A replacement for a Colchester lathe part
|Tim Stevens||23/03/2021 21:11:50|
1430 forum posts
My Student lathe has a damaged gear in the thread indicator. All it does is track the lead-screw position for thread cutting, but ... The gear I have is missing one complete tooth and parts of others, so I am minded to have a go at making a replacement. The original seems to be cast iron, and all it does is go round under very little load, so I am minded to use brass* for the replacement. or perhaps Delron.
There are two complications:
But, there are 24 teeth, so easy to step round each 15 degrees.
I am minded to try setting up at the correct angle using a Dividing Head tilted a bit less than 90 degrees. To start the teeth I will use a slitting saw in the Mill, cutting to the correct depth for the teeth. Then I will position the gear on my Spin-indexer, and use a shortened engraving cutter to widen each side of each tooth space to a semblance of the profile to match the Apex form of the lead screw. Further fitting if necessary will be done with escapement files.
The bore of the gear is originally a fine spline, and I propose using brass or plastic as this should take a splined shape when first fitted, sufficient for the small loads involved. Or, I will certainly see if the original gear section can be machined away to leave a splined boss with a flat face where I can bolt on the new-made gear.
My questions are: Does this all sound plausible? Do you see any snags which I have not considered? Brass or Plastic?
Regards - Tim
|Pete Rimmer||23/03/2021 21:28:34|
|1004 forum posts|
A 19DP hob or gear cutter would cut the teeth to a very close profile (ideal would be 18.85DP). I recently cut a 18 tooth thread chasing gear for a 6tpi screw with (I think) a 19.5dp hob, which worked perfectly well.
|Nigel Graham 2||23/03/2021 21:31:41|
|1608 forum posts|
All seems plausible, and since you want the indicator to track the lead-screw properly, mounting the replacement on the splined core of the original would be better than trying to use the spindle as a broach.
Others here have advised my using a similar approach to fitting a new worm-wheel (with matching worm) to the lead-screw on a small horizontal mill I am intending to overhaul. Its worm-wheel is intact but it has lost the feed-drive and worm.
Brass or bearing-grade plastic ought work fine.
I take it by "measure " the helix-angle you mean calculating by diameter and pitch (or lead).
There might be enough un-worn area on the ends of the teeth of the existing wheel to use as a template for grinding the engraving-cutter.
I hate to think what the lathe's previous owner had done to break teeth on the thread-indicator!
|Michael Gilligan||23/03/2021 21:38:14|
18325 forum posts
Reference your  ... Think ‘circular pitch’ instead of ‘diametral pitch’
|old mart||23/03/2021 21:48:46|
|3185 forum posts|
If the number of teeth are the same, it cannot fail to work.
The Atlas 12 x 24 which we are working on at the museum, (with a 6 month co 19 break) did not have its threading indicator. I found out the number of teeth, measured the leadscrew tooth pitch and just bought a gear to match. The threading dial will be tilted to match the helix angle of the leadscrew thread. As there is no power involved, an exact thread engagement will not be a factor.
|not done it yet||23/03/2021 22:52:07|
|6078 forum posts|
Plastic gears seem to cope with rather higher load ans higher speed. I would expect a plastic gear would gently wear to the profile during use.
|Chris Crew||23/03/2021 23:59:41|
88 forum posts
The angle of the teeth will equal the helix angle of the thread. The imperial Student has a 6 TPI lead-screw, mine has anyway but I don't know about that on a metric lathe. You may have to vary the involute cutter number when cutting what is in actual fact a short slice of a spiral to mesh with the lead-screw because the helix angle makes the tooth slightly wider which changes the involute although I doubt if it matters in this instance as the gear is small and not transmitting any power. All this information is contained in Ivan Law's Gears & Gear Cutting book. Law recommends that you cut a tooth space in one pass rather than nibbling away at it, although you may have to gash it if your machine is not robust enough to take a tooth space out in one go. As you have the original gear to copy you are almost over halfway there, count the number of teeth, calculate the gear blank if you know the DP and set up an appropriate cutter over the blank in the dividing head. I don't know the included angle of the Student lead-screw thread but it will equal to a spiral rack so you may have to approximate the pressure angle of the gear, 14.5 or 20 deg. whichever is the closer.
I can't claim to be any expert on the subject or that I have cut a great number of gears and racks but those I have cut over the years seem to function well enough for the purpose they were intended to serve in a back shed workshop, although I have no way of measuring their final accuracy. If you wanted to cut the splines for the boss, you can do this in the lathe with either a slotting attachment or simply mounting a suitable tool in the tool-post and working the saddle back and forth, provided that you have a method of indexing the lathe spindle. Again, I have cut splines using both methods successfully enough for my amateur purposes.
Edited By Chris Crew on 24/03/2021 00:08:46
Edited By Chris Crew on 24/03/2021 00:14:11
Edited By Chris Crew on 24/03/2021 00:18:13
5505 forum posts
I'd make life easy and leave the splines intact. Machine off the old gear teeth with some kind of step and make the new one a ring that can be Loctited etc in position. You could then make sure the new gear is in the correct position vis a vis the marks on the dial before securing it. Otherwise you may have to put packing washers between the the chaser dial unit and the carriage to get the marks to line up when the half nuts are engaged fully.
Yes, cutting straight teeth at an angle will engage with the "worm" thread on the leadscrew just fine. I did the very same on the worm and wheel for the Versatile Dividing Head I made.
As the dial gear will probably see very little use in the typical home workshop, you could even just make the new gear out of a disc of 2mm thick brass etc and it will perform the required function.
|1382 forum posts|
A few years ago I made a 28T gear for the thread indicator on my 290 lathe. I used Delrin and set the dividing head to match the helix angle of the leadscrew, my homemade gear has worked well. So your procedure should work, good luck.
|506 forum posts|
To call it a gear is rather to inflate its status and thus to imbue it with characteristics and complexity far in excess of those required for its functionality.
It is a counter with teeth. So long as it has 24 teeth and so long as only one tooth at a time can engage in any groove of the leadscrew thread, it will work.
About the only importatn thing is Mr Hopper's advice concerning the clocking of the new teeth: they need to match the graduations on the threading dial or you will have to realign the fixed mark.
Hacksaw off the teeth of the existing gear so the splines remain. Glue on a doughnut of plywood (PU glue has good gap-filling abilities if you cannot hacksaw in a circle). Coping saw to get the perimeter roughly circular and roughly the right diameter. Glue in 24 wooden dowels poking out of the perimeter. Report back to us in 5+ years when it wears out, complaining that its life was too short.
You are completely on the right road when you say it has no power transmission requirements at all. There is a direct correlation between its power transmission and the amount of thinking necessary to replace it.
|Tim Stevens||24/03/2021 10:03:17|
1430 forum posts
My question 'will it work' was much more about the process I was suggesting, rather than about the effectiveness of the product. As well as the need to use tools I have rather than trying to grind new versions by eye, when the teeth only need to be all there, rather than an exactly matching shape.
One thing in your responses I don't understand - the need, you say, to ensure that the splines and the teeth fit with each other as the original did. When cutting a thread the indicator is engaged, and the tool starts the first cut. There is no fixed relationship between the indicator and the tool - indeed the tool can be moved in X and Y directions as part of the setting up process. All that matters for the indicator to work is that the relationship between the indicator* and the tool position is not altered once cutting has started. Bearing in mind, of course, that with a sensible ratio of gear teeth and lead screw threads the indicator can work at, say, four or eight positions. So, where I have indicator* it ought to say 'relevant indicator mark'. I'm not sure how we can arrive at a conclusion - are you right, Hopper and DC13k, or am I?
And what the previous owner had done to break teeth might be revealed by other details I have discovered. Like the significant graunches on the chuck side of the saddle ... But it still runs as true as I can measure, and is exceptionally good at running dynamos up to charging speeds. I think its early use was as a workshop stand-by or odd-jobs machine, so it was used, sometimes, by workers with less than the required skill, understanding, or attitude. Or perhaps none of all three.
Thanks, everyone, for your helpful and prompt responses. I will report on progress.
5505 forum posts
Yes I am right -- of course! Having done a similar job before I found this out the hard way.
The critical relationship is between the teeth on the half-nuts, the teeth on the thread indicator gear and the lines on the indicator dial. Once the half nuts are locked onto the leadscrew, there is a set and immutable distance between the halfnut and the leadscew thread engaging with the thread indicator's gear.
The indicator gear must engage with a thread that is X number of pitches from the halfnut thread. The index mark on the indictator body must line up with one of the lines on the indicator dial at that point.
Imagine now the dial is a moveable -- but firm -- fit on its shaft. Rotate that dial say 10 degrees on its shaft, without moving the gear that is engaged with the leadscrew. Start the lathe up and get the leadscrew turning and that dial stays in that same position as the carriage moves along the leadscrew, as is normal.
So if you now disengage the half nuts and try to re-engage them when the mark on the indicator is lined up, it is actually now 10 degrees out of position and you will find you have to wait until the indicator rotates that extra 10 degrees before the thead on the leadscrew aligns with the thread of the halfnuts and allows engagement.
So you should look at your old gear before removal and see if one of the marks lines up with one of the gear teeth exactly and try to replicate that positioning with your new gear. That way your lines will still line up.
Alternatively, you will have to put a spacing washer between the indicator body and the carriage to move the indicator gear along the leadscrew, thus rotating the dial, until the marks line up. Easy way to do this on Myford type indicator is slacken off the pivot bolt nut, move the indictor to the right until marks line up, measure the gap on the stud and make a washer to fit in there exactly.
Had me scratching my head the first time I came across it too. But makes more sense when you can stand there and look at it than trying to explain it. If you have a lathe with a working thread indicator dial, try putting a felt pen mark on it to one side of one of the lines and observe what happens if you disengage the halfnuts and try to reengage with that pen mark lined up with the index mark. Or if the indicator is the pivoting type on a bolt like a Myford, undo the nut and move the indictor along the stud while the gear is engage with the stationary leadscrew and see how the dial turns as the indicator is moved laterally.
Edited By Hopper on 24/03/2021 10:34:29
Edited By Hopper on 24/03/2021 10:36:24
Edited By Hopper on 24/03/2021 10:38:14
Edited By Hopper on 24/03/2021 10:40:00
|Russell Eberhardt||24/03/2021 10:57:48|
2675 forum posts
Have you considered 3D printing? I have used a 3D printed compound gear in my Atlas lathe screw cutting gear train for over a year with no problems. It was made as an experiment and has been left in place as it runs quieter than the Mazac gear.
|Nigel McBurney 1||24/03/2021 11:12:22|
894 forum posts
I have a 1973 Colchester master 2500, Just had a look at the thread indicator dial, the gear is white delryn or nylon,the teeth are at an angle of approx 4 to 5 degrees,the gear is 1/4 inch thick.the gear is fitted to the shaft by a fine spllne more like a coarse knurl, and so is the dial at the other end of the shaft. So anyone with this type of fitting the shaft could be pressed into a plain bore in the nylon gear .The shape of the teeth is not really important,the device is an indicator,so a disc with a number of say brass pins around the periphery would work.tilting the dividing head a few degrees would work I personally would use a gear cutter as I have a large collection of various types ,an alternative would be to make up a flycutter,which could be hand ground by using the broken gear as a guide,I would not bother to do an initial pass with a slitting saw. Now on the metric Colchester Triumph ,the thread indicator has a set of gears as single gear only works on some pitches.from memory these gears were made from brass and were about 2.5 mm thick and did not need the teeth to be set at an angle .
|Tim Stevens||24/03/2021 14:04:11|
1430 forum posts
No, I have not considered 3D printing, as I have no digital drawing skills, and neither a 3D printer nor a mate who has one.
And Nigel McBurney confirms my view - thanks - that a decent plastic like Delron, of which I have some, would be good for the ring of teeth.
And I am obliged, too, for Howard's extra information which I had ignored in my tarradiddle about matching splines to teeth etc. As the gear is now off, I will try to count the splines in the hope that there are 24 of them, but I think it is rather more. In that case I will make up my ring of teeth to fit the turned-off gear boss, assemble it, lock the leadscrew half-nut, put the dial in the right position, and then disassemble carefully to drill the holes for eg roll pins holding the gear in position. Unless, of course, I find on removing the top of the indicator with its 8 lines, that it, too, is mounted on splines.
Golly, this gets more complicated by the minute - but at least I know now what should be the order of play.
|Dave Halford||24/03/2021 14:14:16|
|1590 forum posts|
you dont need to use the slitting saw.
I would mount the old part in your cutting set up and see if your chosen cutter follows the old teeth properly.
|Howard Lewis||24/03/2021 17:05:48|
|5036 forum posts|
Other than friction in the bearings, there will be very little load on then gear. It is there to rotate a dial, intended, in effect to indicate the position of the saddle relative to the Leadscrew, not transmit large amounts of torque..
To have a gear which does not have the same helix angle as the Leadscrew, may not be correct technically, but the line, or point contact between the flank of the Leadscrew and the gear tooth is unlikely to cause any major problems, in my view.
Very probably, the important thing is that the Indicator marks are in the right relationship to the Leadscrew .
With a bit of luck, you won' be wrong 23 times!
|Howard Lewis||24/03/2021 17:17:05|
|5036 forum posts|
IF it not too late, a suggestion.
Before removing the original gear, scribe a line across the centre of a tooth and on to the shaft.
Remove the gear.
Fit the new gear so that the centre line of a tooth coincides with the scribe line.
Cross fingers and test!
|Pete Rimmer||24/03/2021 18:02:30|
|1004 forum posts|
If you want I will cut you a new chaser gear in brass or bronze. You can bore it out and turn your original down so the new one can be bonded to the old hub with loctite.
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