Monday, September 14, 2015

C-Arm Jig

A person isn't who they are during the last conversation you had with them, they're who they've been throughout your whole relationship

Rainer Maria Rilke

Here you can see the C-arm chair and it's corresponding bent arm.  The arm rail is tricky to bend as the plane rotates as it approaches the hands.  It is thinned down ( I use a drawknife and clean it up with a spoke shave ) from 7/8 to 1/2 inch at the elbows but still it is a tricky bend.  I tends to fail at the elbows...roughly where the 1st and 2nd spindle are located above, on the right. 

I used to bend it around the top of the jig and then bring it down and clamp one end then I would move to the other end and usually by that time the other had cooled too much and either the piece would have some compression failure or it would out right break.  Now what I do is I bend the top and wedge it and bring both ends down simultaneously and simply hold them in place for a time, maybe a minute.  Then I release them and clamp them.  I find that once they are bent they can be re bent so long as they are bent hot.  If  you allow them to cool they will sometimes fail.

 I've also started just using the PVC pipe to heat most of my bends.  It is wrapped in an old moving blanket which keeps the chamber temp around 208 which is pretty hot for a non pressurized system.  I use grippy gloves too, which allow me to handle the pieces faster.

And because the arms fail sometimes, I always prep more pieces than I need.  With the last chairs I prepped 4 pieces for 2 chairs.  The pieces bent beautifully and so I had four bent pieces whcih is good. The problem is if I leave them laying around the arms will eventually relax and flatten out rendering the pieces useless so I came up with this little holding device that will hold the arms at the correct angle and also hold the hands at the correct distance apart.  It's just a couple pieces of scraps and a couple angled pieces.
I used to spend an obscene amount and at the risk of editorializing, my advice is beware of fancy jigs. I only use about four jigs in my day to day chair building business.  Most sre pretty simple affairs assembled on the fly like this one.  The siren song of the magic jig can be alluring.  I know.  But usually it is born out of frustration at lack of skills

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