Thread: Mill Gears !

With respect to measuring the bend in the elevating screw, can you put it between centres in the lathe? Then you can indicate it if needed (use a bit of shim stock to ride the thread), or at least get a better eyeball on where it's tweaked.

As far as measuring the gear clearance goes, I think Steves idea of measuring the float is likely to be the easiest way. I assume the shaft with the bronze gear is pulled up 'tight' by a thrust bearing behind the handwheel, so removing the hand wheel should allow you to push and pull the shaft in and out, and the amount of float can then be measured via a dial indicator. Make your new spacer thicker by slightly less than that measurement. If not, as he said you'll have to temporarily chuck a known too thick spacer behind the gear and measure the gap between the housing and the column, and then replace your temporary spacer with one that is thinner than it by slightly more than the measurement of the gap.

As far as repositioning the steel gear - that will probably need to be determined by looking up the gap in the column (possibly a small mirror on a stick will help?), and confirming that the backs (the larger diameter) of the gears align correctly at the contact point. In theory it shouldn't need adjusting, but that assumes it was set up correctly in the first place - never guaranteed with the Chinese. The wear on the bronze gear should have no effect on the required height for the steel gear, the contact faces should be much deeper in the tooth profile than that anyway (I think). If I'm thinking about it correctly, the tips of the teeth should never bottom out in the root of the mating gear, as the flanks (?) should be in contact before then, and thus the wear to the tips also shouldn't have any great effect on operation (for this application, high speed/load applications might be a different story) once correctly setup. This check and any adjustment desired should however be done before measuring the endfloat of the bronze gear, as moving the steel gear up or down will change the required spacer for the bronze gear.

Following on from the above thoughts on the gear teeth mesh, in THEORY I think you could actually leave the remaining 'extra' bits of tooth in place, as the tooth profile of gears should mean that they actually have no effect on the correct mesh. In reality though, I'm sure there is some burring at the transition between worn and unworn that needs taking care of. Combine that with the question of whether the Chinese have actually got the tooth profile spot on, and removing the remaining bits and light deburring is probably the best course of action, to prevent a situation whereby you're running on those tiny remnant tips.

John do you have end float adjustment on both ends of the long shaft, if so you would be able to adjust the gear mesh.

Hi J&H, Thanks for your post !

Originally Posted by Jekyll and Hyde

With respect to measuring the bend in the elevating screw, can you put it between centres in the lathe? Then you can indicate it if needed (use a bit of shim stock to ride the thread), or at least get a better eyeball on where it's tweaked.

Yes that is how I got to measure the existing bend. I've since pressed the bend out, or at least as much as I can without damaging the thread. The steel that its made from is surprisingly soft. No spring in it at all.

As far as measuring the gear clearance goes, I think Steves idea of measuring the float is likely to be the easiest way. I assume the shaft with the bronze gear is pulled up 'tight' by a thrust bearing behind the handwheel, so removing the hand wheel should allow you to push and pull the shaft in and out, and the amount of float can then be measured via a dial indicator. Make your new spacer thicker by slightly less than that measurement. If not, as he said you'll have to temporarily chuck a known too thick spacer behind the gear and measure the gap between the housing and the column, and then replace your temporary spacer with one that is thinner than it by slightly more than the measurement of the gap.

The hand wheel shaft has two ball races in it pressed into a cavity with a small flange on the shaft between them, there is a plate on the outside holding them in place. However the brass gear can move back and forth on the shaft. A key stops it slipping round.

As far as repositioning the steel gear - that will probably need to be determined by looking up the gap in the column (possibly a small mirror on a stick will help?), and confirming that the backs (the larger diameter) of the gears align correctly at the contact point. In theory it shouldn't need adjusting, but that assumes it was set up correctly in the first place - never guaranteed with the Chinese. The wear on the bronze gear should have no effect on the required height for the steel gear, the contact faces should be much deeper in the tooth profile than that anyway (I think). If I'm thinking about it correctly, the tips of the teeth should never bottom out in the root of the mating gear, as the flanks (?) should be in contact before then, and thus the wear to the tips also shouldn't have any great effect on operation (for this application, high speed/load applications might be a different story) once correctly setup. This check and any adjustment desired should however be done before measuring the end float of the bronze gear, as moving the steel gear up or down will change the required spacer for the bronze gear.

The only way to adjust the height of the steel gear is to remove some material from the end of the long steel sleeve or add shims. That whole assembly is hung on a 15203 thrust bearing with those two lock nuts right at the top.

Following on from the above thoughts on the gear teeth mesh, in THEORY I think you could actually leave the remaining 'extra' bits of tooth in place, as the tooth profile of gears should mean that they actually have no effect on the correct mesh. In reality though, I'm sure there is some burring at the transition between worn and unworn that needs taking care of. Combine that with the question of whether the Chinese have actually got the tooth profile spot on, and removing the remaining bits and light deburring is probably the best course of action, to prevent a situation whereby you're running on those tiny remnant tips.

I did very carefully turn off the bits of damage being very careful not to touch the top of the remaining teeth. I would have thought that the brass would have been bronze. Its soft like brass and files easily, so there was no problem cleaning up the edges.

Pushing the brass gear forward on the shaft so that when placed in position and the brass gear pushed back, I could measure the amount of width the spacer needs to be. The new spacer need to be just over 4 mm longer than the original one. Far further than I would have thought. The gear is only 20 mm thick and a new spacer would only leave 14.5 mm to support the gear. The thought that this would risk bending the shaft that the brass gear rides on occurs to me. The steel that this machine seems to have appears to be very soft and easily marks.

Anyway time to machine a new spacer.

Thanks Guys.

Originally Posted by BaronJ

I would have thought that the brass would have been bronze. Its soft like brass and files easily, so there was no problem cleaning up the edges.

If it's a designed failure point, as I think it is, bronze would probably be too tough to fail before anything else does.

Our small Colchester lathe has a brass key on the first gear off the motor and all our Mikron hobbing and threadmilling machines have a resin/fibre gear in the same place for that reason.

Originally Posted by BaronJ

The new spacer need to be just over 4 mm longer than the original one. Far further than I would have thought.

If the depth of wear on the tooth was 2mm, then a 4mm larger spacer sounds about the right ballpark to me by looking at the depth of the teeth compared to the wear. After all, the tips of the steel gear should be relatively close to bottoming out in the root of the teeth on the brass gear when properly meshed - which obviously was nowhere near the case from the factory, hence the reason you are where you are. Also hence my acquired distaste for cheap Chinese machine tools - I know they serve a purpose, especially for those who are short on space, and they often work just fine, but when you start digging into them to fix little niggling annoyances or faults you realise just how poorly they're put together in some places - even the simplest things are often done wrong or not at all.

I don't think the shaft that carries the brass gear is likely to bend, the load we're talking about shouldn't be that great given the gearing created by the elevating screw and the overall weight of the thing. But you can always make a sketch, and machine a replacement out of better material once you're operational again (or of course before if you don't need to do any milling!)

Hi Guys,

Thank you all, there is some very good advice given !

I've spent most of the day trying to get this pair of gears to line up properly. I've discovered that the thrust bearing at the very top of the vertical leadscrew was fouling the shaft and it took me a little while to discover why.

The thrust bearing has one plate with a hole 1 mm smaller in diameter that the other ! The one with the number 51203 on it. What was happening when I was trying to get the teeth on the two gears level the shaft was binding in the bottom bearing plate, so when I thought that it was at the right height, it stuck in a slightly different place.

I ended up setting up the lathe soft jaws and turning them so that I could hold the 3 mm thick thrust plate and then using the Dremal to grind away the inner surface. I made both plates to the same bore size of 17 mm, which gives me half a mm of clearance.

So I have had to start all over again and turn another spacer this time only 2.5 mm longer than the original one.

Success ! The teeth now match for height and mesh nicely without binding. By the way thanks for the dental mirror suggestion, worked a treat.

Now to make a new washer and a wrench for the nuts at the top of the leadscrew. The nuts now don't pull the leadscrew up to the top without a 1.5 mm thick washer. I used a temporary 4 mm thick spacer whilst setting up the gears.

More to come. Thanks Guys.

Originally Posted by BaronJ

The thrust bearing has one plate with a hole 1 mm smaller in diameter that the other ! The one with the number 51203 on it.

That may (or may not) actually have been intentional. I've run into that on the Graziano, there are a few thrust bearings with an enlarged bore on one race (I'll probably put something about those when I get around to the next update). On the Graziano they tend to have obscure part numbers on them which don't match anything, but by measuring the OD, ID and stack thickness you can match up a generic thrust bearing from a catalogue and modify it by boring out that ID. However Bridgeport back in the very early days were known to use off the shelf bearings for the spindle, but grind inner or outer races to get the desired preload - and not mark them in any way as being altered. Ordering the bearing number stamped on the side as a replacement would not give the expected result... Wouldn't surprise me if the Chinese did the same thing, used the easily available bearing and just modified some as needed rather than speccing a whole unique run.

The reason the Graziano has them is to provide clearance for that race against a rotating part. For example, the feed stop clutch uses a reasonably large one. The 'unmodified' race (marked S.B. for small bore) is a press fit into the carrier which is always rotating as it's on the output of the quick change box. This press fit is actually only the ID from memory, think the OD clears, but I'll have to check when I get to reassembling it. Then obviously the ball race, and then the 'modified' race (marked L.B for large bore) goes into place. The reason the bore needs to be larger on the second race is that it's also sitting over the same shaft as the first one, but it needs to be able to spin independantly of it when the feed stop gets tripped. From memory the difference in bore diameter is something over 0.5mm in this case.

Co-incidentally, one of the pair thrust bearings in the back of the cross slide screw (where it becomes a straight shaft inside the taper turning snout) is actually missing a race from someone failing to put all the bits back in at some point, and the generic replacement I require as a base is.... a 51203. In this case, I assemble the small bore race onto the cross slide screw, where it sits up against a spacer behind the actual thread of the cross slide screw. The way it's assembled means that this race needs to turn with the screw, so that's where the small bore goes, then the large bore goes inside the sliding collar inside the snout. The screw is then free to rotate inside the large bore race which will always be stationary. It has previously been put together by someone who hasn't thought about it (as well as left a race out ), and there is a bit of a groove worn into the shaft where they had the 'small bore' as the stationary race with the screw rotating inside it.

I think someone has previously remade the cross slide screw for a couple of reasons, but I actually had to lightly hand polish a tiny bit of taper out of the shaft in order to get that small bore bearing to go home without needing a heavy persuasion stick (not an option here, as it needs to be assembled, and more importantly disassembled in place). It did go home before that when experimenting in the vice, but there was no way I was going to get it apart at any time in the future if needed when it was assembled on the machine in that manner. Might have been part of the reason one race is missing I guess - easier to throw it over the shoulder than actually fix the problem.

Hi J&H, Guys,

Interesting about the thrust bearing, 51203 is the same as the one on my machine. I've not been able to find any info about it, probably just a Chinese number. It is surprising how much difference it made opening up the bore on the tight plate when setting up the gears, obviously sticking in a slightly different place each time I tried to set it up.

I temporarily fitted the vertical slide so that I could test and check the rise and fall using the leadscrew and gears. Just a bit lumpy suggesting that they might be a bit tight or its just the rough bits left on the teeth. But it certainly moves up and down far more easily than it did originally from new. Mind you cleaning all that toffee off the gears and threads and using new lubrication helps.

Anyway some more pictures.

17-02-2021-001.JPG 17-02-2021-002.jpg

These two pictures are the gear and the new spacer that I made. You can see that I had to put a groove in it to clear the key. It also gives you an idea of how much I've had to move the gear forwards. In the second picture you can see the machining where I took the damaged tops off the teeth.

17-02-2021-004.JPG 17-02-2021-005.jpg 17-02-2021-003.jpg

These three pictures show the sleeve that I made to support the DRO readout head. Using an M8 bolt and acorn nut to fit into an 10.25 mm hole, at least now the arm is well supported and straight. As it should be. The sleeve fits into the bracket and passes right through the arm and into the bottom, so that it is supported all the way through.

One of the things that I need to find is three M10 bolts that I can modify to do the three bolt conversion for the pivoting head assembly. At the moment there are only two bolts that secure the head. The conversion gives a more secure support to the head unit and better locking if you want to use it at an angle.

Thanks Guys.

Originally Posted by BaronJ

... 51203 is the same as the one on my machine. I've not been able to find any info about it,...

https://www.skf.com/group/products/r...roductid-51203

Michael

(if the dimensions are the same, might be worth swapping the bearing out for something brand name, just in case that bearing is damaged or from the low quality pile)

John with a thrust brg one of the brg shims should be a sliding fit on the shaft and have clearance at the outer dia, the other side of the thrust brg needs to have clearance from the shaft and have a sliding fit on the OD, this way the shims are concentric to the axis and cant rub.
I asked earlier whether there is adjustment at the bottom end of the long shaft, no answer, i think that is where the gear mesh should be adjusted from.

Originally Posted by shedhappens

John with a thrust brg one of the brg shims should be a sliding fit on the shaft and have clearance at the outer dia, the other side of the thrust brg needs to have clearance from the shaft and have a sliding fit on the OD, this way the shims are concentric to the axis and cant rub.
I asked earlier whether there is adjustment at the bottom end of the long shaft, no answer, i think that is where the gear mesh should be adjusted from.

Hi John,

Thanks for the reminder, I saw the question.

Sorry about the omission ! The leadscrew just hangs from the top There isn't anything supporting it at the bottom. Also the threaded block that carries the milling head is just a round pin with a square block slid over it. The block is bolted to the vertical slide. So the leadscrew just floats, supported by the thrust bearing.

Originally Posted by Michael G

https://www.skf.com/group/products/r...roductid-51203

Michael

(if the dimensions are the same, might be worth swapping the bearing out for something brand name, just in case that bearing is damaged or from the low quality pile)

Hi Michael,

Thank you for the link, most interesting ! Particularly the dimensions.
I've noted the link address.

The SKF data shows that the inner diameter is 17 mm and 19 mm. But mine was 16.0 and 17 mm. Also the over all diameter was 34 mm in a 35 mm bore.

That could cause fun buying a replacement. Other than those slight differences the rest of the dimensions match.

Originally Posted by BaronJ

So the leadscrew just floats, supported by the thrust bearing.

Hi John, so the adjustment at the top adjusts the gear mesh...

Originally Posted by shedhappens

Hi John, so the adjustment at the top adjusts the gear mesh...

Hi John,

Yes if you can call it adjustment !

There is a brass ring immediately under the top plate and a steel sleeve below that that sits on top of the steel gear. In order to adjust the height of the gear you have to turn off a few thou from the sleeve or add shims.

Once I had got the thrust bearing sorted it ended up at the right height.

At the moment I'm fitting the vertical scale on the side of the column for the DRO.