The usual rule in imperial is fractions +/- 1/64", decimal +/- 0.005" unless other wise stated on either. One of those for instance would be hole -0 +0.0005" to indicate a reamed hole – and a reamer that was bang on size. They are generally oversize now but I assume closer tolerance ones are available.

Something similar was tried on metric when it was introduced. Initially 1mm for loose dimensions with wide tolerance and later 0.5 was added to those and some started using 2 decimal places to replace the notional +/- 0.005 which would then be +/- 0.1mm. Again unless other wise stated. 1 decimal place would indicate +/- say 0.5mm but some argued it should be +/- 0.4mm. The aim being to avoid tolerancing everything individually.

It was around this time when I moved well away from areas in which tools and products were drawn. There was a lot on inertia and arguments going on,

Surface finish was also covered by these unless otherwise stated. For instance a hole to 0.0005" needs a much better surface finish for it to be meaningful.

John

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I wasn't looking over your shoulder, honest…..

Think about how you expect the person making the part to work.

I practical terms, dimensions in fractions, measure with a rule, in thousandths of an inch, use a caliper or a micrometer, or at least work as accurately as you are able.

The heigh of your standard is probably not critical, but the positioning of the ports probably is.

Where fractional dimensions need to mate with each other or match then you may still need to work to thou accuracy, but the exact measurement is usually not critical.

Another example, unmachined bright drawn mild steel -specify in fractions. Precision ground mild steel – specify in thou.

Neil

Imperial PGMS is available from most of the ME suppliers such as College engineering, M-Machine , EKP etc

From what I have seen, it is ok to use fractions and decimal inches. Important dimensions are usually stated in a note the particular important dimension. The overall is normally like been mentioned before by others.

The only difference I have seen is that on 4 decimal places the tolerance would be +/- 0.0002 unless stated.

Anything with 5 decimal places is always stated. Like in metric, anything to 4 decimal places is always stated.

Neil

If you are making the drawings for yourself you can use any system that you like. Personally, as an ex draughtsman but now 69 years old, I would dimension all my drawings in millimetres. They are so much better.

From this side of the pond for Engineering drawings: inches and decimal inches definitely not fractions.

Tolerances:

1-place ± .03
2-place ± .01
3-place ± .005

(These vary somewhat according to company and industry)

Fractions may well be used in architectural and woodworking drawings (and by people who don't know any better).

I have worked to fractional dimensioned plans where the terms "Bare" and "Full' are used meaning minus a bit and plus a bit, leaving the fit to the builder. When making a model, often dimensions are a guide line and the engineer may vary such to suit tools and material available. If you were making a production line of the models and contracting out the fabrication of parts, you enter a very different ball game.
If you intend the plans to be universal, then it would be necessary to produce two sets of plans, one in metric and the other in imperial (although nothing is impossible).
For imperial, I would use fractions for holes – like drill 3/8" Dia or drill #43 or drill letter J or ream 1/2" and not add a tolerance. If various shaft / hole fits are required, then use the ISO system. For dimensional lengths etc, use decimal inches – like 1.375" ( 1 3/8&quot and only tolerance the dimension if precision were required.
Other terms may be used, Running fit, Press fit, Shrink fit etc and leave it to the builder to investigate the actual dimension based on material to hand.
BobH

The advent of affordable electronic calculators in the late 1970s made the use of decimals in imperial dimensioning much easier than the use of fractions when addition or subtraction of dimensions may be needed. Add to this that machine tools have scales that are not marked in fractions and fractions seem like a strange and old fashioned way of dimensioning a drawing.

Micrometers, vernier calipers, dial calipers and electronic calipers do not usually show fractions on their imperial scales.

I think you should drop use of fractions on any new drawing.

Martin

Ps.  I have not seen DTI or dial gauges with fractions either. 

Edited By Martin Connelly on 06/11/2016 09:11:17

Hopper. The fractions were generally measured with a rule even on lathe diameter work. That way they don't generate confusion. The main problem is that this takes some practice and just isn't on with the metric system unless things are made to 0.5mm. This is why imperial engineering rules have such a lot of different scales on them. Other than dia I still work that way. If metric I guess to 1/3, 1/4 or 1/5 of a mm.

These days if I need an new imperial rule I have to buy from the USA and hopefully get one with 1/100" on it but the way things were usually drawn 1/64" was fine – even for grinding allowances on turned parts.

My recollection of many model engineering drawings is that the people who did them have never worked for a company that had sensible standards. Some aspects can need construction notes because of the way they will be made. Eg a piston in a bore needs a certain fit. Within reason diameter doesn't matter. A shaft might be supported at each end. The hole size used at each end may be important also the fact that they closely align with each other but actual positioning may be less important. For some people these factors would be obvious just from looking at the drawings others may need constructional articles. The alternative would be to assume people had jig borers, hole micrometers and a number of other things. Ideally thing need to be thought out so that they need minimal equipment. Say a lathe and a vertical slide.

Mmm. The dividing head probably needs a T slotted cross slide as well. Many more complicated model engineering tooling designs did.

John

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Edited By Ajohnw on 06/11/2016 10:01:02

For those that don't use or have not seen fractions on a drawing how do you specify threads?

0.375BSP or 0.25BSF just does not look right.

And what about SWG do you use that or just the decimal thickness?