opinion; old German milling machine

Discussion on all milling machines vertical & horizontal, including but not limited to Bridgeports, Hardinge, South Bend, Clausing, Van Norman, including imports.

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liveaboard
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Re: opinion; old German milling machine

Post by liveaboard »

Those old relays are just huge. Modern ones are about 1/4 the size.
There are a couple of mercury relays too; wild things, a coil counteracts a spring, tilts a glass tube with mercury and 2 contacts inside.

If it all works, I'll leave it be.
If it doesn't, I'll figure it out and fix it.
A wiring diagram would be nice, but in my experience they're rarely accurate.
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liveaboard
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Re: opinion; old German milling machine

Post by liveaboard »

A small problem has been uncovered...
Ruhla 250-710 broken gear.jpg
Ruhla 250-710 gearbox didassembled.jpg
The teeth are standard module 3, and I can get a single gear in C45 for 13 + 25 shipping + tax.
Then I would have to de-harden the old one, cut away the damage down to a collar, bore the new gear, press it, weld it, then re-harden the whole thing.

Or I can get a second hand gear for 390 Euros.
shipping cost not stated yet.
Russ Hanscom
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Re: opinion; old German milling machine

Post by Russ Hanscom »

It would appear to be a matter of the relative value/abundance of time and money. If you are not in a rush, try the economical approach and replace the one gear. If that approach fails, you can always go the expensive route.

I did a roughly similar project to replace the bull gear on a shaper, where the teeth were worn down to spikes. I bought a new gear and turned the center out of it and the spikes off of the old gear and brazed the two together. A simpler job though, as the gears were cast iron and no hardening was involved.
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liveaboard
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Re: opinion; old German milling machine

Post by liveaboard »

These are really hard. I don't know if it has to be, but I presume it's better to stick with what the original engineers decided.

I was also thinking to go cheap first; but if it breaks in a few years, who knows if there will still be a part available?
If the cheap was really cheap, I'd go for it. but with shipping + tax, I'm at $50-$60.

A fellow over on the German machinist site says he might have one and if so I can have it for the postage cost.
So it's anticipation time.
pete
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Re: opinion; old German milling machine

Post by pete »

Being hard would be for durability reasons. A soft mild steel shop made gear may or may not work just fine. But having excess worn tooth particles cycling through that gear box may not be a good idea. Given the machines quality you can bet each tooth was ground after heat treat.If you can get one for the cost of shipping that would be the obvious first choice. On the plus side, that gear box and oil color in the bottom appears like that machine was looked after given how clean it's internals are. It also appears there's internal clutches. Afaik those are fairly specific about what oil type and weight they use so they do work properly. That would be something I'd definitely be considering if I was thinking of using a substitute oil.
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Re: opinion; old German milling machine

Post by liveaboard »

That's the spindle brake.
There are clutches too, they're only for the feed I now see.
I'll use hydraulic oil, which apparently is equal to what was specified.

The problem with repairing the old gear with a new one is the small amount of meat between the top of the internal spline and the bottom of the gear teeth, only about 10mm.

It would have to be annealed, cut, fitted, welded, cut again, and heat treated afterwards.
I have an electric oven, but due the time it takes to heat up and the presence of o2 in there, surface oxidization occurs that can affect the final dimension and finish.
So the best method I have right now is heat it orange with a torch, and drop it into a bucket of water.
Considerable work and expense for a job that might or might not work out.
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Re: opinion; old German milling machine

Post by Harold_V »

Heat treat would be best left to a facility in which a controlled atmosphere is available. Heating the gear by any other means (rapid induction heating excepted), including torch heating, is likely to alter the gear enough to render it useless. Way too many things to go wrong to make it a good idea.

Dropping the heated gear in a bucket of water may spell doom. Depends on the material, of course. If it was made from oil hardening alloy, there's a pretty good chance that the gear will crack. If heated unevenly, it is likely to deform, and if quenched unevenly that, too, can be troublesome. Make your decision only after weighing the risks.

With a controlled atmosphere, the surfaces should remain scale free, and there will be no decarburization. Both of those conditions are important if you hope to have a gear that runs well with the others.

H
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Re: opinion; old German milling machine

Post by liveaboard »

Yeah, it's sort of the 'nothing much to lose' option. If it fails, the expensive replacement gear is available.

Anyway, I think I've decided to go in a completely different direction, and add a VFD.
The broken gear is one of the 3 small ratio set, the 4 big ratio change gears are intact.
A VFD is cheaper than a second hand gear. I'm unsure of the counter- gear that ran on the broken one, it looks ok but it seems unlikely that it's not damaged.
Replacing it as well would cost even more, not replacing it is risky.

So I would block off the triple gear so it always runs at one of the undamaged ratios, keep the 4 main change gears, and use the vfd to do the rest.
The 2-speed motor would be only run on the high speed coils, that eliminates more of the original speed options, so 18 speeds becomes just 4.

It's cheaper than a replacement part, and I'm more confident about the result. I'm not a very good machinist, but I am good with electrics.

I also get soft start and higher top speed.
Some people seem to want that but I have no idea if it's useful.
the original speed range was 45 to 2240 rpm. is it likely I'd ever want more than that?
I've never used a milling machine, nor even seen one in use.
John Hasler
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Re: opinion; old German milling machine

Post by John Hasler »

Sounds like a good plan. You can keep your eye out for a cheap replacement gear and still be able to use the machine.

I have 150 rpm to 1800 rpm. I've rarely wanted to go slower but I've often wanted to go faster.
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Bill Shields
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Re: opinion; old German milling machine

Post by Bill Shields »

be careful about running the motor at low speeds and high torque requirements with a VFD...you may be headed for more trouble that you expect...not the least of which is that you can fry the motor...
Too many things going on to bother listing them.
John Hasler
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Re: opinion; old German milling machine

Post by John Hasler »

Consider adding an external cooling fan for the motor.
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liveaboard
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Re: opinion; old German milling machine

Post by liveaboard »

Low motor speed won't be an issue; it's a 2 speed motor designed to run at 75% power at 1/2 speed, at 45 spindle rpm.
I don't think it will ever need to go slower than that.

I'm feeling a bit thick here; because it took me this long to realize that I can simply block off the broken speeds with little consequence.
I think.

Here's the speed breakdown.
L = motor on low speed, this speed will be effectively lost be adding the VFD, then reclaimed by running 25 hz.
H = high motor speed
X = not working due to the broken gear.

So, you guys who use mills; are those missing speeds worth much?
Is there value in a top speed higher than 2240?
It seems plenty fast to me but I'm fixing a machine I know nothing about using.

45 rpm L
56 x
71 rpm L
90 rpm H
112 X
140 rpm H
180 rpm L
224 X
280 rpm H
355 rpm H
450 X
560 rpm L
710 rpm L
900 X
1120 rpm L
1400 rpm H
1800 x
2240 rpm H
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