The Home Machinist!

I have never seen aircraft skins heated to dimple. I work at an airplane museum. Anything worked above 350 degrees F or so is likely to cause cracking due to hot shortness (explained by jpfalt). Maybe some of the mfgs use heat for dimpling in an extremely controlled procedure, but for most of us hot working aluminum is asking for trouble.

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18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

It is just stupid simple to form when heated to a yellow heat. Once you hit the right heat, it'll just do anything you ask it to. Attached is a pic of my .020 titanium Palm Zire organizer shirt pocket clip I made at work. Sheared the basic shape with a Pexto stomp shear and then finished, including the cutout, with a shear/nibbler. Cutting was less than 30 mins and forming took about 15mins with an OA torch.

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18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

Mike C.

Check this out,

http://www.ustool.com/usstore.asp?WCI=wciViewItem&WCE=94026


Yes, it is a controlled situation. I see from the spec page that the temp range is 0-800 degrees.

Wow, the times I could have used a rivet squeezer THAT big! $22.5K... guess we won't be getting one anytime soon. Again though, I have never seen hot dimpling or riveting used. I'll ask a couple of the old hands at work about it. I still stand by my statement, hot-forming aluminum will probably cause problems due to cracking.

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18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

Mike C,

Regarding hot dimpling of aluminum, I know Boeing does it. Several times I've seen the hot dimplers for sale at the surplus store.

I'll do some research in my texts on metal forming to see if I can find anymore info on hot forming aluminum. I recall reading about magnesium and titaniam being hot formed.

I figure it's more of a production technique. Probably only used on REALLY hard skins, too.

I have never played with magnesium. I'd like to, just haven't run across any. As for heat forming titanium, that's about the only way to form it. It will just go right back where it was if you try cold bending it.... if you manage to bend it to begin with. Usually cold bending titanium results in a break. They form shaped skins and such by explosive forming. They put the mold underwater, put the metal in front of it, then set off an explosive charge near the metal, forming super high pressure shockwaves that blow the metal into the mold. That's how the compound-shaped skins for the SR-71/A-12s were made.

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18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

When I mentioned the subject of heat forming aluminum at work today, I thought I was going to be lynched. All hands agreed, NEVER use heat to bend aluminum. As for the heat riveting and dimple.... nobody had ever seen a shop using a hot dimple punch and nobody had ever done any hot riveting. Apparently this is a production technique using controlled conditions on certain alloys. These are all old time aircraft guys who have worked on all kinds of stuff. You can use heat to anneal hard or work hardened aluminum, but heating to bend will result in cracking.

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18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

Mike C,

Regarding the hot dimpling, I'm surprised your older guys don't know about that. The hot dimpling machines I've seen were for low production, one rivet at a time. I assume they use the same technique on automated rivet machines too.

I did do a quick search through about five books looking for info on the subject of hot working aluminum. The only explanation in any depth had to do with deep drawing of thick aluminum plate. They mention hot working in cases where the severity of the drawing caused problems in cold drawing, but hot working allowed the job to be done.

The guys had heard of hot riveting, but none of these guys had ever done it. We have PEMCO Aeroplex, a KC-135/C-130 rework facility over at the airport (formerly Hayes). I'll ask a couple of the airframe guys over there if they have messed with it. Hot riveting/dimpling and bending or forming a piece of aluminum plate are TOTALLY different applications, though. Drawing is a totally different set of rules, too. Yes, heat is used in deep drawing applications.

I am talking about ordinary forming and bending, like making a bend in a bar or a compound bowl shape in a common alloy like 1/4" 6061-O plate. You heat that and hammer on it, or try to bend it, and it will crack.

Aluminum starts losing its strength at relatively low temps and does so VERY quickly, becoming very brittle in this softened stage (kind of like trying to bend a popsicle). It is hot short. If you heat it and bend it, the reduced tensile strength at temp will result in a crack on the outside of the bend. Once aluminum is cracked, it is junk. Even if you don't see a crack, the metal has been injured and will be a source of trouble later. This hot-shortness can also cause cracking in aluminum welding, as stresses are built up from expansion and contraction during the cooling period that end up breaking the part at the last area welded. Likewise, cast parts will be torn apart in the mold even after solidification if the sand cores do not collapse easily during the cooling period (cores must have very low dry strength).

Try it if you don't believe me. Get a piece of aluminum plate and cut four pieces. Put two of them in a vise, side by side. Bend one cold until it starts to crack and then stop. Heat the other and start bending it. See which one goes further without cracking. Then take the other two pieces, heat one and bend it, but not to cracking. Cold bend the other about the same, but not to the point of cracking. Let them cool and bend both of them a little more. See which one cracks first.

I don't think the skid plate in question is going to be hot riveted, hot dimpled, or deep drawn with a 250K lb press. I imagine this will be a hand formed part built in a home shop. If it is to be made out of 6061 or most any other common aluminum alloy, and the metal is heated to form it, it's going to crack.

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18x72 L&S, Fosdick 3ft radial, Van Norman 2G bridgemill, Van Norman #12, K. O. Lee T&C grinder, Steptoe-Western 12X universal HS shaper, 16spd benchtop DP, Grob band filer, South Bend 10L

It took a while but I finally pulled the stock plate. It is about 1/6" steel and beat up. It need to be wider and 3/16 to 1/4" Aluminium. Some of those bends seem pretty dramatic. Can it be done in Aluminium? Should I make a wood "mold" to form it to???

another pic

Good question.
The existing skid plate is bent in several areas so a more robust plate is needed. The shift lever shaft and some of the front engine casing is exposed so it needs to be wider. To prevent damage to the cases a semi wrap around shape is desired at the front. It should be fairly flat on the bottom so you can slide over a log or rock on the skid plate. It should be light and not have a lot of area that can collect mud. Appearance is not important. This bike was sold as a street/ dirt bike in the US. In England, it was sold as an off road only model and came with a more robust skid plate. These are no longer available.