A Guide to Lathe Cutting Angles

An important thing you will want to know about lathe cutting is what geometry you need your tool bit to be in order to successfully cut the type of material you wish. The types of tool bit angles are side and back rake angles, side and end cutting edge angles, and side and end relief angles. A tool must be ground to the correct angle in order to efficiently shape a workpiece.

The angle of the tool bit you will require depends on the material you are using. For example, in cutting brass you will want a side and back rake angle of 0 degrees, whereas if you are cutting aluminum you will need a side rake angle of 15 degrees and a back rake angle of 35 degrees. It is recommended that you always use the minimum angle required for the task you are trying to accomplish because sharper angles make for weaker tool bits.

Angles of tool bits can be positive, negative or have no angle at all.

Rake Angles

The term rake angle refers to the angle located at the top of the tool bit. A side rake angle dips back from the cutting edge. This can be positive or have no angle. This determines the size, shape and direction of chips coming off the workpiece. They may also include chip breakers that break up chips to smaller pieces so they won’t be a safety hazard. A back rake angle combines with the tool holder, which may have it’s own angle, and makes a pathway for the heel of the bit to reach the workpiece. It also provides for the smooth direction for the tool bit, which otherwise naturally curves, to counteract the pressure on the tool bit for sharper cutting. If the tool bit has too severe a rake angle, the edge will no longer be strong enough to cut the workpiece.

Cutting Edge Angles

Cutting edge angles are the angles formed by the cutting edge of a tool bit. End cutting edge angles are on the end of the tool bit, whereas side cutting edge angles are on the side. The end cutting edge angle allows for the tool bit’s nose to come in contact with the workpiece and also facilitates the feeding motion. The side cutting edge angle allows for less pressure to be applied on the tool bit while it cuts. The wedge or lip angle is a combination of the end and end cutting edge angles that allows for cutting to take place.

Relief Angles

Also known as clearance angles, side and end relief angles are the angles behind and underneath the cutting edge that make a pathway for the direction the tool is to travel. The side relief angle is ground into the underside of the tool bit. The end relief angle works with the tool holder angle to provide clearance at the front of the tool bit and keep it from snagging the workpiece.