From time to time we are asked “What is the range that a collet will properly hold an arbor and/or lens blank”. The question depends on whether they are holding an arbor (chuck) or a lens blank/base curve, but the answer is essentially the same. The usual question is about the utility of an existing collet for a size arbor or blank that is different than what the collet was originally sized for. Of course, replacing a collet is expensive and getting more use from one seems to be a good idea.
The two types of collets are the 1. shallow step collet used for blanks, and 2. the deep step collet used for arbors (chucks) both usually with a 12.7 mm bore. (Some manufactures use ¼” diameter (6.5 mm) arbors with the ¼” collet having a 12.7 shallow step for holding the blank.)In both cases, we recommend no more than .01 mm difference in diameter to be inserted into the collet. That is, 12.69-12.71 mm arbor/blank diameter should be used.Background: The concern about collet crimp really surfaced with the move from pitch to soft tool polishing. Pitch would lap away the aberrations caused by blank strain due to collet crimp. Soft polishing requires good optics from the lathe.Some production managers, for a “universal collet”, have requested 12.72 mm diameter arbors that will automatically make the collet grip the arbor more firmly while allowing the 12.7 mm blank to be held with a lighter grip to avoid crimping the blank. While this seems to address the universal utility of the collet, there are three problems with this approach:Inserting a 12.72 mm arbor in a 12.7 mm collet will cause the collet to close on the inside end of the arbor first while the outside end is not gripped tightly.
This can allow the lens end of the arbor to run out causing prism.If you want the collet to grip the arbor uniformly, the collet has to deform, which is not good for the colletSince the collet is ground for a 12.7 diameter, the bore at the slits will contact the 12.72 mm arbor first and make a line contact. For a 3 slit collet this will prevent the middle of the jaw from contacting the arbor. Six split collets do a better job conforming to out size arbors, but can not conform to out of round blanks. This is because the spindle taper constricts the collet to a round bore. Plastic arbors are more forgiving about this, but still they need to be ground to the 12.7 diameter for fit properly.
Many manufactures hold the blank in a step collet. Optics problems arise with out of round blanks and out of size blanks. When out of round blanks are held in collets (regardless of the number of slits) the collet attempts to squeeze the blank to the 12.7 mm (round) diameter. The effect is to crimp the long diameter to 12.7 and allow the smaller diameter to expand. ‘Sound familiar? A 12.68 X 12.72 mm out of round blank tightly crimped will have around ¼ diopter of cylinder, plus bad optics from the slits of the collet.Some manufacturers have adopted the practice of turning the O.D. of the blank to 12.5 mm diameter at the same time the base curve is turned. This is done while the blank is blocked. This provides a truly round blank with the base curve perfectly concentric with the O.D. and does not strain in the blank which prevents poor optics. The base curve is deblocked, polished, and then held in the blocker collet for blocking to the front curve tool. This is a good way to guarantee a precise locating surface. The problem arises when they want to hold the 12.5 mm diameter in a 12.7 mm collet.
Again, problems arise with flexing the collet .2 mm and having contact on the blank at the center of the collet jaws. So, even though blocking machines can be adjusted for the holding force, holding a 12.5 mm blank in the 12.7 mm collet will strain the base curve and yield bad optics. We recommend using a precision ground collet in the blocking machine to match the trimmed blank.
The alternative to the procedures above is to use all 12.7 mm collets and arbors and transfer block.
Make the effort to use the proper size arbor to fit the collet and size the base curve (blocker) collet to the trimmed blank diameter to within .01 mm of the nominal size.