Where's it Going Wrong?

Martin, Christopher has a gearbox machine and doesn't use the simple gear train.

-- Regards,

John Stevenson Nottingham, England.

I did read that he has a gearbox, but the first sentence should be true.

I still th>>

I can assure you that he is not - which is why there's a problem! I have chosen all my words very carefully. You can believe what you read!

--

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

No offence intended Christopher, talking about you in the third person, but I still don't understand how you "run the tool back under power". Are you talking about a power cross-feed, or are you powering the saddle back to the start position? If the latter, how can you do this without interrupting the gear train?

Can't he reverse the motor ?

Peter

-- Peter & Rita Forbes snipped-for-privacy@easynet.co.uk Engine pages for preservation info:

No help to you at all, but I had a Polish AFM lathe for several years, good solid machine (H E A V Y!) but with a bit of wear in it. I had a similar problem to yours, except that it wasn't consistent. I could have made several good cuts on a thread, then suddenly one would be *way* out of alignment, often ruining the thread. I tried all sorts of theories, including several of those mentioned in this thread, but never really got to the bottom of it. It was one reason why the lathe had to go!

Cheers Tim Tim Leech Dutton Dry-Dock

Traditional & Modern canal craft repairs

Martin

Rest assured that no offence is taken! I can run the tool back two ways without interrupting the gear train...via the tumbler gears or simply stopping and then reversing the motor, my machine allows me to do that. --

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

Getting closer. Lets work with 8 tpi as with an 8 tpi screw the train HAS to be 1:1

According to the Gospel as preached by Myford the train does 24 to a 57 to a 19/57 onto a 72. So 24/57 = 0.4210

57/19 = 3.0 57/72 = .7916

multiply these three and you get 0.999 which is 1:1 so that looks OK Now substitute the 24 for the 38 and you get 38/57 = 0.666 x 3.0 x 0.7916 = 1.58319 which is nothing?? Multiply 1.58319 by 8 tpi and you will be cutting 12.6655 tpi No TDI on earth will be able to follow this. It's akin to trying to pick a metric thread up on an imperial lead screw. Christopher, do you have a 24 t wheel ? If not does Mark have one to try ?

-- Regards,

John Stevenson Nottingham, England.

Chris I've been following this with interest and intrigue. Is the offset error you are getting cumulative ie a consistent amount in a consistent direction for each pass or an uncertainty of +/- a constant amount? Could it be swarf in the half nuts?

Watching this thread with interest and just trying a few guesses

1 If the first pass yields the right TPI this eliminates all possible incorrect geartrain setups. 2 The external geartrain is easily inspected so it pretty certain you would have found any missing drive keys or major sticky backlash. 3 The internals of a Norton gearbox are less accessible and its just possible that there is a gear there that is a tight fit on its shaft but that its drive key is missing or damaged. This would produce severe sticky backlash. Cutting in one direction and gently winding back would leave the gear friction locked to its shaft. The shock of power reversal could overcome the static friction and allow the gear to move on its shaft. It should be possible to check this by engaging back gear to lock the spindle and twisting hard on the leadscrew in both directions. 4 Its just possible but less likely that a damaged clasp nut could behave in the same way.

Jim

damaged in a way that makes it possible to

If you leave the clasp nut engaged and wind it back, then surely the only movement can be in the gear train. Then in order to get slippage it must be between the gears on the first or last gear in the train. From gear calc theory the intermediate gears can free wheel on their shafts unless drive is transmitted along a single shaft by two gears (a driven and driver)

In this case the slippage must be between the headstock and the clasp nut. The only non-gear item is the freewheel latch (used for backgear) is this undone? Failing that it's got to be in the gear train in the left hand cover or one of the gears in the gearbox that's come adrift from the shaft.

Hope this helps

Mart

In message , snipped-for-privacy@wurzel.demon.co.uk writes

John

Forgive me, I have to grovel and eat humble pie at the same time. I think I accidentally misled you - I think it's the 20 bit of the 38/20 that should feature in the calculations. Sorry about that. It's the 20 which is in contact with the 57/19, the 38 meshes with the fibre tumbler reverse gears above it. However, substituting 20 for 38 in your maths above gives us 0.831 - equally as bad as the 1.5783? However, the Myford gearbox supplement (dated 1998) which was issued with the lathe last year, shows the tumbler sleeve gear to be a 24 tooth, whereas I have a 20. Unfortunately, I don't have a 24 to try, it's not a straight forward change wheel type of course, its a compound, apparently machined from the solid and is about one and a half inches in diameter. However, after all the debate, I really do think that you might have put your finger on the problem and if somebody out there with a gearbox Super 7, especially a relatively recent one, would report the number of teeth on their tumbler sleeve gear, it sure would help lighten the darkness! Stimulated by Jim's comments yesterday ("Pentagrid") today I actually measured the pitch of the spiral I created after setting the gearbox etc to cut 8tpi....it was about 9.75tpi, a further experiment with

16tpi set on the box produces a pitch of approximately 19tpi. Confused? Join the club! Just another stone thrown in the pool of confusion. Nevertheless, it's another pointer towards a gearing problem in which the tumbler sleeve gear currently seems to be the likely suspect. Martin Akehurst's suggestion regarding the clasp nuts has been taken on board and I will check them when I can. However, I have been ill for much of the year that I have owned this machine and I rather doubt I have created enough swarf to cause any problem, other than with the wife and her carpet's, of course!

This machine, I might add, is Myford's latest, the top-of- the-range Connoisseur. However, for our purposes, it's a just a Super 7 with a variable speed drive and the bigger mandrel bore and I don't think the model, as such, plays any significant part in our debate, other than that it -was- a new model and only the second one that they had made. I think the Connoisseur is to be reviewed in next week's Model Engineers' Workshop...I shall try and contain myself when I read it!

In reply to Bob Minchin's question, the cut shift is progressively and consistently moving along the work towards the tailstock and is of the same magnitude each time ie the second cut is about one 64th behind the first (at 26tpi), the third cut is a further 64th behind the second and so on. Make sense?

Back to the barricades...!

--

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

Chris

At the risk of adding further confusion, I've just checked the tumbler sleeve gear on my Super 7B (which cuts threads perfectly), and the two gears have 24 and 30 teeth respectively.

Mike

(snip)

Thank you very much...I really think we've collectively cracked it. --

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

If I had paid the kind of money that Myford charge for the connoisseur, and had half the trouble you have had, I would have expected a man from Myford on my doorstep to fix it. The excuse that they can't help you because they have an open week seems particularly feeble. With this kind of service it's no wonder people buy Chinese imports.

On the positive side though it's good to see so many people willing and able to help you out -it just seems to me that it's a shame you have to resort to all this trouble for what should have been fixed with a couple of phone calls to Myford.

Regards

Kevin

Any chance of measuring the error on a few other threads?

I have to say I'm enjoying this thread immensely. All the best elements of friends with a positive, helpful attitude and a knotty Sherlock Holmes puzzler to grapple with.

Eagerly anticipating the final solution.

Bill Derby

How long have you got....! If I weaken I'll print the list of problems that I had when the machine was delivered in August 2002. To be fair, they did, eventually, send somebody down to fix most of the snags but one seems to have eluded us all. After ordering, I waited about two months without hearing anything, and eventually threatened to cancel the order if I didn't hear when I would get the machine. I got it week later...need I say more?

To be fair, Myford have not said they can't help, it's just the reality of the situation. They are now appear grossly understaffed, and normally pack up for the week-end on Friday lunchtime. I tried to call them half a dozen times this morning and got only an answering machine. They now seem to be down to two executives, the Managing Director, the big guy, Christopher Moore, who is usually seen selling bits at the shows, and the general factotum Malcolm Townsend, who, though willing, is spread very thinly.

I couldn't agree with you more. It's very gratifying, especially to those of us who don't belong to a club and have to get our help wherever we can.

Regards

--

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

Thanks for the suggestion. The leadscrew end play has been checked and is OK.. I'll follow your other advice over the week-end and report back in due course.

--

Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"

snip

snip

The number of teeth in ANY of the gears in the geartrain (this includes the tumbler gears)is irrelevant. An incorrect ratio will result in a strange pitch but, so long as the clasp nut remains engaged, every cut follows the same path so a single thread will result.

I still think you have a gear that should be keyed to its shaft with the key missing or damaged. Slippage of the gear on its shaft will result in a strange pitch. This will necessarily be finer than the correct pitch which is what you have observed.

Lock the spindle and try using a spanner to torque the end of your lead screw real hard in both directions. If the geartrain is OK, once the backlash in the gear train is taken up, the leadscrew should be locked solid and impossible to move further.

The only other possiblity I can think of is leadscrew end play but this very easily checked and eliminated.

Jim