I have a few projects Id like to make but am a little lost on figuring out how thick to make certain parts based on a materials spec.  So do any of you have a process you use to figure out how thick to make something when looking at the specs of the material.  For example,  If I were to make a slide for a 9mm 1911 and was trying to decide between 7075 aluminum or some type of steel how would I go about that?  The same goes for say a AK47 receiver if i wanted to use a aluminum billet with or without a steel trunion. 

Another project was more for work but I need to make a cylinder from maybe 316 stainless that can hold 7500psi.  If i could just get my head around how to figure the strength per thickness I would be ok.  Is it as simple as taking the tensile strength psi and figuring out how many square inches you have designed.  If so how do you figure that for a cylinder if you have say a 1/8" wall thickness or a square piece with a circle hole bored in it that has different thicknesses at different points?  

It is actually a pretty involved process.

First you have to figure out all of the stresses and loads on the component, not just the one you are most interested in. Many times an anciliary force or load can change what you need.

After that, you have to figure out how each load will affect the part. Is it a compressive or tensile load? Will it induce any kind of torsion or moment in the part?

Next is the part itself. What kind of features does it have? You can usually look at a part with a little practice and get a good idea of where the stresses are highest and the strength is lowest. This is based on the deisgn of the part. Sharp corners see higher stresses than rounded ones. Holes reduce the allowable stress in a part. So on and so forth.

Next you have to figure out how much "padding" you need in your design. You dont want to simply make the part to only carry your calculated loads unless you are REALLY sure of your loads and are REALLY sure that you will never exceed them.

Now you get to the material. Strength and stress are not the only criteria for deciding on a material. You have to consider finishes, fatigue life, wear, hardenability and a myriad of other things.

So, in short, there is a lot to do to determine material dimensions and specs.

 

Now, there are some shortcuts that can be made, and some simplifications that can be done, depending on specific cases and how close you have to get. It all depends on the situation.

I am going answer this by asking the following questions.

Have you ever seen the slide of any pistol made from Aluminium?

Have you ever seen an AK receiver made from Aluminium?

I know I have not, well not a successful one at least.

It is possible to produce the above with Aluminium, but steel was used for a simple reason, it is better than Aluminium for that particular job.

The AR15 is predominately Aluminium, due to the design depending upon the steel barrel extension taking all of the force imparted by the operation of the weapon.

An FN FAL, made from aluminium, would result in a rather nasty accident. Simply due to the design not allowing for it.

Aluminium AK receiver, well this might work, and I stress might, with careful consideration to its operation, a composite Ali/steel design is, I Believe, feasible. But I would have to defer to one with greater knowledge of the design to be sure.

As regards to the maths involved, if you really want to know I will try to explain it for you.

I have tried to keep this short, as this could very easily turn into a continuing saga created by George Lucas.

Pistol slides are usually steel because the weight impedes the blowback to a degree and its not likely to get stress cracks then fail with nasty results

The AK particularly AKM & up when shown firing in slow motion show a lot & I mean lot of torsional twisting Aluminium would not last long under this Titanium might cost could be steep. Or you would need to use thick Aluminum ribbed to strengthen it more time & cost in experimenting.

General grade of steel for gun use is 4140

Usually when I want answers I just ask the supplier a good link for you even though not close they have all the Material Data sheets as PDF there is usually an international equivalent.

www.buau.com.au/