Custom Thread Microscope Objective RMS

[imazursky]

Hi,

I need to make a custom thread for a microscope objective.

It’s an RMS thread (ISO 8038-1), .800" x 36 55 degree Whitworth.  I found the specs below from Thor Labs.

The intended use to 3d print the internal thread.

So far I was able to create a new thread definition but im not sure im putting the correct information where its supposed to go.

The threads aren’t coming out correctly, its too tight and it feels like its off.

The objective will start and wonder off to the side instead of going in straight.

The diameter of the hole is fine, just the thread is wrong.

I have also attached the xml file with the custom thread information.

Any help would be appreciated.

Thanks!

Internal Thread, 0.800"-36.0 UNS-2B

Min Major Diameter 0.8000"

Min Pitch Diameter 0.7820"

Max Pitch Diameter 0.7866"

Min Minor Diameter (and 83.3% of Thread) 0.770"

Max Minor Diameter (and 64.9% of Thread) 0.777"

External Thread, 0.800"-36.0 UNS-2A

Max Major Diameter 0.7989"

Min Major Diameter 0.7934"

Max Pitch Diameter 0.7809"

Min Pitch Diameter 0.7774"

Max Minor Diameter 0.7688"

[MRWakefield · Accepted solution]

Some of the info you got from Thor Labs isn't quite right. You correctly say it's a 55° Whitworth thread profile but the Thor Labs spec states UNS, which is 60°. This might be the main reason for your problems with fit.

Anyway, here's the actual spec:

I tried to post this as an actual table so you could copy & paste the figures but the forum software is so hopeless it just throws up errors!

Hope this helps.

[imazursky]

Thanks for the help!  Turns out that the current version of Fusion lost its ability to do Whitworth from a custom thread.

There is a developer ticket open to fix that.  Hopefully they can fix it soon.

I am just going to filet the thread until the root and crest are "round" enough.

[MRWakefield]

Yes, quite how they managed to break that I'll never know. I mean why does it treat custom thread files differently to the standard ones!? BTW in case you don't know, the radius on the crest and the root of the Whitworth thread is 0.137329 x Pitch.

[imazursky]

From what I gather (from support), they somehow hard coded to only Whitworth in the BSP xml and the others that come with Fusion.

The thread form tag doesn’t do anything.  Hopefully they’ll fix that.  It would also be nice to create custom threads within fusion but that’s a different problem.

Thanks for the radius info.  Looks like im going the Tap route, either in PETG or an insert.

3D printing it turned out to be more trouble.  PETG is too malleable and deforms easily with such a small thread.

Any side to side movement throws the axis off and distorts the threads.  It doesn’t help that the there are only 3 or 4 threads total on a microscope objective.

Not what I wanted to do but it'll have to do.

[MRWakefield]

I agree, it's not as user-friendly as it might be having to build the XML file manually. Every time I have to do it I think about developing an app to make it easier! However, building a universal app that includes all the calculations for allowances and fits for any and all likely thread standards isn't trivial (for me at least!).

I must admit that I thought you were being very optimistic attempting to 3D print such a fine thread. I've got a Formlabs Form 2 SLA printer and the finest thread I can reliably print is 1mm.

Inserts are usually the way to go but I'm not sure how readily available an insert of this spec will be as I wouldn't think it's all that common. Have you considered having one 3D SLS printed in Nylon/Acetal or SLM/DMLS in metal?

[imazursky]

I would have the same problem trying to make a universal plugin.  I hope someone or AutoDesk takes it on.

I can usually get away with fine external threads, internal it turns out, not so much.

I have a mini lathe so making a small puck insert with the tap I bought on the bay should be easy enough (assuming the right one arrives).

The clearance I have on one of the parts might make it hard to fit a threaded sleeve in but not insurmountable.

Realistically this is a one and done, put the lens in and no one should ever have to take it out.

But I plan on the worst, so the insert and maybe some never seize on the threads should future proof it.

[MRWakefield]

Ah! Didn't know you had access to a lathe, that makes it very doable. I wish I had a decent lathe again but I just don't have the space. I've got a small Sherline lathe that I bought 40 years ago but parts of it are broken and worn out, and spares aren't easy to come by here in the UK ☹️. Back in the '80s when I was still living with my parents I had a Willson 18" swing slant-bed lathe which came in very handy. Gaining experience on that enabled me to get a job as a centre-lathe turner which in turn lead me on to being a CNC programmer/setter and a lifetime in engineering & CAD (I started on AutoCAD 2.6)!

Hope you find a satisfactory solution.

[imazursky]

That’s really great you grew up with a lathe.  Nothing like a little machine shop at home to foster the engineering spirit.

I would love a bigger lathe too.  The 7x10 I have now is good but tiny!  Many projects are just too big for it.

For all its shortcomings, its has a good amount of torque (at the expense of some rigidity) and can swing a 5" 4 jaw.

Its definitely a great tool to have and I have learned a lot.  Maybe you can pickup something similar in the UK?

Its compact and there are a ton of after market mods you can (and some you should) make.

They're relatively inexpensive and you might be able to sell the Sherline and use the profits to buy a small 7x10.

[MRWakefield]

Yes I certainly had many hours of pleasure from it apart from being very useful. Unfortunately I'm bursting at the seams here. I only have a small garage that I have to squeeze into as it is! I did have a look at getting a lathe last year but I ended up getting a small diode laser machine instead which is proving a lot of fun too. I need to find a way of earning a lot more money so we can move to a larger place with maybe some outbuildings, then I can have a real workshop (or ideally workshops)! Oh well I can dream can't I? 😢