Cardboard Jigs

1 November 2024

Yesterday was Deepavali, a public holiday, and the family gathered at my place for lunch and mahjong. After lunch, I had a few hours free since I don't play mahjong. So I made a few photocopies of the plan  Oz : Druine Turbulent plan - free download and pasted them on some corrugated cardboard. The jig board is then taped with clear packing tape that was around the house. In less than 4 hours, I completed the horizontal stabiliser, the vertical fin and the two fuselage sides. It was actually quite easy, here are some key points that were different from when I was younger:

• Proper chair and table provides a comfortable work station. Compared to crouching on the floor or working on the coffee table, this is more comfortable. 
• The 1/16" balsa sheet was cut to a length that accommodates the longest member with one inch to spare, in this model, about 11". 
• With a Pilot G-2 07 pen, cross the balsa sheet on one side, and near both ends. This tells me which side I should rotate the 1/16" balsa strip to.
• Cut the required balsa strips of 1/8" and 1/16" width with an NT cutter against a ruler in smooth continuous strokes. The result is much better than making strips from 36" long balsa strip.
• Superglue was used for the joints. The capillary action works the glue into the joints. Balsa parts can be held over the jig board with your fingers before superglue is applied to the joints.
• Pressing an NT cutter onto the balsa strip, the balsa strip can be picked up and transferred to a cutting mat. This makes cutting the strips to exact length and angle easy.
• After one member is cut, align that member atop the balsa stick and cut the second member to exact length and angle by first cutting the angle of one end, and then using your thumbnail to align the ends of the member and balsa stick, before cutting the other end. 
• Fuselage spacers are duplicates. Make all the members in duplicates and set out accordingly at the top of your work space. 
• Dot the packing taped jig board with superglue to temporarily secure balsa pieces, away from the joints. No pins, weights, clamps are necessary. When frame glued, slip in the NT cutter between the balsa structure and jig board to pry off easily (#11 scalpel won't work as well as they are too short and stiff). I didn't need to bother cleaning up the jig board by swiping with solvent to remove the superglue, the jig board can still be re-used without this extra step. I did not use hot glue. If the frame was removed too early and there are still some superglue that is still wet, this will smear the NT cutter blade. The NT cutter blade is cheap and can be replaced, but it appears that it still works well for cutting, slicing and prying.
• If you press on 1/16" square balsa strip while it is on the jig board to transfer the balsa strip to the cutting mat, sometimes it will cut through and your task is done. The jig board seems to be tough enough and the cutter did not harm the jig board; I think this is because there is enough resistance from the packing tape and enough give from the corrugated board.
• Pre-curving of lower longerons was completed without template. Dip in hot water and bend by stroking the balsa strip. 

Next session, I'll have to do up the pair of wings, assemble the fuselage sides into a box structure, add formers to the top of the fuselage box, make the removable noseblock and undercarriage.

The fuselage sides will be temporarily glued upside down to the planform jig board with superglue. Start at the point of maximum curvature. Starting with the fore and aft spacer location, mark/cut the spacers in duplicates. Cut the next pair of narrower spacer and gradually bring in the fuselage sides. To handle the spacers that will be glued "in-air" (i.e. the bottom spacers) and also to help position the top spacers that is on the jig board, make a pair of tweezers or a pin prick. This is because there will not be enough space to put in your fingers. So you can start by making your tool. A pin prick is just a thin pin that is superglued to a 3" length of 1/16" square balsa, and it works by piercing it into the spacer. A tweezer is just 2 pieces of 3" lengths of 1/16"x1/8" balsa, spaced one end with a 1/4"x1/16"x1/8" balsa, assembled and glued with superglue.

30 October 2024

Added how to curve 1/16" square balsa strips using jig board and hot glue in posting of 25 October 2024.

25 October 2024

I often read of using a building board and from what I read, they are meant to be re-usable, needs maintenance and proper storage, relatively big and has a high initial cost outlay. 

The function of these yester-years building boards can be replicated cheaply. You can make it re-usable or disposable, very economically. The only requirement therefore is that it has to be flat. To differentiate from the traditional building boards, I refer them as jig boards.

• Make jig boards from flattish double walled cardboard, steel/plastic panels, wooden boards, glass/mirror by gluing on the section of plan.
• Pre curve the balsa strips over simple templates. Use tap water to wet the strips and hold in position until thoroughly dry.
• Use hot glue to temporary secure balsa strips, wet or dry.

On Hot Glue and Curving balsa strips.

1. On cardboard jig board (or those that are not meant to be used with pins), I think it is feasible to use dabs of hot glue to temporarily secure the longerons. Upon completion, it ought to be possible to pry the completed fuselage sides from the jig board, pieces of hot glue that are still on the jig board or balsa longerons can be peeled clean. Wet balsa can be hot glued to a dry jig board.
2. Spring back of balsa. Accept the fact that after removal from template, the pre-curved balsa strip will spring back a bit. You could make the template with tighter curvature to accomodate a bit of this spring back, but I wouldn't bother too much. How much spring back depends on the size and hardness of balsa, the tightness of curvature and whether we are using a single piece, double ply or triple ply. Held to the correct longeron curvature, wet the balsa again. When dried, the slight stress in the fuselage side will be present, but it has been reduced. This is the case also with tighter curved wing tips and stabilisers, the stress is inevitable and sufficient tension braces are needed to counter those stress and maintain the shape. 
3. Here's an idea for curving the 1/16" square balsa strips. Wet the over long balsa strip, position over the jig board and dab hot glue at the peak of curvature. When the hot glue has cooled, bend the wet balsa strip to conform to the curve line and dab another spot to secure. Alternating between the 2 ends of the first dab, work towards each end. Leave the balsa strip to dry thoroughly before removing from the jig board.

23 October 2024

I bought my wife a new ladies golf set, in came in a stout double walled cardboard box. We could keep the box and next time we want to travel (by air) with her golf bag, we could simply re-used the box. Just seal it up and it will be perfect golf case. She decided she does not want that, so the cardboard box has been cut up and I have a good supply of double walled cardboard pieces that I can use as jig boards, templates and carry boxes for my future builds.

Since it is 'free', my imagination will not be hindered. I can use pins, tapes or hot glue.

Building board = jig board = disposable jig board

Big heavy expensive = small light cheap = cheap and disposable

The jig board can be made of any material, so long as its function is served. 

We want the fuselage side frames be assembled squarely and symmetrically. I don't think assemblying in mid air will be suitable, we need another jig board.

On a piece of cardboard, corrugated cardboard works as well, paste the plan view of the fuselage, cut around leaving 1.5"-2" all round and I'll explain why later. This piece of cardboard will be the other "jig board".

Again, we will need jig pieces, but this time the jig pieces need to be tall enough to accommodate the fuselage sides. You could fabricate the bigger jig pieces, here's how to use the jig board to provide the jig pieces: you simply cut slits at the nose, the mid section and tail and then bend it up perpendicularly. The bent up tab pieces will serve as built in jig pieces for the fuselage sides. Offer the fuselage side frames to this jig board, usually, this means top side down because the top longerons are usually the straightest. Similar jig pieces that were used in the assembly of the fuselage sides can be re-used to 'lock' the position of the fuselage side frames.

Proceed then with cutting the horizontal spacers (in pairs if appropriate), using tweezers if necessary and place them between the 2 side frames. The tail of the fuselage ends as a tip, so proceed from the tail end and finish at the nose.

The assembled fuselage frame can now be removed, but there's an optional step you can take. This involves wetting the fuselage longerons while it is still in the jig, it may reduce the internal stress due to the planform shape. You can repeat this process but do allow a day or so before removing the fuselage frame.  

If your jig board is not destroyed by now, you can re-use it. Place the assembled fuselage frame, top side up, on the jig board. Use the jig board to hold the frame stable while you glue the top formers and stringers.

21 October 2024

Jigs may seem like a tedious step to undertake but it has obvious advantages.

Building equal fuselage sides

On a piece of cardboard, corrugated cardboard works as well, paste the side view of the fuselage, cut around leaving 1" all round for easy handling. This piece of cardboard will be the "jig board".

Jig pieces will be useful for positioning. They are just 1/16" or 1/8" scrap pieces of balsa, something like 1/2"x1/4" size. The jig pieces will be located at the nose, the tail and at each vertical spacer of the fuselage side. Some special jig pieces may be necessary, and these are usually just triangular pieces. It is very easy to make the special jig pieces because all you need to do is the extend the 2 lines, position a scrap piece over it, sight and cut along the lines.

Yes, you can just glue the perimeter's jig pieces onto the jig board. It is advantageous too, because you must not let these jig pieces move.

Once the jig pieces are in placed, the real construction starts. Longerons (the upper longerons are usually straight) are drop in position on the jig board, they can be longer than necessary, because they can be trimmed after the sides are complete. Do two sets of vertical spacers, because we are building a left and a right fuselage sides. These vertical spacers will be glued between the upper and lower longerons, so they have to be accurate. Lay all vertical spacers on to the jig board and glue to the top longeron. Soak the lower longeron pieces in water for 5 minutes, because they usually are curved and we need to soften the longeron. Drop the "softened" lower longeron on to the jib base, against the spacers. Using more jig pieces on the lower edge of the lower longerons to avoid crushing the lower longerons. Do not glue at this stage because the wet balsa has not set yet. Wait until the wet longeron is dried, about 2 hours, and then proceed with gluing it to the spacers.

Do not build the second fuselage side over the first, in case they bond together. Remove the first fuselage and start by placing the longeron, follow by the spacers you already cut, and finally the wet lower longeron on the jig board. Similarly, wait until the lower longeron is dry before gluing them together. 

Sand the 2 fuselage sides so they are identical.

Building the Wing

Similarly, on another piece of jig board, paste the planform of the wing and do up a jig board. This time, use 1/4"x1/2"x1/8" jig pieces for the perimeter. The reason why 1/8" balsa is preferred because the intended leading edge is 1/8"x1/16" on-edge. Start by positioning the leading edge and trailing edge on the jig board. The trailing edge may already be pre-sanded. If the model has tip pieces, these are also positioned and "protected" by the jig pieces. 

You can adapt the wing's plan for the jig board (most ribs are evenly spaced) so that you can use the same jig board for left-hand panel and right-hand panel. In tapered or curved planform, this would not be possible and it is faster just to make handed jig board.

With the perimeter wing frame more or less completed (less the base rib), the next step is to enable placing of the ribs vertically and accurately. Pre-cut ribs are offered first to the inside of the leading edge and then trim to fit the trailing edge The ribs' jig pieces can comprise of the same 1/4"x1/2"1/16" jig pieces previously used for the fuselage's spacers (but not glued on) with additional sticks glued on to the flat edge so that it can provide some kind of a wall to help with the vertical alignment of the ribs. When the ribs are all lined up with straight spar slot, insert the top spar and finally glue them together.

The wing panel is ready for follow up work.

Technology

Thank goodness for photocopier and printer!