Which Steel for a mellotron capstan?

Hi,
I'm in the process of refurbishing a M400 mellotron.

The capstan (a solid plated steel 708 mm bar, about 30mm diameter) has some rust on it. It's officially dead.

It's a very easy to machine part and I might go the DIY road. I want to do it (well, have it done to be honest) in a Stainless Steel piece, like the newest ones are made.
My only unanswered question is :

-Is there a magnetic specificity to respect for this application, or should I just look for "austenitic" stainless steel, or plain stainless steel? The mellotron site just talks about Stainless steel. I kinda remember reading "amagnetic stainless steel" somewhere but can't find it anymore.

-Also the site talks about it being "precision polished". Is there such a thing after you put a steel bar on a lathe?

Oh, yeah, actually, I lied, there were three questions, the third one being: does the metal get magnetized by the lathe? I've read that "cold work" can magnetize steel. Is "cold work" pressing, or drilling, or just anything you do to a cold metal piece?

Any input greatly apreciated!

I recently rebuilt an M400 - most of the parts pertaining to the tape cartridge were missing.  A lot of parts are available from mellotron.com.
Are you sure the capstan is shot?  Can you clean it up with a little steel-wool - maybe have it turning while you clean it up?

The machine I worked on was pretty heavily corroded - everything cleaned up fine.  Even the heads were in bad shape, but all work fine now.
the real trick is getting the motor control board working.  I replaced all the caps and most of the resistors.  The 2 speed control pots can be touchy too - maybe spray with some contact cleaner and make sure they're all snug.
And make sure all your tools are de-magnetized too.

then there's the regulation - and making sure the case is squared up.
Good luck - I hated working on that thing.

Yes, unfortunately, it is already dead. I've tried all the procedures you describe to no avail.
The early capstans were plated steel. As soon as the plating wears off, the steel underneath start to corrode.

Mine was in a very bad shape too.
When he arrived, he looked like that:

And the capstan and head block looked like:


He's feeling much better now, the motor runs again, I had to change some parts on the motor regulation board (the diodes were shot and the pots looked really bad), and the cabinet has been siiting in my living room for two weeks now (my wife is very patient, but...) smelling of paint, sanding machine...

Of course, none of the tools I used were demagnetized... I've demagnetized the heads with a tape machine hand demagnetizer, but I guess I will have to do it all again when everything is done...

Oh, yeah...
...I hate working on that thing too, by the way...

Well, at first sight, using non-magnetic material for the capstan does make sense.
In the other hand, if the earlier models were plated steel, then they were at least as much magnetic as any stainless steel!

I know this doesn't answer to your question, sorry!

"Precision polishing" Yes there's definitely such a thing after tooling on a lathe. You want a mirror finish that won't wear off your precious tapes.
The harder the material, the longer the polishing work.

What about using aluminium? You are sure that it is non magnetic, and it's much easier to polish...

More seriously, I would look for a craftsman who works on cameshafts. those guys must know something about polishing hard steel...

Axel

Just looking at the photos brings back haunting memories.
Don't worry about the tools too much with out the tapes in.

I think you can find people that can re-chrome things - isn't it a chrome 'dip'?  I'm not sure about magnetism with chrome, or how well it'll stay on.  Maybe it needs to be one of those electro/chemical processes.  Either way I'd contact the guys in the UK that deal with Mellotrons - you might just be able to get a new part.

don't get tempted by the road case to actually move that thing from your house once it gets working.

Thanks for the cam shaft makers idea, that might be very helpful.

re: polishing: aw, that's bad news. I was hoping to just get the piece of steel on a lathe. I searched a little more thanks to your answer. It will require some "miror polishing". Done by some professional, because I don't know what would be considered "good enough" and would probably take it too far, or not enough...
Considering mellotron.com sell capstans for 150 USD (plus shipping from canada to france, package being about 5 kg) and Streetly electronics may sell spare parts from europe, I'm not sure it's worth the hassle...

On the other hand, I'm quite sure I could get the metal and machining for free here, via some motorcycle workshops or plane industry people. Do most of the machinists offer polishing? Most of what I find when I google "polishing" is polishing specialists that will work only for your 3000 bathroom taps, but not for one-offs. I don't want to ask for a service from a friend of a friend if in the end I have an unfinished part that will cost me even more than the normal spare...

About magnetics: I found the "amagnetic precision polished capstan" bit again, on mellotron dot com. I think amagnetic simply refers to the capstan being made in austenitic steel, wich is less magnetic than ferritic steel. I don't think there is some rocket science behind it, given the fact that many old capstans were just plated steel.

geoff004 said:

don't get tempted by the road case to actually move that thing from your house once it gets working.

Click to expand...

That thing is actually going to move out, as I hope to fund a recording of my mediocre talents with the money I will make out of it... The road case is gone (yeah that was a stupid move, but mellotron+case= double space, and the case was even too big to go in the stairs to the cellar).
I can't stand its sight, smell of wet paint, dust etc anymore... ;D

And just so you can have a good laugh thinking of those swell memories, let me show you what my living room looks like just now:

Actually if you can find a shop who is specilized in refurbishing hydraulic steering wheels, that would be the best, cause these guys use hard chrome, and their precision is down to a couple of microns.

It could be done on a lathe, if the tools are fine enough and time is not a problem... The lathe could also be used to sand the capstan down to 1600 grade.
From there, just use some chrome polish and that should be perfect.

"good enough" is what a good capstan finish looks like (think Studer rather than some old rusty Realistic car radio!). You should show one to your lathe guy and ask him about obtaining such a finish... According ti his reaction you will know if he can do it, or if you should order from Canada...

Axel

doubleroger said:

Considering mellotron.com sell capstans for 150 USD (plus shipping from canada to france, package being about 5 kg)

Click to expand...

I think the Canadian Mellotron company actually is now run by a guy in Sweden.  At least he's the guy that actually builds the machines.  Cheaper shipping to France for sure.  I have a friend who knows the guy (I think his name is Marcus?) so I'll try and get an email for you. 

Thanks for all those great ideas,
I think I might try to go through the Airbus people first, just to get an idea of what can happen with their tools. It will take some time, but I'm not in such a hurry anyway.
I've already contacted Marcus for a quote, and he's been very quick and helpful. He's in Sweden, wich might make things much easier and cheaper. I need to order parts from him, so I'll just wait till I know more about making the capstan here.
The more I learn about the process, the more I feel I'm going to order it from him... well we'll see... Being broke for the moment I will have to wait a liitle bit before I order.

Thanks for your input, it's just incredible, all the knowledge on this board. Astounding!

Try to find an old cassette bin-loop duplicator. The capstan might be  similar enough in size to use with minimal modification. I recently bought a used Magnefax  for $15.