Step Turning of Mild Steel Rod (130 mm) with CNC Programming – Manual & G71 Cycle Explanation

 Introduction

In my experience teaching and working with lathes and CNC machines, step turning is one of the most important basic machining operations. I have used it many times while training students for practical exams and interviews.

In this example, I will explain step turning on a mild steel rod using a CNC lathe, along with two types of programming: manual (line-by-line) and the G71 roughing cycle. Both methods are important in industry, depending on job complexity and productivity.

Step turning is used to produce different diameters on a single rod. Here, the rod is reduced in size from 18 mm to 15 mm, 12 mm, 10 mm, and 8 mm, with each step 25 mm long.

I prefer this example because it clearly explains tool movement, dimensional control, and machining sequence, making it very useful for ITI, Diploma, and interview preparation.

Workpiece Details

Let me first explain the job clearly.

• Material: Mild Steel (MS)
• Total Length: 130 mm
• Original Diameter: 18 mm
• Machined Length: 100 mm
• Remaining (holding): 30 mm

Step Turning Details:

The rod is stepped into four equal sections, each of 25 mm length:

Step	Diameter (mm)	Length (mm)
Step 1	15 mm	25 mm
Step 2	12 mm	25 mm
Step 3	10 mm	25 mm
Step 4	8 mm	25 mm

So, the diameter is gradually reducing from 18 mm to 8 mm.

Machining Strategy

Before writing the program, we must think like a machinist.

• First → Facing
• Then → Rough turning
• Then → Step turning
• Finally → Finishing

We will first see manual programming, then optimise using the G71 cycle.

Part 1: Manual CNC Program (Line-by-Line Method)

Complete Program

%
O2726; (STEP TURNING MANUAL)

G21 G18 G40 G99;

G28 U0.0 W0.0;

T0101;

G97 M03 S1200;

M08;

(--- Facing ---)

G00 X20.0 Z2.0;

G01 Z0.0 F0.2;

G01 X-1.0;

(--- Step 1: Ø15 ---)

G00 X18.0 Z0.0;

G01 X15.0 F0.25;

G01 Z-25.0;

(--- Step 2: Ø12 ---)

G00 X15.5 Z-25.0;

G01 X12.0;

G01 Z-50.0;

(--- Step 3: Ø10 ---)

G00 X12.5 Z-50.0;

G01 X10.0;

G01 Z-75.0;

(--- Step 4: Ø8 ---)

G00 X10.5 Z-75.0;

G01 X8.0;

G01 Z-100.0;

(--- Finishing ---)

G00 X16.0 Z0.0;

G01 X15.0 F0.1;

G01 Z-25.0;

G01 X12.0;

G01 Z-50.0;

G01 X10.0;

G01 Z-75.0;

G01 X8.0;

G01 Z-100.0;

G00 X50.0 Z50.0;

M09;

M05;

G28 U0.0 W0.0;

M30;

%

Explanation

1. Initialisation

• G21 → Metric units
• G18 → X-Z plane
• G40 →Cancel tool nose radius compensation
• G99 → Feed per revolution

This ensures the machine is in the correct mode.

2. Tool Selection

• T0101 → Tool 1 with offset 1
• G97 S1200 M03 → Spindle ON at 1200 RPM

3. Facing

We first make the front face flat:

G00 X20 Z2 
G01 Z0 

This defines the Z = 0 reference point.

4. Step Turning Concept

Now I explain the logic:

For each step:

• Move safely (G00)
• Reduce diameter (G01 X)
• Cut length (G01 Z)

Step 1

• Reduce from 18 → 15 mm
• Length = 25 mm

Step 2

• Reduce from 15 → 12 mm
• Next 25 mm

Step 3

• Reduce to 10 mm

Step 4

• Final diameter = 8 mm

5. Finishing Pass

We again trace a full profile with a small feed:

• Improves surface finish
• Removes tool marks
• Achieves accurate size

Limitation of Manual Programming

• Takes more time
• Not efficient for production
• Repetitive coding

That’s why we use the G71 cycle

Part 2: G71 Canned Cycle Program

Now I will explain the same job using the G71 roughing cycle, which is used in industries for automatic rough machining.

Complete G71 Program

%
O1002 (STEP TURNING USING G71)

G21 G40 G99 G18;

G97 M04 S2000;

T0101;

G96 M03 S180;

M08;

G00 X20.0 Z2.0;

(--- Facing ---)

G01 Z0.0 F0.2;

G00 X20.0;

(--- G71 Roughing Cycle ---)

G71 U1.0 R0.5.

G71 P10 Q40 U0.2 W0.1 F0.25;

N10 G00 X15.0 Z0.0;

G01 Z-25.0;

G01 X12.0;

G01 Z-50.0;

G01 X10.0;

G01 Z-75.0;

G01 X8.0;

G01 Z-100.0;

N40;

(--- Finishing Cycle ---)

G70 P10 Q40;

G00 X50.0 Z50.0;

M09;

M05;

G28 U0.0 W0.0;

M30;

%

G71 Explanation

First Block

G71 U1.0 R0.5;

• U = 1.0 mm → Depth of cut
• R = 0.5 mm → Retract amount

Second Block

G71 P10 Q40 U0.2 W0.1 F0.25;

• P10 → Start block number
• Q40 → End block number
• U0.2 → Finishing allowance in X
• W0.1 → Finishing allowance in Z
• F → Feed rate

Profile Definition

Between N10 and N40, we define the final shape.

Important Rule:

• Only the final profile is written
• Machine automatically removes material layer by layer

Finishing Cycle

G70 P10 Q40;

• Removes remaining allowance
• Gives a smooth finish

Key Difference: Manual vs G71

Feature	Manual	G71
Programming	Long	Short
Efficiency	Low	High
Production	Not suitable	Best
Control	Full control	Semi-automatic

 Practical Tips (Very Important)

1. Why G96 (CSS: Constant Spindle Speed)?

• Maintains constant cutting speed
• Improves tool life

2. Why Finishing Allowance?

• Prevents rough surface
• Ensures final accuracy

3. Common Errors

• Wrong P and Q blocks
• Profile not continuous
• Wrong tool offset

Real Industry Insight

In industries:

• Manual programming → used for simple or trial jobs
• G71 cycle → used for mass production

90% of turning jobs use canned cycles

Conclusion

In this example, we have taken a 130 mm mild steel rod and performed step turning to achieve four different diameters: 15 mm, 12 mm, 10 mm, and 8 mm, each with a length of 25 mm.

I explained the complete machining using:

• Manual CNC programming (for understanding basics)
• G71 canned cycle (for industrial efficiency)

Note: “I strongly request that all the above programs be carefully checked and dry-run before execution to prevent tool damage or machine errors.”

 Frequently Asked Questions

1. What is step turning in machining?

Step turning is a lathe operation used to produce different diameters on a single workpiece, creating steps along its length.

2. Why is step turning important?

It is widely used to manufacture shafts, axles, and machine components where multiple diameters are required for assembly.

3. What material is commonly used for practice?

Mild Steel (MS) is commonly used because it is easy to machine, has a low cost, and is suitable for beginners.

4. What is the difference between manual programming and the G71 cycle?

• Manual programming → Step-by-step tool movement
• G71 cycle → Automatic roughing (faster and used in industry)

5. Can finishing be done without a G70 cycle?

Yes, finishing can be done manually using G01 with low feed and small depth of cut.

6. Why is facing done before step turning?

Facing ensures a flat reference surface (Z = 0) for accurate length measurement.

7. What happens if G21, G18, G99 are not given?

If already set, the program may run, but it is not safe. Always include them to avoid errors.

8. What is the use of G40 in CNC?

G40 cancels tool nose radius compensation, ensuring the tool moves in normal mode.

9. Why is X given a negative value during facing?

Giving X negative (like X-1) ensures the tool crosses the centre and removes any remaining material (pip).

10. What are common mistakes in step turning?

• Incorrect tool offset
• Wrong diameter input
• Not maintaining step lengths
• High feed causing poor finish