Pantograph drawing: need help pls
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Steerpike
diyAudio Member

Join Date: Jan 2006
Location: Johannesburg, South Africa
Pantograph drawing: need help pls

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***
Attached Images
 panto..gif (7.6 KB, 63 views)
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Steerpike's Toybox

Last edited by Steerpike; 15th December 2009 at 04:44 PM.

 15th December 2009, 09:07 PM #2 Steerpike diyAudio Member     Join Date: Jan 2006 Location: Johannesburg, South Africa 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. __________________ Steerpike's Toybox
 15th December 2009, 09:16 PM #3 neutron7   diyAudio Member     Join Date: Dec 2003 Location: Toronto Canada 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.
 15th December 2009, 09:28 PM #4 MJL21193   Account disabled at member's request     Join Date: Mar 2007 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 Last edited by MJL21193; 15th December 2009 at 09:38 PM.
Steerpike
diyAudio Member

Join Date: Jan 2006
Location: Johannesburg, South Africa
Quote:
 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!
Attached Images
 Panto1.gif (5.5 KB, 48 views)
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Steerpike's Toybox

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