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Anyone know anything about stretch forming aluminum?

YJLOPES

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My dad and I have a small business making a few airplane parts. We are looking at expanding into more complex parts and one of them requires stretch forming. We have a local shop we've done work with in the past that has the machine, we just need to make the tooling for our specific part.

Historically in the small airplane world, a lot of the parts that require this method for manufacture use a form die made of concrete or kirksite, at least based on my limited knowledge of this. I am wondering if UHMW could be used. It would be fairly easy to machine to the shape we need and plenty slick when a piece of aluminum would be stretched over it. I'm just not sure if it would tolerate the compression imposed on it. Anyone ever heard of using UHMW like this?

The other concern I have is availability and cost. the part I am wanting to make is ~3' x ~2', and 10-12" at the deepest part of the crown. My quick inter web search I couldn't find UHMW beyond a 6" thickness. Could I laminate several sheets together and expect them to stay together?
 
What gauge aluminum? I'm fairly certain it will work well as long as it's well supported, and has some confinement or enough material to not deform under load.
 
Since concrete is a million times cheaper than UHMW and, as you stated, the traditional material, why not use it?

Wouldn't a block of cast iron or steel be cheaper than UHMW?
 
What gauge aluminum? I'm fairly certain it will work well as long as it's well supported, and has some confinement or enough material to not deform under load.

The material for this job is 2024 aluminum and measures .032" or .040". The original part was formed in a "0" condition and then hardened to T42. The guys that i have lined up to do the stretching have heat treating available on site. According to them, they can heat treat it, throw it in the freezer until they form it, then let it age harden.
 
Since concrete is a million times cheaper than UHMW and, as you stated, the traditional material, why not use it?

Wouldn't a block of cast iron or steel be cheaper than UHMW?

I may use it, but the UHMW has the advantage of spending 10% of the time being machined that a block or cast iron, so machining costs would be way less. Concrete would be cheap, but I have to mold it to the desired shape, which has some challenges that I think I can beat, but still need to deal with.

UHMW could make it stupid easy to go from measuring via 3d scanner to model to machining a form die, at the price of $$$. I don't mind throwing some money at it if it gets me producing parts in 10% of the time. That is assuming that UHMW can do the job, and I can afford the extra money it requires.

I don't know shit about concrete, so that may be worth discussing as well. If I go that route, it needs to be smooth where the aluminum gets formed over it. What kind of material produces that kind of finish?
 
Me I would just make it out of wood. Just laminate a bunch of solid pine together and sand to shape. 4 - 5 layers of varnish to seal and make slippery and your good to go.
 
You may want to look into 3d printing it. A lot of custom press brake dies are being made that way now.
 
I may use it, but the UHMW has the advantage of spending 10% of the time being machined that a block or cast iron, so machining costs would be way less. Concrete would be cheap, but I have to mold it to the desired shape, which has some challenges that I think I can beat, but still need to deal with.

UHMW could make it stupid easy to go from measuring via 3d scanner to model to machining a form die, at the price of $$$. I don't mind throwing some money at it if it gets me producing parts in 10% of the time. That is assuming that UHMW can do the job, and I can afford the extra money it requires.

I don't know shit about concrete, so that may be worth discussing as well. If I go that route, it needs to be smooth where the aluminum gets formed over it. What kind of material produces that kind of finish?

I'm always amazed at the things I learn on sites like these. Always something interesting to read/talk about. Always different points of view, and different arguments about, well, everything.
 
The aluminum cobra bodies made in Poland had stretched form body panels. I think YouTube as some videos.
 
Me I would just make it out of wood. Just laminate a bunch of solid pine together and sand to shape. 4 - 5 layers of varnish to seal and make slippery and your good to go.

This guy did that to form sheet steel parts:



Some really good design tips for the shape of the die and the fixturing throughout there, but around the 19-minute mark is where you can see the final version work.

He's got some pretty badass creations in his other videos as well.
 
Me I would just make it out of wood. Just laminate a bunch of solid pine together and sand to shape. 4 - 5 layers of varnish to seal and make slippery and your good to go.

Depends on quantity and tolerances to be kept. In my semi-informed opinion, temp/humidity would have to be maintained, (wood form), for consistency across multiple parts. Aircraft have very tight tolerances on every single part.
 
Pen guys use Cactus Juice for stabilizing like that. Pulling a vacuum on a 2'x3' chunk would be fun.

Delrin is a lot harder stronger than UHMW. $$$ too.

3d printing a chunk that big would be a challenge, wouldn't it?
 
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Depends on quantity and tolerances to be kept. In my semi-informed opinion, temp/humidity would have to be maintained, (wood form), for consistency across multiple parts. Aircraft have very tight tolerances on every single part.

This is the biggest issue I would think. I think plastics will wear too fast under the pressures you're using. How many cycles can you get out of a delrin form vs. concrete or steel? Seems like the short life kind of washes out the lower cost of production. I'd think coming up with a cheaper and reliable form to make the concrete ones would be the best approach.
 
What if you used the wood then coated in sheet metal or so layer that was harder than the wood it may also help seal the wood from moisture?

I would think concrete would be your best bet then use some type of mortar or epoxy to smooth out the mold. Could also cast it in a wood mold but not sure how you would extract it without breaking it depending on how big of a mold you are talking about.
 
Wood buck will not last long. Concrete would need to be supported with a really rigid structure or it would crack.

I have done a lot of single die forming using a urethane for the bottom "die". UHMW and urethane in a regular press can make some pretty precise parts.
 
This guy did that to form sheet steel parts:



Some really good design tips for the shape of the die and the fixturing throughout there, but around the 19-minute mark is where you can see the final version work.

He's got some pretty badass creations in his other videos as well.


I've used oak as a buck to form ribs and bulkheads before. Works pretty well for making a few parts, never tried it for making 100.

The first plan was to 3d scan the part to create a model via computer magic that I don't understand. Take that model and use it to cut a negative mold out of wood (MDF laminate maybe?). Seal the mold so it wouldn't get damaged by pouring wet cement in to it. Pour cement to make the form die. Mount it in the stretch form machine and make parts, PROFIT.

The use of UHMW would allow me to go from the model to machining the die. I'm just not sure the juice is worth the squeeze. I'm pretty sure the plastic would stand up to making a part, but that's based on TLAR and not any engineering. I'd like to know that it would work before spending a ton of coin on UHMW.
 
uhmw is soft. it deforms alot. Acetel sp? or by brand Delrin is what you want. that shit is harder than a whores hart. it machines easy. i use it for tube forming dies.
 
How complex is the piece?

aside I had to make a wood form and stretch aluminum sheet over it and trim it for a project in a&p school.
 
uhmw is soft. it deforms alot. Acetel sp? or by brand Delrin is what you want. that shit is harder than a whores hart. it machines easy. i use it for tube forming dies.

2nd this. I've made embossing dies out of Delrin, and that stuff is solid. UHMW is more like milk jugs- it makes great cutting boards (I use it for industrial plating station countertops, and it lasts forever), but I wouldn't count on it's dimensional stability. It seems pretty squishy, comparatively.

https://readingplastic.com/uhmw-vs-delrin/
 
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Depends on quantity and tolerances to be kept. In my semi-informed opinion, temp/humidity would have to be maintained, (wood form), for consistency across multiple parts. Aircraft have very tight tolerances on every single part.

Not a tight as you may think...... especially at the GA end of the market.
Wooden plug could work but I'd probably use a water resistant MDF for greater dimensional stability and coat it all round with polyester/epoxy resin rather than varnish.
 
Not a tight as you may think...... especially at the GA end of the market.
Wooden plug could work but I'd probably use a water resistant MDF for greater dimensional stability and coat it all round with polyester/epoxy resin rather than varnish.

So true, I work on airplanes built by unemployed farmers. Some of the tolerances make me laugh, other times I'm impressed how well they did. At the end of the day, I've flown a ton of different airplanes (same 2 models, different serial numbers) and it is amazing how well they work. Any discrepancies between them is typically found to be poor maintenance rather than manufacturing problems.
 
I've used oak as a buck to form ribs and bulkheads before. Works pretty well for making a few parts, never tried it for making 100.

The first plan was to 3d scan the part to create a model via computer magic that I don't understand. Take that model and use it to cut a negative mold out of wood (MDF laminate maybe?). Seal the mold so it wouldn't get damaged by pouring wet cement in to it. Pour cement to make the form die. Mount it in the stretch form machine and make parts, PROFIT.

The use of UHMW would allow me to go from the model to machining the die. I'm just not sure the juice is worth the squeeze. I'm pretty sure the plastic would stand up to making a part, but that's based on TLAR and not any engineering. I'd like to know that it would work before spending a ton of coin on UHMW.

Do you have an example of the parts you'd be making? The thing I'd worry about with plastic/delrin is any sharp corners or edges getting worn down quickly. If everything is smoothly contoured or radiused, I could see it lasting a lot longer.


Once you get it scanned and have the 3d model, it might be worthwhile reaching out to a couple of the concrete formliner companies. They use a similar process. The material they use for the liner depends on the lifespan you need out of it. For only a couple uses, they have a rubber-foam product. For 100+ uses, they have a urethane-like product. I believe for all, they make make a positive mold our of plastic on a 3d cnc router and then pour the liner to make the negative mold that you'd pour the concrete in to. Depending on how many uses you get out of the concrete, that one mold could last you many years.
 
2nd this. I've made embossing dies out of Delrin, and that stuff is solid. UHMW is more like milk jugs- it makes great cutting boards (I use it for industrial plating station countertops, and it lasts forever), but I wouldn't count on it's dimensional stability. It seems pretty squishy, comparatively.

https://readingplastic.com/uhmw-vs-delrin/

It looks like delrin or UHMW may be cost prohibitive for an experiment. I think I'd be in the $3-4K worth of material for something that may or may not work.

That brings me back to concrete. What kind of concrete do I need for a smooth finish (like a sculpture) AND has the strength to tolerate being compressed? My quick search tells me it has more to do with how its finished rather than what is mixed (meaning concrete from the hardware store, nothing obscure). I assume I'd want sand aggregate and if the inside of my mold is smooth, whatever is poured in to it would mirror that finish. I've read a little about coating the mold with motor oil. After it is pulled from the mold, can I touch up any imperfections? How?
 
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