3D printed parts make aircraft lighter...

When I was working on the advanced models of the 747 and 777 we called this process Stereo Lithography. That was 16 years ago.

 

We're printing some parts for jet engines, like GE is, these are limited to parts that you pretty much can't make any other way.

The problem with printing metals is that it's slooooooow..... And for that reason it's expensive. If you can possibly make something by a more conventional process, then make it that way and be done with it.

Also what the fan boys of printed parts don't want to talk about is that the tolerances that the process leaves you with. If you have a part that requires precision, which means do you require a truly flat surface for mating parts together or a truly round surface for putting in a bearing, you're going to end up machining the mating surface or the bearing bore when you're done printing the part. Been there, seen that. One of our suppliers printed a housing and after failing miserably and requiring endless fiddling to get it to work, backed off, added material to the mating surfaces and bearing bores and came back and machined those surfaces to get it to work.

If you want to make a non-critical part. Say a low tolerance bracket or something like a cleat on the deck of a boat 3D printing is a wonderful process, but it is terribly expensive for what you get out of it.

If you want a real piece of machinery you're going to have to post machine it to get it square and true. This negates a lot of the advantages of 3D printing. If you have a part that is small, intricate and you can't make it any other way, then you get forced into 3D printing. I'm intimately familiar with the GE fuel nozzles that are shown in that article and they've been a nightmare to produce by other methods. The other thing is that this isn't a big part, so for them 3D printing makes sense. GE bought a key supplier (Morris Technologies) who were at the forefront of the 3D printing game because they were doing so much business with them that it made sense to buy them rather than have them as a supplier that everybody else could use.

As I said, I've made some 3D printed parts that were at the cutting edge of technology. It was horribly expensive, but we got it done and it worked, but if there was any other way to do it I'd have gladly done it with more conventional processes..

As Churchill once said, "democracy is the worst form of Government except for all those other forms that have been tried"... 3D printing is much the same... It's a horrible way to make a part, unless you absolutely can't make it any other way...

 

The process results in "striations", because the part is made up of "layers" that can be as thin as a few thousandths thick. If the surface you're looking at is "flat" and that is the surface you've printed flat, then the surface finish is ok, but it's more like a cast surface, it isn't going to ever be like a machined surface. If you want a flat surface like a mating surface on a transmission case or a surface you want to seal, you're going to have to go back and clean it up with a cutting too. If you're at right angles to the printed surface you can get a "wavy" surface based on how close together the layers are. Quite often printed parts are "post processed", which is done by hand with a die grinder and the striations are buffed out, which leaves a pretty nice surface. Morris Technologies had some good technology that really improved the surface finish but that's now proprietary to GE...

The cost is a fact of life, it's strictly a time on the machine thing and the more mass you print and the thicker the part is the more passes you have to make. Cost can be directly related to the thickness of each layer. Thinner layers means more passes and the price goes up. If the part is thin the cost per cubic inch is lower and if it's thicker then it takes more layers and the cost per cubic inch is higher. You can only pan the laser so fast and if you put too much heat into the part it will warp. The tolerances are typically .003 inches per inch of printed thickness. Doesn't sound too bad, but if your part is 8 or 10 inches long you could have a .030" tolerance on a surface, which is pretty sloppy. Usually it isn't that bad, but it's also dependent on the part and how much heat you put in it.

Some folks tout the process as being flexible and that any machine can make anything, but when you look at how slowly the material is laid down, you realize that this really isn't ever going to be a high production process. The real advantage is that you can make a part that normally costs a bundle to tool up for without any tooling. The down side is that each part is about 1/4 the cost of a tool. The rule of thumb is that if you're making just a few parts, or if the tooling is really expensive then you can make the first half a dozen parts by printing them and test and make sure it's all going to work, but if you're making more than that then go for tooling. Another approach is to make sand casting tooling by printing it and we've done that when the part was small and really complicated and couldn't be made any other way.

The process started with plastics, and the quality can be pretty good with them, some machines give you a finish that looks more like it's sand blasted as opposed to smooth. Typically sanding and painting is done to get you something that will look nice. I recently made some complex nylon pieces to make a mold for an carbon fiber layup part. To get a nice finish I had to sand and polish the mold and the resultant parts weren't a really nice polished mold finish. I made the molds out of nylon and it worked pretty well, but after molding I put on a thin coat of epoxy and then sanded and polished the part to get a nice finish.

 

We make rocket engine parts with AM, too. Just looked at some turbo wheels this week in LA, also a bunch of diffusers that are part of a DARPA project. Works well for cooling passages in regeneratively cooled thrust chamber assemblies.

 

Like I said, it's a great process for making stuff that you can't make any other way and avoiding the cost of tooling for the first few pieces. We made a very complex one piece small turbine inlet nozzle vane ring that had very tiny cooling holes in it and we were able to make the holes in the AM process. There was virtually no other way to make those holes since the was no way to get to that area with EDM tools and you couldn't get in there and drill them.

The other thing that is good about AM is that complex parts generally come out without a lot of flaws, cracks or shrink areas with porosity that you can get from a casting. Generally it takes time and money to get the casting gating configuration and the pouring process down so that you can get a consistent casting quality, and that doesn't seem to be a problem with AM.