Metric screw cutting

[Richard Fox 1Participant @richardfox1]

Hi everyone.  firstly I would like to wish everyon a happy and healthy 2010 and thank people who may take the time and trouble to reply to my question.

I have a 1968 Myford ML7 Tri leva fitted from new with a Myford gearbox.  

On looking around the web it seems that I need a conversion set from myford in order to cut metric pitches. 

These are very expensive new and dont seem to come up often secondhand.

However I found a listing on ebay regarding using two gears to generate a variety of metric pitches, certainly the ones I need.   

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=160391564690&ssPageName=STRK:MEWAX:IT

This seems a cheap and easy solution.  My concerns are that if it is so good why I havent found any reference to it elsewhere.   Has anyone else used this method and is there a problem such as very poor fits due to wide pitch tolerance approximations?

Many thanks

Richard

[Richard Fox 1Participant @richardfox1]

Screw cutting with a Myford ML7 with myford gearbox

[Dirk du PlooyParticipant @dirkduplooy98711]

Hi Richard,

An article appeared in Smoke Rings many years ago.

I only have a bad photocopy but I took the data and generated an Excel table for the use of 2 gears (33 & 34 teeth) to fit on the tumbler stud.

The table also contains gears with 24 and 25 teeth to cut M5, M4, M3,5 and M2,5.

There are obviously errors involved when this method is used.

Mail me directly at dplce@mweb.co.za and I’ll mail you the table.

Regards,

Dup.

[Ruaidhri MurphyParticipant @ruaidhrimurphy46564]

As mentioned elsewhere here, I have been woodturning for decades and only started with a Myford Super 7 a couple of years ago. Added the gearbox, and picked up the metric conversion.

 

Recently our chapter of the Irish Woodturning Guild won a Nova lathe at a seminar. Very cool and delight all round.

 

The Nova has a 33 x 3.5 spindle nose, and it would be handy to be able to knock up bits for it, BUT I am slightly concerned at the idea of stepping UP the gearing.

Probably just a beginner’s worry – but I’d rather have some advice from those who know a bit before I destroy my Myford

 

Any suggestions would be greatly appreciated.

 

Regards,

Ruaidhrí

BTW – Dirk – mail sent, thanks

[John HaineParticipant @johnhaine32865]

Richard,

 

When I get home I will copy for you the threading table for the Super 7 without gearbox which gives approximate gears for metric pitches without using the “metric conversion” set (50:127). If you do the sums you find that the errors are insignificant over the length of a normal nut, in many cases probably less than the error in the leadscrew pitch itself. Have you got an email address?

 

John.

[Chris CrosskeyParticipant @chriscrosskey26628]

There’s alots of ways to get metric pitches on a Myford gearbox. My ML7 has the early “low-speed” gearbox which needs the 25:12 pinion to get the stated pitches, however I find if I put a 34-tooth gear on the standard pinion set then I get a 1mm thread to a fairly high degree of accuracy at a setting of 8tpi in fine-feed mode…. I did a little chart of these dodges once and found that most of the small metric and BA pitches are there if you have the correct gears for them…. I do also have the DAG Brown auxilliary banjo and it’s very useful if you are switching bak and forth between a couple of pitches…. Also because I was offered some at a sensible price when I was younger I’ve actually collected just about all the differnt change gears Myford have ever made …. sooner or later I’ll bother trying to fill the gaps, but they atre just oddball prime numbers so of little practical value…..

 

chrisc

[egaParticipant @ega]

Ruaidhri

3.5mm pitch is included in Myford’s table of pitches available with the metric conversion and in fact the table goes up to 4mm. It’s true that they counsel caution when cutting coarse pitches but there is nothing inherently dangerous in gearing up. As always, a correctly-shaped, sharp tool makes all the difference. You could always leave the drive belt on the slack side if you are concerned about breaking something. I would be inclined to avoid using back gear if possible because of the large amount of torque available in this mode.

 

BTW, I’m a woodturner from way back, too.

[LambtonParticipant @lambton]

It is quite a long time ago that Richard asked "

This seems a cheap and easy solution. My concerns are that if it is so good why I haven't found any reference to it elsewhere".

I believe that John Stevenson provides the two gears in question at a reasonable price. The pitch error using the 34 tooth gear is very small at approx.1 in 2222 whereas the error using the 33 tooth gear is rather more at 1 in 464. If a 26 tooth gear was produced this would give better result for 1.25 mm and 2.5mm pitch at 1 in 666. For short threads these errors are probably acceptable.

Any conversion of imperial lathe to cut metric threads that does not include a 127 tooth gear will produce some pitch errors. There are lots of ways of reducing these errors using a change wheel set up (such as the Myford conversion kit) but the options are limited when trying to easily adapt the QC gearbox.

So I reiterate Richard's question and would be very interested in the experience of my fellows who have used these two gears in conjunction with the QC gearbox.

Eric

[Andrew Moyes 1Participant @andrewmoyes1]

So I reiterate Richard's question and would be very interested in the experience of my fellows who have used these two gears in conjunction with the QC gearbox.

I used a 34T gear with the 'box set to 18tpi to cut the 2mm pitch threads for all the backplates for my Myford big-bore lathe (7 in total). Quick and easy to set up and theoretically accurate to 0.00024 over the half inch of thread. Good enough for me.

Andrew

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