Microscope headpiece dovetail dimensions

Rylee Isitt · Post by **Rylee Isitt** » Sat Nov 03, 2012 10:54 pm

Rik,

Regarding thread depth, it seems I misinterpreted a Wiki article on the M42 mount. I can revise the diagram with deeper threads, but I'll probably wait to make a few edits at once rather than just that edit alone.

From what I can tell, the tabs make contact with the dovetail along the *entire* angled surface, not just at 3.2mm from the seating surface. I agree that (especially if we shave off the sharp corner) we need to provide a clear reference diameter along with the angle. But if we shave off the sharp corner in the diagram, we probably need to extend a dotted line down to where it was in order to make it clear what the angle is referring to. If we do that, do you think the 42.80mm diameter would still be a good reference?

My machinist contact has fabricated several parts for me in the past. He asks plenty of questions. On the parts he has made for me, all external sharp corners are slightly rounded. Not knowing how he does what he does, I assume he's been taking a tool to all of the external sharp corners to slightly round them off. His clients include Boeing and NASA and he was recommended to me through the university. Given the reams of advice he has given me and the quality of the parts I've received, he seems like an absolute first-rate fabricator/machinist. So I don't think we'll have problems with him making silly assumptions

But since we've posted these images online, it would be nice if we don't cause any problems for other folks who might machine this independently. So I'm certainly interested in making sure the diagram is clear and not misleading.

rjlittlefield · Post by **rjlittlefield** » Sat Nov 03, 2012 11:20 pm

Rylee Isitt wrote:... if we shave off the sharp corner in the diagram, we probably need to extend a dotted line down to where it was in order to make it clear what the angle is referring to. If we do that, do you think the 42.80mm diameter would still be a good reference?

Yes, that sounds good. If the fabricator wants to measure at a different point, he then has enough information to calculate what the diameter would have to be there.

--Rik

Charles Krebs · Post by **Charles Krebs** » Sun Nov 04, 2012 1:46 pm

A couple additional points.... before finalizing the dimensions let me measure out a couple more Olympus dovetails. My calipers are 3.2mm thick at the machined measuring faces. That's how I arrived at the value of 39.57mm diameter at that distance from the seating surface. Pretty much everything is based on that value and the 60 degree angle. So I would feel much better with several pieces measured out.

As to the through-hole. If you really want to thread it (for flare reasons I presume) why not think about using a standard filter size like 30mm x 0.75. Often when doing polarized work with an adapted microscope it is difficult to find a good location for the upper polarizer/analyzer. And there is a variety of stepping rings from 30mm... who knows when that might prove useful. (Might also be worth asking how much an internal thread like that will add to the cost. Flocking may be a better, and cheaper option for flare control).

Rylee Isitt · Post by **Rylee Isitt** » Sun Nov 04, 2012 2:19 pm

Good points, Charles.

My Olympus trinocular is going to arrive this coming week, too. So I'll be able to take some independent measures.

A standard M30x0.75 inner thread would be great. The part is quite flat all things considered, so I don't think we have to worry about vignetting with a 30mm diameter.

rjlittlefield · Post by **rjlittlefield** » Sun Nov 04, 2012 2:28 pm

I have one more nagging concern. Recall I mentioned that fillet near the bottom of the female tab, the one that might have been pushing my angle measurement a bit high. Well, that fillet also provides a place for interference if the dovetail is a bit too deep or is not chamfered enough on the end.

This morning I tried to get a better measurement on exactly where the fillet is. So I hauled out some ShapeLock plastic and pressed it tight around the tab to get a replica I could measure. Here's a stereo view (shot through both eyepieces of a stereo scope) and a cross-section.

It turns out that the cross section is "too close to call" from the replica. The distance from the top red arrow (seating face) and the bottom red arrow (start of the fillet) is no more than 6 mm and it might be a hair less, maybe 5.8?

It's tempting to suggest making the dovetail shallower, but there's not a huge amount of leeway in that direction either. On my plastic adapter, marks left by the locking screw indicate that first contact occurs about 4.6 mm away from the seating face. At one place where I screwed it in tight and left a dimple, the edge of the dimple is about 5.2 mm away from the seating face.

I suggest carefully photographing a real Olympus dovetail to get an accurate profile, then explicitly specifying a chamfer or flattened section so as to lie within that envelope.

--Rik

Rylee Isitt · Post by **Rylee Isitt** » Sun Nov 04, 2012 2:37 pm

Rik,

EDIT: disregard what I say below... I misunderstood Rik's post.

I think this is sort of a non-issue, though.

I recently measured my BH female mount with calipers (measurements provided in a earlier post), so I could be off by 0.3mm or so in my measurements. I took 3 measures of each metric and used the average to simulate the seating of a dovetail within the mount:

Now, imagine if the tabs were instead angled right up to the surface, instead of having that chamfer. The part would still fit. Since the chamfer removes material, it shouldn't cause problems unless our measurements of the angle are wrong.

Rylee Isitt · Post by **Rylee Isitt** » Sun Nov 04, 2012 2:59 pm

Rik,

I just noticed that the mount you have is different from the mount I have (BHMJ nosepiece). That caused me some confusion about what you were trying to explain. Plus I had a hard time convincing my brain to play nice with the inverted mold

On mine, the tabs do not have a fillet on the bottom. They angle downwards all the way to the lower surface of the mount. Also, I notice that my tabs are slightly recessed. Both of these features are shown in the diagram in my last post. The tabs on your mount appear to be recessed as well.

I understand your concern - we'll have to make sure we put in a chamfer of the proper depth and that the max diameter is 42.00mm.

I am not sure if the seating part needs to fit into this recess (and make contact with the upper surface of the tabs), or if it should rest against the slightly raised surface of the nosepiece that extends beyond the tabs, leaving a small gap between the seating surface and the upper tab surface. This is an important thing to figure out.

Rik - perhaps you can attempt to do a simulation of the fit for your mount?

Nice job with the plastic mold! That worked surprisingly well.

Rylee Isitt · Post by **Rylee Isitt** » Sun Nov 04, 2012 4:00 pm

More changes:

- Deepened threads
- Changed internal thread to M30x0.75
- Put in 0.75mm flange at bottom of dovetail

You can get the PSD here.

rjlittlefield · Post by **rjlittlefield** » Sun Nov 04, 2012 4:30 pm

I'll try to find time to make a cross section diagram later today. For now hopefully these snapshots will resolve a lot of the questions.

I am not sure if the seating part needs to fit into this recess (and make contact with the upper surface of the tabs), or if it should rest against the slightly raised surface of the nosepiece that extends beyond the tabs, leaving a small gap between the seating surface and the upper tab surface.

On my CH mount, it would have to be the latter. The outer raised ring goes all around the mount, while the upper surface of the tabs is only about 1/3 of the way. The place where the third tab would logically go is instead occupied by the locking screw and there's no support there that corresponds to the upper surface of the tabs.

Side comment... Because of the angle of the dovetail, contact between the locking screw and the dovetail is made below center, probably around the location of that polished ring we can see on the end of the screw.

Nice job with the plastic mold! That worked surprisingly well.

Thanks. It worked OK, not perfectly. The plastic remains pretty stiff even when softened. It took me two tries, a lot of pressure, and in the end I still had to pick and choose where to cut the section because the plastic didn't quite fill the whole gap in the middle of the tab. Maybe it trapped an air bubble. On the bright side, it was pretty simple to do and it didn't put the scope at risk.

--Rik

Rylee Isitt · Post by **Rylee Isitt** » Sun Nov 04, 2012 5:43 pm

Rik,

That's good info - thanks! My mount is very similar except that it lacks the "fillets" that your has. Are the CH and BH mounts cross-compatible? Your measurements are otherwise very similar, so I see no reason why not...

It's good that the seating surface does not make direct contact with the tabs. That would just complicate the design and make it even more of a tight fit. As far as I can tell the story is the same for the BH scopes. I'm sure Charles can confirm/deny, and once my trinocular arrives I'll be able to chime in as well.

rjlittlefield · Post by **rjlittlefield** » Sun Nov 04, 2012 8:08 pm

I don't know whether the CH and BH are supposed to be cross-compatible.

I had always thought they probably were, but never had any way or any reason to check.

It's interesting that the tabs have different forms.

-Rik

Rylee Isitt · Post by **Rylee Isitt** » Sun Nov 04, 2012 8:35 pm

I'm guessing they are cross-compatible.

According to Olympus, the BH and CH series use the same "modules".

See here.

If the angle, depth, and max diameter is the same - as they seem to be - then I think it's a given.

Rylee Isitt · Post by **Rylee Isitt** » Fri Nov 23, 2012 4:05 pm

Hi all,

I've been incredibly busy recently so this project fell off the radar for a while.

I've just updated the design slightly, and given it to the fabricator I hope to use, as he has offered to check it over at this stage. As we haven't discussed tolerances yet, they are still not in the design.

My trinocular head arrived. It's actually for an Olympus BH2 scope. It fits to the BHMJ nosepiece holder just fine, although there may be a small amount of centering error (I can't quite tell). I'll measure the trinocular head mount, hopefully some time over the weekend.

So, about centering error... the degree of acceptable centering error will set the tolerances for the angle and diameters of the dovetail. What does everyone here think?

I also assume there is a certain amount of flatness error that we need to decide on. A part that does not sit reasonably parallel may render those planoptic lenses less useful, so this is something to think about.

jin · Post by **jin** » Sun Dec 23, 2012 6:13 am

Hi, just got an Olympus BHM but don't seem to be able to find this dovetail-M42 adaptor from ebay.

kindly advise and thanks in advance :-)

Rylee Isitt · Post by **Rylee Isitt** » Sun Dec 23, 2012 9:43 am

Hi Jin,

This adapter doesn't exist... Yet. When i get back from my holiday i plan to have some machined for myself and forum members who are interested. But my fabricator wants an over-pin measurement so there's still a small amount of work to do. Perhaps these diagrams can be used to have your own machined? It may be fairly inexpensive.