Cutting Uniform Ribs

[stephen.shannon]

For those experienced scratch builders among us. What do you do when looking to cut ribs uniformly? especially when you don't have a CNC machine handy ;-)

Tips and tricks welcome. Assume starting with a model plan and sheets of balsa.

Thanks

Stephen

[billscottni]

Ribs all the same size or tapered Stephen?

[rogallo]

Get a good friend to cut them or else the sandwich method

[Fred]

:?: I don't know, I usually press a small button, go for a quick coffee, come back, and done!
Must be magic or unicorns!
 :lol:  

Seriously, that depend of the ribs, if they are all the same, tapered etc.

Here is a link to an article, in French, from Laurent Berlivet with everything you need to know: http://www.jivaro-models.org/nervures_en_bloc/page_nervures_en_bloc.htm

For a tapered wing, same way, but if it's too "tapered" (?) you can add thicker wood, or depron between the ribs to help.



And finally, if none of them are the same, I usually use a pin to "draw" the contours of the ribs, then cut. You can also use a copier to scan the ribs on the plan, then transfer it on the wood with an iron.

[EI1638]

Hi Stephen,

For those of us without Freds toys Ralph has the right answer, the "Sandwich" method.

This works well for constant chord or tapered wings, curves are a bit of a problem.

To make this work you need a root and tip template, and some long bolts. I usually make my templates from 1.5 mm ply, and depending on the rib size use either long M3 or M8 bolts. Make sure the bolt holes line up in your two templates and that they are in the correct position relative to each other for tapered wings (otherwise the taper will be off. Y axis is important too, otherwise you end up with washin/out.

If you like the templates are the bread of the sandwich. The filling is the number of ribs you want to make.

Drill the ribs to suit your long bolts and their locations and sandwich the ribs between the templates.Check your alignments and start trimming. I usually use very coarse sandpaper (60/80 grit) until I am getting close to the edges of the templates. Then move to finer grades. Cut out your spar and any other slots (leading edge, lightening holes) whilst you have the bunch of ribs together. You did remember to cut them into your templates, right? If the ribs are big enough for lightening holes a dremel drum sander does a very nice job.

Just be careful of doing highly tapered wings. As the ribs come out of the sandwich they will have a champhered edge, and you will remove this during the build. If there is a lot of taper the champfer will be large, so your finished ribs will be a fair bit smaller than what you started with.

You can do one wing at a time, or if you alternate ribs you can sand both wing halves at once (even ribs one wing, odd ribs the other). Doing both at once means that both wings will be slightly different, but it reduces the amount of champfer on tapered wing ribs considerably. On this point you pays your money and takes your chances.

Of course, if the wing is constant chord then no problems with champfer.

I have heard of people using this method to make templates for individual ribs and using the output of the sandwich method above as the templates for custom cut ribs. I can't say I know any such people personally (or maybe I do but don't realise it). This approach eliminates problems with champfer.

Chris

Chris

[stephen.shannon]

Thanks All, appreciate the pointers, in the event of doing tapered wings, (judging by the photo) you need to make up lots of depron templates, is that right ? Maybe a little more detail here would help if any of you gave time?

I was thinking of using 3mm aluminium cut for the templates and drilled for the bolts, the scroll saw will do the job on those.

Thanks

[Fred]

No, you don't need to make lots of depron template. You don't even need them at all if you want to.
They are only here because of the taper, when you are going to cut/sand all your ribs, nothing will be perpendicular if you see what I mean.
Adding "extra" ribs, reduce this a little bit, but basically, once all your ribs are cut, a bit of extra sanding on the edges will sort you out quickly, but once again, not a big issue anyway