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Author Topic: Show me your assembly jigs (please)!  (Read 999 times)
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TWWARCH
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« on: May 09, 2018, 09:00:16 PM »

Hi guys,

I'm working on a Miles Mohawk Kit from EBM and so far so good. The fuselage, wings, and tail group pieces are all built, flat, and straight. Nothing I couldn't figure out because every component could be indexed against the flat surface of the building board. But now I have to assemble the darn thing. How do you make sure the tail group pieces are straight and square to the fuselage? How do you keep the (low) wings parallel to one another and square to the fuselage - a little offset here would mean a death spiral right? Any help would be appreciated.

WW
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tom arnold
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« Reply #1 on: May 09, 2018, 10:08:01 PM »

First off don't be too concerned about flying with different dihedral in the wings. The plane will compensate for it and fly with the fuselage cocked off to one side a bit. It does look funny though. Different angle of attack of the wings is a disaster that no amount of trimming can fix. If your plane has the wings to be mounted separated but the spars glued to a common center former or bulkhead, the best jig is the "shoebox" method. Take a nice clean shoebox, cut out the ends to clear the nose and tail, put a base of anything (I have used a lump of cloth) in the bottom and lay the fuselage in it. Slide the wings in place and make sure the bottoms are flat against the shoebox edge. Now  squeeze a thick layer of Duco type cement over the joining area and fiddle with the wings putting resting surfaces under the tips to give equal dihedral. Quickly look at the front and make sure everything is lined up and symmetrical, make adjustments, pin, whatever it takes. Then step back and DON'T touch until dry. If you screwed up, melt the glue joint with acetone or lacquer thinner and redo.

When that step is done and dry, attach the fin also with Duco and align by looking at it from  the back. Your eye will be pretty good at making sure there is a 90 degree angle with the wing. I use a pin or two to hold it. Finally attach the stab with Duco  the same way lining up the stab from the back with the wing. The Duco can be a real lifesaver for boo-boos. I have found that I screw up most by trying to adjust things after the glue is about set.

If a shoebox is too small/big, a simple jig like it can be glued up with sheets of material found around the shop as the principle is the same. If the model is a low wing and your wing is completed in one piece, again the magic Duco at the front and rear of the wing saddle, pins, and that calibrated eyeball will do it. I guess the shoebox is really the only jig I use.
« Last Edit: May 09, 2018, 11:26:46 PM by tom arnold » Logged
flydean1
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« Reply #2 on: May 10, 2018, 08:56:07 AM »

Only thing I would add to Tom's comments is:  unless you have a moveable elevator, don't glue in the stab!  If possible, on a one-piece stab, make the slot thru the fuselage a little wide and use shims to adjust the stab incidence to trim the glide.  If it's a two-piece stab butt-joined to the fin or fuselage, you are pretty well committed.  However, Duco will save you.  If it is off a lot, you can dissolve the glue and reposition.
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TWWARCH
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« Reply #3 on: May 10, 2018, 12:34:25 PM »

Thanks guys. The stab is a two-piece affair glued the sides of the fuselage... but it has moveable elevators each side too and they can be adjusted independently. The main thing I seem to be hearing is "Don't use CA." I'll get some duco and a shoebox, shim and pin things up the best I can and then take it back apart to glue.
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MKelly
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« Reply #4 on: May 10, 2018, 02:54:26 PM »

I like Tom's shoebox suggestion - simple, inexpensive, and lets you align in multiple directions simultaneously.  I've been using carpenter's squares, wood blocks and small L-squares with the model blocked up over a gridded cutting mat or gridded magnet board (see pictures).  

Basically the fuselage is blocked in place with the sides vertical and the long axis centered over one of the grid lines.  The wings get blocked into position using the L-squares to check that they are vertically and horizontally aligned the same on both sides, and that elevation at each wing corner is symmetrical left and right.  Once the wing has dried and been double-checked to be true, the stab gets assembled and checked using the same method (no glue, as I've been doing adjustable stabs), then (if required) the fin is glued on and aligned vertical using the two carpenter's squares.  If the model fits on it, a magnet board is great for this as it makes it much easier to block parts in position with less chance of bumping things out of alignment.  The magnet board also allows you to get creative in holding the parts during assembly (see the pic of gluing up the F5F fins).

This has worked well for me, but it takes a fair amount of care and attention to detail - Tom's method is a lot simpler.  I wholeheartedly endorse the recommendation to do final assembly with Duco - I've done this on my last four models and have found it to be forgiving in assembly and robust in the flight environment.

Mike
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TWWARCH
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« Reply #5 on: May 10, 2018, 09:06:03 PM »

This is great Mike. The pictures are very helpful. I think I can cook something up. Thanks!
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Returning Builder
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« Reply #6 on: June 05, 2018, 03:44:08 PM »

Getting to this thread a bit late however I do have a contribution. My assembly fixture was made to answer a need I had in building a bulkhead fuselage and eliminate the half shell on keel problem. If anybody wants any details regarding the construction, check here: http://www.stickandtissue.com/cgi-bin/yabb2/YaBB.pl?num=1501438185
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Don't be afraid of perfection - you'll never make it. (Andy Warhol)
fred
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« Reply #7 on: June 06, 2018, 12:59:47 PM »

Nice jig. Couple of points; I used a 1/2" square ali tube as backbone fully trough the full bulkheads of My 1:24 GB R2 Took some care in
 positioning the square holes in the bulkheads.. but this proved a Very stable and Useful method of construction.. Usable all the way through painting.
 By the way  WEIGHT is a real issue on a Scale GB.
 Infill planking may tip the scales a bit too much  Wink
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