How do I accurately transfer CAD drawing to sheet of wood?

[haggis999]

With the help of AutoCAD LT, I designed a rather unusual shape of heavy duty shelf to support AV equipment at a 30 degree angle to the wall. Three such shelves have already been cut from 18mm plywood to roughly the required shape at about 12mm oversize. I now need to cut them down to the exact dimensions on my drawing, a job I am proposing to do with a router.

When marking out the shelves in preparation for my rough cut I started at one corner and drew each section of the shape one after the other as accurately as I could manage using a ruler and a small plastic protractor. However, the final line never quite matched the position of the starting point. This was a good enough guide for making my initial oversize cuts, but will not do for the final cuts. Unfortunately, my printer can only handle paper up to A2, which is not big enough to make a full size master.

I would be very grateful for any guidance on how best to accurately transfer my drawing to the surface of the plywood.

David

 

[Harbo]

Make a full sized drawing then prick through the corners onto the wood.

Rod

 

[Steve Maskery]

It's many years since I used AC, but IIRC there is a dimension option to give you co-odinates. Is that top corner 90 deg? If so, I would rotate your part on the page so that that was at the bottom left-hand corner, then dimension all the other corners as co-ordinates from that. It should then be easy to mark out.

I could do that in Sketchup, but I would need a few more dimensions. How long are the two lines that make up the 120 degrees? And the two that make up the 150?

 

[NazNomad]

... using a ruler and a small plastic protractor... I would be very grateful for any guidance on how best to accurately transfer my drawing to the surface of the plywood.

Buy a better ruler & protractor?

Surely it doesn't need to be any more complicated than using tools with a bit more accuracy?

 

[haggis999]

Make a full sized drawing then prick through the corners onto the wood.

Rod

As I said in my original post, I don't have the ability to make a full size print. It would take several sheets of A2 to do the job and attaching those sheets would be yet another opportunity for inaccuracy.

 

[AJB Temple]

Or you can put your drawing inside a rectangle of 723 by 576 and mark the axis lengths (which you can take off your CAD software) to each intersection point. This will give you a very accurate drawn plan that you can prick through to make a template. Would take 10 mins to draw it given the axis dimensions.

 

[pcb1962]

Make a full sized drawing then prick through the corners onto the wood.

Rod

As I said in my original post, I don't have the ability to make a full size print. It would take several sheets of A2 to do the job and attaching those sheets would be yet another opportunity for inaccuracy.

If you were to use Sketchup you could use Matthias Wandel's BigPrint program which is designed to do exactly what you need - print a 1:1 drawing across multiple sheets of paper.
edit: just realised that you don't need to use sketchup, bigprint can import a jpg from any source

 

[haggis999]

It's many years since I used AC, but IIRC there is a dimension option to give you co-odinates. Is that top corner 90 deg? If so, I would rotate your part on the page so that that was at the bottom left-hand corner, then dimension all the other corners as co-ordinates from that. It should then be easy to mark out.

I could do that in Sketchup, but I would need a few more dimensions. How long are the two lines that make up the 120 degrees? And the two that make up the 150?

That's an interesting suggestion. There's little doubt I can get X,Y co-ordinates for any point on my drawing and that would remove the need for me to draw accurate angles. I will give that a go. Many thanks for that useful idea.

 

[bugbear]

It's many years since I used AC, but IIRC there is a dimension option to give you co-odinates.

This is interesting (if you like maths/geometry). Assuming that a CAD drawing has done all the maths for you, how do you most easily/accurately transfer it to a arbitrarily large surface?

If you (somehow) have already marked a pair of perpendicular axes (X and Y), you can mark arbitrary points at (x,y).
But it's a little tedious; You need to mark y on the Y axis, and x on the X axis, then strike an arc of radius y from the X axis at x,
and an arc of radius x from the Y axis at y. The sequence means you can't even retain the settings on your dividers/trammels.

You could use a baseline, and 2 lengths, striking intersecting arcs from the ends of the baseline. If some of the intersections are near to parallel, (90 degree is ideal) this won't be accurate.

Or you could use a baseline, and 2 angles, like a surveyor with a theodolite, only in reverse. The same "poor quality intersections" problem that we saw with radii applies here.

How did builders and architects lay out complex buildings, back in the day?

BugBear

 

[haggis999]

Buy a better ruler & protractor?

Surely it doesn't need to be any more complicated than using tools with a bit more accuracy?

I don't lack a decent ruler, though my protractor is just a relic of my schooldays. I did attempt to overcome the limitations of a small protractor by printing the two angles on an A2 sheet of paper, but even that did not give me the accuracy I was hoping for. That said, I only tried it once and perhaps I wasn't being careful enough.

I struggled to find a source for a really large protractor, though I might have a go at printing one using http://www.ossmann.com/protractor/.

 

[Steve Maskery]

I've had to guess at those lengths each side of the angles, so this is not accurate, but it shows what I had in mind:

 

[haggis999]

How did builders and architects lay out complex buildings, back in the day?

BugBear

I bought a book called 'How to Build a Cathedral' by Malcolm Hislop after a recent visit to Ely Cathedral. Perhaps it's time I read it!

 

[haggis999]

I've had to guess at those lengths each side of the angles, so this is not accurate, but it shows what I had in mind:

I'm very grateful that you took the trouble to do that, but there was no real need, as I fully understood your original suggestion and hope to put it into practice later today.

Somewhere in my attic there is an old set of drawing instruments that includes a multi-section rod compass for drawing large radius circles. I'm hoping it's long enough to draw intersecting arcs to locate the key co-ordinate positions.

 

[bugbear]

I've had to guess at those lengths each side of the angles, so this is not accurate, but it shows what I had in mind:

I'm very grateful that you took the trouble to do that, but there was no real need, as I fully understood your original suggestion and hope to put it into practice later today.

Somewhere in my attic there is an old set of drawing instruments that includes a multi-section rod compass for drawing large radius circles. I'm hoping it's long enough to draw intersecting arcs to locate the key co-ordinate positions.

BTW, how accurate do you need? Nearest 5mm, 1mm or 0.1mm?

BugBear

 

[Neil S]

Intersecting arcs from the longest base line is definitely the way to go in my opinion. If you use a protractor then extend the line you are just magnifying any errors in the drawn protractor line.
-Neil

Sent from my GT-I9505 using Tapatalk

 

[Ian down london way]

I hope you are asking about creating a single template, rather than attempting to mark up multiple of these patterns independently. Even if the master template isn't 'perfect' at least the multiple supports would then be the same.

But to you question, I'd never use a protractor - they can't hope to get any more accurate than 1 degree. if its right angles, then a compass (home made will do - mark lengths on a strip of wood). + use of Pythagoras will do the job. If you want other angles then you can still use the home made compass, but you will need to use some basic trig (cos/sin/tan).
Its been 30+ years since my maths degree, but I can help on that score if you PM me.

 

[Racers]

I've had to guess at those lengths each side of the angles, so this is not accurate, but it shows what I had in mind:

I'm very grateful that you took the trouble to do that, but there was no real need, as I fully understood your original suggestion and hope to put it into practice later today.

Somewhere in my attic there is an old set of drawing instruments that includes a multi-section rod compass for drawing large radius circles. I'm hoping it's long enough to draw intersecting arcs to locate the key co-ordinate positions.

BTW, how accurate do you need? Nearest 5mm, 1mm or 0.1mm?

BugBear

Accurate enough to find out how far the wall it out.

:wink:

Pete

 

[NazNomad]

How did builders and architects lay out complex buildings, back in the day?

Opression, mostly. :-D

 

[haggis999]

BTW, how accurate do you need? Nearest 5mm, 1mm or 0.1mm?

BugBear

Ideally, I would like to transfer my drawing to the surface of the plywood with an error of no more than 1mm. The job doesn't actually require that level of accuracy, but I'd like to do my best to avoid any cosmetic imperfections. I always find such problems a source of irritation.

BTW, I did some remedial plasterwork several years ago to ensure the wall was flat and vertical before erecting my shelving.

 

[lurker]

Agree with Pete,

I am yet to come across an internal wall that has 90 degree corners.
Nor a traditionally rendered internal wall that is flat & straight.

IMHO, the OP is over thinking this.
Always assuming I have understood what he is trying to do.