Senior S Type Vert' mill repair

Having watched Damian Noble's thread re-building his newly acquired Tom Senior mill, I have been spurred into action to tackle a project I've been ignoring for several years. The pulley driving the quill is supported by a plain bronze bush, and the steel inner drive piece has worn where it runs in this bush. I had diagnosed this as the cause of an annoying rattle/knock coming from the head - but more of this later! The bush inner diameter was scored, but still true to size, but the steel was worn as shown below.

So I decided to go for needle roller bearings, as there was not enough meat in the casting to accommodate ball races. I toyed with making like for like replacements, but using needle roller bearings was actually easier as it meant I could keep the worn inner, and not have to remake this bit with its captive key.

So onto a faceplate with the top casting, and bore the bearing recess to 1.624ins to take two standard needle rollers. There was room for a 0.250 thick spacer between the two bearings. I liked the idea of two bearings, as the pulley runs above the bearings as a cantilever. The change wheels are there as a token counterbalance.

Here's the top casting with the rollers in place, and the inner spindle ready to instal. The PTFE washer is a thrust bearing just to separate the alloy drive pulley from contacting the alloy housing, as the needle rollers have no axial location.

But this didn't solve the nasty knock in the drive. So I decided (with a deferential nod to John Stevenson) to repair the drive end of the motor spindle as the pulley was running about 20 thou' out of true, thanks to the damage of the grub screw coming loose and chewing the end of the shaft. The picture of the damage to the motor shaft is out of focus, so just imagine a 14 mm dia stub with a 5 mm keyway, chewed by a dog point grubscrew which was the location, fastener and drive key. Cheap and cheerful until it came loose! So here's the repaired and lengthened motor drive shaft, with a blind key way cut in it.

A matching keyway in the cone pulley, throw it back together, nice and true BUT IT STILL KNOCKS!!!!!

Stripped the motor again, to find a very small mark on the outer of the rotor, and a corresponding excresence of something hard and nasty that doesn't belong in the stator slots. Clean it out, all back together and runs as sweet as a nut.

So the interesting debate for me is, having agonised over modifying the internals of such a well designed machine, I've eventually decided to alter its design. In the eyes of many I'm sure this is heresy, and repairs should be carried out with as close attention to keeping it original as possible.

Comments anyone?

Nice repair job. Sod originality...

Nice repair job. Sod originality...

Looks like some improvements, perhaps. I've not read (or watched?) The thread you mention.

Seems some were unnecessary, mind!

What took my immediate attention, right at the end was how you missed the frequency of this 'knock' remaining the same, wherever the belt was fitted on the variable speed pulleys!

Time may tell, whether the needle bearings are as good as the original design. I would only make a modification if there was a known design weakness with a machine, obsolescence, or clear improvement in technology. I reckon those that design these things are far more clever, than I, at that.

It's an old thread but I was wondering how the needle bearings are holding up Simon?

The newer tom senior S type heads had the needle rollers in from the factory and I was thinking of modifying mine as the bronze bush needs oiling quite frequently in use.

The design puts radial load on the drive sleeve/bush when the belt is tensioned. Combined with the pulley running ontop of the bronze bush the friction is quite high especially if lacking oil. Another small problem is oil eventually makes it's way up into the belt housing which needs cleaning occasionally.

I notice you fitted a ptfe thrust washer but I have seen these which may be a different option which seem to incorporate axial thrust Needle roller. Although the axial thrust is minimal being only the weight of the drive pulley that's attached to the sleeve.

Thanks for the mention at the start of the thread btw.

The museum's light vertical has the needle rollers fitted, I removed them to be able to clean them better, they were not too tight a fit. There is a bronze washer at the top underneath the pulley, there was little wear on it as the weight of the shaft and pulley is not much. The bearings have been greased with common lithium yellow grease, as is the bronze thrust washer. The mill dates back to the 60's.

Hi Old Mart, I recently read the thread concerning this which was interesting.

I think I would need to fit needle roller inner sleeve's as the original drive sleeve like Simon's above has some scoring and wear. It drives the spindle with the single key slot.

Did you take any measurements of the bearings whilst out?

Thanks

Damian

I might have recorded the needle roller bearing numbers somewhere, but I don't think it will help. This particular LV has a splined, not keyed spindle. Looking at the amount of metal surrounding the needle races, I would think that there was enough meat there to find some ball races. Simply Bearings have an easy to use bearing size finder, which I used to find the most suitable bottom spindle bearing for the MT2 to R8 conversion.

**LINK**

Hi chaps, from memory I bought two needle rollers with their external sleeves OD 1.625 inches. They were 0.625 ins long each and were a perfectly standard catalogue item at my local bearing factors.

I also bought two matching hardened inner sleeves, the internal diameter of which was (from memory) 1.000 inch. I chose these because that was the outside diameter of the driver sleeve - with the key way up its middle.

All of these proprietary parts came from my local bearing stockist, but they had to be imperial dimensions because the starting point is that the hardened inner sleeve must be a snug fit over the original (damaged) drive sleeve.

I then made a short spacer 1 inch ID by approx 1.3 OD and 0.25 thick which went between the two bearing inner sleeves to space them vertically, and a PTFE washer which was about 2 inches OD which went over the outside of the needle roller outer below the pulley to make sure the pulley and casing couldn't contact each other. This PTFE washer was about 2 mm thick, it had to be slim enough that the bearings were left with some endfloat when the driven pulley was correctly positioned on the drive sleeve. I know PTFE is self lubricating, but I greased this liberally on assembly. I can't remember what ID this washer was, but it was to clear the OD of the inner bearing hardened sleeve.

Now I bored out the original milling machine top casting to accept the OD of the bearing assembly - I wanted it to be a nice snug fit so bored it out to 1.624 as close as I could get it. This stage is shown in the pictures above.

Now I assembled the bits, bearing (no pun intended) in mind that the driven pulley is an interference fit over the drive sleeve and needed heat to expand the bore. The two hardened inner bearing sleeves were loctited onto the damaged outer of the driven sleeve (with the keyway up the middle) with the 0.250 thick spacer between them.

Pop the casting and driven pulley back on top of the milling machine head, and replace the motor, drive belt and anything else I've forgotton, switch on and admire.

I rather fancy that the factory version of this as shown above uses a double row needle roller bearing, but I used two separate bearings with a spacer to get the same result. I didn't know that there was such a thing as a proper factory fitted bearing, I was making it up as I went along.

HTH, hopefully I kept some sketches of the bits I made in my notebook (and I can find them) so if they would help I go looking for them. It's all pretty obvious once you've got the bearings in your hand.

My modification is still going strong 3 years later, it's done quite a bit of work in the interim though I don't use it every day. I'm reluctant to take it to bits to see if it's holding up - it's not making any inappropriate noises so I've left well alone. I'm a sucker for taking this to bits to see how they work - and a devil for not quite getting round to putting them back together.

Best rgds Simon

Edit - The ID of the PTFE washer must have been a shade over 1.00 diameter as it only needs to clear the outside of the original driven sleeve, not the OD of the inner bearing as described above.  My apologies.

Edit 2 - There is a circlip in the bore of the machined top casting to hold the bearing outers down so they can't "walk" upwards under load, and there must have been another 0.25 inch spacer in this bore between the two outer bearings, though I can't for the life of me remember making or fitting it.  I hope I did!

Edited By Simon Williams 3 on 23/01/2020 19:36:25

Edited By Simon Williams 3 on 23/01/2020 19:39:37

Edited By Simon Williams 3 on 23/01/2020 19:41:39

I have just thought of a possible problem with needle roller inner sleeves.

1. Getting sleeves that slide over the unworn part where the pulley fits. If you could, then one of the Loctite sleeve and bearing retainers would hold them.

2. If the OD of the spindle driving shaft has to be reduced, there may not be much wall thickness where the keyway is. If that was ok, there is now a problem with the bore of the pulley being too big.

If you have the means of producing a keyway, then a complete redesign of the spindle drive shaft is possible, and would be the best solution.

I noticed that Simon Williams has a lathe with enough swing to enlarge the bush hole. I could have also modified that hole, but only because of having access to another, working, mill.

 The two needle races in the museums machine are only held in a plain bore by friction, they are fitted from either end leaving a gap in the middle. So far, there have been no nasty noises to indicate they have walked, despite using a 1hp motor rather than the original 1/2hp one. Because of the extra power driving the R8 spindle, I milled a second flat on the shaft at 180 degrees to the original and have two drive grubscrews, with backup locking grubscrews in the pulley.

Edited By old mart on 23/01/2020 19:55:51

Old Mart - you're quite right to be concerned about the damage to the driven sleeve outer surface from running in its bronze plain bearing. I sat and looked at this for some time before committing to the procedure described above, but decided that I had nothing to lose as making the driven sleeve was beyond me. I can cut short keyways, but this one is the full length of the bore of the driven sleeve, and is about 3 inches long and maybe 1/8 wide. I didn't rate my chances of making a slotter tool to do that.

So I'd read on here of someone using the viscosity of the Loctite to centre two pieces together, and decided to give it a go. The wear on the driven sleeve outer was only a couple of thou, so it worked out fine. I expect I used bearing fit Loctite and left this assembly to set with it sat on its end - all's well that ends well!

As for reducing the OD to clean up the damage - that's not going to happen as the wall thickness over the keyway is tight, if that's the right adjective to use. Not sure it isn't hardened as well. Besides, you'd need to work to commercially available bearing sizes - next size down would be 25 mm dia if such a thing exists, so that means taking 8 thou off the wall thickness.

Rgds to all

Simon

I had no idea what the OD of the drive sleeve was, yours being 1" was a bit of luck, as the sleeves are easier to get. That and the availability of needle rollers to match the sleeves made the project an excellent upgrade. Your bearings are larger and heavier duty than the ones in the mill we have, and unless the smaller ones give trouble, I won't need to do the same mod, but it's nice to know that it can be done. I have found lots of shortcomings in the Tom Senior design and have addressed some of them, and I don't give a monkeys what the purists think. If I had found that a conversion to R8 spindle was not possible, I wouldn't have spent any money on it in the first place.

Thanks for the replies to the old thread àfellas and all the info.

I'm really glad the mod is still going strong Simon and the R8 conversion looks a great job.

I will look into getting some bearings and sort the conversion out as soon as possible. There are various types of bearings available so may end up with something slightly different to above.

I may be able to chuck the upper housing up on the lathe but more likely will bore it on the bridgeport.

I've enjoyed using mine and it has produced some really accurate work with the 3 axis dro kit.

Hi Damian - just one last thought about the thrust washer.

I don't think it needs to be anything fancy, as it doesn't carry any thrust in normal service anyway. If the driven pulley floats a bit vertically, it will take up the position imposed upon it by the tension in the drive belts, so the belts run true between the motor pulley and the driven pulley atop the quill. That pulls the driven pulley upwards, taking any load off the "thrust washer" which now sits idly in the gap. Hence the thrust washer is only there to ensure that the cast aluminium pulley can't contact the cast aluminium housing, which would make a horrid noise and introduce debris into those pretty little needle rollers.

So a needle roller thrust bearing under the driven pulley would look nice, but I don't think it'll be of any benefit. Another mark against it is that there is precious little spare length to get the bits in the available space, so my thrust washer had to be as thin as I could sensibly make it.

If you do decide to modify your vertical head and fit roller bearings, do please take piccy's and post them here. I'd love to see how you get on.

HTH, best rgds Simon

Regarding the bearing sleeves fitted to the drive shaft, I would have turned the thickness of the large end down and fitted a larger diameter thrust washer of the same thickness before fitting the sleeves.