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Undimpling Question

Stockmanreef

Well Known Member
I have decided to use the Skybolt fasteners along the top cowl along the firewall. At this point, I have the skin, the FW, and the spacer dimpled, since i did not plan this orginally. So should I undimple everything to match drill, and then redimple. Or is it OK to match drill with the dimples in place?
 
If it was me, I?d try to construct some sort of jig to get the holes in the right place, using the dimpled holes.
 
I assume the hinges were part of the stack up when initially drilling all of this. I would used the hinges as a drill guide for the new part. Unlike dimpling, countersinking does not expand the original hole size and can easily be clamped to the new part, as there are no dimpling protrusions. If I remember correctly, there is still a decent amount of original hole ID on the hinges after countersinking.

Larry
 
Match Drilling

I’m assuming you want to locate holes in the Cowl for placement of the fasteners, not vice versa. The technique I propose works either way with a little adjustment.

Good chance for getting an acceptable result “over the arch” if you pay attention to what the material is doing while you work through the process of matching the dimples.

Drilling through the dimples, before or after "undimpling," is a bad idea as it will expand the holes.

I'd consider using a “transfer strip” to achieve your matched hole goal.

There is an article in SA archives by John Thorp in his "Building the T-18" series (Article #2 or #3 back in the ‘60s) that describes the technique of “Matched Hole Tooling.” I’ve taught this technique at forums given at SnF and OSH.

Cleecos, by themselves, for this task, are inadequate for achieving close tolerances. If working “in the flat” John recommended making “holes at the corner 1/8" (0.1250") and using an AD-4 rivet as a locator.

The fact that the holes are already dimpled denies you that advantage but I include its reference for understanding my point and for ease and convenience in other circumstances.

A finnicky builder, if very careful, may avoid the shifting of the transfer strip by handling very carefully and using lots of clamps along and across the transfer strip.

Matching the holes is problematical because accurate transfer of the position is difficult using the “standard 0.125" transfer punch. Consider (machining preferable to hand) creating two or more "custom transfer punches" reducing the diameter of the shaft of an old drill to ( say 0.145" or so) and carefully adding (if by hand) a center point.

Once you transfer punch all the holes, go back, viewing perpendicular to the holes around the “arch”, verify the strip hasn’t shifted and the punches are concentric to the holes.

You then punch/drill the transfer strip with #30 or #40 drill. If you went light on the forces while using the transfer punch to locate hole center, the shallow dimple can be increased to better locate the drill for the next step - transferring the hole location to the Cowl. If this is the case, you could consider using a Whitney Punch. Let the “nib” of the punch center into the transfer punch depression and very carefully, increase the dimple to the depth of the point on the punch.

I’d also consider initially drilling (in my case) the cowl with #40 and take it over to the firewall, carefully position and check concentricity across the arch. If something was “off” you can adjust by angling the drill when you go to “full” size.

Hope this was both "on point" (just read the reference to hinges) and clear!
 
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Definitely do not undimple.

It will make a mess. I got picky and made a drill guide for this purpose on the lathe. Just a small length of 3/8" bar stock, drilled a hole in the center and made a point to align into the dimple. #40 and #30. Just make it long enough to hold it!!
 
Get a drill stop of the appropriate size and slide it down close to the end so the drill is sticking out very slightly. The drill stop centers the drill in the dimple and a perfect hole every time.

Dale
 
Dale--

that is a great suggestion. I just used some plastic sheet and painters tape to make sure that the drilling mess doesn't reach the engine. Drilling is up next.

ken
 
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