Intro to the metal lathe

Metal lathes are versatile tools that we use to make precise round parts. These kinds of precise round parts show up in everything from small consumer goods to heavy industrial machinery, and they make smooth and efficient movement possible. Metal lathes achieve this through accurate linear movement and rigidity, which allows us to make parts with exacting dimensions and concentric features. In this class, we'll learn how to safely operate the manual metal lathes, produce some simple parts, and explore some of the many ways we can fabricate parts with a manual metal lathe.

How Does a Metal Lathe Work?

Overview

Major components of a metal lathe. The lathe holds the work in a chuck affixed to the headstock or between centers (as shown above). The carriage moves the tool parallel to the spindle bore, the cross slide moves the tool perpendicular to the spindle bore, and the compound slide (sometimes called the top slide) can move at an adjustable compound angle. Several controls for moving the carriage and slides are located on the apron.

The lathe holds material securely and spins it along the axis of the spindle bore. As the material spins, operators can move a cutting tool against the work parallel to the spindle bore with the carriage, which travels along the ways, the prismatic rails on the bed of the ways. Users can also move the tool perpendicular to the work with the cross slide, and at a compound angle with the compound (often referred to as the top slide). Users can adjust the depth of cut by adjusting these controls in unison, allowing them to make round, concentric features with precise dimensions.

Concentricity: the Superpower of a Metal Lathe

The spindle of the lathe spins in an accurate circle, so every cut will produce a feature concentric with the spindle axis. The lathe has precisely machined axes, so the carriage will move parallel to the spindle axis, and the cross slide will move perpendicular to the spindle axis. Turning cuts with the carriage will produce precise cylindrical features, while facing cuts with the cross slide will produce flat surfaces perpendicular to the spindle axis.

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  The circles in this bullseye share the same center, so they're concentric.

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  The circles in this bullseye do not share the same centers, so they're eccentric.

Rigidity: the Corollary to the Metal Lathe Superpower

The ways of the lathe are parallel to the spindle bore, and the cross slide movement is perpendicular to the spindle bore. Ideally, all cuts would follow these parallel and perpendicular constraints, but the intense cutting forces cause all of the components -- the work, the tools, and even the lathe itself -- to flex to some degree. We try to minimize this by building the lathe out of sturdy materials; the South Bend 9A, the smallest lathe in the machine shop, weighs over a half a ton. We also reduce the amount of flex by limiting both tool and material stick-out and controlling the aggressiveness of our cuts.

Metal Lathe Safety

The biggest safety hazard with the metal lathe is getting snagged by the spinning chuck or the work it's holding. Do not wear rings, watches, jewelry, gloves, or loose clothing that could be caught in the chuck. Tie back long hair before using the lathe, and keep your hands clear of the headstock while the lathe is spinning.

One lathe specific hazard is the hazard of an unattended chuck key flying out of the chuck when the lathe turns on. Pay attention to where you pick up the chuck key and where you put it down after adjusting the chuck; unless you're actively adjusting the chuck, the key should be outside of the chuck and away from the headstock.

Several common free-machining materials contain lead to improve machinability. These materials are nice to work with, but the presence of lead makes them somewhat hazardous. You should wash your hands after using the lathe, especially if you're working with these kinds of materials. Common lead-containing materials include 12L14 steel, a free machining steel alloy, and C360 brass, which is likely the most common brass alloy.

Grinding lathe tool bits creates metal and abrasive dust. This dust is a respiratory hazard, so you should use the dust extractor connected to the grinders and wear a respirator or mask when grinding.

General shop safety rules apply too. You must wear safety glasses and closed toed shoes in the machine shop, regardless of whether you're using any machinery. Review the safety guidelines on the metal lathe overview page for more guidance.

Kicking the Tires on the Lathes

Refer to the dedicated pages for the South Bend and Logan lathes for detailed instructions.

Moving the Carriage and Slides

Use the handwheels on the apron to move the carriage towards and away from the headstock. Note the relationship between carriage movement direction and handwheel movement direction. Repeat this movement for the cross slide and the compound slide.

Adjusting the Spindle Speed

Move the carriage away from the chuck and the headstock. Determine a spindle speed and turn on the lathe to that speed. On the South Bend, you pick the spindle speed by moving the belts to a specific position. On the Logan, you change the spindle speed by turning the speed adjustment handle with the lathe powered on.

Try several different speeds on the lathes. What sounds does the lathe make at a high speed? What about a lower speed? Do you notice the mechanical, gear-meshing noises when running the lathes in back gear?

Engaging Power Feed

Both the South Bend and the Logan have power carriage and power cross feed. Power feed moves the tool at a steady, consistent speed, and an appropriate feed rate can produce an excellent surface finish. We'll get to cutting with power feed eventually, but for now, enable power feed with the controls on the headstock, then power on the lathe. Listen for the extra mechanical, gear-meshing noises and watch for the leadscrew movement.

First Cuts on the Lathe

We're going to install a tool on to the lathe, then take some facing cuts, then take some turning cuts.

Anatomy of a Single Point Cutting Tool

Most lathe cutting tools are single point cutting tools; they remove material from the stock at a single point instead of plowing away material along a long cutting edge (these are often called form tools). Single point cutting tools have several surfaces that contribute to their cutting behavior

Front and Side clearance

Imagine a square cutting bit with a square front face held square to the work. Turning down an outside diameter (an OD) with this setup would create a new, smaller OD and a perpendicular shoulder where the new OD transitions back to the original OD. Because the tool is square in all dimensions, the left side of the tool would rub against this shoulder, and the front of the tool underneath the cutting edge would rub against the new OD. Tool rubbing creates friction and chatter, which leads to poor cutting behavior. We can fix this by grinding away material from the front of the tool and the left side of the tool.

[view of shoulder] [view of square cutting tool from the top] [view of square cutting tool from the tailstock] [view of the front of the tool highlighting clearance surfaces]

Nose Radius

We've given clearance to the tool, but the newly ground surfaces of the tool come together at a sharp, beveled point. This tool is sharp, but it's brittle. It's like a really sharp pencil; it has a nice point, but it breaks quickly and is not durable. We can fix this by grinding a nose radius on this corner. I recommend grinding this by hand with a bench stone, since a grinder can remove too much material too quickly.

[show the nose radius before and after]

Rake Angle

Nose radius measures how sharp the front of the tool is, while rake angle measures how sharp the top of the tool is. The tool we just ground has a flat, horizontal top, perpendicular to the front of the tool. This tool has zero rake, and it works well for brass and plastics, but other materials -- typically aluminum and steel -- cut better with positive rake tools. These tools have a <90º angle between the front of the tool and the top of the tool, and we can grind this tool by grinding the top of the tool.

[image of front of tool with zero and with positive rake]

Getting Ready to Cut

Holding the Stock

Loosen the three jaw chuck, then place a piece of 1" delrin bar stock in the chuck with about an inch of stickout. Tighten the chuck jaws and remember to remove the key from the chuck.

Installing the Tool

Install a tool into one of the quick change tool holders and adjust the angle of the toolpost to make sure the tool is cutting on its nose radius and not rubbing against the stock. [image of what that looks like]. With the toolpost lever loose, adjust the thumbscrew on the tool holder to raise and lower the tool; we'll use this screw to get the tool centered to the work.

There are several ways to center the tool. One method is to pinch a small ruler against the side of the stock with the tip of the tool. If the tool is too high, the ruler will be tipping forward. If the tool is too low, the ruler will be tipping backwards. When the tool is on center, the ruler will be vertical. [picture of too high, too low, on center.]

The best method for proving that the tool is centered is to take a facing cut. If the tool is centered, there will not be a nub in the center of the stock after facing. If the tool is too low, there will be a top-hat shaped nub. If the tool is too high, there will be a pointed nub, and you'll notice more resistance against the handwheel as the tool tries to bulldoze through the material instead of cutting it.

Picking a Speed

We often express lathe cutting speeds in surface feet per minute (SFM) or inches per minute (IPM). We can use a target SFM value and material diameter to determine an RPM for our operation.

RPM = SFM (feet/minute) * (12 inches/foot) * (1/pi) * work diameter (inches)

Substituting 4 for pi (which is totally close enough), this gets us

RPM = SFM (feet/minute) * work diameter (inches) * 3

Or if our feed is in IPM:

RPM = IPM (inches/minute) * work diameter (inches) /4

300 feet/minute for delrin is a reasonable cutting speed, so 900 RPM is a good place to start for 1" delrin.

Consult a feeds/speeds chart or an app like FSwizard for determining cutting speeds for other materials, but bear in mind that these feeds and speeds are geared towards production machinists trying to get their work done as quickly as possible. Time is money in these settings, so these speeds tend to be pretty aggressive. Err on the side of lower speeds while you're learning to use a new machine or work with a new material.

Check for Clearance

Before making any cuts with the lathe, make sure the spindle can spin without hitting anything like the tool or the toolpost. Clearance issues are more likely to come up with turning cuts than with facing cuts, so figure out where the deepest turning cut will be. Move the carriage to this position and manually spin the chuck to make sure the chuck jaws won't crash into anything. Install a hard stop onto the ways to prevent the carriage from moving too close to the chuck and crashing.

Take a Facing Cut

Power on the lathe and move the tool towards the face of the material. Use the cross slide to move the tool inside the cross section of the stock, and move the carriage towards the stock until you see chips form. Use the cross slide handwheel to wind the tool away from the stock, then move the carriage a small amount towards the headstock, and then wind the cross slide in towards the center of the work. Continue turning the handwheel until you reach the center of the work, then wind the carriage away from the stock. Power off the lathe and take a look at the newly flat surface you've created!

Repeat this a few times. Try taking shallower cuts and deeper cuts. Try winding the cross slide out instead of the carriage when you've reached the center of the stock. Try adjusting the tool height to be a little too high or a little too low.

Facing with Power Feed

Engage the power feed lever at the headstock and pick an appropriate feed rate (0.005"/revolution is a good place to start). Then set the power feed selector lever on the apron to power cross feed. With the tool clear of the stock, power on the lathe and engage the power feed clutch. The tool will move slowly, so it's easier to see the handwheel movement. Disengage the power feed clutch, move the tool close to the stock, and take a facing cut with the power feed.