Friday, April 4, 2014

Making 1:1 Scale Patterns

I do a lot of hollow form work and need to make patterns which can be assembled to create hollow objects when forged.  I can’t economically take a prototype pattern to the water jet shop and have them cut a single test piece.  The first piece is the most expensive one as it includes the “set up” or computer programming charge.  

Let’s say my suitable pattern eventually makes a 14 gauge blank.  The blank costs  $0.97 when I eventually order them in a quantity of 50.  That would be $48.50 plus the set up charge of perhaps $20.  So to cut the prototype piece would be about $20.97 and the subsequent ones $0.97.  If the test piece didn’t work I’d have an expensive piece of scrap and need to start over.

For practicality I must cut the prototype pieces myself from scrap and fool around until I am very sure the plan will work.  OK.  I might have invested more than $20.97 of my time and materials in the process, but, it doesn’t show up as an expense on on my checking account.

I like to draw my patterns on SketchUp.  To actually test a pattern I need to have a “life size” or 1:1 pattern.  What I really need is a pattern I can print and cut out that is true size so I can create it in the shop and see how it finishes.

SketchUp gives me dimensioning options so I know exactly the true dimensions I need.  However, I’ve never figured out how to print a 1:1 scale pattern from that program.  I’ve followed the instructions given in two tutorials and it just doesn’t work with my printer.

So I have a work-around solution.  I get my pattern oriented properly, set the Camera to Parallel Projection (instead of Perspective) and do a screen capture.  That comes out as a tiff image which SketchUp won’t import so I just convert it to either jpg or pdf formats.  I open the file with my GraphicConverter program and closely crop the pattern image.  Then I select the Image Size option and set the dimensions to the ones I need.  I would think that would be the end of it but, not so.  I still have to fiddle with the custom scale to get the exact image I need in print.

When the printout is finally the true size.  I can cut out the pattern, outline it with my Presto pen on the sheet scrap of the thickness I want to test and torch cut the blank.

Then I work the blank and see if the result is what I want.  Sometimes it works, sometimes it doesn’t.  Sometimes I just can’t get there from here and have to change the plan.  I’m working on one of those designs now where I may have to change from a pattern based on a long axis forming plan to an equatorial forming design.  It’s the most complicated one-piece construction I’ve tried.  I need 15 or 20 of these so I have to get a strategy that will work in a reasonable amount of time.

The components for this project would be much easier to forge as solid forms but the weight would not be acceptable.

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