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Author Topic: Scale and/or imperial vs metric conversions.  (Read 1297 times)

Kevin.Hutch

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Scale and/or imperial vs metric conversions.
« on: July 06, 2021, 02:07:41 am »

I am old school and a new boy on this forum, I have been using CAD for over 40 years have not mastered the conversion from 2D to 3D, I think the learning part of the brain has slowed down too much. I been using Autosketch for some 15 years and done many boat drawings for survey work and boat operation manuals, I have just completed a project of digitizing a very scratchy copy of Naval Architect General Arrangement drawing to a scale model build drawing and enjoyed the exercise so much I am looking for a similar project.

Those intrigued with the prospect of converting a set of 1/2”:1’ plans to create a model of 1':1' scale of choice or imperial 1:12 to metric 1:20 size conversion, that are not already CAD operators, the prospect may seem daunting. Let me tell you that, as students of CAD will testify, scale conversion is as simple changing the units and resolution, (fractions to decimals) from metric to imperial conversion and/or converting the scale to fit the paper available in the output device.

The only confusing factor is the scale of the original vs the scale of the CAD drawing vs the scale of the model vs the scale needed to fit on the paper if any dimensioning is needed to be relative to the final output. The paper scale may vary if it is plotted as “fit to printer page” size vs full scale.

If you are not a CAD user and are not intrigued to learn, then provide a good resolution file copy of the plan that can be scanned by many office support shops that run plan printing services for under $2 and enlist a CAD operator’s assistance.

The CAD operator has then only to pull the scanned image into the CAD program of choice and trace the detail you require for your build, then scale it to your chosen scale and output it to paper. The time-consuming part is what level of detail you want in your plan.Alternative obtain a set of offsets for the boat and a CAD operator can create a digital image of the boat to what ever scale you wish, then scale it to fit on a piece of paper. Of course you could “loft” the dimensions directly onto the material to be used for frames/bulkheads.

If the model is larger than the printer available, then most CAD packages have an output capability to scale print a “tiled image” on multiple pages that are simply assembled after printing, or you could have smaller sectioned prints of parts to work with.

Alternative a trip to the local office support plan printing company, for around 10cents for A3 (297mm x 42mm) plan print up to $2.50 for A0 (841mm x 1189mm) they can even print it on tracing paper or laminate it.
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Kevin Hutchison
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BrianB6

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Re: Scale and/or imperial vs metric conversions.
« Reply #1 on: July 06, 2021, 04:18:43 am »

Many Plan Printing firms have printers with continuous rolls of quite wide paper so print size should not be a problem.
The printing ratio is finely adjustable so knowing the imperial / metric ratio should be no problem.
The only problem, as with all prints, is the movement of the printed material.   Not even film is immune!   The old instruction on drawings was always "DO NOT SCALE"
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derekwarner

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Re: Scale and/or imperial vs metric conversions.
« Reply #2 on: July 06, 2021, 04:54:55 am »

Many of these newer Industrial/Commercial printers have a True Scanning Facility, so scan the plan........increase the scale ratio........save the image as an upscaled .pdf file......then print out the selected files............  O0

Keep this electronic file backed up to a few different locations..
:-))


Courtesy of a previous named  'the big Australian'........... %)


Derek
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Derek Warner

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Kevin.Hutch

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Re: Scale and/or imperial vs metric conversions.
« Reply #3 on: July 06, 2021, 05:01:47 am »

During the digitizing of my project the original plan was for a 1:10 scale that migrated to 1:20 and this is still a requirement. Due to the accuracy of the original data there is a question as to some lines and a half model is under construction and now a prototype of 1:35 is intended to iron out the differences before embarking on the large scale. All available information is in imperial and the builder prefers to build in metric. All draft plans were printed on A4 paper so tiling was needed for the few copies at full scale size.


It occurred to me there may be others not familiar with CAD that wish to change plans dimensions from imperial to metric or scale from the odd 1:12 to 1:10 for their plans that may wish to print sections or all on their home printers that usually run A4 paper with a few up to A3.
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Kevin Hutchison
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grendel

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Re: Scale and/or imperial vs metric conversions.
« Reply #4 on: July 06, 2021, 05:51:59 am »

as with everything there is a best practice way of doing things, in CAD everything is best modelled at full size (whatever the units) once you have the full size model in CAD theyn prints can be made at any scale you wish, as brian B6 says though, if printing remember the stretch in the paper, and that with varying humidity the paper print can grow or shrink, on a really good printer the printing stretch may be under 0.5% and can be calibrated out (though not may printer owners bother to do this, and a bad printer can be several % out) but the humidity stretch can be as much as 1% or even more, this can lead to up to a 2mm discrepancy on an A0 printout (5-10mm on a bad printer), if you can live with this then fine.
Paper stretch used to be countered (pre CAD days ) by drawing on draughting film (before that on waxed linen ) this was dimensionally more stable, but could still stretch a little during the printing processes.
the main problem is if your deawings are printed across several sheets, then orientation will matter, if a part is oriented with the direction of printing, then any stretch will be consistent across sheets, parts oriented ar 90 degrees to the same part in another drawing may well disagree dimensionally. as long as you remain aware of this then you can work around it, and it comes down to how fussed you are by the dimensional accuracy in the finished model,
a good example is the frame stations, they may disagree with the widths on the plan because they have been drawn at right angles to the plan and side views for onvenience, and whichever way they are printed, they will be at right angles to either the beam or the vertical so may get stretched in one plane.
As a drawing office manager, and modeller, I spent a lot of time looking into ways to reduce these inconsistencies, until I realised that unless I wanted to continually run calibration prints before every print this just wasnt going to be fixable.
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derekwarner

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Re: Scale and/or imperial vs metric conversions.
« Reply #5 on: July 06, 2021, 06:01:14 am »

Kevin......I had the pleasure of being in the 1st Year of the Wyndham Scheme in Australia.....yes we started with Rods, Perches & Lbs + Oz's  and converted these to mm and grams     %)


So converting Imperial dimension to Si or Metric is not really all that difficult  :-))


The issue is accepting the result......by example your Dimensional detail of the vessel LOA was 63' [ft]

This dimension is really just a whole number O0
However when we convert this dimension to a Metric unit, the LOA becomes 19202.4 mm >>:-(


If this were  an original European design, the Draftspersons would have drawn this as 19200 mm


When cutting wood, I prefer mm, when marking out a brass component, I use mm


However when I think of a class of fit between metal components, I remember the 0.001" clearance per 1" of diameter  {-)


Derek
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warspite

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Re: Scale and/or imperial vs metric conversions.
« Reply #6 on: July 06, 2021, 09:48:54 am »

I am a majority metric man, most of the work we did was metric and the best company I worked for, it was only the oldest drawings that were imperial and the old 1" = 25.4 rule would be used and we would only round down from 19202.4 to 19202 though in the first year of my time in a drawing office at GEC, I would try to get the .4 in if at 1:1, but most of the drawings of the casing we drew were at scales that when we scaled the 1:1 down it came out as say 125.6, I would drawing the item as close to 125.6 on the film as possible, used to drive the others mental, but as the chief draughy would say " if you draw it to scale and can prove its to scale then no one can complain you did not draw it correctly"


Used to drive some foreman nutty when I went to inspect a part that we had drawn from German equipment manufacturer Siemens pressure tank and our manufacturing had built one that I had found they had missed a bracket off or put it in the wrong place, ahh those were the days.
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Re: Scale and/or imperial vs metric conversions.
« Reply #7 on: July 06, 2021, 11:04:55 am »

Metric all the way but even that has confusions.

I think millimetres but others seem to think in centimeters.

My newish car still has a miles per gallon dash readout even though I buy petrol in litres & we have done so for many years!
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Kevin.Hutch

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Re: Scale and/or imperial vs metric conversions.
« Reply #8 on: July 06, 2021, 11:05:43 am »

Guys whilst all the comments are factual but, this is a model makers forum and I posted in the beginners area, siting advice for non CAD students, it was not my intention to open an in depth discussion on engineering precision as it applies to drawing.

In all my time in drafting it has always been practice to select a suitable scale to fit the subject of my drawing on an available piece of paper, I certainly would never considered a 63 foot 1:1 drawing and this option did not come in until the introduction of CAD and even then most good CAD programs honored the drafting practice of drawing a scale drawing from scratch. AutoCAD was the exception breaking tradition and setting the expectation that a drawing be done as 1:1 and only scaled when a printout was required.

Conversely beginner model makers, I believe start with a paper plan of a suitable scale of the model intended, marked up in inches. I merely offered an understanding that a simple 12 based scale can easily be converted to a 10 based one to build a model in a metric scale and if it came in imperial scale/measurements it could easily be converted.

In all my experience in boat drawings, I have never seen a plan purporting to be more accurate than the nearest 1/8" in real dimensions and conversion to metric rounded to the nearest 1 or 2 millimeter is a realistic compromise. There is molded length and breadth, versus overall based on the arbitrary hull thicknesses that is rarely precise to less than the variations in paper sizes. I always relate to boat length in feet yet always draw in metric, drawing a boat with all the information in feet / inches - fractions  halving measurements to mirror image them has really exercised my brain.

A boat length is open to so many interpretations Beneteau even measure their length along the gunwale, so 2 mm conversion is really academic especially for the beginner modelmaker, maybe to a 3D CAD specialist but not to most.
 
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Kevin Hutchison
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grendel

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Re: Scale and/or imperial vs metric conversions.
« Reply #9 on: July 06, 2021, 12:46:41 pm »

Actually AutoCad was only one of the first, their direct competiton bentley microstation did the same at the same time, though the autodesk system was slightly easier to use.
the big problem for the modeller is the reproduction of old plans, they can have been scanned and printed several times compounding any of the aforementioned errors, at least if they are imported back into Cad they can be differentially scaled to return them to their original sizes, as long as you have some fixed dimensions in each axis of the drawing to scale them by.
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ChrisF

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Re: Scale and/or imperial vs metric conversions.
« Reply #10 on: July 06, 2021, 06:09:57 pm »

Kevin, I had to smile at your comment about 1:12 being odd as that scale and its multiples are used for many model builds. It's 1:10 etc. that is odd!


As you say most beginners tend to build from existing model drawings and if they do want to change the scale then the easiest way for them to do it is to have the drawings copied, adjust the scale to produce the size of model required and then print them out - all done by a copy/print shop. I doubt that a beginner would want to convert from imperial to metric and change scales etc. unless they already had draughting or CAD experience. And the use of a CAD operative would be cost prohibitive given the amount of work involved unless you have a friend.

 
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Building Fairey Marine boats: River Cruiser 23, Huntress 23, Huntsman 28, Huntsman 31 and Swordsman 33 and two more to come! All to a scale of 1:12

Colin Bishop

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Re: Scale and/or imperial vs metric conversions.
« Reply #11 on: July 06, 2021, 06:19:09 pm »

I build my models in an empirical way. I use metric or imperial whichever is the most convenient for the particular measurement in hand. It gives lots of extra flexibility.

As has been said earlier, for the size of models most of us build, a few mm error in something like hull length is neither here nor there. Many of us are incapable of building to that degree of precision anyway. And who will notice when the model is out on the pond?

Colin
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Kevin.Hutch

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Re: Scale and/or imperial vs metric conversions.
« Reply #12 on: July 06, 2021, 10:36:45 pm »

Thanks guys the discussion is good, when I used to maintain early photocopiers I remember that the copiers were set to copy 1.1:1 to prevent a black edge sticking to the copy drum and commonly copied forms used to "grow" after being copied several times. I recall drawing offices requesting we set theirs at 1:1 to prevent drawing scales "growing" and we thought sheet feeders would never last, let alone bubble jet printers.

It was not my intent to call 1:12 odd as I realise it is the most common basis of model sizes, but when I cut out the waffle I had about 1:72, 1:35 etc I left the word odd in.

All my previous boat drawings have been done with access to the full size boat and able to confirm dimensions so scale has only been to fit the drawing on paper.

Attached is the best drawing I could find for my latest project and though it is from a good original, it has lost much definition and scale over the many times it has been copied. This was my introduction to scratch built plans from kit built. All the other drawings I have found have been drawings to illustrate and lacked scale, many even had aspect ratio distortion, hence my search for original plans to confirm my offset guesses. I have more information in feet/inches/fractions than drawings, but mostly just photos, hence my need to switch back and forth from metric to imperial.

I thought cutting and thinning a cedar blind slat for planks would be easy and that has delayed my half model build as well, I guess the longer we live the more we learn, but I have changed subjects and careers too often to be a specialist in any.

Thanks for a great forum I still have lots to learn and only too willing to help others.
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Kevin Hutchison
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grendel

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Re: Scale and/or imperial vs metric conversions.
« Reply #13 on: July 07, 2021, 06:20:29 am »

I used blind slats as the basis for my planking, in my experience 12mm wide planks can be got from venetian blind slats, and better still whole blinds can be found in dumpsters or at car boot / garage sales for just a few £ making them a cheap economical source of planking timber.
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Kevin.Hutch

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Re: Scale and/or imperial vs metric conversions.
« Reply #14 on: July 07, 2021, 06:29:05 am »

That is what I have 4mm cedar blind slats but cutting them down has proved more involved than I counted on.

I Tried a thicknesser, but it only went down to 3.7mm, a bit thick to bend the radius I need.

Tried a bench saw but had trouble maintaining a straight line within 1mm tollerance on the small 5mm planks.

Was convinced most band saws would not cut straight enough.

Have a friend experienced with a scroll saw who is yet to prove what he believes he can achieve.

Now I have purchased a bench router mount for my dremel and yet to master small plank control, but it is promising.
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Kevin Hutchison
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grendel

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Re: Scale and/or imperial vs metric conversions.
« Reply #15 on: July 07, 2021, 06:41:00 am »

I ended up clamping a sheet of ply to my thicknesser table and running them through to get them 2.5mm thick, this had the aded advantage of removing the finish, I did find most blinds here are actually just boxwood / linden stained to a colour for other timber types, or painted white.
most timber I use I resaw down from bigger stuff, my cabin sides are burmese teak resawn from a replaced hull plank from the original boat to 3mm thickness the owner of the boat provided me with 2x 3'6" lengths of original teak, 1 1/2" thick and 4" wide, most of the decorative timber has originated from these. resawn down.
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Re: Scale and/or imperial vs metric conversions.
« Reply #16 on: July 07, 2021, 08:44:36 am »

Guys whilst all the comments are factual but, this is a model makers forum and I posted in the beginners area, siting advice for non CAD students, it was not my intention to open an in depth discussion on engineering precision as it applies to drawing.



But to be honest - the subject title is then misleading as it purports to be trying to start a discussion on whether which is better imperial or metric and therefore we all took it as being a preference or experience lead discussion, maybe you need to rename the thread  %)
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Kevin.Hutch

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Re: Scale and/or imperial vs metric conversions.
« Reply #17 on: July 07, 2021, 09:13:34 am »

Only too happy to change the topic name, I originally thought "scale and/or imperial vs metric conversions was what I felt I was offering to address. It was never on my mind to start a debate on what was better only that plans may not be in the required format and it is quite easy to convert, easy I did not say quick.
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Kevin Hutchison
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Re: Scale and/or imperial vs metric conversions.
« Reply #18 on: July 07, 2021, 10:39:07 am »

I just make up a "scale rule" to take model dimensions directly of the plan I'm using.  Perhaps not accurate enough for those obsessed with the ultimate in accuracy, but my models seem to look OK when sailing.  It also avoids me spending more on plans than the model costs to build!


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ChrisF

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Re: Scale and/or imperial vs metric conversions.
« Reply #19 on: July 07, 2021, 11:25:16 am »

I haven't got one myself but I understand that the guys who do cut down blind slats do use a band saw and it can be done with accuracy.

I'm pretty experienced with a scroll saw and would say it would be impossible with one. Even with a guide the blade would be too small and flex and follow the grain.

As well as building I get a lot of enjoyment from producing the drawings and doing the research and like you draw in metric even though all my builds are 1:12. I use Microsoft Visio but did buy a CAD package with the intention of learning 3D for printing but doubt that I will as it will take a lot of learning and I'd rather be doing other things! We shall see.
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Building Fairey Marine boats: River Cruiser 23, Huntress 23, Huntsman 28, Huntsman 31 and Swordsman 33 and two more to come! All to a scale of 1:12

Kevin.Hutch

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Re: Scale and/or imperial vs metric conversions.
« Reply #20 on: July 19, 2021, 05:04:18 am »

I ended up clamping a sheet of ply to my thicknesser table and running them through to get them 2.5mm thick, this had the aded advantage of removing the finish, I did find most blinds here are actually just boxwood / linden stained to a colour for other timber types, or painted white.
most timber I use I resaw down from bigger stuff, my cabin sides are burmese teak resawn from a replaced hull plank from the original boat to 3mm thickness the owner of the boat provided me with 2x 3'6" lengths of original teak, 1 1/2" thick and 4" wide, most of the decorative timber has originated from these. resawn down.

I have had success with my cedar 4mm venetian blinds on a drum sander taking them down to 2mm.

The sander is calibrated as 1/4 turn for 0.4mm (.01"), so a repeatable thickness is easily obtained, I even tried as far as 1mm but the conveyor feed belt has been scored before by users being too ambitious, so I settled for 2mm.

Now it is full steam ahead, planking the 1:35 half model to clarify the lines before we build the large one.

BTW does anyone know where the standard 1:35 came from as it doesn't seem metric or imperial.
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Kevin Hutchison
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BrianB6

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Re: Scale and/or imperial vs metric conversions.
« Reply #21 on: July 19, 2021, 05:15:26 am »


BTW does anyone know where the standard 1:35 came from as it doesn't seem metric or imperial.

It's another of those strange scales just ask any model railway enthusiast.
1:87, 1:76, 1:45 etc.  %%
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grendel

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Re: Scale and/or imperial vs metric conversions.
« Reply #22 on: July 19, 2021, 06:50:43 am »

i did my half hull from cardboard, at 1:24 scale.
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