Making drawings in millimetres of parts which have always been imperial rather negates the 'clear and easy to follow' idea.

If by 'every part' you include the fasteners, what will be your metric description of a 2BA x 1/2" long socket capscrew?

It would be much clearer and more logical to make all the dimensions Imperial but with decimal rather than vulgar fractions, and the fastenings all BS or BA, since if you make accessories for a machine built to British dimensions it is easier and much better practice to make them conform to the machine itself.

If you are making accessories for a Myford ML7, it is very likely you will be turning the parts on the said lathe, which was normally fitted with inch-fraction lead-screws and calibrated in thous.

Keeping the fastenings consistent also avoids the bad situation of needing three ranges of spanners for operating and servicing it! (Some of the commercially-sold Myford stuff now seems to have various inconvenient hexagon sizes.)

If constructors wish to use a rare ML7 fitted with metric screws and dials then obviously they'd need convert the drawings to mm; but printing the drawings in mm means most users will have to convert all the turned sizes to decimals of inches anyway.

It depends on the people you aim at

An imperial set would be for the more purist traditional customers, and there are a lot of them

10ths sounds like good solid advice (decimals) and modern measuring kit uses decimals

A metric set would suit more practical off-the-shelf get-it-done people

I'm happy to bang off the shelf metric stuff into my myford-drummond gear but there may be a faint sound of wailing and gnashing of teeth from the ghosts of bygone past

Edited By Ady1 on 15/09/2023 23:52:24

Duncan –

True, and I think BA was based on the Swiss, Thury thread, though codified in British Standards in inches. The larger fastenings on Myford lathes are BSF or BSW though, so true rather than derived. inch fractions.

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Ady-

Nothing to do with "purist traditonal", while if your are a "practical get it done type" you will be able to use both systems. (I use Imperial or metric according to what I am making for what purpose.)

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Jason –

"…. suit the oldies…"

You youngsters…..

Suit those who will be making the parts on the same lathe with inch-pitch lead and feed screws and decimal-inch dials, more like…..

Nothing to stop you measuring the original made in inches, directly in mm and making the accessories to fit but themselves measured in mm, if you are using another machine that is metric or has a dual-range DRO, but you'd be working to irregular numbers.

Holes for screws are not too problematical because many screw-thread charts show tapping and clearance diameters in mm anyway for metric, BS and American screws; suiting the commonly-available drill sets made to 0.1mm (~0.004" ) increments.

Whether I use inches or mm, I try to use nice round numbers, even for inch-fractions; especially for something entirely new.

The one to watch is a counterbore intended for a socket-spanner rather than a socket-head screw, and clearance for the appropriate spanners generally. 

The biggest bug-bear in model-engineering has anyway been the way drawings' dimensions have long been laid out, not their units. Though sticking to vulgar fractions even for parts intended to be made on decimally-calibrated machine-tools (mm or inch) parts is a secondary nuisance.

Edited By Nigel Graham 2 on 16/09/2023 08:00:05

I'm fairly old (73) but find metric much more user friendly.

Drawings are useful if you want to do your own upgrades or see how an add-on project will interact

You do however need CAD skills in the first place to take full advantage of it

Handy for missing bits too, as Jason said

Saves you going into the workshop and scrabbling about

As Myfords are imperial machines then dimensions in metric will throw up some awkward figures and some rounding if the accurate conversion is not used. 1/64” is 0.396875mm or 0.015625” . The problem with rounding dimensions is that the errors can accumulate. Is your idea to produce a dimensioned and toleranced drawing to make a part to original spec. or just a general drawing with dimensions? I would think most people would be fitting the parts to their machine so just the nominal dimensions would be adequate.

Mike

Mike –

I think your last sentence summarises it admirably!

Harry –

We all do our best to produce decent-quality work as accurately as we can but model-engineering has never worried about stating tolerances and formal standards on drawings because we make things to suit our workshops and our own skills, for ourselves, not for interchangeable production; so tend to be fitter-turners rather than production-machinists.

Obviously we should and usually do endeavour to make, for example bearing-race fits as specified for the bearing types, O-ring and circlip grooves to the manufacturers' recommendations, etc., and these are not difficult to find (often from the part makers); but not all of us have full, ISO9001-registered metrology facilities.

By all means put those dimensions on the drawings, as they should be; but a model-engineering workshop drawing does not need meet full, formal ISO industrial standards. It's more important that the drawing is clear and unambiguous, the details and dimensions are correct and the parts made to them do actually fit each other. (If it's any comfort, even professional trade draughtspeople occasionally make mistakes – or design details that work as CAD images but can't be made!)

Why has your post appeared twice? This seems to happen fairly often (with different people!).

Likewise though – I am used to both metric and imperial but by work rather than the inch-based model-engineering.

Many of the trade drawings I encountered carried tolerances of +/- 1/64 for fractions of inches, but 0.01 or 0,005 inch for decimal, u.o.s; depending on the dimension's function. I forget the mm tolerances but 1/64" = 0.4mm.

I used metric measurements and fastenings for the travelling hoist I built for my 16-foot X 6 ft workshop – after all the materials all came in mm sizes and this is a new item not a scale model, part of the workshop equipment.

On the other hand my model steam-wagon is in inches as the original was English-built more than 110 years ago – though contains an unfortunate mixture of BS and BA with Metric fasteners. This is poor practice, I know, although sometimes for reasons like simulating rivets or to be stainless-steel.

Similarly I try to make any machine-tool accessories compatible with their host machine, be they of metric or Imperial dimensions, including threads.

The original question though was about drawings for fittings for Myford ML7 lathes, and many tools and accessories for, or designed to be made on, these lathes, have already been published in ME and model-engineering reference-books over many years. So new ones are unlikely to be at all "new" even if you suggest alternative materials or dimension them in millimetres.

The original machines were all built to inch sizes so while some constructors may well prefer to make fixtures and fittings in mm, at least some important dimensions need match inch-based ones on the lathe: spindle and tailstock diameters, the T-slots, bed, etc. So somewhere along the line someone needs convert them, be that the designer or the maker; and the conversions either way will not be nice neat integers.

In making the drawings, it matters not a jot whether CAD or manually drawn as long as they look proper but more importantly, are clear and free from frustrating mistakes!

It may be possible to transmit a CAD file to the buyer electronically – but he needs the same or compatible CAD software and a printer for the drawing's A-number sheet size. Squeezing drawings created for an A2 or A3 sheet onto A4 for example, risks dimensions and other annotations, and fine details, too small to read. I'd rather buy paper drawings in their proper sheet sizes, from the start.

It happens fairly often on different fora so must be some quirk of how the site works.

Hi Jasonb 1,000 gb connection so not speed. Fat fingers possibly but I usually use a stylus.