Rest machining for remaining material in CNC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is rest machining in CNC programming?

Rest machining is a process where the CNC machine removes only the leftover material from a previous roughing operation, focusing on areas that were not fully machined.

beginner

Why is rest machining important in CNC operations?

It improves efficiency by targeting only the remaining material, reducing machining time and tool wear compared to re-machining the entire part.

intermediate

Which toolpath strategy is commonly used for rest machining?

Common strategies include using smaller tools or specialized rest roughing cycles that detect leftover material and machine only those areas.

intermediate

How does the CNC program identify the remaining material for rest machining?

The program compares the stock model with the previously machined model to find uncut areas, then generates toolpaths only for those leftover regions.

beginner

What is a practical example of rest machining in a workshop?

After roughing a metal block with a large tool, a smaller tool is used in rest machining to clean up tight corners and small pockets missed earlier.

What does rest machining primarily focus on?

A Polishing the finished surface

B Machining the entire raw stock again

C Changing the tool automatically

D Removing leftover material from previous machining

Rest machining targets only the leftover material that was not removed in earlier operations.

Which tool is typically used in rest machining?

A A larger roughing tool

B A smaller finishing tool

C A measuring probe

D A grinding wheel

Smaller tools are used to reach areas missed by larger roughing tools during rest machining.

How does the CNC software find leftover material for rest machining?

A By comparing stock and machined models

B By guessing based on tool size

C By manual operator input only

D By scanning the finished part visually

The software compares the original stock with the machined shape to detect remaining material.

What is a key benefit of rest machining?

A Reduces tool wear and machining time

B Increases machining time

C Requires more manual labor

D Removes all material again

Rest machining reduces machining time and tool wear by focusing only on leftover material.

Rest machining is usually performed after which operation?

A Polishing

B Finishing

C Roughing

D Drilling

Rest machining follows roughing to clean up areas missed by the roughing tool.

Explain rest machining and why it is used in CNC programming.

Think about how leftover material from roughing is handled.

Describe how CNC software detects remaining material for rest machining.

Consider the difference between the original and current part shapes.

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