CAD to CAM best methods for sheet metal

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Monty

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I started this in response to a post in the CNC plywood aircraft thread because the question and response were specific to metal construction and I thought it would be a good idea to have a metal specific discussion.

I need a good sheet metal design package so I can design my own matched hole airplane. Solidworks? Anyone have definete preferences?
I'm doing this right now. I gave up on matched hole construction. Even with a sheet metal package, you are going to have to guess at the bend allowances. Unless you are going to build multiple prototype parts and tweak things to get it to be "matched". I concluded this was too much trouble for a one-off.

My solution is to layout the rivet holes on the skins. The flanges on the bulkheads and ribs will not be pre-punched. That way all you do is line things up with clamps. Then start the drilling/cleco routine. There is enough slop in the flange to soak up slight errors in bend allowance.

I'm using Rhino. I use the offset surface, and unroll developable surface command to generate my flat patterns. Rivet lines can be made by splitting the surface along the line and unrolling it in pieces. Then you can use the divide curve command to place the desired number of rivets along the rivet line. Make sure all your tab and cut radii are larger than your cutter bit dia. and its easy to export a .dxf and generate your tool paths.

I have Solid Works at work, it is a very capable package and has become a sort of industry standard in the mid range space. I have used a LOT of cad packages and I do not find Solid Works to be as user friendly as something like Alibre...which will do 90% of what SW will do. I find that Solid Works is Similar to Solid Edge in trying to "help" you by dictating workflow....well that's fine unless you don't like to work the way they think you should. I much prefer an open ended programming structure.

I have the sheet metal package for Alibre......I wouldn't even think of using it to design an airplane.

It's much better to understand conic lofting, and developable surfaces and use a package like Rhino than to try to rely on some cumbersome software tool that gets you all tangled in your underwear.

I'll post some examples of flat pattern generation later.
 

Topaz

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...My solution is to layout the rivet holes on the skins. The flanges on the bulkheads and ribs will not be pre-punched. That way all you do is line things up with clamps. Then start the drilling/cleco routine. There is enough slop in the flange to soak up slight errors in bend allowance...
"Matched Hole" construction as it is usually meant in airplanes (at least homebuilts) is really closer to this than pre-drilling all the holes and somehow getting them to line up in assembly. The basics of 'matched hole' construction in homebuilts is essentially as you describe it here - a few guide holes are pre-drilled as locators to cleco, and then the remainder are drilled through the sheet and the substrate, ensuring that the holes "match".

I'll agree it's a bit of a misnomer.
 

oneturboneeded

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hi Monty,
im building me a 12'x5.5'x2.5' router it will be a 5 axis eventually. I currently run alibre and google sketchup. I was wondering what you thought the problem with alibre is in respect to the sheet metal operation? Is this a weak point? I really dont see myself using my router for sheet metal much mostly for fiberglass molds. Do you think alibre will support this well? I just got in to using alibre so im not sure of its limitations yet.
 

Dana

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I'm not familiar with Alibre, but most sheet metal CAD packages (or sheet metal add-ons) are aimed primarily at simple shapes like boxes, with planar surfaces and simple bent flanges. They're quite good at making the design process easy for such things, with automatic unfolding. The kind of shapes you have in an airplane are a completely different thing.

A freeform direct modeler like Rhino (Orion's choice), KeyCreator (which I use) or CoCreate is a lot easier to use, especially for the kinds of shapes airplanes have, than a parametric history based modeler like Solidworks or Solid Edge.

-Dana

We all know that engineers love to play with expensive toys. Especially when someone else is paying for them.
 

flugtek

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I guess I'll stick with rhino then. Good I can spend my money on metal instead :) I think it would be great to get matched hole for the ribs of the Sonex but I guess it's not that big of deal. I was thinking of the RV-12 and another design they reported about in Kitplanes that were matched hole design. I figured it would be nice to get to the level of skill to be able to do it.

The RV-12 claims to not need to deburr the holes because of thier cnc punch process that leaves a really nice finnished hole in the first place. So it's just cleco a few and get right to riveting. I can tell you the laser cut parts I've bought for the Sonex have some slag leftover sometimes but not much. I think laser matched holes are completely doable with a little inspection and end mills are doing a fine job on my cnc router but need inspection and occasional deburring. I am going to make some ribs with the straight flange that rivets to the shear web with final sized 1/8 holes. I think it will work fine if against todays BKMs. The pop rivets are supposed to be more tolerant of burrs but I still deburr. At 09 oshkosh there was a guy building a WWI airplane that never deburrs anything and has been doing it like that for 20 years. Up to the individual.

I'm going to start messing with the Sonex and see if I can get it but I want to finniosh the plane too so I'm not going to stall on it. I think the problem is in the form block needing to be perfect as well. Or at least repeatable if was ever to want someone else to use my same cad/form block combination.

I just though of that if you get a rib made that's repeatable with the rivet holes in the flanges it would be easier to micro adjust the wing skins holes to match. That the spacing may be different my a millimeter or two would only have a cosmetic efect but would also be uniform from rib to rib. May not even be noticable. Quite possibly not worth the effort as well :)

I had thought of using some pins on the form block so I tweek and form at the same time. The only trouble is they never come out perfect from the form block and have to be tweek on a flat table. It could be done though. It's just work and tweeking. Getting better at Rhino would help too :) I tell you - that's what I need a Rhino tutorial on. Pretty please.... :) I need lofting tutorials for bulkheads from a solid model too. I'm trying but making a cutting plate and boolean difference doesn't always work for me. But I am a ginner really.
 
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Monty

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I guess I'll stick with rhino then. Good I can spend my money on metal instead
I think it would be great to get matched hole for the ribs of the Sonex but I guess it's not that big of deal. I was thinking of the RV-12 and another design they reported about in Kitplanes that were matched hole design. I figured it would be nice to get to the level of skill to be able to do it.
The guys a Van's have a lot of experience and I bet the first parts off the tooling don't quite line up....They probably have to tweak things to get it right. That's fine if you are going into production, but not practical for a one-off. The problem is on the flanges where you are shrinking or stretching material. predicting where the hole on the flange will wind up is impossible with hand formed parts, and difficult with pressed parts.

Another thing to understand about sheet metal packages for SW and Alibre. They are mostly for doing flat patterns for simple stamped and tabbed pieces, not for compound formed parts where you are stretching or shrinking metal. They work for developable surfaces only. You still have to give it a bend allowance, and that is tricky.

I'll get around to posting some examples that will get the point across, probably later this week. Too much going on this weekend....
 

Monty

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hi Monty,
im building me a 12'x5.5'x2.5' router it will be a 5 axis eventually. I currently run alibre and google sketchup. I was wondering what you thought the problem with alibre is in respect to the sheet metal operation? Is this a weak point? I really dont see myself using my router for sheet metal much mostly for fiberglass molds. Do you think alibre will support this well? I just got in to using alibre so im not sure of its limitations yet.
Alibre is a great value if you need to do parametric solid modeling. If you know what you are doing it will do about 90% of what much more expensive packages will do. The sheet metal module is fine for what it is meant for, but it isn't going to give you flat patterns for an airplane. For 3D plug carving it should work as long as you can model the shape you are after and have the professional version of the CAM package.

Alibre's achillies heel is that it does not have conic curves. Instead you are stuck with trying to use splines and sketches. What a nightmare for free form parts...don't even bother. Then you have the problem of trying to generate flat patterns from this...not going to happen.

I don't know about Sketchup, I don't have any experience with it.

Rhino is great because it supports conics. If you understand developable surfaces, you can insure that your model is composed of surfaces that can be turned into flat patterns. Then you can easily generate flat patterns from your surfaces.
 

wildblue37

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One thing you have to remember is match-hole kits are production pieces. That means the producer can spend money on tools to locate the holes after the sheet metal parts are formed. With typical sheet metal tolerances, putting the holes in the flat-pattern is a bad idea. A better way is to make a forming tool, hydropress the part, and then move the formed part to a routing fixture to trim the profile and then drill the matching holes. Then the holes are tool-controlled (ie accurate) and match from one part to another.

Being a homebuilder, that's a lot of investment in tools. You might be better off with careful measurements and laying out rivet patterns with a marker (NOT pencil, graphite + aluminum = corrosion). I'd recommend getting one of those 6" rulers with 1/100'ths increments. I think TheYard.com sells them.
 

Jman

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"Matched Hole" construction as it is usually meant in airplanes (at least homebuilts) is really closer to this than pre-drilling all the holes and somehow getting them to line up in assembly.
I have to respectfully disagree with this statement. Although the term Match Hole drill has been used to describe the act of drilling down through a pre-drilled skin into a substructure that does not have a pre-drilled hole in it, it's not the standard any longer. When talking about "Matched Hole" construction we are really talking about what Vans and Zenith have done with the RV-7, 8, 9, 10 and the CH-750 and 650. Holes are pre-drilled in both the skin and substructure. When the holes line up the structure is straight and requires no jigging. Vans has now taken Matched Hole construction to the next level with the RV-12 and the holes are pre-drilled to exact dimensions ready for riveting. Revolutionary really.

The RV-3 and 4 as well as the Zenith 701, 601, 801, and 640 still use the old method of jigging the structure on a flat table and clamping the substructure in place and then using pre-drilled holes in either the spars, or skins as guides to drill through an un-drilled structure. Works great but not really "matched hole" as I believe it has now come to be known in the kit industry. Just my take on it.

What is MadRocketScientist using to design his parts? Is he pre-punching his skins and substructure? Maybe he can weigh in with what software he is using.
 

Dana

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If you're a big outfit like Van's with tooling, jigs and fixtures, etc., you can develop your production to where you can predrill all holes and everything will fit together. For a homebuilder making his own ribs on a form block, it's almost impossible to get things to line up unless you match drill on assembly.

Both are valid techniques... just suited to different production methods.

-Dana

Always keep clothes and weapons where you can find them in the dark.
 
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