A rotary table is a milling machine accessory that lets you cut arcs, circles, and evenly spaced features by rotating your workpiece a precise number of degrees. It bolts to your mill’s table and uses a worm gear mechanism, typically at a 40:1 ratio, meaning 40 full turns of the handwheel rotate the table exactly 360 degrees. Once you understand how to center it, secure your work, and calculate divisions, it becomes one of the most versatile tools in your shop.
How the Gear Ratio Works
Most manual rotary tables use a 40:1 worm gear ratio. Each full turn of the handwheel advances the table 9 degrees (360 ÷ 40). The handwheel dial is graduated in degrees and minutes, so you can dial in precise angular positions by combining full crank turns with partial turns read from the dial. Some tables use a 90:1 or 60:1 ratio instead, so check your specific model before doing any division calculations.
This gear ratio is also the starting point for indexing, which is the process of dividing a circle into equal parts. More on that below.
Horizontal vs. Vertical Mounting
A rotary table can be mounted in two orientations, and the job determines which one you need. Horizontal mounting sits the table flat on your mill, with the rotation axis pointing straight up. This is the most common setup and works well for cutting circular features on flat parts, drilling bolt circles, and machining round workpieces from above.
Vertical mounting stands the table on its side, usually bolted to an angle plate or clamped directly to the mill table. This gives you access to the side of a workpiece, which is useful for machining cylindrical parts, cutting slots around a circumference, or making angled cuts. If your table has mounting holes on its back face, vertical setup is straightforward. Some tables include a vertical mounting bracket.
Centering the Table on Your Mill
Before you can cut accurate circles or bolt patterns, the center of the rotary table must line up exactly with the mill spindle. This is one of the most important steps, and skipping it will throw off every feature you cut.
Start by inserting a test indicator or dial indicator into the spindle using a collet. Lower the indicator tip into the bore at the center of the rotary table. Rotate the mill spindle by hand (not the rotary table) while watching the indicator needle. If the needle moves, the table center is offset from the spindle axis. Adjust the mill table’s X and Y axes to split the difference, then sweep the bore again. Repeat until the indicator reads close to zero all the way around.
Once centered, zero out your digital readout or lock your mill’s dial collars. This gives you a reference point so you can return to center later or measure distances from it.
Workholding on a Rotary Table
Rotary tables have T-slots across their face, just like a mill table, and a center bore that accepts accessories. You have several options for securing work:
- T-slot clamps: Step clamps, studs, T-slot nuts, and step blocks bolt directly into the T-slots. This is the most direct method for irregularly shaped parts. Position clamps so they won’t interfere with the cutter as the table rotates.
- Lathe chuck: A three-jaw or four-jaw chuck can be mounted to the center of the table using a backing plate. This is ideal for round stock and makes centering the workpiece much easier.
- Bolting through the center bore: For parts with a center hole, you can use a bolt and washer through the table’s center bore to hold the work directly.
Whatever method you choose, make sure the workpiece is secured tightly enough to resist cutting forces without shifting. A part that moves during a cut will ruin the work and could be dangerous. Also think through the full rotation before you start cutting. Clamps that clear the cutter at 0 degrees might collide at 180 degrees.
Simple Indexing With the Handwheel
The most basic use of a rotary table is dividing a circle into equal parts, called indexing. For any number that divides evenly into 40 (the gear ratio), the math is simple: divide 40 by the number of divisions you need. The result is the number of full handwheel turns between each position.
For example, to drill 8 equally spaced holes: 40 ÷ 8 = 5 turns of the handwheel between each hole. For 4 holes: 10 turns each. For 10 holes: 4 turns each. Lock the table at each position before machining.
You can also use the graduated dial for divisions that don’t require full turns. If you need 16 divisions: 40 ÷ 16 = 2.5 turns. That’s 2 full turns plus half a turn (4.5 degrees on a 9-degree-per-turn dial) between each position.
Using Dividing Plates for Odd Divisions
When the number of divisions doesn’t work out to a clean handwheel reading, you need a dividing plate. This is a disc with rings of evenly spaced holes that mounts on the back of the handwheel. A spring-loaded plunger pin indexes into the holes, giving you fractional-turn accuracy that the handwheel dial alone can’t provide.
The formula is: crank turns = 40 ÷ N, where N is the number of equal divisions you want. Reduce the fraction to its simplest form. The denominator tells you which hole circle to use on your dividing plate.
Say you need 13 equally spaced divisions. Divide 40 by 13, which gives you 3 and 1/13. That means between each position, you turn the crank 3 full turns plus 1/13 of a turn. To get that 1/13 fraction precisely, you need a hole circle divisible by 13. A 39-hole circle works (39 ÷ 13 = 3), so you’d advance the plunger 3 holes in the 39-hole circle after each set of 3 full turns.
Another example: dividing a circle into 360 equal parts. 40 ÷ 360 reduces to 1/9. You’d make zero full turns and advance 1/9 of a hole circle per division. A hole circle of 18, 27, 36, 45, or any multiple of 9 would work. On a 27-hole circle, you’d advance 3 holes per division (27 ÷ 9 = 3).
If your calculation doesn’t produce a whole number of holes on any available circle, that plate won’t work for that division. You’d need a plate with a different set of hole circles, or a different approach entirely.
Cutting Arcs and Circles
Beyond indexing, the rotary table excels at cutting curved features. To cut a circular arc, center the rotary table under the spindle, then offset the cutter to the desired radius using the mill’s X or Y axis. Lock the axis you offset, then rotate the handwheel while the cutter is running. The workpiece swings through an arc at a consistent radius from center.
For a full circle (like a circular pocket or a ring), you simply rotate through all 40 handwheel turns. For a partial arc, calculate the number of degrees you need and convert that to handwheel turns. A 90-degree arc on a 40:1 table requires 10 full turns.
Feed the handwheel slowly and steadily. Unlike CNC, your hand controls the feed rate, so take light cuts and keep the rotation smooth. Jerky movements leave visible marks on the finished surface.
Dealing With Backlash
Every worm gear mechanism has some backlash, which is the small amount of free play when you reverse the handwheel direction. On a rotary table, this means the table doesn’t immediately respond when you change direction. The handwheel turns a tiny bit before the table catches up.
The simplest way to manage backlash is to always approach your target position from the same direction. If you overshoot, don’t just back up to the correct number. Instead, reverse well past your target and come back to it from the original direction. This keeps the gear teeth loaded consistently on one side.
To measure backlash on your table, place a dial indicator against the edge of the table. Rotate the handwheel in one direction to fully engage the gear teeth, then zero the indicator. Reverse the handwheel slowly and note how far the indicator moves before the table starts turning. That reading is your backlash. On a well-maintained table, it should be only a few thousandths of an inch at the table’s edge. Most manual tables have an adjustment mechanism, usually a gib or set screw, that lets you tighten the worm gear mesh to reduce play.
Lubrication and Maintenance
The worm gear inside your rotary table needs regular lubrication to stay accurate and smooth. For manual tables, a quality way oil or gear oil works well. Haas recommends a synthetic 75W-90 gear oil for their CNC rotary units, with a 15W-50 synthetic motor oil as an alternate. Manual tables from import manufacturers generally do fine with similar weight gear oil.
Check your oil level periodically, especially if the handwheel starts feeling stiff or gritty. Most tables have a fill plug and a drain plug. Drain old oil, flush with clean oil if it looks contaminated, and refill to the specified capacity (which varies by table size but is often just a few ounces). Keep the T-slots and table surface clean and lightly oiled to prevent rust. After use, wipe off coolant and chips promptly, since coolant left sitting will corrode bare cast iron.
If your table has a tailstock for supporting long workpieces in the vertical position, give its moving parts a stroke of grease every six months or so to keep the quill sliding freely.