Eccentric Turning: Two Workholding Methods and Three Ways to Check the Offset

Eccentric turning is two jobs, not one: holding the part off centre, and confirming the offset is the figure on the drawing. The second is the one that gets skipped, and without it the offset is only a guess. This article covers four-jaw and between-centres workholding, and three ways to measure the eccentricity.

1. What eccentric turning is, and where it is used

Eccentric turning means one section of the part has its centre away from the spindle axis. The distance between the two centres is the eccentricity, e.

It is the geometry that converts rotation into reciprocating motion. On the machine side it appears in crank presses and shapers; on the product side, in rotary engines, motorcycle crankshafts, bicycle pedals, knitting machines, air compressors, vibrating screen decks, laminators and treadle sewing machines.

🔑 The hard part is not the cutting, it is confirming the offset is the number on the drawing. Any hold that grips will let you start cutting, but an offset that was never measured is still unknown when the part comes off. Hence two holding methods below, and three ways to check.

2. Four-jaw chuck: move only two jaws

An offset can be turned in a three-jaw or four-jaw chuck, or between centres. The four-jaw is the most direct, because each of its jaws adjusts independently.

The method: leave jaws 1 and 3 fixed and adjust only jaws 2 and 4, shifting the workpiece axis away from the chuck centre by the eccentricity e. With no offset, the four jaws sit symmetrically.

📌 Keep that split in mind — one pair fixed, one pair adjusted. When the measurement in section 6 calls for a correction, the same pair moves, so the setup converges instead of wandering.

3. Between centres: long parts and crankshafts

When the part is long and needs support at both ends, work between centres:

⚠ The offset is fixed at step 2. The centre hole positions decide everything, and nothing downstream can adjust them — the opposite of the four-jaw method, where you can measure and correct.

4. Choosing between the two

📌 In one line: short part with one offset, use the four-jaw; long part, or both ends offset, work between centres. For how the finishing pass itself should be run, see Why a Finish Turning Pass Must Run the Contour in One Go: The Continuous Cutting Principle and Two Uses for a Parting Tool.

5. The indicator check: one formula, two situations

The dial indicator is the method used most, and it rests on a single formula:

e = 1/2 × (reading at the high point − reading at the low point)

Take the indicator reading on the outside diameter at its low point and at its high point, half a turn apart, then apply the formula.

Worked example: high point 6 mm, low point 2 mm, so e = 1/2 × (6 − 2) = 2 mm.

⚠ Why the half: through half a revolution the offset section travels from its low point to its high point, so the indicator sees twice the eccentricity. Take the difference as the offset and the answer is exactly double.

6. What to do with the reading

The value of this check is that it names the jaw to move. Taking a target of e = 1 ± 0.04 mm:

📌 Corrections are always made in pairs, one slackened and one tightened, and always on the adjusting pair from section 2. Slacken without tightening, or the reverse, and the part simply moves somewhere else.

7. Off the machine: V-block and gauge blocks

Once the part is finished and being inspected on its own, the chuck is no longer the rotational datum. A V-block on a surface plate takes over.

V-block with indicator: sit the part in the vee, bring the indicator onto the high point of the outside diameter, turn the part half a revolution and read the low point. The formula is the same one. Only the datum has changed, from chuck to V-block and plate, which suits an inspection bench.

Gauge block method is static instead: sit the part in the V-block and stack gauge blocks on the far side of the offset section until they sit level with the top of the diameter, then read the stack height H2.

Y = R + H2 (R is the workpiece radius, Y the centre height of the offset section)
e = Y − X (X is the datum height from the same setup)

🔑 All three are the same idea: establish a known datum, then read how far the offset section sits from it. The indicator reads the swing between extremes; the gauge blocks measure an absolute height and subtract.

When the same part also carries a knurled grip, see Knurling on a Lathe: Pattern Grades, Undersize Allowance and Spindle Settings.

For the full reading guide on this topic, see Turning: The Complete Guide.

Published: 2026-09-21 | Last updated: 2026-09-21