Cutting metric threads on an imperial lathe

I'm pondering lathes ... I started out thinking about a new Warco machine, but have now moved on to thinking about secondhand Super-7s.
Any Super7 I find is likely (I suspect) to be an imperial machine - how well will this cut metric threads? I did find this comment on a webpage -
"You really only need electrical reverse when screwcutting Metric threads on an Imperial machine, or visa-versa, and have to leave the leadscrew clasp nuts engaged"
Is electrical reverse a necessity or just something that makes the job go faster?
I expect to be cutting 0.75 mm pitch metric threads - is this within the range offered by standard Super7 change-wheels? I've been surfing for info on the threads it can cut, but no luck so far!
Brendan
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cutting a metric thread on an imp lathe you have to keep the clasp nu
engaged as theres no "next line" on the indicator dial to pick th thread up from( so you electricly reverse the tool) . for a .75m thread , its 45 t driver. ist stud 40 and a 45driven ,2nd stud is 5 and a 21 teeth driven and a 70 t on the leadscrew
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On Mon, 3 Jul 2006 16:14:21 -0500, willowkevin

Sorry you have a typo here It's 45T driver to 40/35 driven to 50/21 driven to 70 leadscrew
You have the first driven incorrect at 45, it should be 35. -- Regards,
John Stevenson Nottingham, England.
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You can cut metric pitch threads on a machine with an imperial pitch leadscrew if you use a suitable train as 2 x 127 %4 and there are 25.4mm in an inch a 127 tooth changewheel makes life easier. I have seem such a wheel on a Myford but don't think it is standard. I used to cut lots of 1mm pitch LH thread on a Harrison L5 with imperial 4tpi leadscrew using such a train.
The electrical reverse issue is just that you must leave the half nut engaged when cutting such an odd pitch (in the imperial world); therefore you can't disengage, wind back and then re-engage using the thread indicator. If you don't have electrical reverse then there you just have to pull the belt by hand. I'm always somewhat cautious about using electrical reverse on a machine with a screwed mandrel nose lest the chuck decide to unscrew itself (especially when you apply the spindle brake). But I'm sure there are others better qualified to advise on that as bizarely a Myford is one lathe I have never used!
Alan Bain
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On Mon, 3 Jul 2006 14:50:15 -0500, anotherid

All Myfords whether they be imperial or metric will have an imperial leadscrew, only the cross slide and top slide have different threads.
As Alan replied the electrical reverse is needed to wind back to the start for the next cut. The worry of unwinding a chuck at screwcutting speed is very slight. I abused one commercially for a few years and only had it come loose a couple of times and I 'do' try hard <g>
Many people now are swapping the single phase motors over for three phase but running off an invertor or VFD. This gives you smoother running, instant reverse and controlled acceleration and braking. These still run off a 13 amp plug.
Myfords come as standard with no screwcutting box, it's a specified extra. Without the screw cut box you have to manually swop the change wheels at the end to the pitch you want from a table supplied. With the exception of two 21T gears which are extras and needed for metric you can cut the whole range of imperial and metric threads. 0.75 pitch is one of the standards.
If you get the much sought after screwcutting box model you will need a quite expensive metric add on kit so if you do aim to make full use of metric and imperial screwcutting the non box model is actually easier to use.
Here's a link to an online Myford manual http://www.pilotltd.net/super7.pdf and the screwcutting box http://www.pilotltd.net/s7gbox2.pdf
The Myford use of 21 tooth gears isn't mathematically correct, only the 127 wheel can do that but the errors of using the 21T wheels are that low they are probably better than the leadscrew accuracy anyway.
One disadvantage of the 127 wheel is that it has to be run without the end guard on as it's too large to fit.
21 tooth and 127 wheels are available on Ebay far cheaper than from Myfords, if you can get them to answer the phone, from a delightful seller called Marypoppinsbag, - Hiya Gert <bg>
-- Regards,
John Stevenson Nottingham, England.
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I've bought thread wires and a wiggler from Marypoppinsbag but I never twigged it was one of you two. Since when does Gert begin with an "L" though ?
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Boo

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anotherid wrote:

There are several published charts, from Myford as well as others, that cover in detail the available metric screw pitches available. The full metric conversion set is not really a requirement, as you simply need to check the chart for the particular lathe (gearbox or not) and aquire those few gears that you do not yet have. Looking at the chart shown here http://www.pilotltd.net/s7gbox2.pdf it appears the quadrant would be required as well for the gearbox equipped lathe. If one were inclined to be cheap, a quadrant or a reasonable facsimile thereof could be cobbled together without too much fuss, though you might keep it out of sight when the purists come visiting. For the change gear equipped lathes it is less of a burden as you only have to add a few gears to the collection that should be with the lathe already. 21t, and 63t come to mind, though I don't have a chart in front of me.
If you know anyone with a pile of Model Engineer back issues, you have better than average chances that the subject is covered in some detail in there, as well as several different strategies for dealing with the half nut issues. The booklet from the Workshop Practice Series of books on screwcutting is good for the charts therein, though I was hoping for more than that when I bought a copy. It still rates as a keeper.
Model Engineers Workshop (or perhaps it was ME) ran an article on a metric coversion that entailed a rework of the gear drive train to include a dog clutch. The benefit of that setup was that the clutch could be disengaged and the carriage would back to the starting point of the thread using the crank handle on the leadscrew, then re-engaged without losing reference.
Cheers Trevor Jones
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anotherid wrote:

Brendan, as you know I'm quite a supporter of the imported machines because of their terrific value for money but for someone who can afford a little more and has the confidence and knowledge to buy secondhand, a Myford S7 will rarely disappoint.

You can see from the comprehensive answers to your question one of the major advantages of going the Myford route. Not only do you get a first class machine but a wealth of knowledge and advice is instantly available. The machine has been so widely used over the past 60 years that if it can be done it has been done and by some very ingenious minds.
My own machine is an imperial S7 with gearbox for which I have a metric change gear set. As mentioned before the major difference is the quadrant which has a longer slot to accommodate a wider range of gears. I bought my set from RDG as at the time they were the cheapest, it needed a little cleaning up but works fine. I see no reason why something suitable couldn't be sorted out fairly easily. There are so many gear charts about that produce extremely close approximations of metric threads that it is not really an issue for the common metric pitches.

In my opinion the electrical reverse is necessary but is also not an issue for a S7 fitted with a clutch (they all are), as the clunk produced by single phase on start can be isolated by the use of the clutch. This just disengages the motor from the drive system so has no affect on the gear train. Once the motor is reversed the clutch can be eased in to reverse the direction and return the tool for the next cut without disengaging anything. Reversing the motor electrically is just a switching issue and can be sorted easily without the expensive (but very good) Myford switch. Change gears are available from a number of sources and can be found quite cheaply.

D A G Brown wrote a series of articles in Model Engineer (4145, 4147, 4149 and 4151) about metric thread cutting on Myford lathes. It included some fairly straight forward modifications and covered a comprehensive set of threads including BA and Module. Unfortunately my ME collection is incomplete and I can't help with copies but I'm sure someone will have them.
You already have a comprehensive answer for non gearbox lathes but if you are lucky enough to find a S7 with the standard gearbox the nearest approximation for .75mm pitch is a 28 tooth driver with the gearbox set at 40 TPI. This gives an actual pitch of 0.741mm which is an error of 1.2%. Unless you are cutting long precision threads this is fine for most practical applications. In my opinion having the gearbox is well worth the slight complication when it comes to cutting metric threads.
Best regards
Keith
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jontom snipped-for-privacy@hotmail.com wrote:

Brendan, I should have added that a chart of the possible metric/BA threads available with a S7 and standard gearbox was published in MEW No 42, in a letter from John Walford. It appears that the only common pitch for which there is not a suitable solution is 0.5mm. all the others seem to be covered and the 1.2% error on the .75mm thread is the largest error. Of course if you are only cutting a limited range of metric pitches then I'm sure that the additional of a couple of change gears could reduce this error further. Since this chart was published in 1997 the availability of reasonably priced change gears has improved greatly, particularly the "conversion" 21, 63 or 127 gears. The D A G Brown article explains how to calculate the necessary ratios.
Best regards
Keith
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On 4 Jul 2006 02:52:24 -0700, jontom snipped-for-privacy@hotmail.com wrote:

Keith, I'm sure the error with standard wheels is finer than that on the 0.5 pitch. You can't get an exact match but 35 - 38/25 - 45/20 - 65 will give you 0.4998875
and 20 - 75/65 - 60/30 - 55 will give you 0.5003030
Using two 21's set as 21 - 30/21 - 70/45 - 60 will give 0.5000625
The most exact is a 20 driving a 127 with an idler in between, this does give 0.5 exactly -- Regards,
John Stevenson Nottingham, England.
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John Stevenson wrote:

John thanks as always. In your examples there are more than enough zeros before the error figure to keep me happy. This quest for exact pitch always makes me smile when for typical use you only ever have a few mm engaged at any time anyway.
Hope this post appears only once but I seem to be having a lot of trouble with the Google servers today.
Best regards
Keith
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On 4 Jul 2006 05:22:31 -0700, jontom snipped-for-privacy@hotmail.com wrote:

Same applies here, I have to smile when I see these sorts of figures being quoted as gospel and I remember when I worked at Raglans the lead screws came off the machine like a banana because of the relieved stress's.
I wonder if they mapped any errors after straightening on the fly press <bg> -- Regards,
John Stevenson Nottingham, England.
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Thanks for all the useful info - it's been filed.
What's the feeling towards ML7s rather than Super7s? I did bid on Super7 but was outbid (instantly, and I don't want to start offerin more than I can afford in a frantic bidding war) ... ML7s seem that bi cheaper! For a reason I guess, but ...
Brenda
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anotherid wrote:

Brendan
You are in danger of closing the circle here, the challenge of finding a good Myford for your money drives many into the hands of the likes of Warco. There is nothing inherently wrong with the ML7 it is just an earlier/simpler version of the later Super 7. In fact the specifications cross over with the ML7R which is really an original S7.
Like anything that has been about for 60 years there are good, bad, brilliant and terrible examples to be had. A well equipped and well looked after ML7 will provide many years of enjoyable and productive service. A "dog" (old or new), ML7 or S7 will be an expensive lesson in machine re-building. The ML7 has evolved and improved with the years. The headstock bearings are a good example of this evolution, although all versions are good some are better than others. If you find an old ML7 with whitemetal bearings that's in good condition it will perform well for years. If you find one that is shot you will not get spares from Myford but have to replace it with expensive bronze and a new spindle. Later models like the ML7R have the same bronze/ball bearing arrangement of the S7. However, many of the ML7/S7 assemblies are inter-changeable as the bed changed very little up to the introduction of the power cross feed model. Old machines can be worth more as parts so many are broken and sold on E-bay. If you can do a bit of hand fitting, scraping etc there is no reason why you can't re-build one that suits you. Not necessarily the cheapest way of doing it though. The top speed of an ML7 (except the ML7R) is lower than the S7 and because fo the spindle bearing design it is not recommended that you drive them at the 2000rpm of the S7 for long periods. This again can be greatly affected by the condition of the particular machine, a well looked after and well adjusted ML7 will certainly run faster than standard but possibly not as fast as the S7.
Many complain about the cross-slide, top slide, index dials etc on the ML7 being shorter/simpler than the S7 but they are solid reliable and have served many successfully for a lifetime. The one part I would miss on my S7 is the clutch which is not fitted to a standard ML7. However, there are many articles on how to make one and with a modern 3 phase motor and VFD system it's benefit is greatly reduced anyway.
In short, rather than worry about the name tag (ML/S7) you need to look at the specific machine. Many have been modified and the differences are distinctly blurred (G). I would much rather have a good ML7 than a clapped out S7. I would not hesitate for a second if I found a good ML7 for a reasonable price. They don't have the bells and whistles but they do have they Myford build and feel. Pay particular attention to the bed as although a (Myford) re-grind will only set you back a couple of hundred pounds it could indicate much more work is required (saddle re-fit etc). All of this can be done but it demands patience, knowledge and development of some hand skills if an accurate machine is to be produced. Having said that, many beds are re-ground/replaced needlessly. The physics of lathe design mean that accurate work can still be produced with a fair bit of bed wear.
Brendan, I won't go on as there is plenty to read on the net about the Myford. In my opinion the ML7 is a fine lathe and there are many good ones about. But and it's a big BUT, the design has been so successful there are thousands out there some of which have been worn out/abused and neglected and of course a few have been cared for and loved. Sellers will ask similar prices for both because "it's a Myford". You will need to learn enough about recognizing the differences even after they might have been hidden, or find someone you trust, to ensure that the one you buy is a goodun. In may ways the ML7 is the "bargin" Myford, a very good and capable lathe that suffers in the shadow of an outstanding brother. This is where many turn to a new import as there are less (but still plenty) of unknowns.
Good luck
Keith
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On 4 Jul 2006 09:03:23 -0700, jontom snipped-for-privacy@hotmail.com wrote:

<snipped lots of useful stuff>
I'm a case in point for much of what Keith said. I got an old-ish (1967) but very well used and worn ML7. I got it for the price of a drink and petrol money and have been slowly restoring it whilst still using it.
It's probably about 90% finished now, and has cost me about 400 in parts and services and a lot of my own time. However, it could very well have cost me double this to restore it if I hadn't managed to get a couple of under the counter deals or had no access to other machine tools.
I had the bed re-ground by Myford but ground and scraped the saddle and gibs myself, fitted a new hardended spindle and bearings (definitely an under the counter purchase<g>) and made a new leadscrew from Acme stock with the loan of a NG members lathe. Made new pins and bushes on the Myford and various other bits on the mill & surface grinder. De-rusting, sandiing, & spray painting I did myself
The final step (for which I shall no doubt be pilloried for not heeding earlier good advice on here..) will be replacing the single phase motor with a 3-phase and VFD. This is primarily so that I can get a higher top speed than available at present.
When it is finished I will have a very nice small lathe, in 'as new' condition, albeit with limitiations (the cursed small spindle bore!) in what I hope will be an accurate working condition.
For me it has been as much fun restoring and making parts for it as it will be using it, and make no mistake, they are a very nice lathe to use compared to some of the chinese ones. But I have been lucky, and getting one cheap enough in the first place was a big plus.
You pays yer money...
Peter
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All of this is excellent advice with which, I fully agree. The only thing I can add is that if you are buying at auction (especially eBay) it's good to work out what you might have to pay for a lathe in excellent condition and including all of the bells and whistles that you think you may need and then set your price based on the possible need to totally rebuild and re-equip the machine to achieve the required specification.
eBay items vary (like any market place) considerably from totally knackered to loved and cherished but the prices don't vary much to compensate.
I bought a very well equipped Super 7B on eBay which turned out to be very well worn but I paid what I considered reasonable price if I had to pay for a bed/saddle regrind and refit so, as I got the lathe for perhaps 600 to 800 less than a warranted good item, I was not out of pocket having fully reground and restored the lathe (actually, I did the work myself so I was not out of pocket at all but you get the idea).
One thing I learned the hard way was to beware of the incredibly well equipped machine. Mine came with the lever-collet system and the lever tailstock attachment as well as practically ever other accessory available which at the time I thought was a bonus. In fact, quite obviously in retrospect, the lathe had been used in a production environment and therefore was 'tired' to say the least. A full regrind, total clean, strip and paint, replacement of all bearings and the tailstock barrel (all available from Myford), new chuck and way too many hours to think about, plus the VFD, poly-V belt conversion and DRO has now given me a superb, accurate and near silent machine. It hasn't fixed the small spindle bore problem though :)
Overall, they are a superb machine for small work and are nice to use but caveat emptor, as ever, is very applicable.
Mark
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On Mon, 3 Jul 2006 14:50:15 -0500, anotherid

<snip>
Jusrt spotted a Warco BH600 on the homeworkshop site for 975 ono:
Warco BH600 Gap Bed Lathe (6" x 24")in excellent condition. Owned since new in 2003 & has seen light use, complete with stand, 3 jaw & 4 jaw chucks, faceplate, fixed & travelling steadies, centres, drill chuck, halogen light, T-slotted cross slide. Dual dials - imperial & metric with imperial leadscrew. (I can load onto a pallet if required for ease of collection).975 o.n.o.
Peter
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