Toleranced and open dimensions on drawings

Tony...

You don't mean to say my B-grade in Geometric and Engineering Drawing O=level is next to useless

Neil

In the end Model Engineering is what you want to make of it, you're working away in you own little shed making something for you're own enjoyment, if you want to work with industrail quality drawing correct to BS308 with everything correctly toleranced thats fine, if you want to work off the back of a fag packet making things to fit as you go along thats fine as well, both methods will result in wonderful models, that the builders can quite rightly be proud off.

You can be a Mr Maticulas or a Mr Bodger

Whats realy important that we all work safely and enjoy our modeling.

Stew

I agree with Stew that what we do in the privacy of our own sheds is entirely up to the individual. Tony's point is the quality of the drawings that we share with others. He has a valid point that the drawings that we purchase should be stand alone and not need the words and music, which may not be available in the future. I only disagree with him on the standards that we use. My view is that the drawing should be intelligible to the relatively inexperienced and the modeller should not have to learn a new language before starting to cut metal. In order to achieve this though, the originator must be clear what is intended, using copious notes if necessary.

It's a useful discussion. As I have mentioned before, It would be nice if we could come to some sort of consensus on drawing guidelines for publication in the magazines.

cheers,

Rod

This is the only case where the phrase.

"Fit for purpose REALLY applies.

I think Stew summed it up pretty well

Can I just say, because someone doesn't understand all the jargon of industrial engineering it does not mean they cannot make and even design there own things and more importantly if they have a brilliant idea should they be to afraid to share it because they cannot do industry standard drawings.obviosly good clear drawings are an advantage but  I think its horses for courses.

Edited By Brian Warwick on 26/01/2013 10:30:03

When I design my models I work to a simple system: Mating parts are either Slide fit (SF), Drive fit (DF) or Run fit (RF). A slide fit is just that; it slides with no perceptible slackwhen lubricated; drive fit is lubricated to avoid tearing and forced together at 0.001 to 0.002 per inch diameter interference (which requires a good surface finish) and run fit requires a clearance to allow for lubrication and expansion where applicable. I write other tolerances as I think necessary and always dimension from one datum corner to avoid accumulated errors. colin

Proberbly, at least with models being built for the first time, the drawing and building should continue in parallel, so that the drawing matches the model, and if modification is required to a part, then that drawing should be altered as required.

Colin seems about right to me, but some like to have a dimention for that clearance, or interferance, they might try to fit a 1" OD ball race with a -.010" fit, and if it doesn't crack the bearing, wonder why its siezed up. Ian S C

I think that there are some misunderstandings happening here. In my experience there are several stages to the production of a 99% correct, data pack, or drawing pack if you prefer.

Stage 1, the inspiration lights flickers in the originators head. The Originator (TO) considers the concept, then (maybe) doodles it on the back of a fag packet. No drawings yet, but maybe some very basic sketches.

Stage 2, If necessary TO discusses his concept with the holder of the purse strings. There may, or may not be the go ahead to cut metal. or whatever. TO contacts an appropriate draftsman or techical advisor, who then produces a "Level 1" or "Level 2" data pack. A Level 1 data pack is a recognised and specified level of drawing, at least in Australia. I seem to remeber that Level 1 specifies "engineers or conceptual sketches". Essentially it is a set of drawings which almost certainly would not be appropriate for a Production area, but would be for a Prototype area. There would be a lot of "to-ing and fro-ing", and the time required might be many many times that required when and if it finally makes it into production

Stage 3, If the concept works and the decision is made to go ahead, the next thing is a consolidation of all the data, sketches etc. A Level 2 data pack is then prepared, this may include some toleranced drawings and is probably 90% of the way to looking like a finished product. This enables for the first time people from the Planning/Production areas to become formally involved and start planning/designing tools and fixtures for instance. As a result of this effort a Pre Prototype would be made.

Stage 4, As a result of a successful Pre Prototype the decision to "productionise" the Level 2 data pack might be made, where experience gained would be incorporated in the data pack, dimensions changed as required, tolerances added etc. This would produce a Level 3 data pack. Production plans and tooling and fixtures could then be developed to enable a specified production rate to (probably!!) be achieved. After many reviews and much placing of heads on blocks, the approval would be given to produce a set of prototypes, on production tooling at a reduced rate, i.e. proving the production plans, data pack, tooing etc etc.

To achieve a data pack at Level 3, for almost anything which has to work, without going through the initial stages in some form or another is asking for trouble. This may explain why there are so many "issues" with some ME drawings, they are nearly all at something like Level 1!!!! Just because they have been drawn using CAD doesn't mean they are right.

Sorry to have taken so long.

cheers

Bill

A point that I mean't to make previously.....developing a drawing or pack of drawings is an iterative process. Anyone who imagines that a fully developed data pack leaps from the drawing board, or CAD station, fully formed at the first attempt are deluding themselves.

Imagine a coil spring, tapering, with a smaller diameter coil at one end.

The start point is the larger diameter end, progress is made by travelling along the coils of the spring, with inputs from everyone who matters, not necessarily everyone who has an opinion. Identifying opinions that matter is a key talent. Eventually in a well run process, the end point is arrived at, when the coil will have straightened out....bingo, a well sorted design, a well sorted and proven data pack, ready to push the button.

cheers

Bill (again)

Many years ago, when i built a "Simplex" engine with precise tolerances set in The late

Martin Evans drawings, this good looking engine performed very well except that it would

derail for some reason. A elderly club member undertook to fix my problem.

When he brought the engine back, he commented the the axle boxes were too accurately

fitted and more "slack" should be allowed for so that the engine can "walk" on the track

for no track has ever been built on a billiard table

Confession!

I have identified a missing dimension on my canal crane drawings - the sides of the jib are 3/8" (10mm is close enough) wide.

Any feedback on the drawings will be more than welcome, so I can do better in future. For instance, dimensioning the jib from a datum used alot of space. I try to dimension in the way I mark out - major items from a datum, but a cluster of measurements around a part like a hornblock which just has it's centreline marked from the datum.

Neil

Shocking - I can see the book now, 'The Imperial Case of the Missing Dimension', featuring Sherlock Holmes versus the metric Moriarty.

Oeeeeer, mixing units, that's likely to lead to another controversy. I think I'll keep my head down and go back to my machining.

Regards,

Andrew

Well, a very interesting thread. i have only just re-started model engineering but I have served an apprenticeship and spent 32 years in a modern project/ design office. I have worked on very precise parts that would fit in your hand through to complete plant facilities. So I have seen one or two drawings of various types in my past.

The dimensioning of the drawings that I have seen for modelling are absolutely fine but it is a shame that there is no agreed system to record the modifications on the drawings we buy.

They must be ok because every time I go to an exhibition - there are hundreds of locomotives, stationary engines - you name it - all built to the many old and new designs. It would appear that the people who built them were able to do it without the special tolerancing tables or dimensioning from datums etc.

PhilH

Neil I had spotted that but you did mention 3/8" in the text. Also missing the fact that the top end is reduced down to 1/4" but I worked that out based on deducting the hole spacing from the OA length.

Also quantities missing from the main parts although you have them for the small bits, but again with teh photos its easy enough to see two of most things are required.

Maybe thats why I got a grade A and you only a B

Main thing is that there is enough info there to make the item and that is what really matters

J

Edited By JasonB on 27/01/2013 12:36:37

I haven't read through all this thread, but I still feel it would make sense to have an area on this site where up to date drawings can be deposited and subsequently downloaded (PDF, or DXF etc) by us to help keep things a bit simpler.

Martin.

When I was an apprentice all of the drawings had a grid reference system. Any changes were entered into the box provided for the purpose on the drawing template giving date of change, grid reference, new dimension and previous dimension. This seems to be a sensible way of noting any changes. Colin

Oh no, please don't start on change management.

cheers

Bill

I'll second that...

having seen two DIFFERENT issue 5 drawing for a part were the last (on subtle and slightly underhand request for ALL back issues) common ancestor was issue 2...god help 'em...

.In this I have been on both sides in my time..both poacher and gamekeeper....

As to drawings..above all else two things on a drawing please..

.Issue/date

and a border so that when copying you can tell you have it all....