Centre Punch Aluminum?

[JC_Avery]

Hi folks,

I've been devouring various videos on YouTube such as the Metal Magic series from Kitplanes, as well as a few other aircraft structure videos from BCIT and the like.

When drilling a new hole in sheet aluminum, Paul Dye from the Kitplanes series recommends using a centre punch when locating the new hole. Given his wealth of knowledge and experience, I'm willing to bet that this is an acceptable technique when fabricating the various bits and bobs for our airplanes.

However, I've also seen comments in various places that recommend against using a centre punch on aluminum, but they don't explain why. I'm assuming it has something to do with the brittleness of the aluminum, but I'm no metallurgist.

What does the brain trust say here?

 

[Paul Thomas]

I use a centerpunch; I have not seen the recommendations against it. The use of a centerpunch ensures that the center of the hole is where I want it to be.

 

[Taltruda]

Whenever I center punch aluminum, I back it up with a bucking bar so it doesn’t distort the whole area.

 

[Luke D]

Also, a good automatic center punch works well on soft metals. I used to have the cheap style you can get at the big box stores. On a recommendation, I got one of these from Starrett. I'm very frugal with tool purchases, but this tool is worth the cost. I'm happy I got it every time I center punch something.

(PS: I soaked mine with Boeshield when I got it, and wiped it down. Still looks brand new after years of use)

https://www.starrett.com/details?p=335417&s=18A-Automatic-Center-Punch

 

[Ironflight]

Whenever I center punch aluminum, I back it up with a bucking bar so it doesn’t distort the whole area.

Backing up sheet metal when center punching is definitely the key! I’d have to go back and look at our video, but I expect that I was doing it against the workbench or something else firm - you can’t do it in mid-air, or you’ll just deform the sheet.

I’d be curious to know if you find a written reference against doing it - and I’d love to read the reasoning.

Paul

 

[JDeanda]

Here’s the Reason Why Not

And turns out, not a very good reason. In my A&P course back in the 70s and in some of the very old timey literature like the pocket-sized guide books that A&Ps (well, A&Es back then) and engineers used to carry, they recommended against center punching aluminum because it would work harden the metal and cause the drill to wander off center. Has this ever happened to you? Me neither. I think those old manuals were wrong.

 

[jrs14855]

Center Punch

John Thorp was using punches in the early 60's for his matched hole/no jig construction. The duplicator punches he used are basically center punches with different size shanks. Properly used they are very accurate in locating holes.

 

[JC_Avery]

Backing up sheet metal when center punching is definitely the key! I’d have to go back and look at our video, but I expect that I was doing it against the workbench or something else firm - you can’t do it in mid-air, or you’ll just deform the sheet.

I’d be curious to know if you find a written reference against doing it - and I’d love to read the reasoning.

Paul

Hi Paul,

Thanks so much for chiming in! You were indeed doing the centre punching on the workbench. Makes total sense regarding having a solid backing when doing so.

My question basically stemmed from just browsing other similar type videos and forums, and I came across a few old-time hands that off-hand commented against centre punching aluminum but they never explained why. I haven't been able to find anything concretely recommending against it, but it was enough to make me wonder all the same if I've been damaging parts. I'm new to the game so I've basically been sticking to the builder's manual and reading through AC 43.13.

And turns out, not a very good reason. In my A&P course back in the 70s and in some of the very old timey literature like the pocket-sized guide books that A&Ps (well, A&Es back then) and engineers used to carry, they recommended against center punching aluminum because it would work harden the metal and cause the drill to wander off center. Has this ever happened to you? Me neither. I think those old manuals were wrong.

This might have been where that advice was coming from. I've never had the bit wander, either. Having never built a plane before, hearing that bit of outdated wisdom made me start wondering if I was doing some sort of damage to the parts.

Thanks all! I know it was probably a silly question, but the more you know . . .

 

[RVDan]

My .02. On thin sheet, say .040 or thinner, if not done properly, center punching can easily distort the sheet in a meaningful way. After all, that is how the punch works, by pushing a divot into the metal. I completely support center punching when a precision location is required. However, with #30 and # 40 drill sizes, a 135 deg split point drill will start really close to where you place it. You just need to hold the drill perpendicular to the surface and start slowly. This is typically adequate for most thin sheet metal drilling.

 

[Ironflight]

However, with #30 and # 40 drill sizes, a 135 deg split point drill will start really close to where you place it. You just need to hold the drill perpendicular to the surface and start slowly. This is typically adequate for most thin sheet metal drilling.

I agree…and truth be told, after fifty years and a couple hundred thousand holes, I don’t center punch everything I drill either - I’m guessing I got good enough after the first ten thousand holes that I could start them accurately (by teasing the trigger) without the divot. But I don’t recommend mend that for beginners.

And if you want REAL precise centering, forget the automatic center punch and get an Optical Center Punch…. Ultimate luxury (but not as fast…)!

Paul

 

[wirejock]

Optical Center Punch

I agree…and truth be told, after fifty years and a couple hundred thousand holes, I don’t center punch everything I drill either - I’m guessing I got good enough after the first ten thousand holes that I could start them accurately (by teasing the trigger) without the divot. But I don’t recommend mend that for beginners.

And if you want REAL precise centering, forget the automatic center punch and get an Optical Center Punch…. Ultimate luxury (but not as fast…)!

Paul

Love that thing. If you want precision, it's the tool for the job.
Kinda fun too.
Optical Center Punch
I always have a bar or back riveting plate underneath the material being punched.

 

[koupster]

My A&P sheet metal instructor taught a no center punch approach for location critical holes. He acquired the work hardening from center punching theory from his time as a quality control inspector at an aerospace manufacturing corporation.

He taught starting the hole with a handheld very sharp drill bit just to the depth that a center punch would produce. The power drill would then follow this indent precisely.

I present this approach as a way to locate a hole in an enclosed structure where a center punch can't be backed up, rather than an every day practice.

 

[rv8ch]

As someone that struggles to drill a hole where I want it, I've turned to creating "jigs" to guide the drill bit when the hole absolutely needs to be in a particular spot.

As mentioned earlier, using good quality split-tip bits makes it much easier to drill holes where you want them.