Thread: Mill Gears !

Hi Guys,

I've been fitting the DRO parts to the mill and doing some maintenance work that should have been done a long time ago.

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I got the bracket fitted to the side of the column for the DRO read head arm. Once I had decided on a position it was just a matter of drilling and threading a couple of holes. I had to drill another hole in the bracket so that I didn't hit the outside edge of the column. I used M6 cap screws to secure it, they didn't provide any screws so I had to use my own. All the ones supplied were black japaned finish mine are all zinc plated ones. They did provide an M8 bolt but the pivot hole in the bracket is for an M10 bolt, so you end up with a 10 degree slope on the readout head arm.

Now onto one of the issues that I need to fix !

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These pictures are of the geared drive to the mill head. Some time ago it started skipping teeth on the gears. One gear is steel and the other is brass. I've given them a clean but the grease on them and the threads was like thick brown treacle, no wonder that the head drive was hard to turn. I've also discovered that the leadscrew that lifts the head has a 3 mm bend in it. Which doesn't help either.

There aren’t any spares for the brass gear so I am hoping that some kind person can come up with some suggestions as to how I can sort this problem out. I can sort the bent leadscrew but I'm at a loss with the brass gear short of getting a new one.

I propose doing some other modifications as part of this maintenance work.

Thanks Guys.

G'day John
The thought that springs to mind is to get a bead of bronze weld put on the top of the teeth (where the wear is) so that you can file it back to profile.

I can't quite imagine how this is set up. Is the lead screw stationary and the nut rotates, or is the nut stationary and the screw rotates? Either way, I'm guessing that the steel gear goes on a sleeve that sits 'somewhere'. Is it possible to extend that sleeve and put a bearing or two either side of the gear on the sleeve? Just something to stop the screw deflecting away from the brass gear.

Michael

(Reading your post again, I think I'm way more confused than I thought I was in the first place. Feel free to ignore what needs to be ignored)

AliExpress has a good range of bevel gears at reasonable prices, I've bought 3 sets so far.

If you count the teeth and measure the outside you may well find an exact match, if not buy a new pair as most sellers give center to center distances.
Some also offer machining of the bore, keyway etc.

No idea of the size of yours but here is an example.
The postage was $19, so around $30 all up

Fill and file or try to find a cheap replacement pair; it's a simple 1:1 mitre, so the pitch/number of teeth won't matter as long as they fit. Try to keep it brass as it's likely designed as a known failure point should something cause the head to jam (but you probably knew that already).

The fact that it's skipping when so little of the tooth is missing might be an indication that they're not meshing properly in the first place. Once you have fresh parts, it might be worth coating one in layout die and giving it a spin by hand to see the contact pattern and making up some spacers to go behind them if needed to make sure you have full depth contact (with a little bit of backlash of course).

Originally Posted by elanjacobs

The fact that it's skipping when so little of the tooth is missing might be an indication that they're not meshing properly in the first place.

My thoughts exactly - doesn't look like anywhere near enough missing to cause them to slip in a proper setup. I would have thought the engagement depth should be at least 75% at an absolute bare minimum in the crappiest setup, although I haven't looked in the bible.

I assume that the 3mm bend in the elevating screw can't cause this, as I'd imagine the part where the gear goes is located by a sleeve and bush, and shouldn't be able to move side to side - unless it's a typical Chinese 'precision' fit, in which case that needs addressing.

My guess is that the brass gear needs to be spaced out a bit? Either by a turned spacer behind it, or if it's retained by a grubscrew just slide it out a bit. If you can do this and get the mesh nice and close, another slightly dodgy option may be to file off the remaining original height portions of the brass teeth and just set it up to mesh as close as you can get away with. To my mind, it shouldn't be loading that point of the gear significantly anyway, the main contact should be deeper into the tooth profile - but I'm often wrong...

Originally Posted by Michael G

G'day John
The thought that springs to mind is to get a bead of bronze weld put on the top of the teeth (where the wear is) so that you can file it back to profile.

I can't quite imagine how this is set up. Is the lead screw stationary and the nut rotates, or is the nut stationary and the screw rotates? Either way, I'm guessing that the steel gear goes on a sleeve that sits 'somewhere'. Is it possible to extend that sleeve and put a bearing or two either side of the gear on the sleeve? Just something to stop the screw deflecting away from the brass gear.

Michael

(Reading your post again, I think I'm way more confused than I thought I was in the first place. Feel free to ignore what needs to be ignored)

Hi Michael,

I'll take some more pictures this morning !

Originally Posted by elanjacobs

Fill and file or try to find a cheap replacement pair; it's a simple 1:1 mitre, so the pitch/number of teeth won't matter as long as they fit. Try to keep it brass as it's likely designed as a known failure point should something cause the head to jam (but you probably knew that already).

The fact that it's skipping when so little of the tooth is missing might be an indication that they're not meshing properly in the first place. Once you have fresh parts, it might be worth coating one in layout die and giving it a spin by hand to see the contact pattern and making up some spacers to go behind them if needed to make sure you have full depth contact (with a little bit of backlash of course).

Bingo ! Thanks Elan. That thought hadn't occurred to me. Yes they do have what appear to be loose sleeves behind them. Not having any grub screws in either did make me wonder why.

Originally Posted by Jekyll and Hyde

My thoughts exactly - doesn't look like anywhere near enough missing to cause them to slip in a proper setup. I would have thought the engagement depth should be at least 75% at an absolute bare minimum in the crappiest setup, although I haven't looked in the bible.

I assume that the 3mm bend in the elevating screw can't cause this, as I'd imagine the part where the gear goes is located by a sleeve and bush, and shouldn't be able to move side to side - unless it's a typical Chinese 'precision' fit, in which case that needs addressing.

My guess is that the brass gear needs to be spaced out a bit? Either by a turned spacer behind it, or if it's retained by a grubscrew just slide it out a bit. If you can do this and get the mesh nice and close, another slightly dodgy option may be to file off the remaining original height portions of the brass teeth and just set it up to mesh as close as you can get away with. To my mind, it shouldn't be loading that point of the gear significantly anyway, the main contact should be deeper into the tooth profile - but I'm often wrong...

Hi J&H, Yes I agree with your comments !

The bend in the leadscrew causes loosening and tightening when the milling head is wound up and down. The leadscrew is hung on a thrust race so the only play there is between the top of the gear and a retaining nut.

I've just posted to Elan that there appears to be collars behind the gears. I can very quickly adjust or make new ones. Thinking about it I suspect that at the factory it was a case of assemble it and if it runs, it works, no attempt to set the gears up properly.

I'm going to take some more pictures ! Though I must admit that I was a bit flustered last night.

Just a note on meshing bevel gears, they MUST be properly aligned with each other or you just end up with even more damage. Here's one I prepared earlier...

This is a properly meshing pair, note how the back bevel (AKA back cone) on both matches up evenly and there is even contact across the teeth

PXL_20210216_082549557.jpgPXL_20210216_082658649.jpg


This is what happens when they're not aligned (exaggerated due to the large mismatch relative to the face length, but you get the idea). As you can see, there's massive gaps between the teeth and all you have is point contact because you're basically trying to mesh the fat end of the teeth on one with the skinny end of the teeth on the other; it don't work

PXL_20210216_082719161.jpgPXL_20210216_082846756.jpg

What you need to know is the mounting distance (which you should be able to measure fairly accurately by setting them up on a couple of pins to mimic how they should be in the mill), then make your spacers to suit.

miter-gear-drawing.jpg

Dang..I was too late to offer some wisdom....lol

Hi Elan, Guys,

Thank you for all the information, its very helpful. Here are the pictures I promised...

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These pictures show the arrangement of the lifting mechanism at the top of the mill column. That hole on the side is a close fit on the spigot. There is a ball race in that housing, so the amount of play there is nil.


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These pictures show the arrangement of the two bevel gears inside the top of the column, along with the top plate and securing nuts. I pulled the brass gear off the shaft and found the little spacer, seen just in front of the gear. That brass gear is 41 mm diameter and 20 mm thick. The steel gear is 45 mm diameter and 25 mm thick.


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These pictures show the block and pin that is bolted to the mill head and used to lift the head.

It is very difficult to see into the cavity where the gears are when the parts are in place. I estimate that I need to move the brass gear towards the steel one by about 2 mm and maybe lower the steel gear by about the same. I'm going to turn off the tops of the damage on the brass gear on the basis that there is no point in getting them to mesh properly otherwise.

I've straightened the shaft as best I can, there is only about a half to one mm of bend there now, but I suspect the bit at the top has a bend as well. But it is how to measure it and if so straightening it.

Thank you all for your advice.

Originally Posted by Sterob

Dang..I was too late to offer some wisdom....lol

Hi Steven,

Its never to late to give good council !

One way of measuring clearance inside an assembly is to add a packer or shim of known thickness that will result in negative clearance. In the case of a flange plate with a gear on it that negative clearance will show up as a gap between the housing and flange. Measure that, subtract from the spacer known thickness and you have the clearance you want to measure.
Clear as mud

If it’s possible to remove whatever is on the outer end of the brass gear shaft then you should be able to move the shaft in/out and measure the play.

Steve

Hi Steve,

Thank you for your post.

If I turn off the damaged edges of the brass gear will that prevent me from measuring the gap produced and should I drop the height of the steel gear by the amount of damage. In this case about 2 mm. ?

Thanks: