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cutting a notch in tiny ribs

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denis parkinson06/03/2017 18:23:19
145 forum posts
66 photos

Hi, I'm not new to modelling, but I have a question about cutting notches in ribs without splitting the rib when its cut.

I normally make ribs using the sandwich method, but these ribs were just way too small to drill holes in, so I have had to make each one individually.

As you can see from the photo, there is not much material left after cutting out the notch..

Does anybody have a method of cutting these out without breaking them?

Thank you in advance.


Denis Watkins06/03/2017 18:29:35
4656 forum posts
132 photos

Still block them into a sandwich Denis

They need support with little strength left in them

For example, 5 of those clamped in between plywood, with your slot marked or preformed

Then routed out, or sanded having cut slits either side with a fine saw etc

You know the way, but even at this small scale they need it

If there are so few, them why not make them from stronger material

Edited By Denis Watkins on 06/03/2017 18:32:57

Martin Whybrow06/03/2017 18:30:11
884 forum posts
33 photos

I would sand / file the slots, Permagrit make sanding tools for that very purpose.

denis parkinson06/03/2017 18:32:18
145 forum posts
66 photos

Cheers Denis and Martin..I'll give it a go.

eflightray06/03/2017 19:35:03
626 forum posts
132 photos

If the ribs are very small, why not notch the spar as well, that way you only have a small notch in the rib, less chance of breaking. Hopefully the spar isn't expected to take high 'G' loads.


Former Member06/03/2017 20:47:42
8090 forum posts

[This posting has been removed]

Dwain Dibley.06/03/2017 20:57:01
1574 forum posts
1558 photos

Ditto the dope and tissue idea !!

You defo need to put some strength in there before you take it all out again.


Plummet06/03/2017 22:03:16
1422 forum posts
41 photos

Razor blades are thinner than scalpel blades. (Blood risk warning) Take a double sided razor blade - break or cut it in half lengthwise and then wrap the broken edge with masking tape (to reduce blood risk and increase gripability.). You can use the ends of the cutting edges to slice the sides of the rib slots. Then you can cut out the notch with a scalpel.


stu knowles06/03/2017 22:13:45
616 forum posts
53 photos

Is it really worth bothering with ribs like that?

Why not a full depth spar with half ribs on either side. The only thing to watch for is making sure that they stay tight to the board.

Just a thought

Robin Colbourne06/03/2017 22:51:31
724 forum posts
18 photos

Could you taper the spar, so the thickness of the remaining material below the notch matches that in the spar? Notching the spar would create a stress-raiser which is not good news.

If using a double edged razor blade in its unadulterated form, don't get distracted and press on the other edge (speaking from experience...cryingblush).

Braddock, VC06/03/2017 23:57:29
1661 forum posts
82 photos

I'd either go with stu knowles idea or if your ocd kicks in make the blanks so that two ribs back onto each other in a symmetrical section, one piece rib. File or cut the slots THEN cut systematically through the centre line to create two ribs.

Personally I'd make up a sandwich of the symmetrical ribs and use a razor saw to make the vertical cuts then poke a pointy scalpel blade through the wood of each individual rib to remove the waste piece.

I think stu knowles' idea is best though.

Piers Bowlan07/03/2017 08:00:56
2224 forum posts
57 photos

I haven't seen the permagrit spar slotters before, must get some. I normally tape several old hacksaw blades together to achieve the correct width cutter for the spar. Very quick and effective - cheap too! Not my idea though.

Personally I would cut the spar notches in the bottom of the rib to about 60% depth. Increase the depth of the spar to full rib depth but cut slots in the spar for the ribs to a depth of 40% of the spar depth. Pin the spar to your building board and then pop the ribs over the spar, 'egg box' fashion. You now have a full depth spar with notches in the top so that the spar is not weakened by cutting the lower surface of the wood, which is under tension during flight. Several Boddo and other vintage designs incorporate this layout to good effect and is quite adequate for a lightweight model if not a pattern ship!

Peter Miller07/03/2017 08:05:29
11593 forum posts
1393 photos
10 articles

How about using a DRemel cutting disc. You can do very deicate work with one of those.

Just one thing suddenly occurs to me, with a tiny wing like that the spar looks like overkill, i..e. really huge.

Edited By Peter Miller on 07/03/2017 08:07:12

Barrie Dav 207/03/2017 09:18:53
1012 forum posts
14 photos

thumbs up

NormB07/03/2017 11:37:23
57 forum posts
5 photos

I'd go with Pier's suggestion - this 'egg box' arrangement for spars and ribs is very common to Pat Tritle's lightweight designs, some around 60 inches span. You can stack and wrap with masking tape for the slot cutting.


Mike T07/03/2017 16:16:52
582 forum posts
41 photos

Per stu knowles, plus if the object is to have the underside covering not sticking to the spar, then make the 1/2 ribs undersize and put 1/16 cap strips underneath. Stronger all round!

PatMc07/03/2017 19:54:40
4485 forum posts
548 photos

I think a bit of re-design would possibly be the most practical.

Is the rib shown for a wing or tailplane & what's the model ?

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