CNC machining basics

How to Determine X Y Step on CNC Machining

Two different things get called step: the lateral stepover between passes, and the smallest X Y increment the machine can move. This guide shows how to set the first and how to check the second. Written for engineers and buyers who need to judge a part before it is cut.

Get Instant Quote Talk to an Engineer

Stepover 5–75% of cutter ØScallop height drives finishResolution is a hardware limitTest cut before full run

X Y step on CNC machining setup on a milling table

On this page

1
Key takeaways
2
What X Y Step Means in CNC Machining
3
How Stepover Sets Scallop Height and Surface Finish
4
Checking Machine Resolution Before You Program
5
Matching Stepover to Material and Cutter Size
6
Step by Step: Setting X Y Step on CNC Machining
7
Stepover by Pass Type and Finish Target

Quick answer

Key takeaways

Stepover is a programming choiceYou set it in CAM as a distance or a percentage of cutter diameter. It controls cycle time and finish.

Resolution is a machine limitServo motor, ball screw, and encoder set the smallest reliable X Y increment. You cannot program below it.

Finish decides the numberA 0.8 μm Ra target usually needs a stepover under 10% of cutter diameter on a finishing pass.

Check with a test cutCut one pocket, measure the scallop height, then adjust before the full run.

The two meanings

What X Y Step Means in CNC Machining

In a CAM program, X Y step is the lateral distance the cutter moves between two parallel passes. It is measured perpendicular to the toolpath direction. You write it as an absolute number, such as 1.2 mm, or as a share of the cutter diameter, such as 40% of Ø10 mm.

On the machine side, X Y step means the smallest movement the drive system can make and repeat. That value comes from the servo motor, the ball screw pitch, and the encoder count. It is fixed hardware. A programmer cannot command a move smaller than it.

The two numbers live on different scales. A programmed stepover of 1 mm might sit above a machine resolution of 1 μm. That is normal. The programmed value shapes the surface. The resolution shapes how smoothly the machine follows the path.

When a drawing says step, ask which one is meant. Most of the time it is stepover, because that is what changes the finish and the cycle time. Resolution only matters when the geometry is very small or the tolerance is tight.

1
StepoverCAM setting, in mm or % of cutter diameter.
2
ResolutionMachine limit, in μm or in. Not editable in CAM.
3
Rule of thumbKeep programmed stepover at least 10× the machine resolution.

Geometry

How Stepover Sets Scallop Height and Surface Finish

A ball nose cutter leaves a row of ridges. The height of those ridges is the scallop height, and it follows directly from the stepover and the cutter radius. Halve the stepover and the scallop drops by roughly a factor of four. That is the fastest lever you have on finish.

For a roughing pass, stepover usually runs 50–75% of cutter diameter. The tool is removing bulk material, so finish does not matter yet. Chip thinning and tool load set the limit here, not the surface.

For a semi-finish pass, 25–40% is common. It removes the ridges left by roughing and leaves a uniform stock allowance for the finishing pass.

For finishing, 5–15% is the working range on a ball nose cutter. Below 5% the cycle time grows fast and the gain in finish gets small. If the drawing calls for Ra 0.2–0.8 μm, plan on a separate finishing pass at a small stepover and then a polish or lap step if needed.

1
Roughing50–75% of cutter Ø. Cycle time matters more than finish.
2
Semi-finish25–40% of cutter Ø. Set even stock for the next pass.
3
Finishing5–15% of cutter Ø. Watch cycle time below 5%.

Machine side

Checking Machine Resolution Before You Program

Resolution is not one number for the whole machine. It is a property of each axis. A 4,000 mm X travel with a 10 mm ball screw pitch and a 10,000 count encoder gives a different X resolution than a compact 500 mm axis. Read the axis spec sheet, not the brochure headline.

Backlash and thermal drift also matter. A machine can resolve 1 μm yet hold only ±0.005 mm over a long part because the screw grows as it warms. Warm up the spindle and run a few air passes before you trust the first measurement.

On a 5-axis machine, the rotary axes add error. A Ø400 mm rotary table amplifies angular error into linear error at the part edge. A small tilt error becomes a visible step between passes on a tall wall.

The practical check is a test cut. Cut a 100 mm × 100 mm pocket, measure the wall straightness and the floor scallop height, then compare with the CAM prediction. If the measured scallop is larger than predicted, the machine is not following the path as tightly as the model assumed.

1
Per axisGet resolution for X, Y, and Z separately.
2
Warm upRun the spindle and air passes before the first cut.
3
Test cut100 mm pocket, measure scallop and wall, compare with CAM.

Material and tool

Matching Stepover to Material and Cutter Size

Aluminium 6061 and 7075 allow a larger stepover because the material cuts easily and the tool load stays low. On a Ø10 mm end mill, 60% stepover in roughing is normal at a healthy feed. The same stepover in 17-4PH stainless will overload the cutter and chatter.

Stainless 316L and titanium Ti-6Al-4V work harden at the cut. Keep the stepover moderate so the cutter stays in the cut and does not rub. For a finishing pass on Ti-6Al-4V, 5–8% of cutter diameter is a safer start than 15%.

Cutter diameter sets the absolute step. A Ø3 mm tool at 40% stepover moves 1.2 mm per pass. A Ø12 mm tool at the same 40% moves 4.8 mm. Always write the stepover in the setup sheet as both a percentage and a distance, so the operator can check it.

Small cutters also have a floor. A Ø1 mm ball nose cutter at 5% stepover moves 0.05 mm per pass. That is fine for the machine but slow for production. If the part is small and the finish target is loose, raise the stepover and accept a coarser surface.

1
AluminiumUp to 60–75% stepover in roughing is workable.
2
Stainless and titaniumKeep finishing stepover at 5–8% to avoid rubbing.
3
Write both unitsGive the operator a percentage and a distance on the setup sheet.

Procedure

Step by Step: Setting X Y Step on CNC Machining

1
Read the finish calloutFind the Ra value on the drawing. Ra 3.2 μm is a rough pass. Ra 0.8–1.6 μm needs a finishing pass. Ra 0.2–0.8 μm usually needs a small stepover plus a polish step. Do not skip this step.
2
Pick the cutter and the radiusChoose the largest ball nose cutter that fits the smallest internal radius. A larger radius at the same stepover leaves a lower scallop, so you can run a bigger step and cut faster.
3
Start from a known percentageUse 5–15% of cutter diameter for finishing, 25–40% for semi-finish, and 50–75% for roughing. These are starting points, not final values.
4
Cross-check machine resolutionConfirm the programmed stepover is at least 10× the axis resolution. On a machine that resolves 1 μm, a 0.05 mm stepover is the practical floor before the path gets noisy.
5
Run a test cut on scrapCut a 100 mm × 100 mm pocket in the same material. Measure the scallop height with a profilometer or a surface comparator. Compare with the CAM prediction.
6
Adjust once, then lock itIf the finish is too coarse, reduce the stepover by 25% and recut. If it is acceptable, write the value into the setup sheet and keep it for the run.
7
Watch the cycle timeA stepover below 5% can double or triple the finishing time. If the finish target allows it, raise the step and save the cycle.

Reference

Stepover by Pass Type and Finish Target

Starting values for a ball nose cutter in aluminium. Adjust for harder materials.

Pass type	Stepover (% of cutter Ø)	Expected finish	When to use
Roughing	50–75%	Ra 3.2 μm or coarser	Bulk removal, no finish callout
Semi-finish	25–40%	Ra 1.6–3.2 μm	Even stock for the finishing pass
Finishing (standard)	10–15%	Ra 0.8–1.6 μm	General machined surfaces
Finishing (fine)	5–10%	Ra 0.2–0.8 μm	Sealing faces, bearing seats
Very fine	2–5%	Ra 0.2 μm or better	Optical and sealing surfaces
Hardened steel	5–8%	Ra 0.8–1.6 μm	Avoid rubbing and work hardening

The practical rule

Set stepover from the finish callout, confirm it sits well above the machine resolution, and prove it with a test cut. If the finish is acceptable at a larger step, take the shorter cycle.

Get Instant Quote Talk to an Engineer

FAQs

Questions engineers ask about X Y step

Is stepover the same as step distance?

No. Stepover is the lateral distance between two parallel passes in a milling path. Step distance, or resolution, is the smallest movement the machine axis can make.

One is a CAM setting you choose. The other is a machine property you measure.

What stepover gives Ra 0.8 μm?

On a ball nose cutter in aluminium, 5–10% of cutter diameter usually lands in the Ra 0.2–0.8 μm band. The exact number depends on the cutter radius, the feed, and the spindle speed.

Cut a test pocket and measure. Do not assume the model is exact.

Can I program a step smaller than the machine resolution?

No. The control will round the command to the nearest resolvable increment. The resulting path will not match the CAM model.

Keep the programmed stepover at least 10× the axis resolution so the path stays clean.

Why does the finish look worse than the CAM simulation?

Common causes are tool runout, a worn cutter, machine backlash, or thermal drift during a long cut. Check the tool holder first, then the machine.

A test cut on scrap will show which one is at fault before you cut the real part.

Does a smaller stepover always mean a better part?

No. Below about 5% the cycle time grows fast and the finish gain gets small. At some point the cutter rubs instead of cuts, which can make the surface worse.

Pick the largest stepover that still meets the finish callout.

How do you handle stepover on a 5-axis cut?

The same rules apply, but the effective stepover changes as the tool tilts. On curved surfaces, the local stepover can be smaller or larger than the programmed value.

Check the scallop on the steepest and the flattest areas of the part, not just one spot.