bcjolly » Wed Jul 17, 2019 12:33 am

Dear Sir,
Thank you for your advice. I had a little time to work on the microscope today. I was not able to get the fine focus working as it should. However, I noticed that the fine focus knob worked well when I lifted the nose piece. I am now thinking that maybe the problem is within the part that makes the nose piece go up and down. Is there a rack and pinion in the focus train? If there is, could old lubricant impede the focus mechanism enough to cause this problem? Thanks for your help!
Barry C Jolly

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Re: Fine Focus Problem

#4 Post by apochronaut » Wed Jul 17, 2019 11:59 am

Perhaps first, we should clarify exactly which knob you are referring to, since the microscope can be used with the head reversed 180 degrees. In such a case left becomes right.
So, when you said the right side, you meant the knob without any graduations on it, correct?

bcjolly
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Joined: Sat Jul 06, 2019 2:50 am

Re: Fine Focus Problem

#5 Post by bcjolly » Wed Jul 17, 2019 3:05 pm

Yes! The other knob, with graduations, works fine.

apochronaut
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Re: Fine Focus Problem

#6 Post by apochronaut » Wed Jul 17, 2019 11:46 pm

If you put a small screwdriver in each of the screws that hold on the two knobs, and turn each clockwise as they face the body of the microscope, you will be tightening each of them down on their respective clutches. Since the graduated knob appears to be alreadytight enough to activate the mechanism, the other knob will tighten until the two obtain equal tension, at which point you will feel that you are trying to untighten the other one. You don't need to tighten much, just enough to snug the clutches; done with fingertips only.

Both knobs should work the mechanism equally well then. If the ungraduated knob still slips, then I suspect that 1) either the bowed spring washer is missing or 2) possibly an unknowledgeable repair person has put some lubricant in there, allowing the clutch to slip a little.

Since the focus works fine with the other knob, then the fault must be in the clutching mechanism of the ungraduated knob. Both knobs are clutched to the same shaft.

bcjolly
Posts: 23
Joined: Sat Jul 06, 2019 2:50 am

Re: Fine Focus Problem

#7 Post by bcjolly » Thu Jul 18, 2019 12:01 am

Sir,
You are correct in your assessment of the problem. I had time to take the fine focus knobs off both sides of the microscope stand. There was grease on the washer, the spring washer, the acrylic washer, and the knob and screw. I cleaned the grease off and re-greased the parts with a synthetic grease of the consistency of Vaseline. The results were the same as before. I did notice that the right fine focus knob now works much better in one direction than the other. Did I make a mistake in greasing the inner mechanisms? I can go back and remove it very easily now that I know how to get the screws off. Thanks for your help.
Barry

apochronaut
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Re: Fine Focus Problem

#8 Post by apochronaut » Thu Jul 18, 2019 2:49 pm

Grease in that part of the system is o.k. because a small amount of slippage is required when the focusing limit is reached. There would have been a specified viscosity, which I don't know but it was a fairly heavy damping grease type. Heavier is better than lighter because the only time the grease is functional as a lubricant is when the system arrives at it's limit. At that point the knob being turned is supposed to slip.

If a knob is slipping prematurely, then most likely one screw is too loose, since you are saying that the side with the graduations is working o.k. Your problem would appear to be a case of uneven adjustment between the two slip clutches. You need to gently turn both screws clockwise. The side that is slipping should tighten, while the other side holds the shaft. At a certain point, they will reach equilibrium and they will both be very slightly tightening. Time to stop.

when turning the fine focus knobs, you should be able to rest your hand on the table and spin either of the knobs quickly by just winding it with a finger.

bcjolly
Posts: 23
Joined: Sat Jul 06, 2019 2:50 am

Re: Fine Focus Problem

#9 Post by bcjolly » Fri Jul 19, 2019 12:07 am

Got her fixed. Those screws are touchy. Thanks for all your help. Delivered the microscope to my daughter this afternoon. She was thrilled!
Barry

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Re: Fine Focus Problem

#10 Post by apochronaut » Fri Jul 19, 2019 12:57 am

Very pleased, Barry. I hope your daughter enjoys it immensely.

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