Some Thoughts on the Obi ANH Transistor Washers

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schrodinger555

Sr Member
RPF PREMIUM MEMBER
Hey everyone! So a bit of backstory to what I'm about to show. I work in an industry where we make very high end x ray equipment for materials research. We use spherical washers and recessed washers for various things, like aligning and clamping different parts of the system just so. So when I got back into prop related stuff I took another look at the Obi saber and realized what we have been looking for are what's called "recessed washers" or "countersunk washers."

They're used in a variety of applications, but a common one is aerospace engineering. You find them on military vehicles and assemblies, and they are often made of specialized alloys with various coatings for durability in various environments. A lot of planes use them, and some modern companies, like Boeing or the Bell Helicopter company make them or have them made for specific uses. I reached out to coolhanluked and we discussed the washers, I sent a link to some I found on eBay, which actually ended up being quite close, just a bit too thick, and the diameter is ever so slightly large based on my measurements. A few months later 2Cell reached out to us and shared a site that has a collection of military style washers, which are made to fairly tight tolerances. I'll get back to this part later. I think the goal now, is finding a vintage part, or a manufacturer working to a different standard than these washers. These washers are made specifically for a purpose, or to match a specific sort of tolerance, so there are a large variety of them, and there is a possibility the ones used on the saber came from one of the innumerable military and aviation engines the props department had access to.

What I have found is a part made by a modern manufacturer to a very similar tolerance to the washer we see on this saber. There's a possibility the part we are after is no longer made, or was, just like the upgraded large assembly on the Derwent engine, a relatively small run of parts made for a specific purpose, and they just happened to be a close/tight fit over the transistors.

Anyway! With some ideas in mind I decided to finally nail down some dimensions. I know a few members of the community have made washers in the past, but I didn't have any of those in hand to judge against, so I used an op amp and the length of the graflex clamp as my scaling tools. We're all familiar with this photo:
l3INylp.jpg

There's a small amount of lens distortion, though thankfully the clamp is smack dab in the middle. So I printed the photo out, with the clamp to scale, at 50mm or 2 inches wide, giving the transistor/opamp a diameter of 8.23mm/.324in. So I figured from here I could get the outer diameter of the washer pretty close, and using another photo, I could get the approximate thickness of the washer itself.
B+W OBI.png


Using these two photos and measurements as a guide, I went to Fusion360.
Screen Shot 2020-08-07 at 10.34.21 AM.png

I scaled the photo, and began drawing directly onto it, outlining first the transistor, and then the washer. My measurements were basically the same as when I did it on paper. The OD of the washer is 13.585mm/.535in, and the recess diameter is 10.58mm/.416in.

Screen Shot 2020-08-07 at 11.33.07 AM.png

Using this next photo, I tried to suss out the approximate thickness of this washer. 2mm seems about the perfect number here.



So given this exploration the dimensions we are looking for are as follows:
13.6 mm OD
8.23 mm ID
2 mm thick
10.6 mm recess diameter

.535 OD
.324 or 5/16 ID
.078 thick
.416 recess diameter



The actual dimensions of the found part from the military washers source are as follows:
14.45mm OD
8.04 mm ID
1.6 mm thick
10.4 mm recess diameter

.569 OD
.316 ID
.063 thick
.41 recess diameter

So as you can see the found parts isn't an exact match, or at least not by my scaling, which is of course subjective and done with grainy photos that are washed out. The photo where you can see the height of the opamp/transistors seems to show the washer has a slight rounded section to the top of it. That could affect the perceived diameter of the washer as seen from above slightly. As you guys can see, the difference in overall OD is .85mm as per my measurements, which is of course pretty small.

That being said, on my personal saber I have turned the washers down ever so slightly on my lathe, so the OD better matches the part found on the prop.
When looking for this part, the most important part for me was the recess diameter and the thickness, as the hole can be enlarged easily with a dremel, and mine are just friction fit to the op amps. The OD can also be trimmed easily, so I was happy enough to find a part that is only .85mm off in OD, which can be easily corrected. I'll attach some more photos of my actual washers on my saber as comparison between the real photos and what we have here.
IMG_6893.JPG

0334213E-A77A-4A1C-A822-0230927A50BE.jpg



IMG_6892.JPG



Screen Shot 2020-08-07 at 10.51.08 AM.png



From left to right:
A washer I made from aluminum based on my measurements while I waited for the washers to arrive.
A washer "as is"
A washer with the finish removed
A machined washer
And below, a machined washer on an op amp
IMG_6853.JPG



So this is pretty much everything I have related to these washers that hasn't been a dead end. I strongly believe that this is what was used on the original prop. The washers that arrived had a coating on them that took maybe 30 seconds with some scotchbrite to remove, and enlarging the hole with a dremel was easily 5 minutes of work for both washers. I do believe that op amps were likely not used, and instead it was to-39 package transistors, as the casings are the same size and the op amps have a ceramic disc on the back that is a nightmare to break down and drill through. If I was a prop maker working to a deadline, I would try drilling one of those, stop, and then drill a regular transistor instead, as they are much easier to drill. I also recently took a long dive through the rabbit hole of spherical washers, as they are often sold in male and female portions, or 'convex' and 'concave/dished' respectively. The closest I was able to find is what is called DIN 6319-D which is the bottom portion, though the ones I bought, which were spec'd close to what we are after, did not match what so ever. I am not convinced spherical washers hold a promise, but they tend to have geometrically similar features to the recessed washers. The issue with these parts is that they exist in the exact proportion that we expect to see, just much larger than what we need.

So! exalted ones! I think given the history of incredible detail oriented work on this forum, perhaps with some concrete (I think) details to look for, and a better idea of what we are after, maybe an exact part can be ID'd if it exists. So have at it my friends, good hunting!

I do have a bunch of these left over, and if people are interested, message myself or 2Cell who also has some extra! If people are interested I could machine down more of the ones I have to better match the OD on the original prop as well. Hope this helps!

-Emiliano
 
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2Cell

Well-Known Member
RPF PREMIUM MEMBER
Woohoo! Glad to be a part of this find. Like Emiliano said, I've got a bunch from our buy as well. These definitely play the part for your needs if you want the look easy. Vintae is always better but for now these are pretty nice. Great write-up too Emiliano. Much better than I could've explained lol.
 

Mouse Vader

Sr Member
Outstanding work. Well done. Really does look like the right type.

I know this may mentioned somewhere else but in your ref photo the tranny with the washer looks to have been bashed slightly. Whether this was deliberate or not it's likely effect is to swell (tech term upset) the tranny casing to grip the washer which may account for it not falling off. As there other one hasn't been upset I would suggest this is accidental.

Totally agree likely orig source is the military / aviation surplus they had.
 

teecrooz

Master Member
RPF PREMIUM MEMBER
94394E94-AEC9-4A3B-A9E4-1B29BED9AF37.jpeg

All kidding aside, I think you’re on the right track. I’ll happily add these as possibilities. mugatu and I purchased different aviation washers a few years back. Some have a nice reflective coating that gives the washer the same “washed out” look that we see in the pictures.
 
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thd9791

Master Member
RPF PREMIUM MEMBER
Yes I agree, very well thought out and informative. That side view looks a lot like the saber sitting on the cloak in the Obi Death scene too
 

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Mouse Vader

Sr Member
Just been having superficial search on these & note 13.5mm = 17/32"; 14.28mm = 9/16"; 7.94mm= 5/16" . If these are vintage surplus (in 1975/6) then imperial is the likely unit.
 
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schrodinger555

Sr Member
RPF PREMIUM MEMBER
Just been having superficial search on these & note 13.5mm = 17/32"; 14.28mm = 9/16"; 7.94mm= 5/16" . If these are vintage surplus (in 1975/6) then imperial is the likely unit.

Yes I think that's right! My issue has been finding a place I can search by fractions. Other than the ID, which sometimes is listed as 5/16 for washers of approximately the right size, they don't seem to really be labeled by the fractional imperial sizes. Though it would be really nice to come across a site that sells vintage 17/32 OD washers :) The other big issue is the recess diameter. These are made with usually a 90 degree chamfer/recess being machined into them to hold the screw head, which means that based on size, there is a natural thickness to size ratio. Some of them are built to odd specifications based on individual purpose, but most that I have seen have a diameter that is larger than 17/32 if they have a recess diameter that matches the ones on the prop. So my thought with the washers I decided on was that the recess diameter was right, and the OD could be machined down to match the prop, with the inner diameter only needing some enlarging to fit an op amp or transistor, so the biggest visual detail on them, the bevel, was in the right spot.

Though now that you mention it, perhaps searching using the proper fractional size would help, we'll see! If you come across anything promising please let me know :)
 

Mouse Vader

Sr Member
I will.
So far the nearest have been on ebay UK, listed in fractional sizes as NOS countersunk aircraft washers. Visually they look good proportion wise but they were either much too big or much too small. I live in UK which is possibly the originating country for these. I'm pretty sure there will be some sitting forgotten somewhere over here it'll just be a question of finding them.

I just checked so as to post the ones I'd found & turned up these that are getting quite close. What do you think ? :


Typical they're not in UK but USA & postage to me $40 !

p.s. these are what I 1st found but are too small :

 

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schrodinger555

Sr Member
RPF PREMIUM MEMBER
Yeah those are the same ones he had, they're not quite right, the recess diameter is fine but the outer diameter is too big!
I also came across those smaller ones, they look excellent proportionally, as do a lot of the larger ones, but unless you're doing a .45 scale version or something they're way too small! It's a shame too they look gorgeous.

I did some looking using 17/32 as a search term and didn't really come across anything new or helpful, but it seems you can find regular washers in that size with no problem. I imagine it's just that recessed washers in that size are rare
 

Mouse Vader

Sr Member
Yeah it's a pain. The MS20002c5 has the correct i/d of 5/16 but wrong outer. I've checked the next size down, the MS20002c4, which has the correct o/d of 0.5312 (nominal) but wrong i/d at 1/4" (see link). So not this series of washer. I think the MS part stands for 'military specification' I'll keep looking. What's the betting it came off a Derwent ?
 

schrodinger555

Sr Member
RPF PREMIUM MEMBER
The biggest issue with that one is the recess diameter is basically the size you would need to open it up to in order to make it fit a transistor. Yeah I think it's likely the part came off an engine! I found something on eBay that looked promising but there's no reference to the size of the part which is troubling. I'm imagining it's a series of washers made for a specific assembly on the engine.

Does anyone have exploded diagrams or anything to go off of for the Derwent engines? Might be a good place to start
 

schrodinger555

Sr Member
RPF PREMIUM MEMBER
I have a feeling his results might mirror mine! Using the OD of the transistor and the length of a graflex clamp as guides is how I found my way to the 3d models. Using that, the ID of 5/16 and OD of 17/32 is pretty much all we would need to go on!

I wish these parts were even more exacting and particular, like the balance pipe or booster, but the issue, same with the transistors, is that these are kind of 'normal' parts. Matching what they came off would be impossible as they don't exist on only one object like the graflex clamp or any of the other major components do. That makes IDing an exact component impossible, but finding something vintage in the right size is probably the best we'll ever do!
 

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