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Machine Tool Basics - Lathe Operations by SMITHY GRANITE

Learn about the basics of metal lathe operations using our Smithy Granite combo lathe/mill/drill machines. You can visit them at www.smithy.com for more information. 

Lathe Workholding

 

A metal cutting lathe is used to make cylindrical shaped parts like spindles, pins, bolts, all manner of wheels and pulleys, shafts and bushings.

With this machine you'll have all the controls you need to turn outside and inside diameters to precise dimensions. Do facing cuts, taper turning, thread-cutting operations with inch or metric threads, and bore larger, more precise holes than could be done with a drill or reamer.

With a lathe you'll create parts that would be difficult, if not impossible to do on any other type of metal-working machine. That's why they're the most essential piece of equipment in every machine shop. You can't do machine work without one.

 Lathe Cutting Tools

  The selection of a cutting tool is probably the most important decision a machinist can make. We are going to cover some of the basics, but I urge everyone who intends to use the lathe, to learn as much about cutting tools as possible. They are the key to the quality of your work. Lathe tools are made from many different types of materials, in many sizes and styles. The three types we are using today are, from the left, high-speed steel tools, carbide-tipped tools, and on the right, carbide-insert tools.

High-speed steel tools can be used on many turning operations, on all but the toughest alloys. The tools are sold as blanks in many sizes and grades. The machinist grinds the point of the tool depending on the type of metal he is cutting and the type of turning operation he intends to do. 

A good machinist's handbook will give you the standard guidelines for grinding high-speed steel tools. The cutting edge of a carbide-tipped tool is brazed to the end of a steel shank. The shape of the cutting edge, it's profile and cutting angles, are manufactured into the tool. A little cleanup on a green wheel, a silicone carbide grinding wheel, and the tool is ready to go. Cemented carbide will handle much more heat than high-speed steel, so carbide tools are able to cut tougher metals, and cut at higher speeds.

Cemented carbide is also used to make small inserts that are clamped to a tool shank. This style insert is manufactured with a cutting edge on each of its three sides. After an edge is dulled, the insert can be unclamped and rotated to the next side. After all the edges are dulled, the insert is thrown away and replaced by a new one. Although this type of cutting tool was designed for the demands of the high production work, it's really a great tool for the beginner because it allows even the person with little or no tool sharpening experience the ability to easily maintain a properly formed, sharp tool.

 

No matter what type of material the cutting tool is made of, it's the shape of the tool that determines how it can be used. The profile of the tool's point determines the type of cut the tool is designed to make. The location on the cutting edge on the point of the tool determines the direction that the tool will cut. Turning and facing tools are either right- or left-hand tools depending on which side of the tool shank the cutting edge is ground. Right-hand tools cut from right to left because the cutting edge is on the left side of the tool's point. Facing from the center of a part to the outer edge cutting right to left, requires a right-hand facing tool. Left-hand turning and facing tools cut from left to right, cutting away from the headstock in a turning operation, or from the outer edge to the center, in a facing operation. The cutting edges on these tools are on the right side of the tool's point. 

Sending a tool straight into a work piece is called a plunge cut. This type of cut requires a tool that has the cutting edge square on the nose of the tool.
Here we are using a high-speed steel cutoff blade and holder.

The position of the tool in the tool holder is very important. Tools should always be placed as far into the tool holder as possible. To give the cutting edge adequate support, and to reduce the possibility of the tool vibrating. In the case of a boring tool, only the length needed to clear the hole should extend out beyond the tool holder. The height of the tool should be set to match the center of the lathe axis. A lathe center placed in the tailstock can be used as a gauge to set the height. On this tool holder, shims and spacers are used to raise the tool. The tailstock is also used as a tool holder for drilling, reaming, counter-sinking and thread-tapping operations on the lathe.

Lathe Controls

  In general terms, all lathes have two basic groups of controls, the controls that govern the spindle speed and the controls that move the cutting tool. On combination machines, they use one motor to power both the lathe and the mill. There is a third control. This control is the clutch mechanism that transfers power to either the lathe or the mill. On this Granite 1324, the control lever for the clutch is here. On other machines, it's located inside the gear box on the end of the main drive pulley. These clutches can only be shifted when the motor is turned off and the machine is totally at rest.

Now, determining the correct spindle speed and RPMs depends on the type of metal you are cutting, the type of material the cutting tool is made of, and the diameter of the work piece you are cutting. Harder metals, metals that are tough to cut, are turned slower than those that are soft and easier to cut. Large diameters are turned slower than small diameters. 

Charts that outline the recommended cutting speeds for various metals can be found in a machinist's handbook.

The carriage assembly and the powerfeed system control the movement of the cutting tool. In a facing cut, the machinist will use the hand wheels on the carriage and cross slide to position the cutting tool to take the first cut. 
The depth of the cut is then set by advancing the compound. Before the motor is turned on, the powerfeed system has to be adjusted so that the tool travels in the right direction and the right speed. The speed at which the tool travels is called the feed rate. When everything is set, the motor is turned on, and the cross slide is engaged to make the cut. Here we are cutting steel with a right-hand facing tool.

On a lathe, feed rates will vary, depending on the depth of the cut and whether you are doing a rough cut or a finish cut. In a turning operation, the depth of the cut can be set by advancing the cross slide. The carriage is then engaged to feed the tool along the work piece and then make the cut. For most lathe operations, the machinist will make a few rough cuts, to bring the material down to about 1/32nd of an inch of the final dimension. Rough cuts are usually heavy cuts, with no real concern for the finish of the part. The object is to remove as much material as possible, in the shortest amount of time. When the rough cuts are done, the dimensions are checked, then the machine is set to take the finishing cuts. Finishing cuts are usually done with a separate tool, designed just for that purpose. The depth of the cut is usually small and the feed rate is slow. The time it takes to make the cut is less important than the dimension and the finish on the part.

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