Microscope headpiece dovetail dimensions

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Rylee Isitt
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Joined: Fri Apr 13, 2012 3:54 pm
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Post by Rylee Isitt »

Yup, it works. It is ever so slightly off center but this is pretty hard to spot. It's just a shame that the machine shop botched the inner threads, he usually does great work though.

I've found a very good use for this adapter recently... Converting a finite scope to an infinity corrected scope by mounting a Raynox DCR-150/250 to this adapter and using it with a direct connection to a camera. I was about to sell my low-end compound scope until I figured this out.

genera
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Post by genera »

Rylee Isitt wrote: . . . . It's just a shame that the machine shop botched the inner threads, he usually does great work though.
You didn't say in what way the adapters were botched; the pitch was off or maybe the diameter? I'm just pointing out another possibility.

As drawn your threads come to a sharp point at both the peaks and valleys. This is probably OK in the valleys but the sharp points at the peaks could be a problem depending on how the mating lens's threads were cut. If the mating lens also has sharp peaks and valleys it should be usable but it's not common for threads to be made this way. More likely the lens has rounded peaks and valleys. The problem in mating the two different forms is that the pointy peaks on the adapter, as drawn, may not have sufficient clearance in the possibly rounded valleys of the lens. If this statement isn't clear see the wiki link below and imagine that if you extend the dark grey thread to a sharp point it extends into an area occupied by the light grey thread.

On a relatively soft aluminum adapter the points can often be forced into the space available. They get rolled over and form barbs that will, with enough use, eventually break off and end up . . . who knows where. I've run into this several times on items I've purchased from one particular eBay seller. On a steel adapter the threads are less likely to conform to the space available resulting in a situation where you simply can't mount the lens.

See page 30 here for some examples of more fully specified threads:
http://www.krebsmicro.com/MitutoyoE4191-378.pdf
or this Wiki article:
http://en.wikipedia.org/wiki/ISO_metric_screw_thread

BTW, a good machinist should have recognized the shortcoming in the drawings and asked for clarification. There's a company, SK Grimes I think, that specializes in making custom adapters, lens boards, and shutter mounts and adaptations. I've inquired about having them make some custom adapters for me. They said they could do what I wanted and provided a price range and delivery date. They also requested that I send the lenses to them because, despite their extensive experience, they couldn't guarantee a good fit unless they could determine the exact thread forms for each lens. It seems there's some art to it beyond dialing in the diameter and pitch.
-Gene

Rylee Isitt
Posts: 476
Joined: Fri Apr 13, 2012 3:54 pm
Location: Canada
Contact:

Post by Rylee Isitt »

The drawn threads are purely to indicate the presence of threads. The full specification of threads is available for the indicated thread types, M30x0.75 and M42x1.0. Those may seem to only provide diameter and pitch but the actual metric standard also provides other information as well. However, the guy who made these for me did comment on how there are different grades of threads, with different tolerances, and some grades won't fit well with others. Apparently the lower grades are looser fits. I've received cheap M42 stuff from eBay that did not mate well even with second copies of the same part. The cheapest M42 extension tubes seem to occasionally (but not often) have this problem. And unfortunately that's exactly the type of equipment that my adapter needs to mesh to, haha. So far, though, I've not experienced any problems.

In the drawing, note the complete lack of specific angles, depths etc for the threads. That's intentional, because they aren't drawn in to be used for measurements. If you look at other drawings, it appears to be common practice to rough in threads and simply indicate the specification on the drawing.

My machinist set up his lathe improperly, somehow, and the threads got messed up near the dovetail end of the adapter, so he bored them out. The result is that the threads don't go all the way through. Threads were mostly just for anti-reflection, although M30 was chosen as a possible mount for filters and such. Fortunately, enough thread remained on the adapters at the camera-end to thread things through anyway, and I dealt with the section of missing thread by lining it with telescope flocking. In retrospect I should have asked them to be redone, but I accepted them as-is.

I think two adapters were perfectly made... but I forgot who got them!

Edit: misremembered exactly what was wrong with the threads. I haven't looked at mine in a long time since I've lined the entire inner surface with flocking material.

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