Polamco TUM-35 Replacement Gear Design

Hi all,

Sometimes, you just get lucky. I was fortunate enough come across a Polish-made TUM-35 14x40 lathe this week for the low low price of $350. I'm a home shop guy and have been waiting patiently for a nice machine to come along, so this made my day.

Here's the rub. The motor gearbox had experienced a crash and no longer transmitted power from the motor to the output pulley. Seller advised there was a broken gear, and an inspection confirmed that. Now that the gearbox is home, I've disassembled it partially and extracted the damaged gear.

I was originally hoping to braze a new tooth on and have a local shop machine a new profile. Now I realize that because the wall is so thin, wire EDM is my only choice. Fortunately a friend has access to a wire machine, so my next major task is designing the new part.

Two major questions:

• I believe the spline is a 6x21x25, according to web research. How can I find the details on that spline design? Primarily I'm wondering about angles of the 6 teeth.
• How can I determine the tooth design? I do have access to an optical comparator, if that will help my case. Any pointers here would be helpful.

I have access to Solidworks at work, so I can design the new part once I can figure out these dimensions.

Managed to get the gear under the optical comparator tonight. It was my first time using this tool -- incredible!

I've read that experienced gear guys can identify the relevant stats just by seeing the image. What do you think?

Take out your credit card and order one from Toolmex.

berga said:

I've read that experienced gear guys can identify the relevant stats just by seeing the image.

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Oh. Ahem. I guess that's my cue...

berga said:

What do you think?
View attachment 221351

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SteveBausch said:

Take out your credit card and order one from Toolmex.

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Yep. Seems legit. Spot on.

Oh... you wanted more...? Okay...

( looks at picture ) ( looks at picture again ) ( looks at picture one more time for surety )

Yep. No doubt about it. That is almost assuredly an actual Involute. Also a straight sided Spline.

And Steve is decidedly correct. You paid $350 for the lathe. What do you reasonably expect it will take in time to figure out the gear's details, purchase stock, turn the blank, bore the blank, Hob or Shape the teeth, Shape or Broach the Splines, Heat Treat and Temper the whole affair will take? Now multiply that by your favorite shop's rate.

Think about that.

Go purchase the replacement gear from the manufacturer's importer. Not only will it most likely cost much less, but you will be up and running in faster time. And you can always have the story to tell of how they raped and robbed you to fix the lathe that you stole.

Good luck.

P.S. There is positively ZERO reason to EDM any part of that part. And I say that as someone that makes Gears every single day and also WEDM's things ( mostly Surgical, but other things as well ) almost every single day.

SteveBausch said:

Take out your credit card and order one from Toolmex.

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I've bought gears from Toolmex to repair quick change boxes on Polish lathes. They were reasonably priced IMO.

Going by the comparator image, there's actually a good reason to remake the gear from a premium material - the overlap of the spline and tooth leave two relatively thin "flexures" connecting the inner and outer features, which means those areas are distorting when under load. Note that this is how the missing tooth failed, at one of these overlap zones.

But the reality is that the OEM replacement gear will be cheapest and fastest to get, and if the price isn't too high consider buying two of them, so you're not waiting if the design flaw rears its head again.

Before jumping in head first I would definitely take a good look at the mating gear to that one and also check to see what caused the damage in the first place. I like to remember that mating gears are usually pretty close to equal in design so many times when a gear breaks the mating gear has some damage, could be a crack that is ready to shed a tooth at the first opportunity or may be just some nicks and risers that a little work with a stone will blend out. Also important to me is to try to figure out why it broke in the first place, did a crash occur or is a bearing failing allowing excess play between gears. Replacing the one gear and putting it to work might be the ticket but if something else is amiss in the works it could be disappointing?
I have to agree that getting a gear from the manufacturer is where the smart money is.
Dan

SteveBausch said:

Take out your credit card and order one from Toolmex.

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Yes, I do have an RFQ in with Toolmex already. But the lady on the phone painted a pretty grim picture about availability of parts. She's checking with Poland now to see whether it's available.

Danny VanVoorn said:

Also important to me is to try to figure out why it broke in the first place, did a crash occur or is a bearing failing allowing excess play between gears. Replacing the one gear and putting it to work might be the ticket but if something else is amiss in the works it could be disappointing?

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I agree. What I didn't mention is that the shaft housing this broken gear lost one of its two supporting ball bearings. When I got to the machine, the shaft was completely loose inside the housing

I'm not sure whether the gear tooth failed first causing the bearing to fail, or vice versa. The PO did mention that it had been making a knocking sound for a while before the catastrophic failure. Maybe it lost the tooth first, then eventually torpedoed the bearing? In the photo below you can see the inner bearing race on the right-hand side of the shaft.

Zahnrad Kopf said:

P.S. There is positively ZERO reason to EDM any part of that part. And I say that as someone that makes Gears every single day and also WEDM's things ( mostly Surgical, but other things as well ) almost every single day.

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I'm sure there are simpler ways to make this part, but when the only tool in your toolbox is a hammer, everything looks like a nail

If it's wire EDM you'll be using, perhaps you can fixture the bad gear and use the EDM wire to "touch off" the profile of the teeth by noting changes in current when you short. A small enough grid for measuring and some curve fitting will probably get you close enough, considering you're not shooting for a AGMA 12 class gear. Or measure off the comparator.

Some grinding with a cut-off wheel for the lead chamfers and you're good to go. The spline you can probably direct measure for its geometry.

For the steel, ask Zahnrad what he'd recommend. And make two, or a dozen, then call back the nice lady and see if she'll buy some...

OK. While I'm waiting for the quote, I've finished up the CAD work. I went with Milland's advice and captured the gear profile using the comparator to generate a 2D point cloud. I took about 200 points to define one tooth, then imported the points into AutoCAD and redrew the profile using arcs. I made a couple minor changes, such as smoothing over the crests with radii and I deepened the roots slightly.

As for material, I'm going with case-hardened 8620 on the recommendation of a friend. This material is commonly used for transmission gears, so I think it should be a good fit here.

Make several. It's a shit design and it's just going to break again so you may as well have a few spares. Rule of thumb is, under the teeth you need at least a full tooth thickness and preferably one anda half. Two is even better. You've got teeth on both sides but nothing supporting them.

Put a bigger radius on the inside corners of your internal spline, then knock off the corners on the male part with a Dumore. That'll help.

And 8620 is going to go goofy when you quench it. The part is too thin and the cross-section changes too much. If I were going to try to make it better I'd do the basic shape in 4130, quench and temper to maybe 36-38 Rc then machine the teeth hard then nitride. That might give you an extra couple of weeks before the skinny part between the corner of the spline and the root breaks again. At least it wouldn't go kaka on you in heat treat.

4130 nitrides better than 4140 but if you can't get it, then 4140 will work. 4142 is what you usually get tho, and every step up in carbon is worse, sigh. In this case you could probably skip the nitriding entirely and it'd be okay. I just can't help myself, middle name is "overkill".

If you insist on 8620, make up a cast iron plug that's a slight interference fit cold. They can heat the part red, drop it on the plug, then quench. That helps it stay round a little bit. But not much. The bore is going to shrink on you, plan on a good .001" per inch. And the splines will tighten up. Easiest way to fix that is a bead blaster with silicon carbide media. Steady hand and you can take a thou or so out evenly. More if you have to. Makes a little lightning show in the cabinet, kind of cool. Also zaps your hand if there's any holes in the gloves. Feels like about 20,000 volts

For a carburizing steel you could do the spline first then copper plate the semi-finished blank then cut the teeth. That'd give you a case on just the teeth. When you add .035" case from both sides, that area between the spline and the roots gets even worse. At least if you plate the bore then you only have a brittle layer on one side. Not much help but in this case you need all you can get. Or you can just mask off the bore, might be easier. Plating bores can be a little tricky. Heat treaters usually have a masking paint-like stuff.

9310 would last better than 8620 and the amount you're talking about is tiny so cost would be same. About the only thing you can do with a rotten part is try to make up for it with 3x stronger material. 9310 is a whole bunch stronger than 8620. Generally when 8620 parts fail, if you change to 9310 that's the end of the problem. Good stuff.

Or you might even try 17-4. It's not as strong but much easier to work with. It heat treats beautifully, you can even do it in the self-cleaning oven if your wife doesn't catch you and it should be adequate for your part if you make spares. In fact, I think I'd go that way, personally. You can even nitride it if you want. If this wasn't the pinion I wouldn't be concerned about tooth hardness but ... it is.

(If you want to go nuts, grab a chunk of Vascomax 250. The fatigue in the corner is what's killing you and the second-best thing Vascomax is good at is fatigue resistance. The first best thing is emptying your wallet )

Very poor design.

Besides your $350 bargain you're going to get an education

Sounds good, SeaMoss. Aside from specific materials suggestions, what I mentioned in #6
I like the idea of using 9310, while they'll rape you on the cost, places like this have small amounts for sale: 2" ROUND 931 VAR ALLOY STEEL - Online Metal Store

The OP could even make his gears like NASA do: https://www.shotpeener.com/library/pdf/2002104.pdf Some nice details in the paper.

Milland said:

Aside from specific materials suggestions, what I mentioned in #6

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Yup. Just thought I'd throw him a few bones that might be helpful. It's all obvious after you've done it once or twice but that first time can be full of surprises :-)

Before you give up on finding a gear call this fellow -

Rob Cowan
847-902-1568

He used to work for the importer and once called me offering to be able to supply spare parts, said he still had connections to the manufacturers.

Before you choose 9310, determine who is going to heat treat it for you. Not everyone can process 9310 properly. I'd think 8620 or 4XXX would be plenty, my $0.02 is that the bearing failed first, and caused the gear to fail when the shafts moved apart.

Mud said:

.... my $0.02 is that the bearing failed first, and caused the gear to fail when the shafts moved apart.

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I don't think so, because



It looks like there's another crack over at the next thin area between the spline and the tooth root. Also, the existing crack is quite long to develop in just a few cycles from a bearing failure. And it shows shiny spots, where the metal rubbed against itself, as opposed to the clean break where the next tooth tip was knocked off (from the failure of the missing tooth ?) So that crack has been there a while. There's also a chunk out of a neighboring tooth that is old so this thing has been in use long enough to develop fatigue failures. The crappy bearing for sure didn't help things but the teeth didn't fall off any other gears. It chose this weak point.

The teeth on this pinion see a lot more cycles than any of the other gears, plus that skinny area between the i.d. spline and the roots of the teeth ... I've seen similar parts in other boxes and they always let go eventually.