Mixing fractions and decimal units in an imperial drawing

Hi Guys,

I'm after a bit of advice please.

I'm making some drawings for a small oscillating steam engine and though I've done quite a few drawings in metric, I have little experience in imperial drawings.

Following on from looking at some Stuart drawings I've dimensioned most components in fractions (a few where necessary to 64'ths). I have a couple of instances though where a dimension of 0.1" is more appropriate.

Question 1: Is it okay to mix fractions and decimal units on a drawing

Question 2: How should I tolerance the fraction dimensions? For a drill size for instance and also for a dimension of two shafts, say centres 2" apart. The steam engine is being built from castings.

Many thanks,

Sarah

Hello Sarah, personally I would forget the fractional measurements. In the early days of model engineering people would use a rule and calipers but these day most people will use a digital caliper, they are so cheap these days.

Be interesting to your design when it is finished.

Brian

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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All the drawing I was given were always dimensioned in fractions .............. If no other tolerance was given then the dimension was to a 1/64th, perhaps the material left after spotfacing a bolt hole on a casting. I most cases however there would be a tolerance in thous , I can't present it as on the drawing with one number over another but it would say ( lets look at our spotfaced hole again) 3/8" +000 -005, so between 0.370 and 0.375, just a drilled hole then , if it had said 3/8 +000 -001 then that would be a reamed hole. It always seemd fairly straight forward to me.

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

What I see on a lot of teh American drawings where Imperial still rules the roost is this

2 decimal places +/- 0.01

3 decimal places +/- 0.001

4 decimal places +/- 0.0005

So for instance a dimension of 0.50" would not be to such a high tollerance as one shown as 0.5000"

Fractions are not often used now, but if the drawings are for your own use then use what you are happy with.

If you are showing a hole that needs to be exact there is no reason not to put "ream 3/16" and for a less critical one just put 3/16" which most people will take as a drilled hole.

J

Edited By JasonB on 05/11/2016 20:04:43

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

Neil, so you saying 20mm Dia. BMS should really be noted as "1/50 Mtr Dia." !!! Oo-er that's wot I been doin' wrong all this time.....

Mark

PS. Where do you get imperial ground stock - import it from USA or have it made special?

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

Jason, just looked at the prices!

I buy my stock from a mainstream stockist (account) and generally buy stock lengths and they don't do imperial unless it is stainless or Al (which depends on the source these days as to what units you will get). At that price, it would be cheaper to buy a grinder....

Mark

PS. I have recently bought a small pile of Vanadis (about 100 kg) - now that was really expensive.

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

Sarah

Re your Q2 about two shafts 2" apart. Regardless of how you show the tolerance the more important thing is to establish what variation your finished product will tolerate.

If the two shafts are coupled by a rubber belt on pulleys then the distance between the two shafts is not going to matter very much. On the other hand if they are fitted with gears with fine teeth then the centres need to be exact for the gears to mesh properly.

What Jason said about the number of digits after the decimal point defining imperial dimensions resolution, also applies to metric annotation. I have seen drawings where the draftsman (sorry draftsperson!) had put notes in the title block stating the relationship between tolerance and shown digits applied to the whole drawing except where explicitly shown otherwise.

Ian P

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.

Being relatively new to machining I prefer using metric units, but if I were asked to make something to imperial it would be nice to see drawn dimensions in whole inches and/or thousandths. I find thou' easier to visualise than fractions.

Ed.

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 am currently building GH Thomas's dividing head from the old-time drawings in his book "Workshop Techniques" and his use of a mixture of fractions and decimals is driving me nuts! Please don't do it. It's awful. You spend half your life thinking "now what is 27/32 as a decimal and looking it up on charts or performing mental gymnastics, and getting it wrong.

He tends to use fractions for lengths of shoulders or sections on turned shafts that are not critical, so +/- 1/64" I suppose, but not always. Then the critical measurements, mostly diameters of shafts, pins, bushings etc are in three-point decimals, which indicates +/- .001". Anything finer than that is specified as "0.3750 - 0.3755" etc. as is commonly done.

Another trick ISTR from my few months as a pimply yoof in the drawing office many decades ago is that instead of specifying, say, an unmachined piece of 1/2" BDMS used say as a dowel pin or an unmachined section on a non-critical stud etc was that instead of speccing 1/2" diameter it was rather written on as 0.5" Nom.  Nom standing for nominal, so indicating it is the unmachined original size.

Edited By Hopper on 06/11/2016 02:10:33

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" 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

Do your drawings in metric so much easier to work with. I draw in metric and machine in imperial it sound daft but it works for me. Most model engineering drawings are not toleranced mine certainly are not but I know for my application what fits are required from a performance requirement. Also I very rarely do a complete drawing just put on important centres and dimensions, they would not make much sense to anyone else.

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

During my working life I never used or even saw fractions on drawings. I have only used them in model engineering drawings. There is something nice and satisfying about them, mathematically, but I would not recommend their use to anyone. They are a relic from a bygone age.

Obviously make use of fraction size drills etc where convenient.

JA