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Myford ml7 21 tooth change gears

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Michael Gilligan29/12/2015 17:52:04
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This 21 tooth Timing Pulley may be of interest.

I bought 'the last one' today, but presumably there will more availale sometime.

MichaelG.

John Stevenson29/12/2015 21:09:47
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Michael,

Think you are going to have problems running that alongside a 20 DP Myford change wheel wink

Michael Gilligan29/12/2015 21:35:29
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Posted by John Stevenson on 29/12/2015 21:09:47:

Michael,

Think you are going to have problems running that alongside a 20 DP Myford change wheel wink

.

John,

Running alongside shouldn't be a problem ... meshing-with would, of course cheeky

MichaelG.

.

P.S. ... it may, or may not, end-up on the Myford.

John Stevenson29/12/2015 22:17:20
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Something like this ?

Michael Gilligan29/12/2015 22:22:05
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Posted by John Stevenson on 29/12/2015 22:17:20:

Something like this ?

.

Yes, very much like that [except mixed with the changewheel set], which is why I was rather surprised that you [of all people] didn't immediately guess what I had in mind.

MichaelG.

.

P.S. .... Martin Cleeve's use of a reduced pitch 127 tooth, mixed in the Myford train, is maybe my source of  'inspiration'. 

 

Edited By Michael Gilligan on 29/12/2015 22:44:52

Roderick Jenkins01/01/2016 12:28:33
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Posted by Roderick Jenkins on 29/12/2015 16:33:35:

Confusion reigns. The metric conversion set is for the the gearbox version, which is why it includes a new banjo to replace the fixed geared version on the imperial gearbox. We are talking about cutting metric threads using the changewheels, aren't we? For this, you just need to get an extra 40T and 2 x 21T gears to add to the standard set to cut all the threads shown in my table above.

In response to ega, the actual threads cut using the gearbox conversion set are:

... Still very good in my view and no use of the 63t gear, which is only used for DP and BA pitches. You can't wriggle out of it that easily John cheeky.

Apologies to John . The gearbox metric conversion set does use a 60/63 pair together with a 50/45 pair permanently on the banjo for metric threads. To get all the threads shown in the table you only have to change the tumbler stud gear and the gearbox input gear. But you still get a very close approximations to the true metric pitches so, presumably, the 50/45 pair refines the translation. I blame the (self inflicted) jet lag. Anyway, sorry - that will teach me not to be sarcastic (maybe) blush

Rod

John Stevenson01/01/2016 13:38:22
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Posted by Roderick Jenkins on 01/01/2016 12:28:33:

. I blame the (self inflicted) jet lag. Anyway, sorry - that will teach me not to be sarcastic (maybe) blush

Rod

.

Never, ever leave the sarcasm out. It makes life far,far more interesting smiley wink

And no need at all for the apology.

Rufus Roughcut03/01/2016 16:34:57
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Hi All

Pleased to see I created a bit of fuss of exactness, has anyone tried jacking up a Myford 7 and extending the quadrant (non gearbox version) and fitting a 256T to the lead screw?

Pleased to see that even the professors and computers are rounded to best guesses exactness.

Ed, Did you achieve the thread and was it significantly cheaper than the use of the correct Tap/Die

Happy new year

Rufus

John Stevenson03/01/2016 16:42:26
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Why would you want to use a 256T ?

Think you mean 254 ? and as it's not a prime 127 is exactly the same.

Michael Gilligan03/01/2016 17:44:05
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Posted by Rufus Roughcut on 03/01/2016 16:34:57:

... has anyone tried jacking up a Myford 7

.

Rufus,

as I mentioned earlier; Martin Cleeve had a tidier solution:

MichaelG.

cleeve_dog_fig5.jpg

Martin Cleeve's use of a reduced pitch 127 tooth, mixed in the Myford train, is maybe my source of 'inspiration'.

Rufus Roughcut13/01/2016 23:28:31
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20 photos

Hi Michael

I see the different pitches and DPs of gears can be mixed without direct meshing throughout a train by use of the stubs, out of sad interest I have a T104 like the one on the lead screw which meshes fine with the myford gears but have absolutely no idea what use it may be, along with a small box of similar rogue number tooth gears, it's also worth saying I do check mine against the myfords and my hobs before running them together I'm not quite that rough yet.

Great concept though, I'm getting tempted to get my spare ML10 out and play

Rufus

Tendor14/01/2016 06:34:38
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John Stevenson’s comment about 60/63 being used “instead of” 120/127 might, for some, leave the impression that the approximation comes about because 63 is the closest integer to one half of 127 and is, in comparison, compact and hence more convenient. But not so – the error would be one part in 126 (or 127 for their reciprocals). As stated in other posts, a 127 tooth gear will produce a mathematically exact metric pitch on an imperial lead screw, being the exact number of mm in 5 inches and the lowest integer pair that expresses this exact relationship.

For metric pitches using an 8 tpi leadscrew, the exact desired conversion ratio is 25.4/8 mm per one leadscrew pitch, or 25.4/8 = 127/40 = (127/100) x 2.5 = 3.175 (exactly). It just happens that this is closely approximated by 200/63 = (80/63) x 2.5 = 3.174603…, just 1 in 8001 smaller (12.5 microns per 100 mm, or 0.0005” per 4".

Notice though, that whereas the 127 tooth gear forms the numerator (a driven gear), the 63 tooth gear forms the denominator (a driver gear). It is not performing the duty of an approximate 127 tooth driven gear (i.e. not 63 x 2 approx. equals 127). The actual comparison is between the ratios 127/100 and 80/63 (ignoring the 2.5 common multiplier).

Since 21 and 3 are factors of 63, a 21 tooth gear can also serve as a driver, with the factor of 3 formed by other train gears. For example, a 1 mm pitch from an 8 tpi lead screw requires a lead screw to spindle turns ratio of 63/200 = for example (21/40)·(30/50),  or, as per Roderick's table, (45/40).(21/75)

Edited By Rodney Entwistle on 14/01/2016 06:36:48

Edited By Rodney Entwistle on 14/01/2016 06:37:28

Edited By Rodney Entwistle on 14/01/2016 06:58:45

Edited By Rodney Entwistle on 14/01/2016 07:00:14

Michael Gilligan14/01/2016 08:41:27
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Good summary, Rodney yes

MichaelG.

Roderick Jenkins14/01/2016 09:26:10
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That's the first really clear explanation I've seen of why a 63 (or 21) tooth gear works so well, as a driver, for metric conversion.

Many thanks,

Rod

Neil Wyatt14/01/2016 09:47:56
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Posted by Roderick Jenkins on 14/01/2016 09:26:10:

That's the first really clear explanation I've seen of why a 63 (or 21) tooth gear works so well, as a driver, for metric conversion.

Shame on you Rod, you must have missed the excellent article in MEW 204

Neil

Roderick Jenkins14/01/2016 10:05:58
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Posted by Neil Wyatt on 14/01/2016 09:47:56:

Shame on you Rod, you must have missed the excellent article in MEW 204

I did. As it concerned a mini lathe I merely skimmed it - that will teach me not to be parochial blush

Rod

Michael Gilligan14/01/2016 10:31:30
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Posted by Roderick Jenkins on 14/01/2016 10:05:58:
Posted by Neil Wyatt on 14/01/2016 09:47:56:

Shame on you Rod, you must have missed the excellent article in MEW 204

I did. As it concerned a mini lathe I merely skimmed it -

.

... and therein lies the importance of indexing, cross-referencing,

'allocation to forum topic', and all that tedious stuff. devil

.

MichaelG.

Edited By Michael Gilligan on 14/01/2016 10:33:18

Neil Wyatt15/01/2016 10:45:25
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Posted by Michael Gilligan on 14/01/2016 10:31:30:

... and therein lies the importance of indexing, cross-referencing,

'allocation to forum topic', and all that tedious stuff. devil

I thought you would be the champion of search engines which allow us to abandon all that 20th-century stuff

Neil

Michael Gilligan15/01/2016 10:59:46
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Posted by Neil Wyatt on 15/01/2016 10:45:25:

I thought you would be the champion of search engines which allow us to abandon all that 20th-century stuff

.

dont know

That, Neil, is the whole point ... There is no search engine that can reliably find things that have inadequate references.

MichaelG.

Neil Wyatt15/01/2016 17:18:48
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Posted by Michael Gilligan on 15/01/2016 10:59:46:
Posted by Neil Wyatt on 15/01/2016 10:45:25:

I thought you would be the champion of search engines which allow us to abandon all that 20th-century stuff

.

dont know

That, Neil, is the whole point ... There is no search engine that can reliably find things that have inadequate references.

MichaelG.

I won't keep this going beyond one response, as we are wandering OT, but I think the joy of search engines is that when references are inadequate they provide just about the only way of finding things.

Several years ago the vaguely remembered phrase 'The queech obulates' returned no hits... it now gives many relevant results and last year allowed me to finally tack down the source.

I was also able to find the origin of the 'Green Meat Saga' from my childhood - in an RAF yearbook.

They may not do so reliably but I am convinced they do so better than any other way.

Neil

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