automatic center punch... friend or foe?

While learning to rivet at Sun-n-Fun 2005, I used a neat little spring loaded center punch. Seemed like a great idea at the time. Recently, I have seen several reports that they shouldn't be used on our aluminum planes. Any thoughts?

Carter in NC
finishing training kit, RV-8 empennage pending

I used one, Joe Schumacher used one. What seems to be the danger? If the mark it makes is too small for the drill size, just repunch with a regular center punch. It's nice to mark off hole locations, and as I mentioned, repunch with a bigger center punch and hammer, if needed.

JMHO
Roberta

I used mine for my HS-908 brackets on my Horizontal stab. I wouldn't really want to use it on the skins though, it might mess those up a bit especially the .016" Mine takes quite a bit of pressure before it pops, but its also the $4 special from harbor freight.
Jeff

They work great. Probably best to back the work with something if it's thin, otherwise you'll get a dent.

Steve Zicree

Automatic center punch

I could be wrong, but I seem to recall being told not to use one of these punches on sheet aluminum. The impact from the punch work hardens the area right around the hole which can lead to cracking later.

stress cracks...

During Tom Emery's WPARV building class, he mentioned that using a center punch on thin aluminum will cause stress cracks that extend beyond where the drill bit will remove the material. In addition, as metioned below, there's a high probability of making a large dent in the thin skin. Just another viewpoint. -Jim

Center Punch Tricks

Here's what my tool 'n die maker father and some experience have left with me:

The only reason a center punch dents, instead of making a nice pin-prick crater, is because you did not back up the material with some mass.

The caution of not using a center punch on aluminum is if you don't intend to drill it out. Think of all those pre-punched holes!; they're undersized so that drilling removes the stresses in the shear line. Center-punching does cause a bit of stress (it would on any material), but still can be used to identify parts and orientations on aluminum if you don't bludgeon the part to death. (These last two points are in the builder's manual, also.) Use a 2-ounce machinist hammer, gently.

The auto punches are a disaster waiting to happen. Since you have to apply pressure before they actuate, there's an increased possibility of skidding before they ding. And you can't control the force without fiddling with adjustments. It's far easier to position and control a sharp, fine point punch tapped with a light hammer. Also, you can walk a slightly mis-aimed punch across the surface by tipping the punch.

The secret: massive backup behind a well restrained or clamped part.

John Siebold

deburring pre -punched kits

Am I to understand correctly that the 'newer' pre punched style RV kits still require each hole to be drilled out & therefore deburred , or, have I misunderstood and actually there is very little deburring required because it has already been done?

Still have the joy of deburring

Each hole still gets drilled to size and deburred during assembly. Wouldn;t want ot miss out on all that fun now would you?

Cartaire said:

While learning to rivet at Sun-n-Fun 2005, I used a neat little spring loaded center punch. Seemed like a great idea at the time. Recently, I have seen several reports that they shouldn't be used on our aluminum planes. Any thoughts?

Carter in NC
finishing training kit, RV-8 empennage pending

Click to expand...

Carter--

I'm biased--my father-in-law invented that tool and now sells it through Noxon, MacTools and I think Sears. I used to mock him for inventing something that my normal hammer in punch did just fine, but it is great for those rivets that need to be drilled out and I also just used it to start holes on my very hard stainless steel aileron counterbalance tubes.

I'm not sure about how a tiny center-punch like that could start a significant problem. Read any of the Tony Bingelis or Ron Alexander articles on the EAA Homebuilders HQ web site and they will be adamant that you start all holes with a hole punch--particularly because using a centerpunch will prevent the drill bit from wandering which could cause a problem that IS definitely real. Beyond that, I don't know what I'm talking about, and I am biased, so take this for what it's worth.

Auto Punches

Ok Guys,
I am an ex-machinist and have done a lot of aluminum aerospace work. I have NEVER seen a recomendation against using a center punch. Even on mil-spec missile work. In the auto punches get a good one as the quality (and pressure) varies tremendously. Most of the good ones are adjustable. The best set up I've seen for aluminum is a spring punch. It has a punch hardened end on both ends and a spring connects them. Pull back one end and release and bingo a punch mark. VERY CONTROLLABLE. Usually the two ends are different weights so you can have different force. I've never used anything better, and no hammer needed.
Bill Jepson

Those spring-loaded gizmos are adjustable. if you're making a dent....or leading to stress...you've got it set to apply too much pressure before it "gives." Dial it down.

The other option is simpler, however, put a drill where you want to drill and spin it a couple of times by hand.

voila!

By the way, those spring loaded things tend to break after a short period of time. In my SportAir class, THAT was the warning of why not to use it. And, as it turns out, they were right.

In general, another unneccessary tool.

But...Harbor Freight sells the automatic center-punches on sale quite often, for like $1.99 each. I love using them.

From Rosie: I built my plane with this $1.99 punch, and with 2262 hours on the airframe, it's holding together fine!