This is the key to my process. On some cheaper grinders, the tool rests are not angle-adjustable. For practice, get some mild-steel square stock similar in size to the tool bits you want to grind.
Mild steel grinds quickly and makes it easy to learn. I recommend cutting three pieces, and practicing on each end. By the time you have made six ends, you will be competent enough for the real HSS stock.
When grinding the mild steel, it will overheat and turn blue. First, set up your bench grinder. As I said, you want 60 grit and grit aluminum oxide wheels. Grinders make a cloud of fine grit in a five foot radius, so operate it away from your machine tools and your coffee, for that matter.
The grit of grinding is bad for precision machinery and your mouth. This trick will eliminate the need to think in compound angles, which hurt my primitive primate brain. Time to grind a tool bit. Using the grit wheel, grind the long edge first.
When it gets hot enough to burn your fingers, dunk it in the water and continue. The more dead the nerves in your fingers are, the faster this will go. How long you can go between dunks is the measure of true grit. I just made a grinder pun. How often does a writer get to do that? As you go along, dress the wheel any time it gets loaded up with dark grit. The grinding will slow down the further you go, because the material is getting wider.
Patience and nerve heat-death are machinist virtues. I wear a heavy glove on the hand holding the tip near the wheel. This increases the time between dunks quite a bit, making the process go quicker. Those are the two main angles. We have created the approach angle, the front clearance angle, the front relief angle, and the approach relief angle, all with two simple grinding operations and without understanding what all those words mean.
The last two angles we need are the side and back rakes. Since this is a right-hand turning tool, the cutting edge is on the right when the tip is facing you. Place the cutting edge side of the bit down on the tool rest. If you want really nice surface finishes, at the expense of being able to get into tight corners, the last operation is to put a nose radius on the tool.
Make sure to double-check the tool height any time you grind a bit. It probably changed! Without it, you will go crazy trying to set tool heights all the time.
Skip to content Search for:. Home Hacks Lathe — Grinding tool bits. An easy method to get started. Here are my very first six attempts to make tool bits, saved for posterity.
Chronologically left to right, you can see how I got better and better at it. I have no skill to speak of, so if I can do this, you definitely can. Apply layout fluid to the top surface, and allow it to try completely. This process is pretty forgiving. To the grinder! Holding the bit flat against the tool rest and sliding it back and forth on the wheel, grind the long side first. The remaining three faces require grinding along two axis each, however they can still be created within three grinding operations.
On most tools there is also a fourth grinding operation which is a radius on the tool tip. This radius increases tool life and improves surface finish. The numbered faces in this image also indicate the order in which the three faces are cut. Although there are three faces to cut and each of them has two angles to set, the front and side faces both have two critical angles and two non-critical.
The top face has two angles, both of which are important. Tool geometry. The relief angles are needed to stop the tool rubbing. A tool with a greater relief angle usually has a lower rate of wear, but because there is less material to support the cutting edge the tool can break more easily , it also cannot conduct heat away so efficiently.
The top face has two rake angles because it can cut both 'into' and 'along' the work piece. These angles are identified as a 'side rake' angle and a 'back rake' angle. The rake angle sets the angle of shear for the cut. A greater rake angle reduces cutting forces and gives a better tool life, but too much rake can make the tool fragile. The diagram below shows the terminology used to describe cutting tools click for a larger image.
Different materials are best cut using slightly different angles and the table below gives some suggested angles for the critical faces; but in summary harder materials have a smaller rake angles and softer ones greater rake angles.
The exception is Brass and bronze which are usually cut with zero or negative rake to prevent 'digging in' of the tool. A Tool Grinding Rest. Accurate grinding of the required faces can more easily be achieved with an adjustable grinding rest. The photo on the right shows a simple grinding rest which is only angle adjustable not height and it has a sliding fence which can be used to hold the cutter at a set angle as it travels across the edge of the grinding wheel.
Some notes on the construction of the grinding rest can be found here. Throughout this sequence the tool temperature was kept reasonable by frequent dipping in a pot of water. Also - always ensure that the safety guards are in place on the grinding wheel, and always wait for it to stop before adjusting the rest. The first face to be cut was the side face. In this example we are making a normal right handed tool for the lathe. Grinding the side face of the tool. The grinding rest was tipped to give the required side clearance relief and the fence adjusted to give the side cutting edge angle.
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