Choosing & Using the Right Milling Machine

Introduction

There was a time, say 75 years ago, when a model shopper would speak proudly of his metal lathe as the centerpiece of his model engineering capability. It would likely have at least an independent 4-jaw chuck and a tailstock chuck for drilling and tapping, and be able to cut screw threads predictably. The lathe might even be adaptable for light milling operations, by attaching a vertical slide with vise to the cross-slide.

In model shops today you will usually find both a lathe and a vertical milling machine, which might be as small as a 300-lb benchtop mill drill or as large as a one-ton Bridgeport-style professional mill.

If you are new to all this, a definition of lathe work versus mill work might be helpful. Both have to do with removing material from a block of metal—the workpiece. The result will be a functional element that may stand on its own or be part of an assembly. The key difference between lathe and mill is in how the workpiece is handled. On a lathe, the workpiece rotates, and it is cut away by a knife tool moving along the axis of rotation or at right angles to it. Typical products of lathe work are turned parts such as spindles, bearings, screws, washers, and circular blanks for gears.

On a milling machine, the cutter rotates. The workpiece is clamped to a rigid table, which can be moved left to right (X axis) and front to back (Y axis), in precise increments. The Z axis is represented by the headstock, which can be moved up and down on a vertical column. A motor on the headstock drives the cutter, which is most often a cylinder-shaped end mill, with cutting edges on the bottom face and also, helix fashion, on the outer surface. The end mill removes metal from any exposed surface of the workpiece, resulting in flat, cleanly cut sides parallel to the X, Y, and Z axes. Other cutters often used on the mill are slitting saws (miniature circular saw blades) and drill bits. For drilling operations, the headstock comes with a pinion-driven quill that functions in exactly the same way as a drill press.

Typical mill products are flat-surfaced blocks of metal, often with all six sides at right angles to each other (think cube), sometimes drilled for spindles or dowel pins, often tapped for screws.

The lathe and the mill are amazingly flexible machines, but neither is capable of doing useful work right out of the box: Both call for a number of work-holding accessories—chucks, vises, and clamps—and a selection of cutting tools, drills, reamers, end mills, and so on. This book, and its companion book on the lathe (Choosing & Using the Right Metal Shop Lathe), will introduce you piece by piece to the add-ons that get you operational with the least delay and expense.

The mill drill is a fairly recent innovation. Starting in the late 1990s, the first mill drills—all from Taiwan and China—were beefed-up drill presses with vertically adjustable headstocks, and with tables that could be driven in the X (left-to-right) and Y (front-to-back) axes by lead screws with handwheels. The key difference between the earliest mill drills and those sold today is that their round drill press–style columns have mostly been replaced by more rigid dovetailed columns that allow the headstock to be raised and lowered without affecting spindle position relative to the workpiece (a significant shortcoming of the round column type).

Aside from its greater weight and rigidity, the Bridgeport type of mill is quite different in several ways. In the first place, its headstock is not vertically adjustable; instead, the table and workpiece are raised to meet the cutting tool. Second, the table is supported on a massive casting—the knee—that runs up and down on dovetailed ways machined into the base, hence the general descriptor for this type of machine as a knee mill.

Knee mills like this have been a fixture in practically all general-purpose machine shops for the past 75 years. The many variants of the Bridgeport design have been widely copied, with varied success, by many manufacturers in Pacific Rim countries. This has resulted in dozens of similar-looking machines on the market.

When choosing a mill, one thing you will notice as you survey the catalogs is that Asian mills of a given size tend to have similar features. They might just be more than similar, even identical, thinly disguised by the importer’s paint job and label. But that’s not always the case. Some distributors call for special features and quality control that won’t be found on other machines. Also bear in mind that there are dozens of manufacturers making what seems to be the same product, but that doesn’t mean that parts will be interchangeable, or of uniform quality. There is literally no standardization, only the similarity that comes, certainly in the case of knee mills, from having copied Bridgeport machines over the years.

Unlike lathes, which have not changed fundamentally in the past 100 years, vertical mills—especially bench mills—are a recent addition to the small model shop. This may account for the relatively small amount of published how-to material—aside, that is, from the thousands of videos on the internet. However, take care: Some are misleading, and a few are just wrong. On the plus side is Tubal Cain, a one-time machine shop instructor and now publisher of many beginner-level videos, all of them reliable it seems to me. For more current advice, there are a few thoughtful machinists making helpful videos. Three examples (2021): Stefan Gotteswinter, This Old Tony, and Joe Pieczynski. Those new to mill work should find this book helpful when working on any size and type of vertical mill. Experienced users may also find useful reminders and new material.

Another publication it’s good to have on hand is Machinery’s Handbook, now in its 31st edition, available in print and on CD. It is recognized throughout the United States as the reference for every conceivable machining question. With its almost 3,000 pages of fine print, Machinery’s Handbook is perhaps over the top for anyone starting out, but there may come a time when you really do need the official word.

Examples of Frequent Mill Operations

In this book, the word mil is sometimes used to signify 1/1000″ (one-thousandth of an inch).

CHAPTER 1

Choosing a Milling Machine

CONTENTS AT A GLANCE

1-1 A Selection of Milling Machines

1-2 Bench Mill Versus Knee Mill

1-3 Machine Size and Weight

1-4 Additional Factors to Consider

1-5 Mill Versus Drill Press Similarities

1-6 Mill Versus Drill Press Differences

1-7 The Bench Mill

1-8 Spindle Drive System—Bench Mills

1-9 Table Structure

1-10 Backlash

1-11 Other Bench Mill Features

1-12 Table Power Feed

1-13 Power Downfeed (Quill Auto Feed)

1-14 Digital Readouts (DROs)

1-15 Quill DRO

1-16 The Knee Mill

1-17 Plus Features of the Knee Mill

1-18 Knee Mill Drive Systems: 4-Speed Pulley

1-19 Knee Mill Drive Systems: Variable Speed

1-20 Auto Quill Feed

1-21 Bench Versus Knee Mills Summarized

1-22 Which Mill to Choose?

1-1 A SELECTION OF MILLING MACHINES

Figures 1-1 through 1-6 show a selection of milling machines. Machines similar in appearance to those in the figures are in some cases available from more than one supplier, but their overall quality and specifications may differ. All the machines shown in this section are usually offered with R8 spindles.

FIGURE 1-1 Small bench mill. A 7″ × 17″ table, 0.7-hp brushless dc motor, belt drive, 100–2500-rpm spindle, weight about 170 lbs.

FIGURE 1-2 Large bench mill. A 7″ × 27″ table, 1-hp brushless dc motor, belt drive, 50–2500-rpm spindle, weight about 275 lbs.

FIGURE 1-3 Intermediate bench mill. A 7 × 28 table, 1-hp brushless dc motor, belt drive, 100–4000-rpm spindle. Usually mounted on a stand as shown. Weight, not including stand, about 400 lbs.

FIGURE 1-4 Large bench mill. Same basic configuration as the bench mills in the preceding figures—movable headstock, not movable knee. Usually supplied with a cast-iron stand, as shown. Measuring 9″ × 40′, its table is larger than many Bridgeport-style knee mills. 2-hp ac motor, 6-speed gearbox, 90–2000 rpm spindle. Weight, including stand, about1,300 lbs.

FIGURE 1-5 A series of Bridgeport-style knee mills with Vee-belt drive, table sizes 9 × 35 to 10 × 54, 3-hp ac motor, 4-speed Vee-belt drive with back gear, 80–2700-rpm spindle, weight 1,500–3,000 lbs.

FIGURE 1-6 Knee mill series similar to Figure 1-5, but with a mechanically variable flat-belt drive, 70–4,000-rpm spindle, weight 1,500–3,000 lbs.

1-2 BENCH MILL VERSUS KNEE MILL

Vertical milling machines of the sort described in this book are found in almost all model shops and industrial labs. The two main categories of vertical mills are the bench type and the knee type.

Sometimes called a mill drill, the bench mill is what you might expect—a machine that can be bolted onto any rigid work surface, ideally 30″ or more above the floor. Stands, some welded steel, some cast iron, are available for most bench mills. Bench mills are similar in concept to the standard drill press. Both have a work surface—the table—and a vertical column that supports a headstock and motor, but that’s it for similarity.

The main differences between a mill and drill press are the mill’s heavier, more robust construction and the way the workpiece is handled. On a drill press, the workpiece is usually moved into position by sliding it on the table. On a mill, the workpiece is firmly clamped to the table, which is moved by lead-screw action in precise increments, left to right (the X axis) and front to back (the Y axis). Another key difference is in the Z axis, the height of the headstock relative to the table. This is adjusted on the drill press by moving the table, and on the bench mill it is adjusted by moving the headstock. One final difference: The drill press spindle is always exactly at right angles to the table. On a bench mill, the headstock (and with it, the spindle) can be rotated and set up to ± 90° left or right relative to the table.

The Bridgeport-style knee mill is functionally similar to the bench mill in most respects, except that its headstock cannot be adjusted vertically (it has no column). Z axis motion comes instead from a movable knee, a robust platform for the table, weighing several hundred pounds in itself. Unlike the headstock on most bench mills, knee height can easily be adjusted in small, precise increments, allowing the cutting depth to be finetuned within ± 0.0005″, even less.

Most knee mills come with a number of other features that add to their overall capability. For instance, the knee mill headstock is usually mounted to the front of a ram, which allows it to be repositioned front to back (in some cases, the ram sits on a turret that allows full-circle rotation in the vertical axis). Also, just as on a bench mill, the knee mill headstock can be rotated and set (trammed) at ± 90° relative to the table. On all knee mills—but never on bench mills—it can also be rotated forward and backward by a few degrees, in some cases as much as ± 45°.

Finally, perhaps the most obvious distinguishing feature of the knee mill is its weight. Even the smallest of knee mills is significantly heavier than any bench mill. Why is that important? The answer can be summed up in one word: rigidity. This is the key factor that determines maximum depth of cut and, to a lesser extent, smoothness of the cut surface. (That said, all bench mills can do a comparable job, simply by using less aggressive cuts and lower traverse speeds.)

1-3 MACHINE SIZE AND WEIGHT

A mill is usually measured by its table size, but that can be misleading. Many bench mills have tables as large as the most popular knee mills. The other key metrics are weight and (of course) purchase price. Prices vary widely from one distributor to another, but here are some 2021 statistics (shipping is usually extra):

Bench Mills

•The smallest bench mill has a 7″ × 27″ table and weighs about 300 lbs. The largest bench mill has a 9″ × 40″ table and weighs about 1,000 lbs.

•Bases are available for most stands, which eliminates the need for a dedicated bench. Some stands are cast iron, weighing about 300 lbs.; others are welded metal, about one-third of the weight.

•Prices go from about $1,500 to $4,000. Essential accessories cost about $500.

Knee Mills

•Knee mills range in table size from 8″ × 35″ (1,400 lbs.) to about 10″ × 45″ (3,000 lbs.). All are floor-standing.

•Prices go from about $4,500 to $6,500. Essential accessories same as for bench mills.

1-4 ADDITIONAL FACTORS TO CONSIDER

Two more factors need to be considered:

•Ceiling height. For bench mills, be sure there’s enough headroom in the shop for the headstock at its maximum height. Knee mills have no height adjustment. For both bench and knee mills, be sure there is headroom for the hoisting/rolling equipment when installing.

•Floor loading. Before installing any milling machine, knee mills in particular, be sure the floor can take the machine weight plus a 500-lb allowance for workpiece and workholding items.

1-5 MILL VERSUS DRILL PRESS SIMILARITIES

There are similarities between all small vertical milling machines and the traditional drill press, such as the vintage example shown in Figure 1-7.

The main similarities are:

1. A vertically movable quill that encloses the spindle.

2. A drill press lever that propels the quill downward.

3. A quill clamp to lock the quill firmly in position.

4. A variable-speed spindle drive system .

5. A headstock that can be moved up or down on a vertical column. Note, though, that this feature applies only to bench mills. A key question regarding the column: Is the column round or dovetail? Headstock alignment can be a serious problem with a round column. Most of today’s bench mills have dovetails—much better.

FIGURE 1-7 Drill press.

1-6 MILL VERSUS DRILL PRESS DIFFERENCES

Every vertical mill is a part-time drill press, but there’s more to it than that. Here are the main differences:

1. Massive, rigid construction, a lot more cast iron.

2. Heavy T-slotted movable table on dovetail ways, with precise position measurement capability. Optional digital readout (DRO) on left-to-right (X), front-to-back (Y), and up-down (Z) axes.

3. The workpiece usually doesn’t slide on the mill table; it is firmly clamped to the table, which can be moved in precise increments.

4. A mill spindle is designed for both axial down load (like a drill press) and also side load (radial). That is why a mill spindle runs in tapered roller bearings (or deep-groove ball bearings) inside the quill.

5. The spindle isn’t just for drill chucks—use any R8-compatible device —end mill holders, collets, slitting saws, etc. (The R8 taper was originated by Bridgeport in the 1950s.) Note: Some mills have non-R8 spindles.

6. The headstock can be swiveled from left to right and (on some knee mills) front to back.

1-7 THE BENCH MILL

Sometimes called a mill drill, the bench mill is what you might expect—a machine that can be bolted onto any rigid work surface, ideally 30″ or more above the floor (Figure 1-8). Stands, some welded steel, some cast iron, are available for most bench mills.

Most bench mills have these basic features (Figure 1-9):

1. A saddle on dovetailed slides that can be moved in precise increments forward and backward by the Y axis lead screw .

2. A flat table on a second set of dovetailed slides, on top of the saddle, that can be moved in precise increments left to right by the X axis lead screw (X and Y axis dovetails in the saddle are at right angles).

3. Three or more T-slots on the table for clamping the workpiece or vise.

4. A column at the back of the machine, right angled to the table in both the X and Y axes, that supports the headstock and allows its vertical position to be adjusted, usually by a hand- wheel and lead screw. This is the Z axis (on some bench mills the Z axis is powered).

FIGURE 1-8 (a) Bench mill, 9″ × 40″ table, with cast iron stand. This example includes power feed motors on the X axis (table) and Z axis (headstock). (b) Bench mill, 7″ × 27″ table. This example includes a DRO unit for precise indication of table and headstock positions.

FIGURE 1-9 Bench mill schematic. Headstock (Z axis) lead screw omitted.

5. A motor-driven spindle running in a sleeve (the quill ) that slides up and down about 4″ within the headstock casting. Quill position is independent of the Z axis headstock setting (unless the quill is locked to the headstock). The quill is controlled