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Post by ChrisCrosskey on Oct 17, 2005 21:53:05 GMT
Well that's about the size of it.... you might remember me moaning about how my ML7B was cutting threads twice as coarse as the box indicated when fitted with the standard 24-25 tooth compound gear on the tumbler output. At the time I put it down to pilot error, set to cut twice as fine and all was well... however I've just done a test with the standard 24/25 gear and putting the box into its coarsest setting.... it proceded to cut 4tpi... if I put the 12/25-tooth fine-cutting compound in there, all is well, at least for imperial threads, even if it robs me of the ultra-fine feeds, but I'd calculated up a bunch of metric and BA approximations based on the 24-tooth gear being replaced with other gears (I have nearly every gear Myford have ever made), and reckoned that at 36tpi a 34-tooth gear in place of the 24-tooth one on the tumbler stud would see me cutting a 1mm thread to within a part in rather more than 2000.... unless something is mis-assembled inside my gearbox to make it cut 2x setting I'll have to do something like make a 17-tooth gear for this purpose (will have to be integral to the 5/8" compounding shaft.....)... any Norton Gearbox specialis#ts out there know where something could have gone wrong to do this?
chrisc
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Post by ChrisCrosskey on Oct 30, 2005 22:10:14 GMT
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JohnP
Hi-poster
Posts: 186
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Post by JohnP on Oct 31, 2005 20:27:42 GMT
Chris,
I'm probably not the best person to answer this, but seeing as nobody else has put their head over the parapet....
I've never used an ML7B, so this is straight off the top of my head.
If the problem is not in the changewheels, it must be in the gearbox. Does the selector detent properly, and is there a drive at each position? If there is, are these drives at the right ratios relative to each other? Can you see inside the gearbox without too much dismantling?
Is it worth checking on the net to see what other people suggest for 8 tpi, just in case there's always been a misprint?
The leadscrew must turn at mandrel x drivers/driven, so can you work through the maths?
Sorry, only more questions, not answers!
Cheers,
JohnP
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Post by ChrisCrosskey on Nov 1, 2005 18:19:39 GMT
I've been on to Myford (and posted them the pics) and Malcolm was insistant that there were no internal revisions that would lead to it cutting 4tpi..... it should only do that if you put a 48-tooth gear on teh compound on the end of the tumbler reverse output.... which you can do but is very bad practice..... I will get the top off my gearbox and have a look, but last time I did I didn't notice anything that didn't look like the manual. All the detents engage fine..... that's the point really, when you are ion coarsest cut you know where you are cos everything is over to one side..... Hopefully Malcolm will be able to see what I'm doing wrong (they're clearing up after their open week last week ATM) over the next couple of days and sort it out..... otherwise I have a nasty feeling I'll have to take the lathe to Myford themselves ..... I wasn't planning on doing that for a few years yet (if/when I get soem spare cash getting the ML7 fully refurbed is on the shopping list....)
chrisc
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Post by ChrisCrosskey on Nov 8, 2005 16:20:32 GMT
Right,
I've had the top off the gearbox adn according to my maths with a 24-tooth gear on the tumbler output it IS designed to cut 4 tpi .....
Basically the order of gears is...
25-tooth on mandrel output
idlers in tumbler
25-tooth driven on tumbler compound
24-tooth driver on tumbler compound
57-tooth idler
19-tooth driven on gear-transfer compound
57-tooth driver on gear-transfer compound
72-tooth on gearbox input.
This all means that the gearbox input shaft rotates at mandrel speed. The gearbox input shaft is basically a 16-tooth gearbar with an idler that slides along it to engage different gears on the lower shaft of the gearbox. These start with 16-teeth and end with 28teeth. The drive is then transferred from th elower shaft to the upper shaft via one of three selectable ratios, 16:32, 24:24 or 32:16. This is controlled by a sliding dog-clutch system. This means that in the middle setting it is running the output at 1:1 of input, the coarsest is twice as coarse as input and the finest of the three is twice as fine. THis would be all well and good if there was a 2:1 reduction between the output of the gearbox and the leadscrew, but there isn't, it's 1:1.... therefore this Myford was setup to cut 4tpi with the standard input gear.... For a short-term fix I'm knocking up a 17-tooth compound gear and will prolly also make an 18 and a 19 as well (I have the gearcutter required to do this), longterm whatt I need to do is put a 2:1 reduction somehow on the output of the gearbox to the leadscrew which won't be easy as the shafts are all fixed and there's currently an idler running free on the end of the lower shaft..... It is a very early gearbox, only just after they started hardening the gears so it is possible I've got a special...
chrisc
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Post by ChrisCrosskey on Nov 11, 2005 16:22:47 GMT
Righto.... Malcolms had a look at the input gearing arrangement and says it is correct, everythign inside the gearbox matches the manual, the only bit that isn't mentioned is the output gearing to the leadscrew..... it looks like it is meant to be like it is (no obvious bodges adn it all fits under the standard Myford cover).... it runs at 1:1, maybe it's supposed to run at 2:1...
chrisc
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Post by ChrisCrosskey on Nov 18, 2005 9:37:49 GMT
OK,
as a final posting on this (unless someone else starts posting) .... I worked out a combo of changegear and setting for a 1mm thread..... 34-tooth gear and 8TPI with the fine-feed compound gear set to fine-feed rather than screwcutting...... it's good to within about 1 part in 2000.... seeing as the piece I was cutting (25mm) was aluminium, I touched a carbide threading tool to it, wound it in 20 thou, put the lathe in bottom back-gear adn cut it in a single pass.... fits fine.... I'll have to spend some time with a spreadsheet and see what else pops out for metrics and BA's as I do have all the changegears between 20 and 48 to choose from.... might still get round to making 17T, 18T and 19T gears (do them so they have their compounding thingy built in though....)
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SteveW
Elder Statesman
Posts: 1,397
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Post by SteveW on Jan 14, 2006 16:32:46 GMT
Chris, Just cruising 'uk.rec.models.engineering' and spotted the following exchange: Que: Can anyone help me, I have a old Myford lathe ML7 No. K26243. The Gearbox has no.QC1339. My question is: The tumbler stud gear is only with 12/25 tooth. In my new manual from Myford Quick Change Gearbox is this gear 24/25 tooth. I have try to change to 24/25 gear, but so go the gear cutting wrong. Is it correct that this old gearbox (QC1339) must use tumbler gear 12/25 for correct working ? Regards, Flemming Copenhagen/Denmark Answer: The first 2500 Myford gearboxes were different. Then they corrected the mistakes and started making the version in the manual that you have. Details here: www.lathes.co.uk/page10.htmlCharles. |
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Post by ChrisCrosskey on Jan 17, 2006 9:40:38 GMT
Many thanks for that, I'd sort of figured it out (my gearbox has a low serial number, in teh 1600's)..., been thinking about putting it to current standard, would need the new driving gears and the current model internal shaft as well as a new leadscrew, then I'd have to line-bore the gearbox casing and fit some sort of bearing sleeve to it....
In order to cope with the oddness I'll be doing a big spreadsheet workup of the adjustments one has to make with DAG Browns auxiliary banjo conversion, plus I'm also in the process of making a set of smaller than standard pinions so that I've got 17,18 and 19T available to me. As you're running out of metal to get the 5/8" centre hole plus a keyway in they won't be tot eh stadnard pattern but will incorporate the compounding stub for the otehr gear with them..... Once I pick up/make (file under things to do in very near future) a 41 tooth (re-cut a 45) and 49 (ditto 53) then I'll have every single number between 17 and 66, with repeats of most of the 5's, plus all the standard 5's to 100 pluis 127.... planning on making a stub arbour for my Elliot dividing head (unless anyone has a 30INT 5/8 stub arbour lying around) as it will allow me to make gears pretty much at will.... you might have gathered that I'm a bit strange about gears..... They're like Pokemon.... gotta have them all...... :-)
chrisc
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SteveW
Elder Statesman
Posts: 1,397
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Post by SteveW on Jan 17, 2006 20:59:26 GMT
Chris,
Re: "The DAG Brown Banjo" alternate metric conversion kit.
A chap at the club is in the middle of one of those but hit the stops last night with his attempts to drill the 24T gear to take the n-off 6BA set screws. The thing was just too hard to drill with what he had at hand.
I'm not following his progress too closely so no heavy questioning on this one yet.
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Post by ChrisCrosskey on Jan 19, 2006 1:58:07 GMT
Ooh yes... I remember that bit.... carbide stub drill was how I did it in the end, whenever I find myself neediing a small tapping drill I invest in them.....
chrisc
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