We would first like to emphasize that we believe the prime method of cutting metals on a miniature metal cutting lathe should be high speed steel. It’s inexpensive, easy to sharpen, and can be shaped to make “form” tools. Insert tooling is expensive, can’t be resharpened or shaped, but it can be a lifesaver. The obvious difference between carbide tools and inserted carbide tools is the tip is held on with a screw rather than brazed on to a piece of steel. This fact has a lot to do with the success of insert tools in recent years. Steel and carbide have slightly different expansion rates which can cause premature failure of the carbide tip. It is surprising that a small screw will hold these inserts tight enough to accurately cut metal, but they do. We run 20-horsepower computer controlled lathes at Sherline that can remove metal at a rate of 2 pounds (1 kilogram) a minute with these tools and have few failures. The reasons we believe insert tools should have a place in your shop is they are ready to use, they hold their cutting edge when cutting exotic metals or abrasive materials, and can speed up the cutting process. Normal cutting speed rules don’t have to be considered to the same extent as when using high speed steel. Stainless steel can be cut at triple the rate over high speed steel with these tools. This puts you in a better horsepower range on the Sherline motor. Another interesting fact is you can get a better finish on some steels, such as cold rolled, by turning up the RPM. Insert tools don’t need cutting oils to work well, but we suggest that you still use a few drops now and then. This can be an important fact when working on your kitchen table. We experimented with various tools before making a choice. We wanted a cutting tool that had a positive rake. We don’t believe the tools we manufacture are rigid enough for negative rake tools which cost less and are far more popular.* These tools are designed for machines that weigh hundreds of times more than a Sherline lathe. Positive rake tools have to be sharpened at the time of manufacture, which adds to their cost, but also adds to their performance on a Sherline machine. You can’t use any insert in these holders unless it has a positive cutting edge.

CUTTING EDGE Negative Rake Tip CUTTING EDGE Positive Rake Tip FIGURE 1—Negative and positive rake carbide cutting tips. Negative rake tips can be held upside down giving four cutting edges. Positive rake tips cut from one side only, but cut better. Another choice we made was the .015" (4mm) radius on the tip. A large tool radius can give good finishes on a full size machine, but it can cause havoc on small diameter parts or miniature machines. Large radius tools create high tool loads because of their large cutting surface. A radius smaller than .015" will chip too easily. The difference between the 80° and 55° tools is that the 80° tool is a little stronger at the tip, but the 55° tool can get into corners. If you plan to only purchase one, buy the 55°. The 80° tool is a good choice to rough cut with. To get maximum life out of these tools be sure to increase the “feed” as you increase the RPM. The chip should have a tight curl to it and break off into short lengths. These chips can be very HOT. Remember, your hands are closer to the cutting edge when using miniature machine tools, so use caution. Another plus for insert tools is their ability to cut hard or abrasive materials. Don’t plan on machining down the shanks on end mills with them, but you can cut through a work hardened surface on stainless steel with ease. These tools can simplify many machining operations, but will never solve problems caused by poor machining practices. As with all machining operations,