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Actual size Drawings

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Ricardo29/11/2014 13:52:50
13 forum posts

Hi

Have just received a set of drawings from one of our main suppliers for LBSC Doris.

I assumed that these would be drawn "actual size", they are not and don't seem to be to any consistent scale.

My question is, is this normal? and can drawings be supplied actual size from any other source?

Thanks in anticipation of a response.

Richard

Circlip29/11/2014 14:23:23
1101 forum posts

Drawings are (Hopefully) a scale representation of a required part. By necessity, it might not be convenient to show at full size due to what other parts may be "Fitted" onto a "Standard" drawing sheet. Sometimes it may also be convenient to show a representative shape of a part with dimensions that don't even scale to the finished part, Draughtsmans License (a sometimes too easily obtained qualification, - I pushed a pencil and set square for many years) allows a quick sketch and the necessary measurements to convey the info to make a part without a Rolls Royce representation - Full Size 12BA screw 0.134" long ??????

A more pertinent question - which you SHOULD ask BEFORE starting to cut metal is "Does anyone have a list of known errors in the dimensions for LBSC'S DORIS?" I can guarantee there will be some.

Regards Ian.

Ricardo29/11/2014 15:05:57
13 forum posts

Ian

Thanks for the comprehensive reply and advise. I'll ask the pertinent question in another post.

Regards Richard

Circlip29/11/2014 15:14:12
1101 forum posts

Now that question is going to raise a hailstorm from them as don't know what's involved in "Correcting" a drawing. Suffice to say, it's been discussed many times so don't allow garbage and vitriol to deflect you from the question you need to ask.party

 

Regards Ian.

Edited By Circlip on 29/11/2014 15:14:35

Edited By Circlip on 29/11/2014 15:15:20

JasonB29/11/2014 16:24:51
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18101 forum posts
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Its usual for the parts to be to some form of scale, larger ones may be reduced to fit a standard sheet or very small ones shown larger so you can see the detail.

If there is no obvious ralationship between the part and a scale such a sFull size, half fullsize, twice full size, etc then I wonder if the drawings have been tweaked to fit modern A0 or A1 size sheets from the old double elephant or similar sizes which would be a real pain. Can you tell us what a part that is say dimensioned at say 1" actually measures on teh sheet

Neil Wyatt29/11/2014 19:18:41
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The most commonly used sentence on engineering drawings is 'DO NOT SCALE'.

Neil

Flying Fifer29/11/2014 20:46:32
180 forum posts

Neil,

The one I remember is "Not to Scale" ( I did work on full size aircraft !)

Richard

Many years ago I used to take ME & a very regular query was about drawing dimensions on something or other & I don`t just mean LBSC`s work. I dont think the mag owners at that time had anyone to check drawings so unless there is a box on the drawing stating drawn by Joe Bloggs CHECKED By Fred Smith BUT & its a big BUT the dimensions could still be wrong.

Drawing errors are not unusual I`ve found incorrect dimensions on drawings produced by some very famous companies & the hoops you have to go through to get them corrected would make you weep !! It was "fun" at the time but I`m glad I`m retired.

ALAN

Paul Lousick29/11/2014 23:10:23
1408 forum posts
540 photos

Hello Ricardo,

The quality of drawings for models from well known suppliers is one of my main gripes. After working for more than 40 years as a professional draftsman in the mechanical engineering industry I am very critical about drawings. "Proper" drawing techniques and standards are no longer taught and many of the young draftsmen that I have worked with in the recent years do not have the skills of the old generation.

Drawings for model engines are either copied from the original engine drawings or measured from existing parts. Often the components are simplified for making parts for models. Different scales for drawing parts is acceptable on drawings but they should be labeled if different from the overall scale of the drawing shown in the title block. (This is often not done).

Even if the drawing is drawn to scale, it may not print correctly and could be out of scale unless the printer has been calibrated. The X and the Y (width and height) dimension on the drawing may print at a slightly different scale and the paper may shrink/expand after it has been printed, so drawings should never be scaled to measure critical dimensions. Always calculate any dimensions which are missing.

I have received a set of drawings for my 6" traction engine which contains many mistakes and I double check all dimensions before making them. (Dimensions on some parts do not match the dimensions on mating parts.)

They say that you only get what you pay for. It only cost me $150 for a set of 25 drawings which is very cheap. If they had been drawn by a professional draftsman and checked by another they would have cost many thousands..

Paul.

Ian S C30/11/2014 09:04:30
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7468 forum posts
230 photos

One important reason for "do not scale from drawing" was the use of "blue prints", in the process of making a blue print the paper is wetted, and dried with a heater, at school we had a go at making our own prints, and the paper shrank, and distorted slightly.

Ian S C

Ricardo30/11/2014 10:07:39
13 forum posts

In answer to your question Jason. A dimension of 5 1/4 ins on the main frame is represented by 4 7/8 ins on the drawing. As this is a relatively small reduction I find it hard to believe that LBSC wouldn't have drawn the main frame actual size. Incidentally, I used to build model aeroplanes and plans were always actual size

Maybe I am expecting too much.

Thanks

Richard

JasonB30/11/2014 10:22:59
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Does sound a bit much for the usual slight distortions, what size paper is the sheet that would definately confirm if they have been reduced down to fit modern A size sheets. The proporsons sound about right for an old "Royal" size sheet that has been reduced to A2

Is there a border around the drawing, if so is it proportional around the edges

Edited By JasonB on 30/11/2014 10:23:49

Ricardo30/11/2014 10:28:18
13 forum posts

There is a border and it's a different distance from the edge on each side of the paper.

David Jupp30/11/2014 11:31:06
728 forum posts
17 photos

Model aeroplane drawings are often full size because it is expected that the drawing will be used as a 'template' for cutting and/or bending balsa - the same is not usually true in metal working.

Reproduction methods used can introduce deliberate or accidental changes in scale. Where microfilm has been used, I've even seen straight lines come out very slightly curved. The 'scale to fit' function of most computer printers can introduce slight shrinkage.

Regard the drawing as a tool to convey design intent, not a perfect rendering of the part in question.

If you desire accurate actual size drawings, you'll likely have to re-draw them yourself (but note previous comments about paper size stability).

I've noticed that a number of people have posted in other threads that they routinely re-draw / re-model a design in CAD to both check dimensions, and get a good understanding of how things go together.

Derek Drover30/11/2014 11:59:33
85 forum posts

Cheap photocopy reproductions always change the size/ratio of the copy to the master.. they're never the same.

jason udall30/11/2014 12:15:14
2024 forum posts
41 photos
It might be best to "assume"( with all that implies)
That "drawings" are a set of dimensions , held together by a sketch of where said dimensions apply...
Oftern in production several parts from same drawing with a table of dimensions against part number....
Sorry but as has been said unlike "full size plans"..drawings are often to some arbitrary scale and aspect ratio..often differing in reproduction from the original.
Neil Wyatt30/11/2014 13:15:45
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Unlike balsa airframes model aircraft, metalworking is often to tolerances that are smaller than the width of the lines. Would you measure to the inside, outside or centre of the lines when judging if a drawing is accurately to scale?

Engineering drawings are like maps. The actual scale is unimportant, it just has to be big enough to show the detail you need and small enough to be usable.

Neil

KWIL30/11/2014 17:04:06
3233 forum posts
63 photos

I always scan original drawings of whatever scale as I do not want the originals covered in oil and metal chips in the workshop. That way I can print the whole or extracted parts to suit the job in hand on a suitable covenient piece of paper. Hand annotations and changes can be added in the workshop and transferred to the originals later ie a metric thread in lieu of imperial etc.

Edited By KWIL on 30/11/2014 17:07:51

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