ZB50

Hi all, I am trying my hand at scratch building. I have rib cutting with my scroll saw down pretty good but find myself making a mess cutting the slots for the spars. Any tips or techniques for easy, tight and proper depth cutting?

Thanks,

Jimmy

BMatthews

For small stuff up to 1/8 wide I like to use a stack of fine to medium tooth hacksaw blades. Sand the paint and epoxy them together. They seem to come out about 1/32 wide depending on the tooth set and stuff. Jiggle them a little and the sets will nest together for a nice fit. Epoxy together clamp along the length and add 4-40 safety screws at the holes if you can. Or just thread wrap the ends and add some CA. To fine tune the width of the cut I scuff the tooth set down on a sharpening stone. If 4 blades for 1/8 is too much then include some layers of fine business card like stock between the blades to space them.

Over 1/8 I use a Zona or Xacto saw to cut down the sides and a knife or narrow chisel to chip out the bits. Cut very slightly undersize if anything and fine tune using a 6 inch coarse tooth metal file. Grind or sand off the edge teeth on one edge. This safety edge helps you control where the file cuts. Keep one of these for balsa and wood only. Cutting metal with them dulls the teeth and just polishes the wood. Depth isn't as important as you think as long as there is a firm but not tight grip on the spar on the sides. This lets you push the spar into place just to where it's flush and then hit the sides with thin CA. Well fitted spar slots in conjunction with notched trailing edges to hold the ends of the ribs allows you to almost assemble the whole wing dry and then just touch the thin CA drops to each joint. It's almost like a religious experience when it goes well.... And it's a lot cheaper than laser cutting.

A little cabinet making trick for marking and cutting accuratly. Use a scrap of your spar stock and use a knife blade to mark the vertical side across the top of the rib. Score about 1/64 deep. Now cut away a shallow V notch of the spar slot scrap up to the score line. This will give your Zona saw a nice edge to butt up to and almost guarantees that the saw blade won't drift into the rib side of the cut.

I cringe when I see a box or drawer full of metal files all banging and rubbing together. These are cutting tools every bit as much as your Xacto knife. Buy good ones and treat them with respect.

Hope this helps and welcome to, IMHO, the fun'ner side of modelling.

onewasp

BMatthews

Paragraph 3 is not very clear, at least to me . Are you notching the spar stock to act as a guide when held against the rib? / or are you notching the rib ?

The first option seems right but, I'l ask the source.

Dzlstunter

Still another way is to take a piece of your spar material and glue a strip of sandpaper on the edge of it so that you can sand a width exactly equal to the width of the spar. Then do so on one rib or a stack. This is cheap and "dirty," so make a trial "cut" before you do all the ribs to make sure you have a tight fit for the real thing. Good luck, Dzl

lennyk

My Feedback: (12)

If you have one good rib, use it as a template to cut the notches for the others, it helps to put some thin CA on the notch of the template rib to harden it so it won't deform when you line up your razor blade on it.

BMatthews

I should have added that I always cut my ribs out with no notches and then stack them together with pins or long 4-40 bolts and then sand and notch them as a stack. This ensures that the ribs all match closely to each other and the lengths are all identical....

Or at least, and this is the important part, any minor differences are graduated smoothly along the whole span and the spars, and leading and trailing edges are not forced to bend as they wend their way from rib to rib. I truly feel that preventing such stresses from being built into the wing in the first place is the first major step to a warp free wing.

Now that I've set my stage here's a sketch of what I meant about the scoring technique to mark the spar notches for a snug fit. This is a view of a spar notch looking at one of the end ribs.

Offer the spar stock up to the stack of ribs and score the whole width across the tops of the ribs about 1/32 deep. Then make a shallow slanted cut into the score line from the waste area as shown. This forms a lovely channel for the Zona or other thin saw blade to be guided during that first couple of critical cuts. The channel also ensures that the saw cut is biased into the waste so the notch will be a snug fit.

It DOES take a little practice. I'd try this on some 1/2 inch sheet scrap for a few times first to get the hang of it. It's not HARD, just takes a time or two to get it right. But when you do get this down pat you'll turn up your nose at laser cutting....

onewasp

BMatthews

Thanks for the explanation ---- it is appreciated.

MikeS

My Feedback: (37)

I have experimented also. I tried cutting the notched area with the scroll saw, cutting inside the lines then making a sanding stick that matches the spar width, paper and all. I like to do the final width sanding when the spar is ready to be put in the notch, that way I can make sure it is snug. The q with cutting from a plan is whether the designe intented to include the width of the line inside or outside of the notch. I have built from plans from several MAN plans, all inked by the same guy, Joe DeMarco. When measuring spar width, he sometimes includes the lines, so I cut right on the line, or he excludes the lines, so I cut outside the line. Be sure to measure. Just take the plan and where it calls for a 1/4 inch stick spar, place a 1/4 in. stick in place to see if it is inside or on the line. Also remember that copiers can change that. I have noted that using Kinkko's often.
I have also tried the stamp method. From the plan, locate the leading edge of the spar on the rib. The use a piece of the spar wood to stamp the spar area on the wood. Remove all the inked or paint stamp and you have a tight fit.
The best way I have done when all the ribs have the same spar location is as BMatthews described. Just cut slightly narrow and do the final sanding in situ.
Enjoy, building is therapy.