How Billet Car Parts Are Machined
The Art of Machining Billet Parts
I am a Certified Toolmaker (U.S. Department of Labor and State of Ohio, Department of Labor) and have been in the Machine Trades for thirteen years. Among the extremely varied things I do, for the purposes of this article, I design and engineer parts, draw them in CAD, program CNC code in a CAM program, edit CNC code, and set up and operate a CNC milling machine. By the time you are done reading this article you will know just what Billet Aluminum is as well as everything that is involved in machining parts in a CNC milling machine and understand what makes the price of machined parts what they are.
Just what is “Billet Aluminum” anyways?
If you ask around many will agree that it is the best made part in aluminum that you can get. Beyond that many people don’t seem to really know what is meant by “Billet”.
The way Billet Aluminum got its reputation is from the strength advantage it has overcast or fabricated aluminum. Because of the crystalline structure of cast aluminum and the stresses in welded aluminum the strength of components made in these ways is limited to a certain degree. However, when maximum strength is needed in an aluminum part they are machined from Billet. Basically, “Billet” means a solid “chunk” of aluminum. The part is machined out of a single block of aluminum such as bar stock as it was produced at the foundry. Parts made in this manner are superior in strength as they have a much stronger crystalline structure, or “grain”, and don’t have the stresses of a welded part due to excessive heating and cooling. The structure of the grain has not been disrupted because material has only been cut away from the original piece and no excessive heat has been created.
Billet Car Parts
In the Auto Industry aftermarket billet parts are desirable for two reasons. In functional components the strength enables lighter, severe duty parts to be produced. Also the fast thermal conductivity of aluminum can be a desirable characteristic.
In dress-up components Billet parts have a much more serious, beefy look. The machined aspect adds to the car’s mechanical “feel”. Many find machined parts classy and elegant. Sometimes just being able to tell a part was machined rather than cast gives a feeling of significance to the part. The machined part has a different artistic value to many enthusiasts as well.
Many Billet Aluminum car parts are machined from 6061 T6 aircraft grade aluminum. This is a very good general purpose aluminum and performs well for most situations. Another grade of aluminum used in automotive parts is 2024 T3 or T4. This is stronger than 6061 and more costly as you would imagine. This is used in severe duty parts such as some of the better roller rockers used in race engines.
CNC stands for Computer Numerical Controlled. The simplest programs to run these machines are still written manually but most now are written with the use of a CAM program; Computer Aided Machining. This is because complex code that can be as many as tens or hundreds of pages long can be written in a matter of hours or minutes with the use of a CAM program. These programs work by the programmer specifying tool paths, speeds and cutting feeds, depth of cuts, number of passes, cutters to be used, and every other machining aspect. The programmer still needs to have knowledge of machining while using the CAM program in order to tell it how to go about cutting the part. The CAM program merely makes writing out the code for the CNC much faster than any person could do it, often times in mere seconds. The cost of a CAM program can range from $6,000 to $125,000 depending on complexity.
This code isn’t always perfect though and needs to be run in a simulation on the CNC’s computer before it is actually used to make a part. Sometimes there can be an anomaly when the code is written (glitch) and a specification could be wrong. Sometimes the programmer makes a mistake such as typographical error while specifying information. Even after running the program in simulation mode the part might be machined out of a dense blue wax first. This is to save the cutters should something go wrong. The wax will give instead of the expensive cutter breaking. If the program needs to be changed the person making the change has to be able to read and understand this code, called G-code, to be able to make the necessary changes manually. Here is an example of what this code looks like:
It may appear like gibberish but to an experienced programmer it talks to him, or her. This is another skill required to perform this kind of work.
Setting up the machine takes time and is often the major reason for the expense of getting a single item machined. First the table of the CNC mill has to be set up to hold the metal to be machined. This set up can be as much as 75% of a machining job even in a manual machine. The set up has to hold the metal solid, in the orientation needed for the cuts to be made, and also to accommodate the cutters being used so that nothing is in the path of the cutter such as screw heads, clamps, or vise jaws. Everything has to be mounted square and perfect.
Next the cutters (end mills, drills, and such) are mounted into the machine’s tool holders. Each one of these mounted cutters is then installed in the spindle and “touched-off” the top of the metal stock that they are going to be machining. This programs the “tool length offset” so that the machine knows at which exact height each tool will be touching the top of the metal to be machined. This is referred to as “Z-zero”, which means “0”, or the origin, of the “Z axis” which is the vertical up and down movement in the Cartesian Coordinate System which CNC programming is based on.
So far we have drawn and perfected the part in CAD, programmed the G-code for the CNC using a CAM program, ran the CNC program in a simulation mode to verify it does what we want it to, edited the program for any “tweaks” it needed, mounted the stock to be machined on the CNC mill table perfectly square and solid, set X-axis and Y-axis “zero”, selected and mounted all the cutters into the tool holders, touched off all the tools setting TLO (tool length offset), and maybe ran a trial part in wax. After all this we still don’t have a product, nor have we made the first part. In addition to all these task stock to be milled has to be cut from bar stock. Metal (steel or aluminum) comes in lengths such as 10 feet, 12 feet, and even 20 feet long. This has to be cut to length in a band saw to a size that can be mounted on the CNC table. Often times it is necessary to machine a perfectly square end on one or more sides of the cut pieces in order to accurately mount the stock on the CNC table. How many pieces do we need? Twenty-five? One hundred? Five hundred?
After the part is finally machined on the CNC it is initially inspected. Next it gets deburred to remove all the sharp edges. It is cleaned up to remove chips and coolant from the part and inspected again. From here it may need to go to a polisher (appearance items) for a mirror like finish. After it is picked up from the polisher it gets cleaned again to remove excessive polishing rouge from machined grooves and then the part gets cleaned one last time with aluminum polish and a micro-fiber towel. The part is inspected once more before being wrapped in protective bubble wrap and placed into stock or shipped to a customer.
Getting Custom Parts Machined
This is an example of what goes into machining custom parts. Imagine going through all this for one part! At a typical shop hourly rate it is no wonder why “one-off” single custom parts cost what they do.