Basic Machine Tool Safety and Use
Michael Schippling

I. Overview

A five session class with additional lab periods for one-on-one hands on instruction. In this class you will learn how to safely operate a lathe and a mill, and how to use them to make simple objects.

A. What will be covered in the class sessions.

B. Rules....Always Rules....

C. Demo of lathe and mill operations.

D. Planning a project.

II. Introduction to the Lathe

The engine lathe is the grown up cousin of the potter's wheel and more sophisticated sibling of the wood lathe. All of the above tools spin the material being worked (work-piece) while holding some sort of tool in contact with it. In the case of the potters wheel, you use your hands as the tool to form the clay work-piece into a symmetrical cylinder of some kind. In the case of the engine lathe the tool is a piece of hardened metal that is securely clamped into a tool holder and the work-piece is a bit of metal (or wood or plastic) that is being shaved down into a cylindrical shape. The lathe is used for making concentric circles and rings (and piles of chips) out of chunks of material.

A. Lathe nomenclature

B. Basic Lathe operations

  1. Boring -- cutting or drilling a hole into the center of the work-piece.
  2. Turning -- cutting down the outside of the work-piece to make it a smaller diameter.
  3. (sur)Facing -- cutting material off the front surface of the work-piece to flatten or shape it.
  4. Parting -- cutting a thin slice through the work-piece to cut it off of the rest of the material.
  5. Threading -- cutting a spiral screw thread into the work-piece, either on the outside to make a bolt, or on the inside to make a nut. An advanced topic.

C. Setup for basic operations

0. Fundamentals
1. Boring
2. Turning
3. Facing
4. Parting
5. Threading

C. Lathe tool bits

You may find many other special tool shapes, but these are the basic ones. The general idea is to keep as much material as possible on the tool's working face, because of the forces and pressures involved. The general round nose and roughing tools can take deeper cuts because they are stronger. The finishing tools take shallower cuts and have a finer contact with the work. The notching and threading tools should only be used for their specific tasks because they are fairly weak mechanically.
The left and right shapes may seem to be named backwards, but the direction refers to the edge of the workpiece that can be cut, not the direction that the tool points. Thus, the Left tools are used to cut a shoulder on the left side of the work and the Right tools on the right side.
Tool Grinding:
The various faces and angles of a steel lathe tool bit are shown on the drawing above. Strictly speaking, different work materials require different angles and clearances, but in practice a compromise tool can be used for almost all work. Side and edge angles of 5-10 degrees are usually sufficient. End clearance should be 2-5 degrees in order to keep as much tool material for support of the cutting edge as possible. The back Rake may be modified for the material being cut, generally the harder or more brittle the material the less rake:
Metal Being Cut:   Cast Iron   Hard Steel/ Brass   Carbon Steel   Mild Steel   Aluminium
Top Rake Angle:              0°                   8°                     14°                  20°                40°
The lip at the back end of the rake is called a `chip breaker' because it tends to direct the spiraling cutting (chip) upwards and break it off before it gets a chance to wrap itself around the tool and work.
Good instructions for steel tool grinding can be found at:
Carbide tools are usually used with a 0 degree rake, and are not so easy to grind anyway....

III. Introduction to the Vertical Mill

The Vertical Milling Machine is a really fancy drill press. It has a chuck or other mounting device to hold and spin a cutting tool, and a very heavy duty X-Y table to which you attach the work-piece. Using the table the work-piece can be positioned with great accuracy and repeatability. Like drill bits, end mill cutting tools are usually sharpened on the end so they can drill vertically, but they are also sharpened on the sides so they can cut horizontally as well. Therefore, besides being able to drill a hole into the work-piece, the end mill can cut slots and other shapes, as well as flatten surfaces. The Vertical Mill can also be used to cut exact angles and curves, and for many other operations.

Horizontal Milling Machines are a different type of tool mostly used in production shops. The cutting tools are mounted on a horizontal arbor, sort of like a table saw, and the work-piece is moved longitudinally under the cutters. They are good for cutting slots and grooves, and surfacing material, but are not as versatile as the Vertical Mill.

A. Mill nomenclature

B. Basic Milling operations.

  1. Drilling -- drilling holes in the work-piece.
  2. Milling -- cutting slots in the work-piece.
  3. (sur)Facing -- flattening a surface.
  4. Slitting -- cutting a narrow clearance slot.
  5. Boring -- like drilling, but usually BIG holes. Somewhat advanced topic.

C. Setup for basic operations

0. Fundamentals
1. Drilling
2. Milling
3. Facing
4. Slitting
5. Boring

C. Mill Cutting Directions

In general you want to use the Conventional milling cutting direction because it puts the force of the table feed screw in oppostion to the direction of the cutter. But in some cases this just can't or shouldn't be done, so you use Climb milling. The problem with Climb milling is that, as you are pushing the work into the cutter, the cutter is trying to pull the work along with it, away from the feed screw. If there is any play in the table feed screw (and there always is some) the cutter will grab the work and pull the table along with it. The result will be chatter and a rough finish for the cut. The advantage of Climb milling is that it pushes the work into the clamping surface, whereas Conventional milling tends to pull the work up off the surface. When cutting thin or delicate materials Climb milling may make clamping the work easier.


IV. Measurement

Accurate measuring is the crux of good machining. Even in an art project you may find that you need to make two parts that fit together well. It may be a press-fit, a really nice smooth feeling fit, or just a loose-fit to weld. In all cases you have to be able to make repeatable measurements in order to get it to work right. The simplest measuring tool is the ruler but it is not very accurate or repeatable so machinists use calipers and micrometers. Squares and other angle measuring tools are often necessary too. Many other types of measuring devices with various accuracies and uses are available. Being able to perform simple numeric calculations is an absolute necessity as well. Measuring tools are a black hole for money.

B. Using the Basic measuring tools.

C. Practical measuring.

D. Reading a vernier scale.

Many measuring devices have vernier scales which provide an increased accuracy of one or more decimal places in their readings. Calipers, height gauges, and micrometers as well as angular measurement devices all make use of this technique. The vernier measuring device has two scales. The main scale works just like a ruler. The second, vernier, scale is used to gain the extra accuracy. It is opposite the main scale (sort of like a slide rule, if anyone remembers slide rules....) shorter in length, and is layed out such that 10 divisions on the vernier take the same amount of space as 19 divisions on the main scale (in the case of the example below, although the technique holds in any ratio of vernier to main scales where the vernier has one less marking than the main).
To read the vernier device, the 0-mark on the vernier is used to get the main scale value just like a regular caliper. Be sure to use the value of the main scale mark that is just to the left of the vernier zero mark as is shown in the above diagram. That is, the value is 3 mm, not 4 mm, even though the answer is closer to 4.
Now look closely at the vernier scale. Notice that 10 divisions on the vernier match 19 divisions on the main scale, and that this guarantees that one of the vernier markings will line up exactly with a mark on the main scale. Decide which vernier mark comes closest to matching a main scale mark, in our example this is vernier mark 7. Combine the two readings to give a length of 3.7 mm.
Most SAE (inch) calipers have main scale markings at every .025 inch and use a vernier scale to add the last .025 inch of accuracy to measure thousandths (.001) of an inch. Because of their simplicity, vernier measuring devices are usually a great deal cheaper than dial or digital devices with the same accuracy, but they are slower and harder to read. Unfortunately you will probably run into a vernier at an inopportune time, so it's a good idea to learn how to use them.


V. Tooling

Tooling is the other black hole of machining. Tooling may be divided up into two categories, Cutting and Mounting. Cutting tools are like drill bits, milling cutters, etc. Mounting tools are like vices, chucks, and various positioning blocks and fixtures. There is no end to the variety of either that you can find, or their expense, but you can also make many jigs and holders yourself as you need them.

A. Common Cutting tools and materials.

B. The Mystery of Material Cutting Speeds

Cutting speed is the speed at the outside edge of the tool as it is cutting. This is known as surface speed and is measured in surface feet per minute (SFPM). Cutting speeds depend primarily on the kind of material you are cutting and the kind of cutting tool you are using.

The harder the work material, the slower the cutting speed. The softer the material the faster the speed:

Steel -- Iron -- Brass -- Aluminum -- Gold
Increasing Softness of Material
Higher Speed ----->>>>
The harder the cutting tool, the faster the cutting speed:
Carbon Steel -- High Speed Steel -- Cobalt Steel -- Carbide
Increasing Hardness of Tool
Higher Speed ----->>>>

With faster Cutting Speeds and harder materials, more heat is generated. If the tool bit gets too hot it will loose its sharpness and often become un-useable. You know this has happened to a piece of tool steel if you can see darkened, burned looking areas near the cutting tip. Sometimes these can be ground out and the tool resharpened, but not always. So you need to avoid going too fast. But going to slow can cause problems too. Higher speeds (and sharper tools) tend make smoother cuts and leave less finish polishing to do. Also sometimes the tool chatters on the work and doesn't cut evenly if it is going too slow. And it just takes longer to do anything if you are cutting slowly. So compromise is the essence of machining...

Determining Spindle RPM

The Cutting Speed is the speed that the outer edge of the tool bit travels relative to the work piece. This should not be confused with the Spindle RPM which is the `speed' that the tool bit (or work piece in the case of the lathe) rotates. The Spindle RPM must be set so that the tool will be operating at the correct Cutting speed. As the diameter of the tool bit increases, the distance traveled on each rotation also increases because it is `drawing' a larger circle. This means that the Spindle RPM will have to be lower to compensate:
1/4" -- 1/2" -- 1" -- 2" -- 4"
Increasing diameter of tool or workpiece
Higher tool speed (sfpm) ----->>>>
----->>>> Lower RPM

Therefore, to find the proper Spindle RPM we need to calculate the number of revolutions per minute which will cause the outermost edge of the tool to travel at the desired Cutting Speed. Fortunately there is a quick and dirty Cutting Speed and RPM table included in this package. But if you ever need to know, here's how you calculate the correct speed....

The cutting, or surface, speed changes with the size of the tool. So to keep the cutting speed the same for each size tool we must use a formula which includes the tool's diameter to calculate the proper RPM to attain the correct surface cutting footage. This also applys to the speed of a lathe spindle relative to the stationary cutting tool:

(Cutting-speed x 12) / (Diameter-in-inches x Pi)
Which conveniently enough reduces to the approximately-good-enough:
(Cutting-speed x 4) / Diameter-in-inches

This simplified version of the RPM formula is the most common formula used in machine shops for all sorts of machining operations. Put this formula to work in calculating the RPM for an example drilling operation. Use the recommended cutting speed of 70 SFPM for mild steel, and a 1/2" High-Speed Steel drill:

Cutting Speed = 70 (fpm)
Diameter of Cutter = 0.500 (inches)
70(fpm) x 4 = 280;
280/.5(inches) = 140
Spindle RPM = 140

Although we have calculated the RPM, remember that this is only a recommendation. Some judgment must be made in selecting the actual RPM to use. There are always outside factors that must go into deciding on the proper speed and feed to use. Ask yourself these questions before deciding on a spindle speed. How sturdy is my setup? Go slower for setups, which lack a great deal of rigidity. Am I using coolant? You may be able to use a faster speed if you are using flood coolant. How deep am I drilling? If you're drilling a deep hole there is no place for the heat to go, so you may have to slow the RPM down for deep hole drilling.

See the Cutting Speed Table for some good rules of thumb and you probably won't ever have to do this math for yourself.

B. Common Mounting tools and devices.


A. References

Note that I have not checked these in many years but the Wayback Machine might help if they are missing now...

B. Suppliers  -- MSC Industrial Supply Co. Home of the BIG BOOK catalog.
    phone: 800-645-7270 -- Enco -- Now owned by MSC, but may have some cheaper prices.
    phone: 800-873-3626 -- Grizzly Industrial -- Mostly woodworking and sometimes not as reliable.
     phone: 800-523-4777 -- Harbor Freight Tools -- Really cheap in all senses of the word, but worth many of the pennies.
    phone: 800-423-2567 -- Schip's, more in progress, list of places to look for stuff.

C. Handy tables