Preventing Bucking Bar Damage

When riveting the top skins on my wings, I managed to put couple of small bucking bar dings in the spar flange beside some of the awkward to reach rivets (e.g. near the ribs etc). This was despite the fact that I taped with duct tape the edges of my already rounded tungsten bucking bar (the typical 4x1x5/8" bar). I blended these out then primed them after consulting Van's, however as you can understand, I'm nervous as anything about riveting the bottom skins to the mainspar flange (given that I couldn't get the top right), as any such ding is going to be a pain in the rear to blend out blind. I'm thinking that perhaps I should tape around the rivets with duct tape as additional protection before I put the skins in place, then tear it off after I have riveted. Any thoughts or suggestions here would be greatly appreciated.

Regards,

Tom.

IMO, it's 95% technique. Need to be able to reach in blindly and control the bucking bar with your fingers. Are you keeping the bucking bar edge parallel to the spar flange? When riveting blind, I held the bucking bar with my fingers on the bar right over the rivet. I use my finger tips to hold the bucking bar over the rivet, while also overlapping them to touch the skin surface below the bucking bar. I could feel the gap and make sure the bar was parallel to he surface. As you, I also wrapped my bucking bars with tape to help minimize scratches and gouges.

Of the RVs flying, I would imagine there are many, many bucking bar dings. I bet I managed to ding or gouge something about one out of every 200-300 rivets. Obviously fewer the more experienced I became.

I'm sure someone has come up with a more creative method, and hopefully they'll post.

I have a similar bucking bar, and have found a few tricks which help. Like you, I keep both ends of the bucking bar wrapped with masking tape. I generally use 3+ layers of tape, and carve out the portions which strike the rivet using a utility knife. For hard to reach places the long side of the bucking bar can sometimes be easier to use. If you're using the long side blindly (like with the bottom skins) it sometimes helps to stick little squares of low-med density self-adhesive insulation on the bucking bar to assist with keeping it parallel to the skin.

I think these "artifacts" frequently develop because of nearby structure.

The skin has to flex in response to the rivet gun blows.

The harder the rivet, the more force, then more skin flex.

Pretty soon, you pass the yield point and a crease is left in the skin.

Technique helps a lot. Try imagining that the gun stays stationary and you're pushing the bucking bar into the rivet to form the shop head. [I don't mean this "literally," but if you can reduce the bending load on the skin, you reduce the chance of leaving a "reminder"

The real key is getting a hold of fresh (soft) rivets.

mjb

FWIW

Blind riveting

I also tape the end with Gorilla tape and relieve areas for rivet contact. I hold it similar to Low Pass but before it voes into position, the bar is placed flush with the nearest flat surface to get a "picture" of the correct position. Then slide it over trying to maintain the orientation. Sounds complicated but it's not.
Also, for the rivets along the rear spar, I made a bar shim so the bar would be in the correct orientation. Worked great.

I've had my fair share. While my bucking bar is wrapped in tape, I still seemed to have occasional rubbing/bumping against adjacent structure. The fix for me was to use the structure as a guide and keep the bar separated by a piece of cardboard or scrap wood. In most cases my bar is flat against that rib or spar (keeping it perfectly perpendicular to the rivet) and no damage occurs because of the cardboard spacer.

I have the cardboard taped to one side of my tungsten bar - (1" x 2" maybe).

AND, don't forget to pile a bunch of rags or a pillow in the leading edge in case you drop the bar while doing the wing skins . Assuming they are in the wing stand LE down.

Great point Bill,

mjb

Thanks Guys. Some good points on the soft rivets, judging the surface angles etc. Toward the end of the wings I started driving the rivets in multiple short bursts, which limits how far out of shape I get if things aren't going right, and I think I may continue this for the bottom skins. This is likely to be like every skill on the aircraft, where you become proficient by the time you've completed the particular task. Looks like I'm going to have to build a second one to leverage some of that proficiency.
Any other points would still be helpful, as we can keep adding to this post so there's a bit of pool of ideas on this topic.
Cheers,
Tom.

I made it a practice to practice.

Even if only one day off. The rivets I got from Fullerton Air Parts would get harder, day by day. Over the course of 4 days, my gun pressure would increase from about 60-90# and the bursts would more than triple in length.

Then I got smart and started annealing the rivets.

40# and lightly tripping the gun trigger, plop, plop, plop, plop

" . . . .and never was heard a discouraging word and no "smilies" around structure all day"

mjb

Here's how the pros handle it at the Maule plant in Moultrie GA....jig the bucking bar so it positions itself perfectly, every time.

Hi Marc. What temperature and for how long do you anneal the rivets? I have definitely noticed a variation in hardness of my rivets, so taking them back to a particular set point may be an idea.
Tom.

The preliminary answer is found in Alcoa's Technical Reference on Riveting.

The definitive answer is found in the ISM-4, the ultimate authority.

But neither mentions a critical point, that being, you only anneal rivets with a clear anodize coating, no colored, no alodine.

There are two "annealing" approaches, one, accurately called "W" temper, is in the vicinity of 775 ( I won't say exactly because I wish you to look it up, there's more info that can be related here). The other is the classic "O" temper.

With the "O" temper, temperature control is critical. You only have a 10 degree window at about 935 deg F. (Caveat: Note there is a variance between Alcoa and the ISM-4)

In the olden days, this was critical because exceeding the upper temp scrapped the rivets. The problem was the analog temp controllers had a hard time maintaining +/- 5 deg F at that level.

Nowdays, the digital controllers can easily maintain +/_ 1-2 deg F. so it's not so much of a problem. I have a recording thermometer on my HT oven as a QA back-up.

The difference is remarkable and you can achieve flush rivet installations that won't catch a finger nail dragged across.

Onward and upward.

mjb

DanH said:

Here's how the pros handle it at the Maule plant in Moultrie GA....jig the bucking bar so it positions itself perfectly, every time.

Click to expand...

That's what we did. I called it cheating. I never missed a chance to let the bar "ride the work". It is often as simple as taping a piece of balsa to the bar to guarantee the correct angle and placement.

As I've told many people.......when it came to bucking the rivets.......we were not above cheating

The pro's that don't use jigs and fixtures to hold bucking bars learn to hold the bars so that they always have finger tips extending beyond the end of the bar face that is forming the shop head of the rivet.

This allows you to feel when the face of the bar is square to the structure (which will also make it square to the rivet) and verify that there is equal clearance around the perimeter which will prevent the bar contacting the structure and causing dents.

It is rarely possible to get a finger on all four sides, but if three sides feel the same, the forth usually will be also. Sometimes you can only get fingers on two opposite edges, but if one of those edges has two fingers you can still gauge the gap in both axis.

BTW, every time someone starts a discussion about annealing rivets it makes me very nervous.
It can be done, but the temp control to do it right is extremely critical. With a very narrow temp range that has to be hit. Get it wrong and you could be building sub standard assemblies with no way to know it.

Yes,

The critical importance of excellent temperature control cannot be over emphasized. Not so critical for "W" temper, but "W" temper is usually INTENDED FOR relief from work hardening - something you can't do when the rivet is already in the structure and hasn't been fully driven.

There is no margin to achieving "O" condition when you exceed the upper temp limit, "it was only for 5 seconds" doesn't get the chemistry - one reason why I was purposefully vague on the temp range. Get the reference book or a copy and READ IT.

mjb