With the use of CNC Lathes now common, the reliance on them to produce finished lenses has become a reality. While the lenses still need to be polished, little or no modification is required or desired. In fact, one of the compelling reasons to use a CNC Lathe is the expectation of making repeatable lenses.
This capability requires that no strain is introduced during the fabrication process. Obviously, the holding of the lens can deform (strain) it. This most commonly occurs during cutting the base curve and blocking it.
Apparently, blocking the base curve has been a common practice in Europe. This practice has held my interest for the short time that I have been traveling to Europe.
I will discuss two methods of blocking and their attributes. Both methods involve blocking the blank and blocking the base curve for front surface cutting. The difference lies in when the base curve is removed from the base block (chuck). This decision is based on the inspection and gauging methods available.
First, and what seems to be the conventional method, is to remove the base curve before front curve blocking. This would progress to polishing and checking thickness. Then the blank would be manually mounted on a thin layer of wax and trued by spinning while the wax firmed by cooling. This procedure works well with spherical base curves.
When mounted on a matched curve chuck, the base curve radius has a good chance of centering, even if the outside is not running true. The base curve tends to align itself with the curve on the chuck. This will result in little or no prism even if the blank wobbles. The wobble (runout) becomes a prism problem if the base curve has peripheral curves cut in with the lathe. These curves will show prism even though the base curve has none. Wobble will also make the thickness appear to increase. Since the base curve is spinning and not held tightly in a collet, no strain is introduced in it. This will improve the outcome of the optics. This method allows for polishing the base curve and checking the thickness.
A modification of this method considers the diameter variation of the lens blank. While most blanks are ground to diameter, a diameter tolerance exists and can be +/- .03mm. (This variation can be a serious problem for direct chucking lathes and blockers. Many of these machines use collet systems, which require collet adjustment to fit the blanks.) A convenient way to deal with the diameter variation is to skim (trim) the diameter of the blank completely when the base curve is cut. This guarantees that the reference surface is true with the base curve. It also makes sure that the blank is round. This will reduce strain if a collet is used to hold the blank for blocking or run truer if the blank is spin blocked. This method works well with one blocking machine. Both back and front chucks can be used in the same blocking machine, so no adjustments are necessary.
Deblocking of the blank can present a challenge. I have seen many holding techniques: wax, pitch, double-backed tape etc.
The wax method uses a similar technique as matched radius front blocking. The blank is pressed firmly onto the chuck forcing the wax out. This is sometimes used to ensure that the blank is flat against the chuck. I am concerned that this aligns the blank. A wedge of only .02mm will result in prism and the pressing can strain the blank. Pitch remains much thicker and cannot be relied upon to allow the blank to seat on the chuck. Thus, the blocking machine must provide the alignment precision. Double-backed tape has many limitations: strength, chemistry and precision. However, it is easy to deblock. All the above methods require blank cleaning. This adds another step to manufacturing.
The other method in the blocking process uses the blocking chucks only to locate the blank. This transfer blocking process does not strictly require a machine to block the blank onto the base curve chuck. The blank is positioned on the chuck (even by hand). Also, the length of the chuck must be known or a means to measure base curve thickness must be provided. A known length chuck can be used, and the thickness measured. The length of the chuck is then subtracted from the reading. This assumes that the blank is seated precisely on the chuck.
The chuck can be hollow. This feature allows the chuck with the blank to be placed over a thickness gage probe, which contacts the center of the base curve. The chuck will act as a guide to center the lens over the thickness gauge. Thus, you are always measuring thickness at the center of the lens without scratching it.
Then the base curve and the block go to polishing. A standard base curve polishing holder is fixed to the block to polish the base curve. A radial arm polisher should not require a stroke change while using the block. A horizontal stroke polisher will require about 3 times the stroke to get the same polishing action. Note that the base curve block will add a few grams of mass to the polishing process. Some gyroscopic effect may be noticeable at high polishing speeds.
After these steps are done, the base curve block is mounted in the blocker. Theoretically, the blocker could simply be a precision “V” block. It would have a gap around the blank to allow for wax. Since only the chucks are in contact with the “V” block, precision alignment is assured. A blocking machine with two collets also works well with transfer blocking (if the collets are aligned). Again, the challenge is to deblock the correct chuck. I need to explore this procedure more. Heating the block or breaking the blank off is risky. Perhaps a release screw inside the chuck would work.
The base curve is now only blocked on the front chuck and the front curve can now be cut with the confidence that the base curve and front curves are aligned. This was all done without being concerned about the condition of the blank: whether it was round or off center. No strain was induced by any holding devices. Then, front polishing can be done on conventional polishing machines.
The transfer blocking method provides a means to rotationally orient the two chucks using index features on them. This is useful for controlling prism or cylinder axis. The blocker can have fixed locators or a means to precisely rotate the chuck before blocking. This eliminates the need to rely on marks on the blank to locate the axis. The basic concept of transfer blocking is this: the lens blank does not influence the quality of the finished lens. The outside blank diameter must only be larger than the lens to be centered well enough for the lens to be cut. It is possible that a blank right out of its mold tube could be mounted and processed (providing that faces were flat). This would make the blanks easier to make.