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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › CNC Machining › CNC Setup: Machine Preparation, Tooling, and First-Part Verification › Lesson 8
MFG-CNC14-06 · Lesson 8 · 16 clock hours · lab

Single-Block Cutting and In-Process Measurement

Objective. Students execute a multi-operation milling program in single-block mode on a steel block, pause at designated checkpoints to measure an in-process feature with a micrometer and digital caliper, and adjust a wear offset to bring the next cut within tolerance before continuing.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Single-Block Cutting and In-Process Measurement

Running a CNC program straight through without stopping is fine for a proven job. For a first part on a new setup, single-block mode is your most important safety tool. It executes exactly one block of G-code each time you press Cycle Start, then halts the spindle feed and waits. That pause lets you confirm the tool moved where you expected, read the machine's position display, and measure the workpiece before material you cannot put back is removed.

Why Single-Block Mode Matters

Most scrapped parts on first runs happen in the first few operations: the wrong tool length offset, a sign error in a coordinate, or a work offset that was not zeroed to the part. Single-block forces you to observe each move. On a Haas control, press SINGLE BLOCK so the indicator lights, then press CYCLE START once per block. Feed override at 50% gives you extra reaction time on rapid and feed moves until you trust the program. Once the roughing pass proves out, you may increase override and run finishing in single-block or continuous—follow your shop's procedure card.

Safety Before You Start

Apply lockout/tagout (LOTO) any time you reach inside the machine envelope to adjust workholding, change tools, or clean chips. When the door is open and you are measuring, the spindle must be stopped and the program must be in feed hold or single-block pause—never reach past a rotating tool. Wear safety glasses with side shields throughout the lab; flying chips are the primary eye hazard in milling. Steel-toed footwear protects against dropped blocks and vises. Keep the chip auger off and the coolant nozzle pointed away while you measure inside the pocket to prevent slipping on wet steel.

Measuring In-Process: Pocket Width

After the roughing end mill retracts and the program halts, open the door and measure the pocket width with a 0–1 in outside micrometer. A micrometer gives resolution to 0.0001 in, which is the right tool for a ±0.002 in tolerance. Place the anvil and spindle across the shortest span of the pocket at mid-depth, not at the top edge where the end mill entry burr may throw off the reading. Rock the micrometer gently to find the true minimum reading, then lock the thimble and read.

Record the measured width on your inspection log immediately—memory is not a substitute for a written number. Calculate stock remaining:

Stock remaining = Finish nominal − Roughing measured width

If the finish nominal is 1.000 in and the rough pocket measured 0.950 in, stock remaining is 0.050 in. That matches the expected roughing leave if your program was written for a 0.025 in per-side finish allowance. If the number is outside your expected range, stop and consult the instructor before proceeding.

Wear Offset Adjustment

Wear offsets (sometimes labeled Wear or W in the offset table) add a small dimensional correction to a tool's programmed path without editing the G-code. A positive wear offset on a milling tool moves the cutter away from the part wall, leaving more material. A negative value moves it closer, removing more.

The formula for a width adjustment is:

Wear offset change = (Finish nominal − Measured after rough) / 2

Divide by two because the cutter affects both walls simultaneously. If you need the pocket 0.010 in wider total, each wall needs 0.005 in more removal, so enter −0.005 in (negative because you want the tool to cut deeper into the wall). Enter this value in the correct offset register on the Haas offset page, confirm it is in the Wear column—not Geometry—and double-check the sign before running the finishing pass.

Never enter a wear offset larger than the finish stock you have, or you will overcut and scrap the part.

Running the Finishing Pass and Re-Measuring

Run the finishing pass in single-block at 100% feed override unless your procedure card specifies otherwise. After the finishing end mill retracts, measure the pocket width again with the same micrometer setup. Record this second measurement. Both the rough and finish measurements belong on the inspection log with their values, not just pass/fail checks.

Accept criteria: finish width 1.000 ±0.002 in, meaning any value from 0.998 in to 1.002 in is conforming. If the part measures outside that range and you have not overcut, a second wear offset correction and re-run may be possible—ask the instructor. An overcut part is rejected and must be repeated on a new blank.

Pocket Depth and Hole Diameter

After all milling operations complete, measure pocket depth with a depth micrometer. Seat the beam flat on the top surface of the block and extend the rod to contact the pocket floor; read the depth. Compare to the print dimension and tolerance.

Measure hole diameter with a pin gage set. Select the go pin (nominal minus low limit) and the no-go pin (nominal plus high limit). The go pin must pass through the hole; the no-go pin must not. Record go/no-go result plus any diameter you can confirm numerically. Pin gages verify size but not roundness; if the drawing calls out roundness, a bore gage or CMM is needed.

Documenting Disposition

Every measurement goes on the inspection log with: feature name, nominal, tolerance, measured value, and accept (A) or reject (R). Your instructor will verify the finish pocket width with an independent micrometer. A documented measurement that matches the instructor's reading demonstrates measurement repeatability, which is a NIMS and industry requirement for quality assurance.

Key terms9

Single-block modeA CNC control mode in which the machine executes one block of G-code per press of Cycle Start, then pauses, allowing the operator to verify each move before continuing.
Wear offsetA small dimensional correction entered in the CNC offset table that shifts the tool path without editing the G-code; used to fine-tune a finished dimension after measuring an in-process feature.
Feed overrideA control knob or percentage setting that scales the programmed feedrate up or down in real time, allowing the operator to slow the machine during first-part verification.
Stock remainingThe material left between the current machined surface and the final finish dimension; calculated as finish nominal minus the measured roughing dimension.
Outside micrometerA precision measuring instrument using a calibrated screw thread to measure external distances; a 0–1 in micrometer reads to 0.0001 in resolution, appropriate for tolerances of ±0.002 in or tighter.
Depth micrometerA micrometer variant with a flat beam and interchangeable rods used to measure the depth of pockets, slots, and stepped features from a reference surface.
Pin gageA precision cylindrical gage used in go/no-go fashion to verify that a hole diameter falls within tolerance; the go pin must enter the hole and the no-go pin must not.
In-process measurementDimensional inspection performed on a feature while the part is still in the machine or before all operations are complete, allowing corrections before the feature is finished or tolerance is lost.
Inspection logA written record listing each measured feature, its nominal dimension, tolerance, actual measured value, and accept or reject disposition, providing objective evidence of part quality.

Worked examples4

1. After the roughing pass, you measure the pocket width at 0.946 in. The finish nominal is 1.000 in. Calculate the stock remaining and determine the required wear offset adjustment.
  1. Stock remaining = Finish nominal − Measured rough width = 1.000 − 0.946 = 0.054 in total across both walls.
  2. Each wall carries half the total stock: 0.054 ÷ 2 = 0.027 in per side.
  3. The finishing tool is already programmed to remove the expected per-side stock. No wear offset change is needed if 0.027 in per side was the intended finish allowance. Confirm against the procedure card.
  4. If the intended finish allowance was 0.025 in per side, the rough pocket is 0.004 in undersized (0.946 vs. expected 0.950). To correct, enter a wear offset of +0.002 in (half of 0.004 in) so the finishing pass removes 0.002 in less per side, leaving the pocket at 1.000 in.
  5. Answer: Stock remaining = 0.054 in; if rough pocket should have been 0.950 in, wear offset = +0.002 in.
Answer: Stock remaining = 0.054 in total; wear offset correction = +0.002 in to compensate for undersized roughing cut.
2. After the finishing pass, you measure the pocket width at 1.004 in. The print calls for 1.000 ±0.002 in. Is the part conforming? What is the deviation?
  1. Upper limit = 1.000 + 0.002 = 1.002 in.
  2. Lower limit = 1.000 − 0.002 = 0.998 in.
  3. Measured value = 1.004 in.
  4. 1.004 in > 1.002 in (upper limit), so the part is out of tolerance.
  5. Deviation = 1.004 − 1.002 = 0.002 in over the upper limit.
  6. Because material has been removed, this is an overcut condition. The feature cannot be corrected by re-machining. Disposition: Reject.
Answer: The part is non-conforming (rejected). The pocket is 0.002 in over the upper limit; the feature is overcut and cannot be reworked.
3. Before running the finishing pass, you want to enter a wear offset to move the finish width from the expected 0.995 in (after finishing with no correction) to exactly 1.000 in. What value do you enter?
  1. Total additional width needed = 1.000 − 0.995 = 0.005 in.
  2. Both walls are cut simultaneously, so the cutter must move 0.005 ÷ 2 = 0.0025 in farther into each wall.
  3. Moving the tool farther into the wall means the offset must be negative (the tool path expands outward from the pocket center).
  4. Enter wear offset = −0.0025 in in the correct offset register.
Answer: Enter a wear offset of −0.0025 in to add 0.005 in total width (0.0025 in per wall).
4. Your micrometer thimble reads 0 on the sleeve 25-thousandths line, and the thimble line aligns with the 13th vernier graduation (0.0013 in). The sleeve shows two full 0.100 in divisions and one 0.025 in subdivision exposed. What is the total reading?
  1. Sleeve major divisions: 2 × 0.100 = 0.200 in.
  2. Sleeve subdivision exposed: 1 × 0.025 = 0.025 in.
  3. Thimble reading: 0.000 in (thimble at zero line) + vernier: 0.0013 in.
  4. Total = 0.200 + 0.025 + 0.000 + 0.0013 = 0.2263 in.
Answer: Total micrometer reading = 0.2263 in.

The math in this lesson2

Learners split total stock between two walls and halve the undersize error to get a wear offset correction.
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Brush up in the math halls at newSchoolWorks.com:
In class: Students run the first roughing pass in single-block at 50% feed override, pause after the roughing end mill retracts, and measure the pocket width with a 0–1 in micrometer; they record the measured value and calculate stock remaining. · Based on the measurement, students calculate and enter a wear offset adjustment to hit the 1.000 ±0.002 in finish width, then run the finishing pass and re-measure; they document both measurements on the inspection log. · After all operations complete, students measure the pocket depth with a depth micrometer and the hole diameter with a pin gage set, recording accept or reject disposition on the inspection log with objective numeric evidence.
Materials: Haas CNC mill, 1018 steel block, carbide end mill (roughing and finishing), 0–1 in outside micrometer, digital caliper, depth micrometer, pin gage set, inspection log form, wear offset procedure card
Check: Finish pocket width 1.000 ±0.002 in and depth within print tolerance confirmed by instructor measurement; wear offset calculated and entered correctly with no overcut — any scrapped feature requires repeat with a new blank.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

CNC Setup and Operation
1. What is the primary purpose of running a first-part program in single-block mode?
Measurement and Quality Control
2. After a roughing pass, the pocket width measures 0.952 in. The finish nominal is 1.000 in. How much total stock remains across both walls?
CNC Setup and Operation
3. You need the finishing pass to remove an additional 0.006 in of total pocket width (0.003 in per wall). Which wear offset entry is correct?
Measurement and Quality Control
4. Which measuring instrument is most appropriate for verifying a pocket width held to a ±0.002 in tolerance?
Measurement and Quality Control
5. When using a pin gage set to check a hole, which condition confirms the hole is within tolerance?
Measurement and Quality Control
6. A finished pocket measures 1.003 in. The print tolerance is 1.000 ±0.002 in. What is the correct disposition?
Safety and OSHA/Machine Tool Standards
7. What is the correct safety action before reaching into the machine envelope to measure an in-process feature?
CNC Setup and Operation
8. An operator enters a wear offset of −0.010 in, but only 0.008 in of finish stock remains on each wall. What is the likely result?
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