🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Health & Personal Services › Optician / Optical Technician › Lens Fabrication & Edging › Lesson 6
HLT-OPTI68-03 · Lesson 6 · 17 clock hours · lab

Edging CR-39 & Standard Plastic Lenses

Objective. Students will edge CR-39 lens blanks to a prescribed frame shape, producing a bevel that seats fully in the frame groove with no rocking, gaps, or lens fallout.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. 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.

What Is Edging and Why It Matters

Edging is the process of grinding a lens blank down to the exact shape of a spectacle frame so the lens fits securely in the eyewire. A correctly edged lens seats fully in the frame groove, stays there under normal wear, and keeps the optical center precisely where the prescription demands. A poorly edged lens can rock, pop out, or shift the optical center away from the patient's pupil—any of which degrades vision and risks patient harm. CR-39 (allyl diglycol carbonate) is the most commonly edged standard plastic material; it grinds predictably, but it still requires proper technique and equipment care.

Understanding the Frame Trace and Pattern

Before the edger ever touches a lens blank, you need an accurate representation of the frame shape. Modern automatic edgers use a tracer—either built into the machine or standalone—to digitize the inside of the frame eyewire. The tracer records the full 360-degree shape and stores it digitally. That shape data, combined with the monocular pupillary distance (PD) and optical center height from the prescription, tells the edger exactly where to cut.

Double-check that the traced shape matches the actual frame before running your first lens. A tracing error carried through four lenses wastes all four blanks.

The Wheel Sequence: Rough, Finish, Bevel

Automatic edgers use a series of diamond-impregnated grinding wheels, each doing a specific job:

Selecting the wrong wheel or skipping a stage produces an edge that is rough, incorrectly sized, or missing a proper bevel. Always confirm the machine's program matches the material (CR-39, in this case) before starting.

Bevel Placement

The bevel must sit in the correct position across the edge thickness so it engages the frame groove around the full circumference. For a standard metal frame, the bevel is typically placed toward the front surface—closer to the front of the lens—to present an acceptable cosmetic appearance. Most automatic edgers offer bevel placement options (auto, front-biased, rear-biased, or manual). For metal frames with a defined V-groove, auto placement usually works well; verify with a trial fit.

If the bevel is placed too far toward the back, the front of the lens may sit proud of the frame front. If placed too far forward, the lens may not seat deeply enough and can rock or fall out.

Blocking the Lens Blank

Proper blocking holds the lens blank on the edger chuck without slipping and ensures the optical center is in the right position relative to the cut. Steps:

1. Use the lensometer to locate and mark the optical center (and prism reference point if applicable) on the blank.

2. Set the blocker so the center dot aligns with the monocular PD and the prescribed height.

3. Apply the block cup with the axis mark aligned to the prescribed cylinder axis (or 180° if spherical).

4. Confirm the block is secure before placing in the edger chuck.

A shifted block is the leading cause of optical center displacement in finished lenses.

Running the Edger Safely

Safety is not optional. Follow these practices every cycle:

Fitting and Inspection

After edging, hand-fit each lens into its frame eyewire and check:

If any defect is found, identify the cause before re-edging. If the blank has enough stock, re-edging is possible; otherwise, start with a new blank.

Verifying Optical Center Placement

After fitting, confirm the optical center position with the lensometer. Measure the monocular PD from the frame center to the optical center dot (or to the prism reference point). The finished lens must be within the tolerance specified on the job ticket—typically ±1 mm for monocular PD. Document your measurements and pass/fail determination on the job ticket. The instructor must sign before the job is considered complete.

Completing the Job Ticket

The job ticket is a legal and professional record. Record the prescription values, lens material, frame name, measured PD and height, pass/fail status for fit, and any re-edges performed. Accurate documentation protects the patient, the lab, and you.

Key terms9

CR-39A common optical-grade plastic (allyl diglycol carbonate) used for standard single-vision and multifocal lenses; lightweight, easy to edge, and accepts most coatings.
BevelThe V-shaped ridge ground around the edge of a finished lens that seats into the corresponding V-groove of the frame eyewire to hold the lens in place.
BlockingThe process of attaching a holding device (block cup) to the front surface of a lens blank so it can be mounted on the edger chuck with the optical center correctly positioned.
Optical center (OC)The point on a lens through which light passes without prismatic deviation; it must be positioned at the patient's pupil location as specified by the monocular PD and height.
Monocular PDThe distance from the center of the nose (frame center) to the center of one pupil, measured separately for each eye; used to position the optical center horizontally in the finished lens.
Wheel sequenceThe ordered set of grinding wheels used during edging—rough, finish, and bevel—each performing a progressively more refined cutting operation.
Bevel placementThe front-to-back position of the bevel ridge across the edge thickness of the lens; must be adjusted to match the frame style and groove depth for secure, cosmetically acceptable fit.
RockingA lens defect in which the edged lens pivots or wobbles in the frame eyewire, indicating an incomplete, uneven, or incorrectly sized bevel.
Job ticketA written record accompanying each eyeglass order that documents the prescription, measurements, materials, and pass/fail results; serves as a quality control and liability document.

Worked examples4

1. The prescription calls for a right lens monocular PD of 32 mm (measured from frame center). After edging, you place the finished lens in the lensometer and measure the optical center at 33.5 mm from frame center. Is this within the ±1 mm tolerance?
  1. Identify the prescribed monocular PD: 32 mm from frame center.
  2. Identify the measured OC position: 33.5 mm from frame center.
  3. Calculate the difference: 33.5 − 32 = 1.5 mm.
  4. Compare to the tolerance: 1.5 mm exceeds the ±1 mm limit.
Answer: No. The optical center is 1.5 mm from the prescribed position, which exceeds the ±1 mm tolerance. The lens must be re-blocked and re-edged, or remade if insufficient blank stock remains.
2. A patient's prescription requires a left lens monocular PD of 30 mm and an OC height of 20 mm. After edging, you measure the OC at 29.5 mm horizontal and 20 mm vertical. Does the lens pass the ±1 mm PD check?
  1. Prescribed horizontal monocular PD: 30 mm.
  2. Measured horizontal OC position: 29.5 mm.
  3. Difference: 30 − 29.5 = 0.5 mm.
  4. Compare to tolerance: 0.5 mm is within ±1 mm.
  5. Vertical OC height: prescribed 20 mm, measured 20 mm — no deviation.
Answer: Yes. The horizontal deviation is 0.5 mm, which is within ±1 mm. The vertical height matches exactly. The lens passes the OC placement check.
3. You are setting bevel placement for a right lens that has a total edge thickness of 4 mm at the temporal edge. Your edger allows you to set the bevel apex position as a percentage from the front surface. The standard for a metal frame is to place the bevel apex 40% from the front. How many millimeters from the front surface will the bevel apex be located?
  1. Identify edge thickness at the relevant point: 4 mm.
  2. Identify the bevel placement percentage from front: 40%.
  3. Calculate position: 4 mm × 0.40 = 1.6 mm.
  4. The bevel apex will be 1.6 mm from the front surface of the lens at that edge location.
Answer: The bevel apex will be placed 1.6 mm from the front surface of the lens, keeping it in the front-biased position appropriate for a standard metal frame.
4. After fitting a lens, you notice a 0.8 mm gap between the lens bevel and the frame groove at the nasal edge only. The temporal and superior edges fit flush. What is the most likely cause, and what should you do?
  1. Observe that the gap is localized to the nasal edge, not around the full circumference.
  2. A localized gap at nasal only—with the rest fitting flush—suggests the lens is slightly oversized at the nasal edge OR the bevel was not cut correctly in that region (uneven bevel).
  3. Alternatively, the frame trace may have been inaccurate at the nasal curve.
  4. Check the tracer reading against the physical frame nasal curve with a loupe.
  5. If the trace is accurate, attempt re-edging with a slight size reduction at the nasal region, or re-trace and re-edge.
  6. If the blank has insufficient stock for re-edging, start with a new blank.
Answer: The most likely cause is a localized sizing error or bevel irregularity at the nasal edge, possibly from an inaccurate trace. Re-trace the frame, verify the data, and re-edge the lens if enough blank material remains; otherwise remake from a new blank.
In class: Students edge a set of four CR-39 lenses using the automatic edger, selecting the correct wheel sequence (rough, finish, bevel), and adjusting bevel placement for a standard metal frame; instructor observes each cycle. · Students hand-fit each edged lens into the corresponding frame, checking for rocking, gaps at the nasal or temporal edge, and correct bevel engagement around the full circumference; students record defects and re-edge if needed. · Students measure the finished lenses with a ruler and lensometer to confirm that optical center placement matches the prescribed monocular PD and height, completing a job ticket with pass/fail determination.
Materials: Automatic edger, CR-39 lens blanks (8 total, 4 pairs), frame set (metal zyl, matching prior patterns), lensometer, PD ruler, thickness gauge, edge inspection loupe, job ticket forms, cooling water/lubricant for edger
Check: Each lens seats without rocking or gaps, bevel is continuous and uninterrupted, monocular PD within ±1 mm of prescription, verified by instructor before job ticket is signed; any lens failing inspection must be re-edged or remade.

Practice check8

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

Lens Fabrication & Edging
1. What is the primary purpose of the rough (coarse) grinding wheel in the edging sequence?
Lens Fabrication & Edging
2. A finished CR-39 lens rocks when pressed lightly in the metal frame eyewire. Which defect is the MOST likely cause?
Lens Fabrication & Edging
3. Why is adequate coolant flow essential when edging CR-39 lenses on an automatic edger?
Optical Theory & Dispensing
4. The prescribed right monocular PD is 31 mm. After edging, you measure the OC at 32.5 mm. What action is required?
Workplace Safety & Lab Procedures
5. Which safety practice is MOST important immediately before placing a blocked lens blank into the edger chuck and starting the cycle?
Lens Fabrication & Edging
6. For a standard metal frame, where should the bevel apex generally be positioned across the lens edge thickness?
Lens Fabrication & Edging
7. A student finds a consistent gap between the lens edge and the frame groove at the nasal and temporal areas but full contact superiorly and inferiorly. The MOST likely explanation is:
Professional Practice & Patient Safety
8. What is the function of the job ticket in the optical lab?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →