The successful operation of a lathe and the class of work turned out depend to a great extent on the skill of the operator in grinding his tools. Dull and improper- ly ground tools throw a heavy strain on the feed mech - anism, eat up horsep o wer, and cause the work to spring and the lathe to chatter. Lathe tools are made of carbon steel, high - speed steel and alloys such a s stellite and cemented carbide. The cemented carbide tools are becoming more generally used a s their cost is reduced. There a r e but a few carbon steel tools used, the general practice i s to use high - speed steel tool bits in holders.
Determine the kind of a tool you are grinding, a s carbon and high - speed steel requires different treatment. Tools should be marked to show the kind of material from which they are made. A quick and simple way to tell whether a tool i s carbon or high - speed steel is to grind the end and watch the sparks. If carbon steel, the wheel will throw a light colored spark and if high - speed steel, the sparks will be a dark red. Carbon steel tools should be ground on a grindstone or wet emery wheel o r dipped into water often to keep the tool cool while grinding to prevent the drawing of the temper.
On the other hand, high - speed steel tools should not be dipped into water when hot as it will crack the tool and crumble the cutting edge. Four angles are important in grinding tools and these vary with the material being machined. The back rake (see illustration 127 above) " A " , the side "B", the front clearance " C " and the side clearance "D " . The back r ak e is usually pr ov ided for in the tool holder by the tool being set on an angle (128), which is correct for the machining of steel and cast - iron. On solid steel tools it is necessary to grind the top rake in the tool.
By adjusting the tool in the tool post through wedge o r rocker, this top rake can be varied somewhat to suit the material being turned. The softer the material the less the top rake should be a s the r e is a tendency for the tool to dig in if the rake is too great. The side rake "B" (127) also varies with the material being machined. If this angle is made great enough, the tool will drag the carriage along by feeding into the work of its own accord, especially if the material i s soft. On the other hand, without side rake, the tool would not cut and the feed mechanism would be under excessive strain. The proper angle i s from 6 degrees for soft material to 15 degrees for steel.
The tool i s ground with the side clearance " D " (127) to take care of the feed advance and to prevent the drag - ging of the tool on the shoulder formed by the cut (131). This angle is usually about 6 degrees from the vertical and is constant. The front clearance "C" (127) depends somewhat on the diameter work to be turned. To turn cast - iron or steel it is advisable to set the tool above center. If the tool was ground square without any front clearance, it would not cut, but would rub on the material to be turned below the cutting edge of the tool (132). The front clear - ance is necessary for this reason. This clearance should be less for small diameters than for large diameters.
The clearance should range from 8 to 15 degrees. Do not grind more front clearance than is necessary a s this takes away the support from the cutting edge of the tool. Tool bits can be ground best in their own holders. To prevent grinding the holder, extend the tool beyond its regular cutting position. After a tool has been ground on the emery wheel, it will produce better work and last longer if the cutting edge i s stoned with an oil stone. This takes out the wheel marks and gives a smooth cutting edge. Care must be taken in grinding cut - off tools to see that both sides of the tool have the necessary side clearance (133).
A tool of this kind also cuts better if a lip i s ground back of the cutting edge to curl the chip as it comes off the piece (134). In grinding boring tools, see that the front clearance is sufficient to prevent the tool from rubbing in the hole and dragging at the point " A " (135).