Step 3: Making tool bits
Now you need to make your tool bits for the lathe work. Grab some High Speed Steel, a smaller piece (I had a 3/8 HSS tool holder, so I used 3/8 HSS. It was a good size) and a bigger piece, 1/2 is a good size for the bigger piece.
To make the tools, there are some terms we need to know first. I'll go over them quick here, you can find more information by googling about lathe tools. Here is a nice little video: http://www.youtube.com/watch?v=Mn9jpqI8rao
Side rake: On top of the tool, the angle that the chips slide down after they have passed over the cutting edge. You want it smaller, because if it is too big an angle, the cutting edge gets too sharp and will break. 8-12 degrees is good for this.
Back rake: Is similar to side rake, except it goes back towards the body of the tool, instead of off to the side. Again, 8-12 degrees is good.
End relief angle: This is the angle under the cutting tip, and it is there to make sure the front face of the tool doesn't rub along your cutting surface. 10-15 degrees is good
Side relief angle: Is the angle that is on the side of the cutting tip, which makes sure the side of the tool doesn't rub as the tool is feeding into the material. 10-15 degrees is good.
End cutting edge angle: The end angle that makes the point of your tool.
Side cutting edge angle: The side angle that makes the point of your tool
Nose radius: The nose radius determines how nice of a finish is left behind. It is the blend between the side relief and the end relief.
Now, take a look at picture number 1. It shows the 90 degree chamfering tool, and the boring tool. The boring tool is not the prettiest, I know, but it works. You can see in the paint drawings what it would look like if it was perfect (or as perfect as paint drawings can get....)
When grinding HSS bits, rough grind all the angles first, then finish them up on a finishing wheel. You don't want any metal discoloration, and theoretically you shouldn't if you are using the grinding wheel correctly, but just in case, rough grind everything, then finish grind the last little bits to remove the discoloration (if there is any) at the end. It's also good to use a finishing wheel, and maybe even a honing stone at the end.
First, we'll go through the chamfering tool. It is quite simple. Take your HSS, and place it on a 45 degree. Tilt the backside down, so the tip is lifted. This will create your side relief, end relief, and side cutting edge angle, all at once. Look at picture 2, you can see how the HSS should be tilted. Grind the 2 45 degrees (the black lines on the red HSS) to get your 90 degrees. And that tool is finished! It has no back rake, nor side rake, nor any nose radius. Keep the tip sharp!
Second, we'll make the boring tool. This tool needs end relief, side relief, end cutting edge angle, side cutting edge angle, back rake, side rake, and a nose radius! But don't fret, we'll get it done. For this, it would be nice to set your tool rest on the grinder to about 10 degrees, it makes making the nose radius a lot easier. The paint drawing may not look exactly like the tool in real life, that's because my tool is a little funky looking. Just follow the pictures, understand the concepts, and you should be fine. Honestly, since this shouldn't be your first lathe project, you should know about tools already, so what I'm telling you should be stuff you already know.
Any ways, on we go. Check out picture #3. Here we are grinding the end relief angle, and the end cutting edge angle. Hold the tool off to your right, and rest it on the tool rest.
Picture #4 shows how to cut the side cutting edge angle, and the side relief angle. Place the tool on the tool rest to cut the side relief, and tilt it to the left, to cut the side cutting edge angle.
Now, to cut the nose radius, simply do a quick sweep from the angle in picture 3 to picture 4 (top view) to round off the tip. This is where it is nice when the tool rest is set on an angle, because you can rest the tool there (who woulda thunk it?!) and get a smooth radius. It's hard to make a smooth nose radius when you are holding the tool in the air, and trying to keep it steady.
Last but not least, the back and side rake angles. See picture #4 again. The position that you hold the tool in is very similar, but you rotate the tool 90 degrees, tipping it towards the wheel, so you are working with the right side facing you, and the top facing the wheel, as opposed to the left side facing the wheel, and the top facing you.
Finally, see picture #5 to see the front, side, and top views of the completed tool. The black curves would be good to grind off, because we're working on cutting a circle, and we don't want the bottom rubbing. It will shrink the tool a little bit, and make it weaker, but we are cutting aluminium, so it should be fine. If you are making a cube out of steel, then just don't go nuts and make super heavy cuts. Up to 0.015 (0.030 on the diameter) cuts should be fine.
In picture #6, we can see the tool in real life, with all its grinder marks and dings on it.
The last tool we need is the undercutting tool. It looks like picture #7. It's basically a parting off tool, on the end of a stick. So the first step, is take the 1/2 HSS, and grind a thin section in the middle, so there is a big fat section on one side, and a small fat section on the other. Let's get some specs on this, so we know how big to make it. From the tip of the tool to the big fat bit, it needs to be at least as long as your deepest bore (6). My tool is really long, it's a bit excessive. The amount that the tool sticks out should be at least as big as half the diameter of your biggest undercut (5) (for example, the deepest undercut on this cube is 0.300, so the tool needs to stick out at least 0.150. Mine is about 0.250, so plenty of room.) And, the entire end has to be small enough to fit inside your smallest bore (this shouldn't be a problem unless you make a really small bore.) Check picture #9
Instead of repeating everything I wrote for the boring tool, I'll just tell you what to grind, and hopefully you learned how when you made the boring tool.
1) Start by grinding the very front of the tool, grind the end relief and end cutting edge angle. (2)
2) Next, grind the back rake (4)
3) Grind the end relief angle (3)
4) Grind the relief angle opposite the first one (near 5 in top view, the 2 on the right in side view) Between grinding this and the first step, get your tool width (1)
5) Grind the radius on the bottom (7)
6) Grind a tiny tiny nose radius on the tip. You just want a little one, because how big this is affects how deep you need to undercut. If its a perfectly sharp corner, then you need to undercut just a few thousandths of an inch bigger then your corner to corner distance, but if you have a radius that is say, 0.050, then you need to cut more then 0.050 when you're undercutting, which means the undercut needs to be 0.100 bigger on the diameter. If you check our calculations, the undercuts are about 0.070-0.090 bigger then the corner to corner distance, so we can afford a 0.030 radius (~1/32 on an inch). But even still, smaller is better. Try for a 1/64 radius.
And voila! This was a huge long step, but now you have your three tools. We can proceed with the cube!