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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › CNC Machining › Metrology: Precision Measurement for CNC Machining › Lesson 8
MFG-CNC14-07 · Lesson 8 · 10 clock hours · theory

Surface Finish Measurement and Interpretation

Objective. Students interpret surface finish symbols on engineering drawings and measure Ra and Rz on machined samples using a contact profilometer, comparing results to print callouts.

⚠ 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.

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What Is Surface Finish?

Every machined surface has microscopic peaks and valleys left by the cutting tool. The height, spacing, and pattern of those irregularities determine the surface finish (also called surface texture). Surface finish affects how a part performs: a shaft journal that is too rough will wear bearings prematurely; a sealing surface that is too smooth may not hold a gasket compound. CNC machinists must be able to read finish requirements on engineering drawings and verify that completed parts meet them.

Key Parameters: Ra and Rz

Ra (Arithmetic Average Roughness) is the most common surface finish parameter used in US industry. It is the arithmetic mean of the absolute deviations of the surface profile from the mean line, measured over a defined sampling length. Ra is expressed in microinches (µin) or micrometers (µm). A finer finish has a lower Ra value.

Rz (Average Maximum Height) is the average of the five largest peak-to-valley heights within a sampling length. Rz is more sensitive to isolated deep scratches or high peaks than Ra is. Some drawings, especially those following ISO or DIN conventions, specify Rz instead of or in addition to Ra. Always check which parameter the drawing calls for before measuring.

A useful approximate relationship: for many machined surfaces, Rz is roughly four to ten times the Ra value, but this ratio varies by process and material. Never substitute one for the other without authorization from engineering.

Reading Surface Finish Symbols on Engineering Drawings (ASME Y14.36M)

ASME Y14.36M is the US standard that governs surface texture symbols on engineering drawings. The standard uses a check-mark style symbol placed on the surface line or on a leader pointing to it. Key elements you will encounter:

When a drawing shows 125 µin Ra, the finished surface must not exceed that value. Some drawings give both a maximum and a minimum to prevent over-finishing (which adds cost).

Typical Surface Finish Ranges by Machining Process

| Process | Typical Ra (µin) |

|---|---|

| Rough milling | 250–500 |

| Finish milling | 63–125 |

| Turning (finish) | 32–125 |

| Grinding (cylindrical) | 8–32 |

| Honing | 4–16 |

| Lapping / superfinishing | 1–8 |

These are general ranges; actual results depend on many variables.

Factors That Affect Surface Finish

Feed rate has the strongest effect on turned and milled surfaces. A higher feed rate leaves wider, deeper cusps between tool passes, increasing Ra. Halving the feed rate approximately quarters the theoretical roughness (for a single-point tool).

Tool nose radius on a turning insert or end mill corner determines cusp height at a given feed. A larger nose radius reduces roughness for the same feed rate, which is why finish turning inserts often have a larger radius than roughing inserts.

Cutting speed affects surface finish indirectly through built-up edge (BUE). At low speeds some materials form a BUE on the tool, which tears rather than shears the surface. Increasing cutting speed above the BUE-prone range usually improves finish. Very high speeds may also improve finish through thermal softening, but consult tooling data for each material.

Coolant flushes chips away before they can be re-cut (re-cut chips score the surface) and reduces heat that can cause material adhesion on the tool. Flood coolant or minimum quantity lubrication (MQL) typically produces better finish than dry cutting for most steels and aluminum alloys.

Using a Contact Profilometer

A contact profilometer (surface roughness tester) drags a diamond-tipped stylus across the surface at a controlled speed. A transducer measures vertical stylus displacement; the instrument calculates Ra, Rz, and other parameters electronically.

Operating procedure (general):

1. Clean the surface with a lint-free cloth; remove coolant, chips, and oil films.

2. Place the instrument on a flat, vibration-free reference surface or use the built-in skid to ride on the workpiece.

3. Set the correct cutoff wavelength (λc) for the expected roughness range — refer to the profilometer manual and ASME B46.1 guidance. An incorrect cutoff will give misleading readings.

4. Orient the stylus traverse perpendicular to the lay (perpendicular to tool marks) unless the drawing specifies otherwise. Traversing parallel to the lay will underreport roughness.

5. Initiate the measurement and wait for the instrument to complete its travel.

6. Record the displayed Ra (and Rz if required). Take at least three readings at different locations and average them for a representative result.

7. Compare the measured value to the print callout to make an accept/reject decision.

Safety Notes

Key terms8

Ra (Arithmetic Average Roughness)The arithmetic mean of the absolute deviations of a surface profile from its mean line over a sampling length; the most common surface finish parameter in US industry, expressed in µin or µm.
Rz (Average Maximum Height)The average of the five largest peak-to-valley heights within a sampling length; more sensitive to isolated defects than Ra.
ASME Y14.36MThe American Society of Mechanical Engineers standard that defines surface texture symbols and their placement on engineering drawings.
LayThe predominant direction of the surface pattern (tool marks) on a machined surface, indicated by a letter symbol on the engineering drawing.
Cutoff wavelength (λc)The filter setting on a profilometer that separates roughness (short-wavelength) from waviness (long-wavelength) components of the surface profile; must be selected to match the expected roughness range.
Contact profilometerAn instrument that measures surface roughness by dragging a diamond-tipped stylus across the surface and calculating parameters such as Ra and Rz from the stylus displacement signal.
Built-up edge (BUE)A deposit of work material that welds onto the cutting tool at low cutting speeds, causing tearing rather than clean shearing and degrading surface finish.
Surface finish comparatorA set of reference chips or plates with known Ra values produced by specific machining processes, used for visual and tactile comparison to a machined surface.

Worked examples4

1. A turned aluminum shaft has a drawing callout of 63 µin Ra maximum. You take three profilometer readings at different locations along the journal: 58 µin, 61 µin, and 67 µin. What is the average Ra, and what is the accept/reject decision?
  1. Add the three readings: 58 + 61 + 67 = 186 µin.
  2. Divide by the number of readings to find the average: 186 ÷ 3 = 62 µin Ra.
  3. Compare the average to the callout: 62 µin ≤ 63 µin maximum.
  4. Note that one individual reading (67 µin) exceeds the callout. Check your shop's inspection protocol — many quality plans require every individual reading, not just the average, to pass. If the protocol requires all readings to pass, the part is REJECT. If only the average is evaluated, the part is ACCEPT. Report both the average and the out-of-tolerance reading to your supervisor.
Answer: Average Ra = 62 µin. The average passes the 63 µin callout, but one reading of 67 µin exceeds the limit. Flag the result and follow your shop inspection protocol; do not ship without supervisor review.
2. A finish-turned steel part has a current Ra of 120 µin at a feed rate of 0.010 in/rev. The drawing requires 32 µin Ra maximum. Approximately what feed rate would you try first to achieve the target, given that theoretical roughness for a single-point tool is proportional to the square of feed rate?
  1. Write the proportionality: Ra ∝ f², so Ra₂/Ra₁ = (f₂/f₁)².
  2. Substitute known values: 32/120 = (f₂/0.010)².
  3. Solve the ratio: 32/120 = 0.2667.
  4. Take the square root: √0.2667 ≈ 0.516.
  5. Multiply by the original feed: f₂ = 0.516 × 0.010 ≈ 0.0052 in/rev.
  6. Round to a practical value available on the machine, such as 0.005 in/rev, and verify with a trial cut and profilometer measurement.
Answer: Try a feed rate of approximately 0.005 in/rev. Always verify with an actual measurement; the theoretical formula is a starting point, not a guarantee.
3. A milled surface is measured with the stylus traversing parallel to the milled tool marks (parallel to the lay). The reading is 45 µin Ra. The same surface is then measured with the stylus perpendicular to the lay. That reading is 110 µin Ra. The drawing callout is 63 µin Ra maximum. Which measurement is valid for the accept/reject decision, and what is the decision?
  1. Recall that profilometer traversal must be perpendicular to the lay to capture the full peak-to-valley height of the tool marks.
  2. The parallel traverse (45 µin) runs along the grooves rather than across them, producing an artificially low reading — this measurement is not valid per standard practice.
  3. The perpendicular traverse (110 µin) correctly captures the actual roughness profile.
  4. Compare valid reading to callout: 110 µin > 63 µin maximum.
Answer: The valid reading is 110 µin Ra (perpendicular to lay). The part is REJECT — it exceeds the 63 µin callout.
4. Convert a surface finish callout of 1.6 µm Ra to microinches Ra. (Use 1 µm = 39.37 µin.)
  1. Multiply the µm value by the conversion factor: 1.6 µm × 39.37 µin/µm.
  2. 1.6 × 39.37 = 62.992 µin.
  3. Round to a practical figure: approximately 63 µin Ra.
Answer: 1.6 µm Ra ≈ 63 µin Ra. This is a common drawing callout you will see on parts designed to ISO standards.

The math in this lesson2

Learners average three profilometer Ra readings and compare the mean and each reading to the 63 µin callout.
S-ID.2A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: Match surface finish symbols and values (drawn from ASME Y14.36M) to a set of visual/tactile comparator chips; rank them coarsest to finest and explain what machining process typically produces each. · Run a contact profilometer across three machined test samples (milled, turned, ground); record Ra values and compare them to the drawing callouts provided by the instructor. · Explain in a short written paragraph how cutting speed, feed rate, tool nose radius, and coolant affect surface finish, drawing on experience from the Manual Mill & Lathe course.
Materials: Contact profilometer (surface roughness tester), machined test samples, surface finish comparator set, engineering drawings with finish callouts, ASME Y14.36M surface texture reference (abridged), profilometer operating manual.
Check: Student measures Ra on two samples and selects the correct accept/reject decision based on the print callout; written paragraph is coherent and technically accurate; pass = both measurements correct and paragraph accepted by instructor.

Practice check8

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

NIMS Machining L1 – Print Reading and Inspection
1. On an engineering drawing following ASME Y14.36M, the number placed above the horizontal bar of the surface finish symbol represents:
NIMS Machining L2 – Metrology and Quality
2. When using a contact profilometer, the stylus should be traversed in which direction relative to the surface lay?
NIMS Machining L2 – Metrology and Quality
3. A machined surface measures 85 µin Ra. The drawing callout is 63 µin Ra maximum. What is the correct disposition?
NIMS Machining L1 – Process Knowledge
4. Which machining process typically produces the finest (lowest Ra) surface finish?
NIMS Machining L2 – Process Optimization
5. A turner doubles the feed rate from 0.005 in/rev to 0.010 in/rev while keeping all other parameters the same. Based on the theoretical relationship between feed and roughness for a single-point tool, the Ra value will approximately:
NIMS Machining L1 – Cutting Tool Technology
6. Built-up edge (BUE) on a cutting tool is most likely to occur at:
NIMS Machining L1 – Safety
7. Before placing a contact profilometer stylus on a workpiece that is still fixtured in a CNC machining center, the operator must first:
NIMS Machining L2 – Metrology and Quality
8. An engineering drawing specifies a surface finish of 1.6 µm Ra. A technician's profilometer reports in microinches. Using 1 µm = 39.37 µin, what is the callout in microinches (rounded to the nearest whole number)?
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