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Thread: Gear Problem.

  1. #1
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    Default Gear Problem.

    Hi Guys,
    Some of you will know that I've had various problems with my Chinese mill. Well I would like to solicit comments about this plastic gear from my mill. The gear in question is the duplex speed change gear that lives inside the mill head. From new, the mill has always made a slight ticking sound that varied as the spindle speed changed. Well the other day I was slitting some alloy bar with a 1 mm thick saw and the blade grabbed and the top plastic gear lost a tooth and ground it into the other teeth. I'm not overly happy about it but not too concerned either. Since replacing it is a 20 minute job. I'm more concerned about the other one. After removing the smashed gear I tried rotating the spindle by hand and found that it had a spot where it was hard to turn. Getting to this gear is a major job to remove and inspect. The pictures show what I found. So my request for comments.

    The first three pictures are of the duplex gear. The damaged area is inside the red box. The other picture is the top driving gear. This one is directly driven by the motor. You can see where the initial grab took place and the place where the teeth were stripped off.

    Thanks all.
    Attached Images Attached Images
    Best Regards:
    BaronJ.

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  3. #2
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    Default

    It looks like it will mesh but will be noisy, may jam in the future and will probably affect your surface finish. If it were me I'd be replacing it either with a maker's spare part or preferably with something made from a decent material (we used Nylatron for Simon's worm gear and that seems to be going really well)

    Michael

  4. #3
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    Replace both gears.

    The teeth are already weakened and will let go at some later stage - plus as stated it will be noisy and irregular.

    I would ditch all those poly gears and use alloy instead. There's almost certainly a metal gear or belt kit available, given this is an obvious weak point.

    Rob
    The worst that can happen is you will fail.
    But at least you tried.



  5. #4
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    Hi Guys,
    Thanks for the comments. I'm having difficulty getting the supplier to talk to me about supplying replacement gears. You know the old not here, send Email describing problem, run around.

    Anyway that aside I do have access to a firm that makes gears ! and they have suitable replacements, in steel and bronze, in stock. OK I'm going to have to do a little massaging because the bores are unfinished but subject to their checking my measurements I'll have replacements tomorrow. Not expensive either so I'll know more later.

    I'm still interested in any and all comments about my initial post.

    Thanks Guys:
    Best Regards:
    BaronJ.

  6. #5
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    John

    If those gears are a pair (meshing together) I would suggest one in steel and one in bronze unless the steel is hardened, to ease wear. I believe that dissimiliar metals cause less wear.

    Ken

  7. #6
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    Hi Guys,

    I've now got three of the four replacement gears. The last one is being posted out to me. So I have taken photos of the two main ones that make up the dual gear. These gears are in steel and are 1 mod with 42 and 62 teeth. One of the pictures shows how they will go when in place in the machine.
    Since they need machining to finish the bores, I've some work to do. I've also included a picture of the machine just for reference.

    Here is what I propose to do. Since the original dual gear slides on a keyed hardened steel shaft, I have obtained a short length of brass bar 25 mm diameter and 30 mm long. The hardened steel fork that is used to move the gear from high to low speed is 5 mm thick and runs in the 6.5 mm gap between them requires a 25 mm diameter surface. This works out perfectly for the 25 mm diameter brass bar.

    I will turn down the ends of the brass bar to say 23 mm diameter with a 6.5 mm land in between and bore 23 mm diameter in each of the gears so that they are a good press fit onto the brass bar. I need to bore the brass bar so that it will slide freely on the hardened steel shaft and cut a 5 mm keyway so it cannot rotate independently of it. If I have got all this right then I should have a replacement gear that fits and runs properly.

    Ken, mentioned using dissimilar metals to reduce wear on the teeth. Neither of the gears purchased nor the original spindle gears are hardened ! The advice from the manufacturer of the new gears is that it won't make any difference to the wear pattern and over time and that the gears will eventually bed in to each other, though it may take many years... Or a few weeks if they are not kept lubricated properly.

    I could have had bronze gears from a wear point of view but those gears were more than twice the price even though the lubrication aspect would not be as severe but would still be necessary. So I chose the cheaper option.
    Attached Images Attached Images
    Best Regards:
    BaronJ.

  8. #7
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    John,

    sounds good. Your gears with some lubrication should last lots longer than the original crap. I am over chinese made machinery. Usually just cheaply made rubbish that has to be rebuilt properly. I now look for good old heavy cast iron machines and restore. Oh; and I do tend to over engineer.

    Ken

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    Quote Originally Posted by Toggy View Post
    John,

    sounds good. Your gears with some lubrication should last lots longer than the original crap. I am over Chinese made machinery. Usually just cheaply made rubbish that has to be rebuilt properly. I now look for good old heavy cast iron machines and restore. Oh; and I do tend to over engineer.

    Ken
    Hi Ken,

    I would hope they do. I expect to have good use of this mill for a few years yet ! Even if I have to completely re-manufacturer it.
    Best Regards:
    BaronJ.

  10. #9
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    Hi Guys,
    Next instalment or the work I managed today...

    The photographs show the work carried out in turning and boring the brass bushing, the small gear that will be pressed onto it and the plug gauge that I made in order to make sure I got it right. There are also pictures of the gear being bored out on the lathe and the soft jaws used to make certain that I didn't damage the teeth on the new gear and that the bore was concentric with the teeth. One of the pictures shows how the original gears were set up in the machine and the shaft with the key that the dual gear slides up and down on.

    I'm sure that I will have forgotten to mention various things. So all comments welcome.
    PS. Sorry about camera shake on a couple of pics.
    Attached Images Attached Images
    Best Regards:
    BaronJ.

  11. #10
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    Hi John,

    Nice work with the gears. Thye will last longer than the rest of the machine!

    WRT the soft jaws, did you make them purpose built for the job? I assume it's not a co-incidence that the radius of the jaws is exactly that of the gears?

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  12. #11
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    Quote Originally Posted by simonl View Post
    Hi John,

    Nice work with the gears. They will last longer than the rest of the machine!

    WRT the soft jaws, did you make them purpose built for the job? I assume it's not a co-incidence that the radius of the jaws is exactly that of the gears?

    Simon
    Hi Simon,
    Thanks. See today's post.

    Those soft jaws were made just for this job. Oddly enough they were made from an old aluminium belt drive gear that I had kicking about. Those teeth on the outside have nothing to do with using slices from the gear as soft jaws. The hole is conveniently 16 mm in diameter with a 2 BA grub screw. The grub screws were just sawn off from a 50 mm long bolt with a hacksaw cut across the end. They are very easy to make and throw away when finished with.

    As a point of interest the large gear won't go into the 4" chuck. Before anybody says anything, I don't know where the other set of jaws is, they disappeared a very long time ago. So I have four ways around this problem !

    1/ Use a larger chuck. I do have a 5" one but I will have to make soft jaws for it. I don't want to mare the teeth of my new gears and I do want the bore to be as concentric as it possibly can be.

    2/ Use the faceplate and clamp the gear to it. OK I will have to clock it and make sure that it is running true.

    3/ Same issue with clocking if I use the four jaw chuck. (Preferred option) Use soft packings under the jaws to protect the teeth.

    4/ Clamp a sacrificial aluminium plate to the faceplate and turn a recess into it to take the gear as a press fit then clamp the gear down so it won't move.

    I think I've got all options covered.
    Best Regards:
    BaronJ.

  13. #12
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    Thumbs up

    Hi Guys,
    More got done today. Finished turning the brass bushing and got the small gear pressed onto it. I warmed up the gear in a pan of boiling water. Still had to press it on but it didn't take much effort. Skimmed the very slight protrusion off the brass bush where it came through the other side. Put the gear back into the chuck and checked the bore for being true. I surprised myself. Not a flicker on a .0005" test gauge. Then I turned .050 of the diameter ready for the gear on the other end.

    (See previous post)

    There are photographs of the other two gears that I am going to use to replace the two primary drive gears. One goes on the end of the original shaft that drives the dual gear discussed earlier. The other is to replace the bronze gear on the motor shaft. As you can see there are straight toothed gears and not helical as the original ones. The 7.5 degree helix tends to pull the motor shaft down when running in the normal direction and pushes it up when in reverse.
    So using straight gears won't produce this effect and should make no or very little difference to the smooth running.

    These two gears are also stainless steel ! That wasn't intentional. Simply that these gears had the right teeth numbers and the correct PCD. Whilst a little more expensive, not as much as bronze.
    Attached Images Attached Images
    Best Regards:
    BaronJ.

  14. #13
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    Quote Originally Posted by BaronJ View Post
    As you can see there are straight toothed gears and not helical as the original ones. The 7.5 degree helix tends to pull the motor shaft down when running in the normal direction and pushes it up when in reverse.
    So using straight gears won't produce this effect and should make no or very little difference to the smooth running.
    You'll have to see but it may. With a properly specificed helix angle there is always a tooth in contact with a tooth of the other gear. This makes things quieter (no micro-bang as individual teeth contact initially - gears need clearance) and the transmission a little smoother. It may not be noticeable but going from plastic helical gears to metal straight cut I suspect that you will find things a little noiser.

    Michael

  15. #14
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    Quote Originally Posted by Michael G View Post
    It may not be noticeable but going from plastic helical gears to metal straight cut I suspect that you will find things a little noiser.
    Michael
    Well if that's the case, welcome to the (noisy geared head mill) club, John!

    They are looking good. The soft jaw idea worked well! I would have been a little lazier and just used a 4 jaw with copper sheet in-between each jaw.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  16. #15
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    Quote Originally Posted by simonl View Post
    Well if that's the case, welcome to the (noisy geared head mill) club, John!

    They are looking good. The soft jaw idea worked well! I would have been a little lazier and just used a 4 jaw with copper sheet in-between each jaw.

    Simon
    Hi Simon, Guys,

    Yes I do believe that the mill will be a little noisier. But that was the excuse I was given for the use of plastic gears.
    Michael. Thanks for the information about helical gears. That is something that I didn't know about. However all is not lost because if push comes to shove I can go back to using a plastic helical gear and the original bronze one. That of course assumes that I can actually get one !

    Simon, I'm going for option three with the big gear.

    More in the next post.
    Best Regards:
    BaronJ.

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