Screwing around on the lathe last night, I made a dimple die set to play with. Dimples a 1.5" hole out to a 2" OD dimple.

Got me thinking about making other sizes (found PolyPerformance's dimple dies, and honestly, having already made one of my own, theirs are a good deal anyway), and then about dimple theory and reality.

Is there a point at which a hole is too small to have it effectively matter if you dimple it? How about too big?

It occurs to me that the bigger the hole is, for the dimple to effectively stiffen the material, the dimple needs to be deeper--for example, my 1.5" one, pushes the material about 3/16" from flat. A 3" hole, it seems, might need to be pushed more like 1/2" from flat, to get equivalent stiffening.

Am I overthinking things?

Scott,

Just a little to make the hole stiff. 1/4" is fine for a 2" or 10" hole. Uh,
maybe a little deeper on the 10" one.

The dimple really doesn't need to be that deep! If you make the hole 4" is diameter, then just flange it 1/4", so the hole is 4.5"... you've increased the strength quite a bit!

I'll look tonight in my A&P accepted practices to see if there is a recommendation. Also there using a "U" shaped dimple does seem to make it stiffer. Noticed those on commercial (stamped) ribs. Most home builders just flange the hole.

Example is attached.... Part of the wing rib. :)

http://www.mybearhawk.com/wings/025center.jpg

Yep, airplane wing.

What are you building the dimple set for?

Tom :usa:

So far, just for screwing around with.
Was thinking about dimpling the roof on the buggy I'm working on now, maybe the dash and/or floor, stuff like that.
Truthfully, it's more an appearance thing than a strength thing, at least for what I'm doing with it so far, but it got me thinking... usually a dangerous thing, but every once in a while, interesting.

Got it...

I think you'll get more stength from using a bead roller on the panels.

Admitted I *hate* oil canning panels. So a dimple, bead roll, WELDED
"L" bracked to it, anything but the "thunk, thunk, thunk" of a panel
popping in and out. Then again, nothing a ball peen hammer wouldn't
fix.... but your buggy looks too nice for that! :)

I know you wouldn't want the weight... but also look at those "noise
mats". Forget the name, but a tar mat with super stick adheasive to
apply to the inside of panel. LOL... I didn't realize there were two types,
one for door panels, another for roof panels. Used the on the old Jeep
Grandwagoneer (360 V8) on the roof as it oil canned all the time. Sure
enough the FELL OFF after the first Texas summer... but I had replaced
the head liner by then... so it just oil canned, and the tar thingy would
hit me in the head (sitting the top of the headliner) on bumps. Thunk,
whapp, thunk, whapp. No wonder I hated that Jeep! :)

Tom :usa:

To some degree, that's an annoyance on my big buggy as well--the Lexan roof pops back and forth and rattles a bunch. Haven't tried dimpling Lexan yet, maybe I oughta... hmm... I just really like the tinted roof that's on it, much nicer than the aluminum roof that I'll have to build for this one.

The one I'm working on now, has to comply with comp rules that require an 1/8" aluminum roof. I can also use steel, but aluminum's lighter. Any weight I can save out of that is all the better, obviously, but it'd take a brutal bead roller to roll a bead into that (I'd think). So I'm kicking around dimpling it instead. Further problem there is, however, there's only so big of a piece of anything that I can put in my press anyway, so this could all end up a nearly lost cause as far as the roof is concerned.

Mostly, truthfully, I'm just all geeked up to build tools. Just makes me happy to make stuff that does stuff, and quick-fix stuff, like dimple dies, are an easy thing to make, that do something cool. Someday I'm gonna be that crazy old guy down the block, tinkering away. Maybe I need to build that brutal bead roller next. Hmm... :)

Typically, holes in steel are there to reduce weight or just for looks. The dimpling of the hole just strengthens that area, in turn it will reduce "oil canning", but won't really strengthen the panel as a whole. Also note that anytime you bend steel 90* you effectively double its strength.

roller to roll a bead into that (I'd think). So I'm kicking around dimpling it instead. Further problem there is, however, there's only so big of a piece of anything that I can put in my press anyway, so this could all end up a nearly lost cause as far as the roof is concerned.


Scott - you can always run a center hole through your dimple dies, then use a bolt n' nut in lieu of a "press".. that way you can dimple anything that won't fit in the press.

How thick can the material (mild steel) be with the nut and bolt method? Same thing for a press? One last disjointed question, is there a formula that can be used to calculate capabilities? i.e. die sizexmaterial= load?

A quote from this thread about dimpling: http://www.pirate4x4.com/forum/showthread.php?t=295114

The tonnage required depends on the thickness, material and size of the hole. I have done most everything with a 20T harbor freight press. It's really advantagous to apply flared holes to thin material (less that .090), any thicker than that is mostly cosmetic.

'k, got some pics from my dad, and more info, and now I'm wondering how much stronger it is, having a radiused edge to the dimple, as opposed to the typical racecar stuff that I see, which is a basic conical dimple. But the radiused dimple may be more for fatigue life than ultimate strength.

Dad says that it's very important in the airplane world, because every corner is a stress riser (duh) and that the dimpling in his wing ribs (similar to what Tom posted above) actually rolls all the way to 90 degrees from the panel it's rolled into, and there are no corners to it at all. But that's a somewhat different application than what I'm doing, and governed by rules that allow much thinner materials and whatnot, than what I can play with. Also, apparently wing rib dimples are rolled by a rotary tool, not pressed like with a conical dimple die, from what he told me. That rotary tool is apparently closer to a bead roller than anything else.

Radius of the rolled edge depends on the size of the hole, and as such, distance from flat also varies based on the size of the hole.

I started thinking about trying to press dimples with radiused instead of conical edges, and quickly concluded that it'd only work right, if the dimple die set used was designed specifically for the material thickness it was being used on, while conical dimple dies can be readily used on any thickness up to a certain limit (but per Goat, unlikely to be of any use on thicker materials).

The die set I made has a small hole in each end--I can drill it out and run probably as large as a 1" bolt through this one, which I may ultimately do if it seems like I want to use it in the interior of a panel that I can't otherwise access. I haven't calc'd what the pressure involved is yet, but I can readily dimple 24ga steel in this die, in my arbor press (1t). I can't do much more than slightly indent 1/8" aluminum with the arbor press--that requires the real press. My real press, however, goes from zero to thirty tons, in less than a second.

Anyway, I thought it was interesting stuff. Now that I can get back to work on the tube stuff (finally got more tube in) I'll probably ignore the dimple thing for a while, but eventually get back to it.

If anybody cares to have a look at what I'm tinkering with so far,
http://www.rockstomper.com/images/tools/dimpledie02.jpg

Scott,

Looks good!

Irony is for production the ribs are done on a 250 Ton press. Hydroformed.

For home building either a die like you have, or the flanging tool your dad descibed work. Hmm. Is you Dad building a RV aircraft (those guys love the flange tool... :) Other home builders use a die from MDF and BFG (big friggin hammer) to dimple.

Couple of link for your Dad (aviation related):

http://www.watermark.ie/aviation/Beginnings.html

http://www.watermark.ie/aviation/pl-9.html

Tom :usa:

Actually, he's not really building one at all--he's got a Piper Dakota (I want to say mid-70's vintage) and he's mostly just trying to keep it airworthy. Apparently old airplanes are a little like old cars--eventually, the aftermarket forgets about them, and parts stop being available.

Wanna talk irony? A neighbor of mine has a hydroformer. Well, his employer does, at work... but he's basically the head guy at the shop, so there's nobody at the facility above him, and if he comes up with the dies, he can use it to do whatever he wants.

I was wondering about using the big-hammer method with this--I'd be nervous about my own accuracy with an 8-lb sledgehammer swinging at the middle of a panel that I don't want to mess up. Lotsa practice, I suppose.

Thanks for the links, I'm sure they'll be interesting to him, so I'll forward them along. :)

Furthering the bolt idea, I have a small hydraulic press used for punching holes in steel in electrical applications. The pilot hole can be 1/4", 1/2", or 1'. I would think you could use something of this variety to with a dimple die in the middle of a huge panel. It's a ram, you put one side of the die on one side, and the ohter side screws to the end of the pilot on the other side and you pump it and it draws them together.

I have some Poly's dies and they work great and i just place them in the vise and tighten it down to make the dimple. Works for the thin stuff at least. 1/8" steel will take some real force to get it fully dimpled but haven't done any alu.

Got it...

I think you'll get more stength from using a bead roller on the panels.

Admitted I *hate* oil canning panels. So a dimple, bead roll, WELDED
"L" bracked to it, anything but the "thunk, thunk, thunk" of a panel
popping in and out. Then again, nothing a ball peen hammer wouldn't
fix.... but your buggy looks too nice for that! :)

I know you wouldn't want the weight... but also look at those "noise
mats". Forget the name, but a tar mat with super stick adheasive to
apply to the inside of panel. LOL... I didn't realize there were two types,
one for door panels, another for roof panels. Used the on the old Jeep
Grandwagoneer (360 V8) on the roof as it oil canned all the time. Sure
enough the FELL OFF after the first Texas summer... but I had replaced
the head liner by then... so it just oil canned, and the tar thingy would
hit me in the head (sitting the top of the headliner) on bumps. Thunk,
whapp, thunk, whapp. No wonder I hated that Jeep! :)

Tom :usa:

Goes by the name dynamat or several other makes.

comes in 3 kinds. Tar based. This is the base level stuff. good for floors and firewalls. Next is the aluminum backed geltar stuff. This is for roofs, doors, and motor compartments. Anywhere it gets HOT! Last they have the spray in/paint in stuff. Works great, but you must put it on a bit thicker.

Back when I did a buddies 4x4, we did the floors and cab in the aluminum backed, then did a coat of the spray in to hold it all in place. Worked fantastic. cut the noise down by a good 10db or more.

We just ordered something called Spectrum V2 from Second Skin Audio (http://secondskinaudio.com/). It's a water-based, spray/brush/roll-on coating. They claim it's resistant to oil, gas, hi-temp, etc. Haven't applied it yet, but did a lot of research on car audio forums and it seems like a lot better value than DynaMat's product line.