Member postings for gerry madden

Well done Clive Sir ! I just picked it up and turned it the 'wrong' way with my bare hands and look what happened !

David, its a toolmakers measuring microscope manufactured by OMT. It is VERY robustly made so I can understand the confusion.

G.

Thanks very much Clive. I will look at that.

Re the grease, I purchased a small pot from China recently. There were no numbers in the advertisements indicating its actual viscosity, only references saying "good for camera lenses". I've tried it on a number of things all with good results so far - except on a 10" rotary table. You can imagine what that was like ! Walking in 2ft of syrup would be a good description so off it came and on went a standard grease and all's well again.

Gerry

I never used to be defeated by things until this forum came along but asking a question does save one a great deal of time allowing one to get on with other 'important" things in the workshop

My latest challenge is dismantling my eyepices. In many places OMT have done a great job of hiding the typical 120 deg. grubscrew connections, with a fine push-fit brass sleeves, which were then painting over to make it look solid. I think I have found all of these and I'm down to the coarse focussing threads. These badly need fresh kilopoise grease so I have to get them apart and this is where I'm stumped. I'm hoping that someone will know the secret behind this particular assembly or can suggest something to try that I haven't considered.

The eyepiece assemblies look like this:

The outside 'focussing sleeve' with the straight knurling will currently rattle axially by a few mm's if the focus is opened to its limit. If the focus is closed down the 'focussing sleeve' becomes tight. From playing with the assembly I 'think' the internal construction must be something like this:-

If this theory is correct, to get it all apart, part no.2 the focusing sleeve, has to be separable. But I just cannot see how to do this. My expectation was that the pieces of the sleeve to either the left or the right of the knurling would unscrew from the knurled section. However I see no way to grip them other than by friction. I tried that, and it wasn't successful. In any case I carefully brushed all the paint away from the likely joints and viewed them under a microscope. I could not see any obvious junction.

What I have detected though is that the 'zero mark' on the tapered portion (on the right of the knurling, visible in the first picture) is more than just an engraved mark. It appears to be a joint and if one forces a screwdriver under the sleeve and levers it you can see the joint open very very slightly. But that's all I can make it do, it will not allow itself to be prised out of where it currently sits.

Clearly I'm missing something so any thoughts or experience here would be appreciated.

Gerry

Just for the record, in the event that anyone else decides to refurbish one of these machines, I have "decided" that this should be the correct assembly:-

This arrangement will provide a self-aligning contact on the gauge-block surface and thereby prevent any damage or indenting of the block. There may be initially some brinelling of the micrometer probe tip from the ball but once that's happened it wont grow further and subsequent measurements wont be affected.

Gerry

Thanks chaps for all your comments. And I have some very good news !

As you suggested Michael I did have another microscope with which to look at the 'printing' more closely. What I quickly discovered was that the marking wasn't on either side. Its on the inside !!! The disc is comprised of two glass discs bonded together. One can clearly see this at the periphery and the bore where there are a few trapped bubbles close to the chamfered edges of each disc. .....I wondered why I wasn't able to carefully feel the 'printing' with a pin !

So cleaning worries are no more ,but i will still be careful to avoid scratching.

As for the actual coating, I'm not so sure its an aluminised type Michael. I have a 1970s marching compass and that has what I guess is an aluminised line on one of its windows. When I put that under the microscope its very obviously a metallic material when you get the lighting in the right way. The lines on the OMT disc don't have any of this kind of appearance. They just look like 'black paint' from every possible angle. I'll try to get some pics later and dig into the possible processes that may have been used and let you know if I reach any conclusions.

Gerry

On the end of the probe on my micrometer head there is a small self-retaining sleeve. Inside the sleeve is a ball and a top-hat-shaped disc. My dismantling notes have failed me again and I'm not sure if the top-hat goes between the ball and the end of the probe, or its the ball that goes between the probe and the top-hat !

There is only one small witness mark on the parts and that is a tiny brinnell mark on the large diameter end-face of the top hat.

I can see a good reason for both options. The top hat could be put outboard of the ball to prevent the gauge block from getting brinnelled. On the other hand perhaps the micrometers probe is soft and the top hat is there to protect it from brinelling.

If anyone knows the truth of the matter I would appreciate hearing it.

Gerry

I've finally got around to cleaning up the optics on my OMT microscope. The eyepiece heads, or 'oculars' contain a 3" glass disc which you can see in the picture below. What you cant see in the picture are the myriad of microscopic graticules, thread profile lines and cross-hairs that lie on the surface of the glass.

Also sitting on the glass unfortunately is 70 years of doom and gloom which I would like to try to remove. Having previously successfully dissolved numbers on an old watch face with alcohol I'd like avoid making a similar mistake this time.

So, are there any experts in the house that can enlighten me and comment on:-

a) how were the graticule lines actually created on glass in the 1940s~50s era ?

b) what would be a safe cleaning solution and method to remove the contaminant ?

Gerry

Milton Keynes Metals is very close to me but sadly I have vowed never to go there again. Every visit was worse than the previous one, the culmination being when I went to collect some slices of stainless and there was two pieces missing. Apparently one had been cut but had rolled off the saw and had "completely disappeared". The other missing piece wasn't made because the saw's band broke and he didn't have a spare. The spare took several weeks to arrive. I did eventually get one more piece but just gave up on the last one.

My last 'experience' was about 9 months ago at at the time I could not ever believe it was a viable business. If you are getting no responses, I would fear the worst.

Gerry.

One of the points I was hoping to get across in my earlier post was that when you do the maths you almost don't need to worry about the area. A shim is just a very short column. Very short columns have even smaller compression despite high stresses that might be created by reducing the area.

I know its counter intuitive, but that's the reality. All I would say is make sure the position of the shims gives a stable joint, and not one that has a plane/direction of weakness.

Gerry

Dr_GMJN, I dont think you need worry too much about the elastic qualities of the epoxy.

If you imagine a 250mm bolt torqued to its maximum it will extend by approximately 1mm. That means every 1mm slice the bolt has extended by 0.004mm, or every 0.1mm slice has extended 0.000,4mm. If you now reduce the stress level to a quarter, the deflection will also reduce by a factor of 4 taking it to taking the change in thickness of that 0.1mm slice to 0.000,1mm.

Of course your epoxy shim will have a lower modulus than the bolt but is also likely to be thinner that the 0.1mm slice that I referred to, and your stresses likely to be considerably lower too. In addition, the fact that it will be in compression as a thin film will automatically make it even stiffer than the same material would appear to be under a pure unadulterated tension test. So keep it thin and all will be fine.

Gerry

Well, I tried the 4BA up against the grub screw and it was close... but just didn't feel quite comfortable. I then dug through my tap collection and stumbled across a 1/8BSW which is 40 TPI. Offered it up and it fitted like a glove, at least on pitch. Its just too small, or too late at night to compare the thread forms but hey for £3 I cant go wrong with the 5/16 ME and I'll order one tomorrow.

Thanks all for your suggestions and thoughts. Appreciated.

Gerry

Michael, you've just thrown a spanner in the works now ! Im wondering now as every other thread on this machine is BA.... I'll give this some thought.

Thanks all for your efforts. 40 tpi was all I could conclude too. Working backwards I only need to be a third of a pitch out with the calipers and that would be the difference. I think you are right Tony - even when I magnify the calipers what looked like sharp points in the ends of the anvils turn into very rounded off clubs and it doesnt really improve accuracy.

I dont have any ME taps but I have found a cheap one so I will buy that and see how it looks.

1940's British..

Apologies, its an OMT microscope.

Hoping someone can help me identify a thread. I have several on my OMT that are nominally 5/16 diameter but have a very fine thread which I determine using calipers to be approx. 38.6TPI. I think it must be some kind of instrument thread but cant find anything as fine as this on the web but I probably just haven't looked in the right places !. Just for comparison, in the picture below I've included a 7/32-28 BSF tap.

Edited By gerry madden on 16/11/2020 19:46:29

I visit for the engineering content and its never occurred to me once in several years of use that the slickness of the format of this site might not be up to the latest standards.

But as a user of many other platforms and systems, I know I hate the way operators like to keep changing things in the interests of "efficiency". Making changes is obviously fun and interesting for them, and keeps them employed, but its a pain in the A for the occasional users that suddenly discovers something no longer works the way it did before and needs relearning. When such things happen repeatedly that becomes a big inefficiency for me.

I don't want a digital watch with 15 million functions on it at my age, no matter how much better little ones tell me it is. Lets not turn this site into one of these unless it really is broken. I'm not sure it is at present.

Thanks for your comments chaps. I'll buy the £3.65 stuff because you cant go wrong at this price. But I do like the Newgate info and range of products and I'm sure I will be delving into these also later once I have a baseline to play around.

Stevegtr, vaseline is better than nothing but as a material it has stiffness but not viscosity. So it doesn't separate the moving parts when relative motion is introduced. When you compare the feel of vaseline with these other products they are like chalk and cheese, especially if the mechanism is relatively heavily loaded as in an adjustment rack.

Gerry

I'm wondering if our instrument specialists can advise me on the popular greases for creating that nice viscous damped movements in small hand controlled devices such as binoculars etc.

I can remember many years ago someone referring to a 'megapoise' grease on a radio volume control. I don't see that term on the web at the moment but I do see kilopoise. Thats a 1000 times less viscosity but it might be the right thing of course