Drawing Curves on the Baseboard

[AndyID]

In the August 1964 Railway Modeller, Jack Dugdale described a very useful tool for drawing curves of any radius on the baseboard. It's not difficult or expensive to make one. I made myself one about 50 years ago, but I seem to have mislaid it somewhere

 

If anyone is interested I will attempt to produce a drawing and some information and post it here. 

 

Andy

[AndyID]

Mine is as simple as it gets: a wooden stick (actually, some stray off-cut from unknown heritage) with a large hole (for an 2HB pencil, wrapped in Scotch tape) and some 2 mm holes for the nail in predetermined intervals. (in my case 33, 40, 45 and 51.5cm radius) Simply tap the long nail into the centre for a few mm with a suitable device (a hammer is advised, although other tools may prove almost as effective ), then slightly press on the pencil while you draw the curve. Easy, super cheap, highly effective.

 

Hi Dutch,

 

I've used a similar method and it works very well as long as you have something solid to bang the fulcrum nail into. The method Dugdale describes is "centerless" - no fulcrum required.

 

Cheers!

Andy 

 

EDIT: I should have mentioned that it's also useful for drawing transition curves.

[Ian_H]

Yes, please post the details, I am very interested.

 

Ian

[AndyID]

Yes, please post the details, I am very interested.

 

Ian

 

I suppose I asked for that!   OK, stay tuned.

[BR60103]

I use a meter stick for mine (a yardstick with a bit on the end).

I have a selection of holesdrilled along various of the inch marks, and there is a large hole at one end that takes a pencil.

There are a couple of other holes spaced for drawing either the ends of sleepers or the edges of roadbed. This eases things when the center is tacked down. I also have extra center holes drilled from earlier tries.

I have a stick (1x2 or 1x3) with a piece of subroadbed glued or otherwise fixed to it. This gets clamped under the roadbed on the layout to provide a center pivot point where there is no actual layout.

[AndyID]

 

RadiusTool.pdf

 

Here it is. Please see attached PDF for dimensions. All dimensions in inches for this version.

 

You need a 40" by 2" piece of plywood, hardboard or whatever seems appropriate.

 

It's not very obvious, but there is a notch at X where you hold a pencil to draw the curve. Ideally the center of the pencil will lie at the intersection of lines XY and XZ.

 

To use it, you drive a panel pin A into the baseboard at the start of the curve you want to draw. Now put X (the pencil point) at that pin and position edge XY on the straight leading into the curve. Drive a second pin B into the baseboard at the radius mark you want on edge XZ. Insert pencil at X and slide X between the two pins while keeping the edges of the tool in contact with the pins A and B.

 

When the pencil at X gets to pin B keep the tool in that position, remove pin A (leave pin B where it is) and drive A  into the baseboard at a new position at the desired radius mark along XZ. It can be a different radius from the first arc you drew.

 

Rinse and repeat

 

Obviously you can use it in either direction and you might produce a cut-down version if you are tight for space. There is no reason to use these particular angles. Any triangle manipulated in this way will draw an arc, but this geometry seems to work quite well for typical 00 radius curves. I only show the marks for 30 to 42 inch radii, but you can add as many as will fit on the edges. They just need to be spaced at 0.7 inch intervals for 2" radius increments for this particular triangle.

 

EDIT: This version draws a 20 degree arc.

[Ian_H]

Thanks Andy, makes perfect sense .................. I think

 

I'll give it a go.

[kevinlms]

By amazing co-incidence, this issue is available complete on the web.

 

https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB4QFjAAahUKEwi-7_7jzNrHAhUl5qYKHZ10AeA&url=https%3A%2F%2Farchive.org%2Fdetails%2FRailwayModellerAugust1964&usg=AFQjCNE1c93jegMBLm0Ek-AkQRpAuWECBA

 

No idea who posted it, but there it is.

[brigo]

I wonder if one of those suppliers who make things out of laser cut wood has seen this ?

 

Brian G.

[AndyID]

I wonder if one of those suppliers who make things out of laser cut wood has seen this ?

 

Brian G.

 

Might be a bit expensive to ship?

 

Jack Dugdale never explained why it works. Back then I didn't care much - I was just happy that it worked, but it always intrigued me, so much later on I found out.

 

This is why -

 

 

For any chord (the straight line across the circle, the angle formed by the triangle (three examples shown) is constant. In this case it happens to be 119.82 degrees. If the chord happens to also be the diameter of the circle, the angle is 90 degrees.

 

Hope you are paying attention - there will be a test later

[AndyID]

You might ask (or more likely wouldn't bother) if it's possible to vary the chord length to change the radius for a fixed apex angle, why not do it the other way around and fix the chord length and vary the apex angle to change the radius?

 

You can. The tricky bit is coming up with something that will hold the precise angle because a small variation in angle has a large effect on the radius. For example, if the chord length is 12 inches, for a 30 inch radius the apex angle is 170.05 degrees. For a 60 inch radius, it's 174.26 degrees.

 

An instrument made that way would be a lot more portable because it would be shorter and it could fold in half, but it would need some accurate method of setting the angle - at least at first. Precise scribe marks could be be engraved on it for future use. The pencil would also have to be positioned at the fulcrum of the two arms. That's a bit more complicated, but not that difficult to do.

[Poor Old Bruce]

For transition or gentle curves, I use a piece of nylon curtain rail which can be bent to any reasonable (and variable) radius you require. You may need help to draw the (pencil) line as your other three hands are holding the thing. You could use pins to hold it while drawing of course.