Pantograph drawing: need help pls

[Steerpike]

Does anyone know anything about pantographs?

I made a rudimentary 'test' setup from strips of plastic an wood, before I go all out on making a metal one, but my prototype doesn't work!

I want a scaling factor of around 10:1, i.e., the drawing must be 10% of the stencil size, so that any errors tracing the stencil are almost invisible on the drawing. I made (A+B)=50cm (about 15 inches).

I figured the scaling factor would be
A/(A+B) = C/(C+D)
and to maintain a 'square' aspect ratio A=C and B=D.
(my diagram is a little distorted, but tha actual machine is accurately measured)

But I'm getting some very trippy drawings that look like Salvadore Dali's. So my trigonometry is wrong!

Does anyone know the correct trigonometry for this thing and what I did wrong?
Here's a neat online illutration of operation, but they don't give any info on how to calculate the arm lengths:

*** pantograph Java applet***

[Steerpike]

I've figured out WHY I'm getting distorted drawings - it's because the pen is slightly offset from the fulcrum.
But I still need assistance to figure out a way to compensate for this. It's physically impossible to place the pen right at the fulcrum - unless it involves a very elabrate bearing with a hole in the centre.

[neutron7]

just make an ofset for the stylus which is 10 times (or whatever your scaling is) as much as the pen is offset and in the same direction., in your case D would have 10x the distance between the pen and its closest pivot added to it.

[MJL21193]

Looks to me like dim "C" should be equal to dim "B", forming a square in other words.

Also, moving the pen to here will help:

panto_.gif

[Steerpike]

Originally Posted by neutron7

just make an ofset for the stylus which is 10 times (or whatever your scaling is) as much as the pen is offset

That's what I thought at first; but it doesn't work. Hard to explain what happens in words... the offset required changes as the arm angles change. A fixed offset on the stylus doesn't correct for that.

BUT, I have found an alternative design, that I will try tomorrow. How to set the scaling of this arrangement is also a mystery for now - I'll have to revise some geometry!