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Microscope headpiece dovetail dimensions
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Rylee Isitt



Joined: 13 Apr 2012
Posts: 474
Location: Canada

PostPosted: Thu Nov 01, 2012 9:53 pm    Post subject: Microscope headpiece dovetail dimensions Reply with quote

Hi everyone,

I am designing a part to have machined, which is a 42mm dovetail to male M42 adapter. Specifically, this is to fit an Olympus BHMJ nosepiece, however it appears that this nosepiece has a standard 42mm "universal dovetail" mount, since my generic compound microscope's trinocular head fits properly.

I wasn't successful at finding diagrams online, so I've attempted my best to reverse engineer the dimensions.

Here's what I got:

Largest diameter: 42.40mm
Depth of dovetail: 6.40mm
Angle of dovetail: 60 degrees

If anyone can somehow verify these measurements, or point me towards a technical drawing with these measures, that would be awesome.

Thanks!
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Pau
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PostPosted: Fri Nov 02, 2012 8:27 am    Post subject: Re: Microscope headpiece dovetail dimensions Reply with quote

Rylee Isitt wrote:
...it appears that this nosepiece has a standard 42mm "universal dovetail" mount, since my generic compound microscope's trinocular head fits properly.

Many generic microscopes are copies more or less altered of the Olympus CH, so is not surprising some cross compatibility, I'm not aware of any standardisation of dovetail mounts between makers (unfortunately).
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Rylee Isitt



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PostPosted: Fri Nov 02, 2012 10:59 am    Post subject: Reply with quote

I have an original Olympus BH Trinocular head on the way, the very same model that came with the BHMJ scopes (and other scopes in the series). If its measurements agree within tolerance, that'll be a good sign.
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Charles Krebs



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PostPosted: Fri Nov 02, 2012 2:07 pm    Post subject: Reply with quote

Coincidentally I recently measured three Olympus BH dovetails. I did the large diameter and depth, but at the time did not have a way of determining the angle to the degree of accuracy I wanted. (Intend to re-visit that soon).

Here are the measurements:

A) 42mm, 6.02mm deep
B) 42.05mm, 6.05mm deep
C) 41.92mm, 6.07mm deep

A and B were Olympus parts. C was a high quality BX to BH adapter made by Leeds.

I measured the depth of the receiving part of the dovetail mount and found that there was slightly more variation there. I think your 6.4mm depth measurement should probably be cut back to 6mm to insure it will fit Olympus components. You don't want it "hitting bottom" since, when clamped, the seating surface needs to be pulled tight against the receiving part.


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Rylee Isitt



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PostPosted: Fri Nov 02, 2012 2:28 pm    Post subject: Reply with quote

Charles,

Thanks for posting those measurements - they are certainly useful.

The angle could be anywhere between 60 and 70 degrees. Unfortunately, I have the same problem as you - I lack a blade micrometer so I can't accurately measure the minor diameter. I've used dental floss to get the circumference, converted that to a diameter, and did trig to get an angle of about 65 degrees. An attempt to wedge my calipers in there to measure the minor diameter directly gave me an angle of 60 degrees.

I also photographed the mount and used photoshop's angle measuring tool and got 60 degrees.

Still, those are all error-prone methods. Plus, they are measurements from a third party component.

Regarding pulling the seating surface tight... I assumed that the clamp on the dovetail itself was the important consideration, much like the way the Newport rails or Arca Swiss dovetail clamps work (the dovetail itself is the only surface in contact with the clamp).

I have one adapter already that doesn't fit properly because it's not deep enough (5.25mm) - the result is a very off-center fit, even though the major diameter and angle appear to be correct. So I was planning to err on the side of being too deep in order to ensure proper alignment...

If the depth must be just-so in order that the seating surface makes contact with the receiving part, then the angle of the dovetail becomes much more critical.

I'm going to have to figure out a way to measure it directly...

If you have two calipers, you can use one pair to measure the thickness of the other's blades. Then you can use that to get the diameter of the dovetail at a known distance from the seating surface. You need two diameters with two different known distances from the seating surface to calculate the angle. But you have to be careful - most dovetails don't come to a point but have a flat rim. So ideally you'd measure the depth of that rim, too.
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rjlittlefield
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PostPosted: Fri Nov 02, 2012 5:16 pm    Post subject: Reply with quote

Rylee Isitt wrote:
Regarding pulling the seating surface tight... I assumed that the clamp on the dovetail itself was the important consideration, much like the way the Newport rails or Arca Swiss dovetail clamps work (the dovetail itself is the only surface in contact with the clamp).

I have one adapter already that doesn't fit properly because it's not deep enough (5.25mm) - the result is a very off-center fit, even though the major diameter and angle appear to be correct. So I was planning to err on the side of being too deep in order to ensure proper alignment...

It seems to me that referencing the major diameter and the total depth is not the right thing to do. As shown on Charles' diagram, the end of the dovetail is typically chamfered so that the major diameter is reduced even though the total depth remains the same. If somebody machines a new part with the same measurements but with a different chamfer or no chamfer at all, then it will be off-center when mounted.

Quote:
If you have two calipers, you can use one pair to measure the thickness of the other's blades. Then you can use that to get the diameter of the dovetail at a known distance from the seating surface.

These two measurements are exactly what's needed to make the dovetail the right size, together with the angle of course.

--Rik
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Charles Krebs



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PostPosted: Fri Nov 02, 2012 5:18 pm    Post subject: Reply with quote

Rylee,

The initial measurements I made to determine the angle yielded values between 63 and 66 degrees, but I was not satisfied with the methods I used and the accuracy. I should be able to get it figured accurately, as it would seem like you want this to be rather precise if you want to get a tight, centered attachment.

I'll recheck the depth of the "female" dovetail on a few Olympus parts. I seem to recall they were in the 6.5mm range but I did not record them so I need to remeasure. As you can see, my male dovetail depth measurements look pretty consistent at 6mm, and I have good confidence in that value.

Occasionally you will find microscopes where the female portion of the dovetail mount not only has the locking screw, but uses adjustable set screws in the other two positions that allow for a nice centered fit. None of my Olympus gear has that, so I think the all dimensions need to be determined rather accurately for a good fit. I'll take a second look.
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Rylee Isitt



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PostPosted: Fri Nov 02, 2012 5:37 pm    Post subject: Reply with quote

Charles,

Yup, my Olympus female mount only has the single locking screw.

Your range of values for the angle turned out much more precise than mine, but you probably were using something more refined than dental floss Wink

At some point last night I detached the dovetail mount from my generic trinocular and measured the minor diameter, major diameter, chamfer depth, and total depth with calipers.

My numbers were:
Total depth: 6.4mm (directly measured)
Chamfer depth: 0.75mm (estimate)
Major diameter: 42.40mm (directly measured)
Minor diameter: 36.0mm (directly measured)

So this means the angled part of the dovetail is 5.65mm, transitioning from 42.40mm to 36.0mm. Doing trig gets you a angle of 60 degrees.

Using dental floss I got 36.76mm minor diameter instead, and that led to a angle estimate of 63 degrees.

I think we're narrowing it down... but do keep in mind my measurements are not for Olympus part, so perhaps not of value.

Rik brings up a good point as well... there is that chamfer (I referred to it as a "flat rim" in my previous post) that is pretty important, too. The chamfer is of a known diameter, but its depth will affect the fit of the dovetail within the mount and will change the angle of the remaining depth of the dovetail. On my microscopenet.com trinocular head, the chamfer is roughly 0.75mm.

To measure the chamfer on your olympus parts, I suspect a good approach would be to place a small amount of viscous ink on it, press it against paper, and photograph the ink impression and a stage micrometer with a microscope and camera combination, preferably in the same frame. I don't think you can get a good measure with calipers since you can't "grab" onto the chamfer.
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rjlittlefield
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PostPosted: Fri Nov 02, 2012 7:33 pm    Post subject: Reply with quote

I don't have an Olympus male dovetail to measure. I do have an Olympus CH base. I just now took a series of measurements of its dovetail tabs by pressing a small steel rule against the tab, marking the rule against an index card set across the seating surface, then measuring the marks. With four measurements each for the 2 tabs, I got 60.7, 61, 61, 61.3, and 61, 61.5, 63, 65 degrees. The 65 felt suspect. Taking a second pass on the one tab where I could put a protractor directly on the seating surface, I got 61 degrees. Even so, I'm not confident that I've gotten quite the right measurement, because there's a fillet at the bottom of the female dovetail that could have pushed the rule a bit more vertical than a perfect mating.

Given what I can see and using a bit of judgement based on previous experience, I'd bet that the original spec is 60 degrees. But if you make it 61 and take your diameter measurement near the seating surface, you'll cut the maximum error to less than you can measure anyway. 60 vs 61 degrees with a lever arm of 1 mm is only 15 microns.

--Rik
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Rylee Isitt



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PostPosted: Fri Nov 02, 2012 7:37 pm    Post subject: Reply with quote

Rik,

My only hesitation being that all of the methods I've used give me results that are at *least* 60 degrees. If the actual spec were 60 degrees, I'd expect a few measurments below that spec as well...
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rjlittlefield
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PostPosted: Fri Nov 02, 2012 8:14 pm    Post subject: Reply with quote

Yep. That's why if you can't be sure, the safe thing to do is make it too steep and measure close to the mating surface.

You might even want to make it much too steep, as I will describe next...

On my aus Jena scope, the male dovetail measures 70 degrees. The female tabs might be 70 degrees also, but it's hard to measure accurately because they're cut back so that the mating surface is less than 2 mm deep.

The tabs on my CH base are clearly not 70 degrees, but nonetheless the aus Jena head actually mates well with it except for some centering error.

So, when I wanted to machine my adapter for CH-to-M42, as shown HERE, I made the angle 70 degrees and slowly whittled down the diameter until the adapter was centered on the circular outline of the female dovetail. I never even measured the female dovetail on the CH until this evening because it didn't matter.

So now what I have is an adapter that's 70 degrees, mating nicely with a base that's maybe 61 degrees. Works fine.

The angle of the dovetail is really not important for light duty centering. What's critical is that you get the right diameter at the depth where the female tabs will contact the male dovetail.

--Rik
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Rylee Isitt



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PostPosted: Fri Nov 02, 2012 9:33 pm    Post subject: Reply with quote

Yup, and it's actually quite easy to measure the diameter at the point of contact with the tabs.

The dovetail needs to be 38mm in diameter at the point of contact (37.8mm based on measuring a different tab), but figuring out the point of contact is less trivial. So it's easier, probably, to reverse engineer an actual BH dovetail, rather than trying to work backwards from the female mount.
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Charles Krebs



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PostPosted: Fri Nov 02, 2012 11:35 pm    Post subject: Reply with quote

This should be extremely close. I opened a brand new Olympus U-SPT tube and made some careful measurements. The outside diameter was exactly 42.0 mm (but as Rik mentioned the edge is slightly chamfered/rounded). The diagram, showing 42.8mm, indicates what the measurement would be if the edges were left "sharp" and the 60 degree cut was maintained sharply up to the seating face (not shown). The angle looks to be 60 degrees.

The depth of the dovetail was exactly 6.0 mm

(At a distince of 3.20mm in from the 35.88mm side, the diameter was 39.57mm... green line in diagram)

Hopefully all the old trigonometry neurons woke up and fired properly! Wink


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PostPosted: Fri Nov 02, 2012 11:45 pm    Post subject: Reply with quote

Rylee Isitt wrote:
Yup, and it's actually quite easy to measure the diameter at the point of contact with the tabs.

The dovetail needs to be 38mm in diameter at the point of contact (37.8mm based on measuring a different tab), but figuring out the point of contact is less trivial. So it's easier, probably, to reverse engineer an actual BH dovetail, rather than trying to work backwards from the female mount.

Agreed.

As an aside, it's always an interesting exercise to figure out what I would have made using method A, and compare that with what I actually made using method B. Using measurements from the outer ring of my CH base to the tabs, I get that the adapter diameter should have been 38.2 or 38.4 mm diameter at the contact point. Contact point appears to be about 2.0-2.2 mm below the seating surface, measured by referencing off a blank slide laid across the mount and eyeballing the hole gauge of my slide calipers against the tab.

Measured diameter of the adapter at 2.2 mm from the face is 38.6, so there's less than 0.2 mm difference in centering between the two methods. Well within bounds for my purposes of attaching a bellows, but presumably you can do better with some measurements from an actual BH dovetail.

--Rik

Edit: I see Charlie posted his numbers while I was checking mine. So, now it's off to do some more cross-checking.
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Rylee Isitt



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PostPosted: Fri Nov 02, 2012 11:55 pm    Post subject: Reply with quote

I did a "simulation" to see if the measurements we've collectively obtained so far actually work.

I measured my BHMJ female mount in as many ways as I could using calipers, and then created a side-on diagram where 1 pixel corresponds to one one-hundredth of a millimeter (note that the image below was downsized, but if you're interested you can get the original here). You can get my spreadsheet of measurements here.

I used Charles's measurements for the male dovetail (42mm major diameter, 6mm deep). I used 60 degrees as the angle for both the male dovetail and female mount.

I also assumed the chamfer on the male dovetail is 0.75mm - this is about equal to the chamfer on my generic trinocular head's dovetail but is a pretty rough estimate. Finally, I assumed that we want the part to press right up against the female mount.

When I sketched out these shapes in photoshop, they nested almost perfectly (a few pixels of overlap in the full size image).

So, if we go on the measurements we have so far this is what we can expect the fit to be like:



So what we still need is:

- The depth of the chamfer on the BH male dovetail
- The angle of the BH male dovetail
- The angle of the BH female mount

Once we have those three values nailed down, I can modify the diagram. Ideally the dovetail (grey) should fit within the mount (black) quite nicely. So far the measurements we have, even though many are just estimates, look pretty promising.
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