When starting a lot of projects on the a vertical milling machine, it will be necessary to true up the sides of the workpiece before you can begin milling in earnest. This is due to inconsistencies in the surfaces and angles of rough-cut materials, even extruded bar stock can have enough variation that you need to clean it up if you're doing precise work.
This instructable will show how to get the six faces of a rectangular workpiece all perpendicular to each other. The X, Y, and Z dimensions of the stock can all be different. My photos are of a near-cube shape, but the process is the same for any rectangular workpieces. I made mine at TechShop.
A small, flat scrap of wood
Endmill for machining the faces of the dice
Marker (an oil pencil is good, or use a permanent marker if you have a solvent with which to clean it off)
Step 1: Rough Cut the Stock
Whenever possible, you should cut your stock down to fairly close to what the finished product will be before you take it to the mill. This will cut down both on the time your project will take (a bandsaw is definitely faster than a mill for cutting bar stock to length) as well as reduce wear-and-tear on both the mill and tooling. Your goal should be to be able to use the bandsaw to get the stock to a size where you will be able to make just one rough cut with the mill to to square each face of the stock, and then one finishing cut (around .001 inch) to bring it to the proper size.
After making rough cuts on the bandsaw, none of the sides will be flat, nor at a 90% angle to any adjacent faces.
Step 2: Cut the First Face
Since there are no true dimensions which can be trusted to be used as a reference for creating the other faces, we can be somewhat cavalier about the positioning of the piece as we prepare to cut the first face of our workpiece. Of course, we don't want the piece to be at completely the wrong angle, but this is the one time where eyeballing it will be good enough.
Select a parallel that will permit a large portion of your workpiece to be held by the jaws of the vise, but will also permit enough of the workpiece to protrude above the vise that you can machine material off of the top.
Use a brush and clean out any chips from between the jaws of the vise. Place one parallel in the mill's vise, against the stationary jaw of the vise. Rest your workpiece on top of the parallel and close the jaws most of the way. When you get close, insert the small scrap of wood between the workpiece and the movable jaw of the vise, then close the vise firmly. The wood will deform against the uneven edge of the workpiece and help prevent it from wobbling between the vise jaws.
Turn on the mill and lift the table until the top of the workpiece touches the bottom of the end mill. Raise the table another couple thousandths of an inch and then mill away all the material on that plane. Using climb cutting around the outside edges to help prevent burrs. Once you have faced the piece, double-check to make sure that you have milled away all of the band saw marks. If not, raise the table another couple of thousandths and repeat the process until you have a flat surface.
Turn off the mill and remove the workpiece from the vise. Use a metal file to remove any burrs, and mark the newly machined face with the number '1'. We will be trying to machine all of the other faces so they are in proper orientation with relation to this face, which will be our most 'true' (all others being subject to some small amount of error due to imperfections and misalignment in the mill.
Step 3: Cut the Second Face
Place the workpiece back into the vise resting atop the parallel with side '1' facing the stationary jaw of the vise. Use the wood scrap again between the movable jaw and the workpiece. It is important that side '1' is flush with the jaw, so make sure that it doesn't cam at an angle as you close the vise.
Repeat the facing process from the previous step.
Remove the workpiece from the vise, remove any burs. I've marked mine with a '2'.
Step 4: Cut the Third Face
Clean any chips off of the parallel and place the workpiece back in the vise with side '1' facing the stationary jaw of the vise and side '2' facing down (touching the parallel). Close the vise using the wood scrap between the movable jaw and the workpiece.
Tap the top of the workpiece with a mallet to seat it firmly against the parallel. You will want to do this any time a machined side of the workpiece is facing down. Remember that the front edge of the die is unsupported underneath, so only tap on the back half. The die should be seated flush with both the face of the stationary jaw and the top of the parallel.
Machine the top of the workpiece flat.
Remove the workpiece from the vise and remove any burs. I've marked this as side '3'.
Step 5: Cut the Fourth Face
Clean any chips from between the jaws of the vise. Place the second parallel into the vise against the movable jaw. Place the workpiece into the vise with side 1 facing down (touching the parallels), side '2' against the stationary jaw of the vise, and side '3' against the movable jaw.
Close the jaws of the vise and tap the top of the workpiece with a mallet to seat it firmly against both parallels.
Machine off the surface of the workpiece.
Remove the workpiece and remove any burs. I've marked this as side '4'.
Step 6: Cut the Fifth Face
Brush any chips out from between the jaws of the vise.
Place the workpiece with side '1' facing the stationary jaw of the vise and side '2' facing to the left.
Use a machinists square to align the workpiece so that side 2 is perpendicular to the top of the stationary jaw of the vise. In the photo the square is pointing up, but you will get more surface contact if the square points down into the vise (assuming it is short enough to not bottom-out in the vise).
Clamp the vise while holding the workpiece at 90 degrees with the square.
Machine the face flat.
Remove the workpiece from the vise and remove any burs. I've marked mine with a '5'.
Step 7: Cut the Sixth Face
Remove any chips from between the jaws of the vise.
Clamp the workpiece into the vise with side '1' against the stationary face of the vise and side '5' facing down against the parallels. Use a mallet to seat the workpiece.
Machine the face flat.
You will sometimes want to use the 'Z' depth of your final cut as a starting point for whatever the next step in your machining project will be. If so, set your 'Z' axis indicator dial to zero to preserve the position.
Remove the die from the vise and remove any burs.
You now have a precision milled piece with 90 degree angles between the faces.