trimming ribs for the spar

[75Fox]

Hi everyone, I have found a fun way to trim back the ribs using a guillotine shear. It does a great job very quickly, with precision and almost no burr cleanup. But it has to be done while the ribs are flat blanks, and this means the ribs are cut to size (to accommodate cap strips) ahead of time. However, from what I've read, I am supposed to cut them back later, with hand shears after the spar is made. Before I commit to this technique- am I in for an unpleasant surprise doing it this way? Is pre-sizing a big no-no?

Thanks,

Tim

[sjt]

Do not trim early.

The ribs will not be the same. The spars for the right and left wing are not the same. The ribs are cut from a sheet of material at arbitrary angles referenced to the grain pattern of the sheet. They will not bend identically. Some bigger then others.

When it is time to put the ribs onto the spars - you may find that you would prefer a rib be placed at a certain location along the length of the spar. It is possible to optimize rib locations along both wings. They will fit better that way. Notch the ribs as the last step after selecting the best rib (on average ) for that location

Also, you can get a better rivet to edge distance if you do not cut early. All that is required is a notch. You do not have to cut all the material.

In my case (patrol) I cut all blanks at 31.66 inches. That distance accounts for the two 032 sheets to achieve a 32 inch separation.
Because both sides of the ribs blanks are cut with a router to the same distance, they will be parallel. When final assembly is done the rear and front spar will be square to the rib if it was notched instead of trimmed.

[sjt]

My preferred way of trimming the notch is a couple of drilled holes and a sander. This rib fit best at location 8 on the left wing. Notches take seconds. This was only possible because I had not committed early.


The notch method preserves the 31.66 parallel distance of the original blanks that are made with a form block and a router. I can butt the 31.66 edge against the main and rear spar for increased accuracy of build.

sander.jpg

[75Fox]

Do the Patrol plans employ vertical spacers between the upper and lower cap strips where the ribs join the main spar?

[geraldmorrissey]

Trin on assembly. This insures a perfect fit. Why would you want anything else?
Gerry
Patrol #30

[sjt]

The spacer offsets the angle bracket from the spar. From the picture it should be possible to see the bracket mounted on a spacer mounted on the spar. If a rib is not cut short, it will have additional distance between the rivets in the angle bracket and the edge of the uncut rib face that is touching the spar.


I looked at the picture the spacer is behind and supporting the angle bracket. It can not be seen.

[75Fox]

Many thanks! I will go with your slotting technique. Once I studied the photos, and got a better idea of the placement of the angle attachments, I could then visualize how all the ribs can actually contact the spar right at the zero line. Silly me.