Rest machining for remaining material in CNC Programming

Introduction

Rest machining helps remove leftover material after a rough cut. It makes the final shape more precise and saves time.

After rough machining leaves small areas of material that need finishing.

When using different tool sizes to clean up hard-to-reach spots.

To avoid re-cutting large areas already finished.

When you want to improve surface quality by focusing only on leftover material.

Syntax

CNC Programming

G71 P<start_block> Q<end_block> U<allowance> W<allowance> F<feedrate>
G70 P<start_block> Q<end_block>

G71 is used for roughing cycles with rest machining.

P and Q define the start and end of the machining block range.

Examples

This runs a roughing cycle from block 100 to 200 with 0.5 mm allowance, then a finishing cycle on the same blocks.

CNC Programming

G71 P100 Q200 U0.5 W0.5 F0.2
G70 P100 Q200

Rest machining with smaller allowance and slower feedrate for finer finish.

CNC Programming

G71 P50 Q150 U0.2 W0.2 F0.1
G70 P50 Q150

Sample Program

This program rough machines leftover material between blocks 200 and 220 with 0.3 mm allowance, then finishes it.

CNC Programming

N100 G71 P200 Q220 U0.3 W0.3 F0.15
N110 G70 P200 Q220
N120 M30

Important Notes

Rest machining saves time by only cutting leftover material.

Allowance values (U and W) control how much material is left for finishing.

Always verify tool paths in simulation before running on the machine.

Summary

Rest machining targets leftover material after rough cuts.

It uses block ranges and allowances to focus cutting.

This method improves efficiency and surface finish.

Practice

1. What is the main purpose of rest machining in CNC programming?

easy

A. To remove leftover material after rough machining

B. To perform the initial rough cut on the raw material

C. To polish the surface after finishing

D. To drill holes in the workpiece

Solution

1. Step 1: Understand rest machining concept

   Rest machining focuses on removing leftover material that rough machining did not clear.

2. Step 2: Differentiate from other machining steps

   Rough machining removes bulk material; rest machining cleans remaining parts for better finish.

3. Final Answer:

   To remove leftover material after rough machining -> Option A

4. Quick Check:

   Rest machining = leftover removal [OK]

Hint: Rest machining targets leftover material after rough cuts [OK]

Common Mistakes:

• Confusing rest machining with rough machining
• Thinking rest machining is for polishing
• Assuming rest machining drills holes

2. Which of the following is the correct way to specify a block range for rest machining in a CNC program?

easy

A. G71 P100 Q200 U-0.5 W-0.3

B. G71 P100 Q200 U0.5 W0.3

C. G71 U0.5 W0.3 P100 Q200

D. G71 P200 Q100 U0.5 W0.3

Solution

1. Step 1: Identify correct block range order

   In G71, P is the start block number, Q is the end block number; P must be less than Q.

2. Step 2: Check allowance values

   U and W specify allowances and must be positive for rest machining.

3. Final Answer:

   G71 P100 Q200 U0.5 W0.3 -> Option B

4. Quick Check:

   Start block < end block and positive allowances [OK]

Hint: Start block P < end block Q; allowances positive [OK]

Common Mistakes:

• Swapping P and Q values
• Using negative allowance values
• Placing parameters in wrong order

3. Given the following CNC code snippet for rest machining:

G71 P150 Q180 U0.2 W0.1
N150 G01 X50 Z-20
N160 G01 X55 Z-25
N170 G01 X60 Z-30
N180 G01 X65 Z-35

Which blocks will be used for rest machining?

medium

A. Blocks N150 to N180

B. Blocks N100 to N150

C. Blocks N160 to N170

D. Blocks N180 to N200

Solution

1. Step 1: Identify block range from G71 command

   G71 specifies P150 and Q180, meaning blocks from N150 to N180 are selected.

2. Step 2: Confirm blocks exist in code

   Blocks N150, N160, N170, and N180 are present and will be used.

3. Final Answer:

   Blocks N150 to N180 -> Option A

4. Quick Check:

   Block range P150-Q180 matches blocks used [OK]

Hint: Use blocks between P and Q numbers inclusive [OK]

Common Mistakes:

• Choosing blocks outside P-Q range
• Selecting only middle blocks
• Ignoring block numbers in code

4. Identify the error in this rest machining code snippet:

G71 P200 Q180 U0.3 W0.2
N180 G01 X40 Z-15
N190 G01 X45 Z-20
N200 G01 X50 Z-25

medium

A. G71 command is missing

B. Allowance values U and W are negative

C. Block numbers are missing

D. Start block P is greater than end block Q

Solution

1. Step 1: Check block range order in G71

   P=200 and Q=180 means start block is after end block, which is invalid.

2. Step 2: Verify allowance values and blocks

   U and W are positive; blocks N180, N190, N200 exist, so no error there.

3. Final Answer:

   Start block P is greater than end block Q -> Option D

4. Quick Check:

   Start block must be less than end block [OK]

Hint: Ensure P < Q in block range for rest machining [OK]

Common Mistakes:

• Reversing P and Q values
• Assuming negative allowances are allowed
• Ignoring block numbering order

5. You have a rough machining program that leaves a small amount of material on the surface. To optimize the finishing process using rest machining, which approach is best?

hard

A. Skip rest machining and do a full finish pass over the whole part

B. Apply rest machining over the entire rough machining block range with large allowances

C. Use rest machining with block range covering only leftover areas and small allowances

D. Use rest machining without specifying block ranges or allowances

Solution

1. Step 1: Understand rest machining goal

   Rest machining targets only leftover material to save time and improve finish.

2. Step 2: Choose block range and allowances carefully

   Select block range that covers leftover areas only and use small allowances to avoid overcutting.

3. Final Answer:

   Use rest machining with block range covering only leftover areas and small allowances -> Option C

4. Quick Check:

   Target leftover with precise range and allowances [OK]

Hint: Focus rest machining on leftover areas with tight allowances [OK]

Common Mistakes:

• Using large allowances causing excess cutting
• Applying rest machining to entire rough range wasting time
• Not specifying block ranges causing full part machining