A Comparison of lens manufacturing techniques
I have followed contact lens manufacturing techniques since entering the industry in 1974. This is a short article about what changes I have seen and even some things that have come full circle. I will cover the basic steps of polishing and blocking comparing the traditional process with what is considered current. In referring to traditional and current manufacturing methods, I don’t mean to say that one is in common use while the other is fading out. It is important to look at the reasons that particular methods are in use. This is especially true when process or tooling changes are considered. When something is changed, it usually affects other parts of the process.
Probably the most important step in making lenses was “polishing in” the optics with pitch. This allowed the lathe set-up to be “close” and the lathe finish to be “pretty good”. I still hear these terms rather than “ten microns of runout” for example. The blank was held in a collet to turn the base curve. Depending on the size of the blank and if it was round, the lathe collet would force it to the collet size and shape. This resulted in a distorted base curve. But the pitch would grind the distorted base curve to a sphere.The test of a good polish was a suction of the lap and the lens after polishing. This meant that there was a good match of the curves (spheres). Crisp optics and a predictable radius were expected. Typically, the lathe was adjusted a few hundredths of a millimeter to yield a particular radius after polishing. If the lathe lateral and tool height were set up well, this compensation was fairly accurate over the range of radii being produced. However, it usually didn’t work to turn base curves and fronts curves on the same lathe. So it was common to have a lathe for each side.
This process yielded the two sides of the lens; peripheral curves and CN bevels were done by a skilled technician by hand. For specialty lenses, basic spherical lathe turning was employed and then the lenses were modified at the finishing station. Toric lenses were generally crimped (some labs used fly cutters) and polished in the spherical shape. Again, the peripheral curves were done by a skilled technician.
The advent of the CNC air-bearing spindle lathe has caused a change in polishing methods. Along with superior precision and finish from the lathe, the CNC control offers a capability to make a wide variety of lens designs. To polish these designs, various “soft” tools are used to polish non spherical surfaces. Since it is no longer necessary to polish in the optics, pitch has given way to the soft tool. In setting up the lathe, the finishes are so good that the optics can be read in the radiuscope. In fact, poor optics on the lathe cut surface usually indicates collet crimp which, ironically, can be corrected by polishing with pitch. Actually, many labs have continued to polish the spherical base curves with pitch with good results and shorter polishing times. An alternative is to block the blank and turn the base curve. This eliminates collet crimp distortion and provides subsequent tooling and blocking advantages such as transfer blocking and blank truing.With the improved surface on fronts, cloth covered drum tools filled with sponge are the preferred polishing tooling. The main difference being that the surface from the lathe has to be the one that you want on the finished lens. So, the lathe set-up is critical and the soft tooling buffs it out.
A later lathe development was the oscillating or fast tool servo. This feature coordinates the movement of the diamond tool with the spindle rotation to offer an almost unlimited lens design capability. Of course, this complicated polishing. Pitch doesn’t work on non spherical (free-form) surfaces. Solid soft tools molded in increments worked on small (.2 mm radius) departures from a sphere. For toric base curves of many diopters, the molded solid tools didn’t conform well enough to polish the flat meridian. This is because of the stiffness of the solid tool and unequal contact pressure. The bladder tool polisher system was developed to address this problem. In particular, the bladder works like a tire in that the contact pressure is uniform. The use of a single bladder tool for all lens designs (including peripheral curves) simplifies and shortens the polishing process.
The next major procedure was to block the base curve. This was usually done on a jewelers lathe or on a few special machines made for the industry. They were the spindle type usually with tapers (some had collets) to mount the blocking tool (arbor). The combination of the lathe collet, base curve, arbor, and blocking machine made for a challenge to make a prism-free lens. It was incumbent that every component process was of high quality and well maintained. Alignment adjustments were not done and repair meant sending the machine in for reconditioning or replacing collets, and/or arbors. The condition of these components is still a very important part of lens manufacturing. A particular blocking method was to block directly on the front cutting lathe. This actually has a lot of merit. This method eliminates how the precision of the lathe spindle/taper or collet and the arbor affect the runout of the base curve. This is because the base curve can be trued to the lens diameter shoulder while mounted on the lathe spindle.
A feature of blocking arbors offered by the Geo. Nissel Co. was an incremental sizing of the radius and a black surface. From my background (USA), I didn’t understand why it was important to match the radius of the base curve to the arbor. In the UK and Europe they used soft wax to block lenses. The arbor was heated with the wax, the base curve pressed on and trued on a spindle while the arbor and wax cooled. The thin layer of wax gained support from the arbor. Also, the radius was concentric with the internal taper and the12.7 mm (.5000”) shank. This concentric radius was useful in providing alignment to the base curve. I imagine some of my customers were concerned when I would question the need for individually radiused arbors since I wasn’t aware of the soft wax that they used. Toric lenses were generally flycut, so the lenses were not crimped and the soft wax worked fine. My introduction to blocking was with Kerr (1) impression compound. This is a heavy bodied material which is very stiff at room temperature. It was common to see 1 mm of this compound between the base curve and the arbor and 10 times more along the side of the arbor. This is where I got the idea that the condition of the top of an arbor wasn’t important. Any imperfections would be absorbed by the Kerr compound. It has excellent adhesion properties especially to hold crimped base curves to arbors for front toric lenses. The spin blockers dominated the industry because they provided an easy way to test the prism (runout). I have seen countless times where a technician will put their fingernail against the side of the blank to test the runout. This method was better than nothing even though it was time consuming and was not verifiable. I haven’t seen data that shows how this test affected the prism reject rate. I often see the fingernail test being used today. If there is runout, rather than look for the cause, the operator re-chucks the arbor a few times to try to minimize the runout.
There is not an inherent need to spin block to attach a base curve to an arbor. Two well aligned collets will block a base curve with consistent good results. Alignment needs to be checked periodically. The periodic monitoring (with recorded results) of all lens holding devices on a regular basis makes the lab operate smoothly. The records help identify where problems that cause prism are occurring. This makes it easier to correct these problems. This data is very important when considering automation.Static blockers were introduced in the early 90’s. Some were variations on the spin blocker frame and had some means of adjustment to radially align the two collets (no face tilt adjustment). This usually involved a gage pin inserted in the collets, then securing the collet positions. While this sounds like a good approach, the proof of “no” prism is taking the blocked button to the lathe and checking the radial and face runout. The problem is that there are no gauges on these blockers to indicate what the adjustment is. When we use a gage pin to align the blocker, it is for roughing only, and results in about 50-100 microns of prism. After roughing in the blocker with the gage pin, we calibrate the blocker to lathe spindle/collet that has less than one micron runout. It happens to be a Hardinge HSL 59. This involves taking runout readings on the side and face of the blank relative to “0” degrees on the blocker, and then making the indicated adjustments on the blocker. In the lab, the usual procedure is to block some blanks and record the runout on a few arbor/blanks. If the results are random, then clean the lathe and blocker collets and inspect the arbors. This usually fixes the runout problem. If the runout is consistent in amount and location, the blocker needs to be adjusted. This should be monitored every week.
Two other developments have occurred in the past few years that are being adopted throughout the industry: solid arbors and water soluble blocking compound. We introduced solid arbors to standardize tooling, much like more mature industries standardize fasteners and pipe fittings. This standardization reduces the cost and fits in with automation. A detail is to keep the arbors the same height. Reducing variation is a cornerstone of manufacturing.
The other less apparent benefit is that the plastic is more durable than the brass or stainless steel arbors. Plastic arbors also lend themselves to modifications for different lens styles (sclerals) and/or job tracking devices.
Water Soluble Blocking Compound has changed the industry and even the patient’s comfort. Labs have documented cleaner lenses being shipped and fewer returns. My introduction to WSBC was a bit of a challenge and reminded me of the European practice of using a thin layer of wax. The first WSBC’s were very thin and would not work on a spin blocker. At that time, our Autoblocker was working with Kerr or Perfectblock (2), the heavy bodied compounds. As the top chuck of the blocking machine came down, the compound dispensed in the base curve was thick enough to support it. This allowed for a free operating system where arbor length or center thickness variations were cancelled out.This all changed with WSBC. It was determined that a thin layer of WSBC was needed because of its softness. This requirement meant that the blocker had to position the base curve and arbor to a .1 mm precision while allowing for CT and arbor length variations and not touching the BC. This needed to be compensated for on each lens/arbor combination.This challenge lead to the development of a Wax Thickness Control on the Autoblocker that measures each base curve/arbor and yields a .2 mm compound thickness. So, the Nissel method of blocking with the thin wax layer has returned. It was also determined that, unlike the Nissel matched radius arbor, for 90% of base curves a 7.00 mm arbor radius works fine. The over-all result of the WTC is better support of the base curve yielding better optics and cleaner lenses. I’m not saying that this has been an easy transition, but many labs have made the effort to use WSBC. With the number of WSBC suppliers, there is a lot of support for the labs. An off shoot of the WTC was the requirement that the arbor is fully inserted into the upper collet. An arbor sensor was added to the Autoblocker to ensure full insertion in the collet. This greatly reduces prism caused by the arbor not being fully engaged in the collet. The advent of WSBC caused several accommodations to tooling and machinery which is a good example of the effects of process changes.
First, I am not a contact lens fitter. I have the highest respect for the technicians who bevel lenses and roll the edge. My understanding is that the edge determines the comfort of the lens. My impression of hand beveling lenses was that there were many adjustments to the lenses made at this point in the process. So while the lathe work was done and production had a certain yield, the finisher “saved” many lenses by the end of the day that would have been rejects otherwise. The result was a very high yield rate with a high reliance on the finishing technician.
The CNC lathe can generate almost any lens design and make that lens over and over. Inspection of the base curve, for many labs, has become automated with the results given on paper or directly downloaded to the front lathe. So, theoretically, the lens should be finished when it is de-blocked and the edge polished. If all the tooling and machinery is setup and calibrated properly, then the last step would be final inspection without modification. In reality, some power adjustment may be needed.
I see the industry evolving with some labs just getting their first CNC lathe and peripheral equipment, while a few are automating the process to a high level. In both extremes, there is an ongoing need to monitor the processes with good housekeeping and investment in key personnel. The manufacturing process is changing, yielding repeatable results and lower costs. This is possible while improving quality. This is the prescription for success.
(1) Kerr Dental impression compound
(2) Perfectblock a product of Polychem