Starburst Countersink, Wing Main Spar

[Riverside3]

I used a guide fitting to countersink the wing main spar fuel tank screw holes. While working the inboard most hole on the top, the fitting came loose resulting in a jagged, starburst shaped hole...a very short, quick chatter cut. The #6 fastener will sit flush in the space but I am concerned that cracks might develop in the tank overlap at the fastener point due to the jagged fit that the hole offers. I'm thinking, better to finish the hole out to a concentric shape and then introduce a semi-rigid spacer, rubber washer, during assembly to make up the space. That, or just over dimple the skin and have the fastener run in the extra distance.

[Rick6a]

Originally Posted by Riverside3

I used a guide fitting to countersink the wing main spar fuel tank screw holes. While working the inboard most hole on the top, the fitting came loose resulting in a jagged, starburst shaped hole...a very short, quick chatter cut. The #6 fastener will sit flush in the space but I am concerned that cracks might develop in the tank overlap at the fastener point due to the jagged fit that the hole offers. I'm thinking, better to finish the hole out to a concentric shape and then introduce a semi-rigid spacer, rubber washer, during assembly to make up the space. That, or just over dimple the skin and have the fastener run in the extra distance.

I see this is your first posting...welcome to VAF! It would be nice if you could post a picture of the specific problem you speak of but short of that, let me toss out these GENERAL ideas. Fuel tank screws are #8 so if you countersunk those holes for #6, you have no worries, just bring the holes and countersinks up to the correct size. If on the other hand, the holes are already #8, the subject hole and other chattered countersinks can be easily smoothed out while removing a minimum of materal by using a pilotless open 100° countersink and applying low RPM to it. Such open countersinks are available in many styles, here's one:

http://www.aircraftspruce.com/catalo.../averyHSS3.php

Chuck the bit directly into a drill motor and free hand the work using a low RPM. You can use a handle if you prefer:

http://www.aircraft-tool.com/shop/de....aspx?PageNo=1

A little lube is optional. I also find non-piloted type open countersink bits to be ideal for fine finishing the countersinks in Plexiglass without worring about chatter and open countersinks are also an excellent choice for many deburring tasks too.

Good luck.

[n8zg]

I've had precious little success removing chatter marks from a countersink with the fluted cutters that Rick mentioned. I have had some success using the single-flute variety, similar to these:

http://www.cleavelandtoolstore.com/p...sp?number=CCSF

I've heard that there are also some 100-degree countersinks out there made with PermaGrit (but a cursory search didn't reveal any at Avery or Cleveland).

Given a choice - meaning that if all three styles were readily avilable in the tool box - for this operation, I'd reach for the PermaGrit cutter first, the single-flute countersink (or de-burr) second and the multi-flute variety last.

As an alternative, I'd make a new hole in the guide block, clamp it securely in place, lube it up and use high RPM & very low tool pressure / feed speed to clean up the chatter. Then final-size the hole and countersink.

[Transporter]

Avery has the 100 degree countersinks:

http://www.averytools.com/c-104-perm...ntersinks.aspx

Mike