Blocking tool styles and lathes
Over the years we get requests for quotes for setup new CL labs. We would request production information regarding number and types of lenses that a customer what's to produce. Two decades ago, it was common to hear that the customer was projecting 200-400 custom lenses per day in both RGP and SL material. The big ticket item was the lathe which for these numbers meant at least two lathes. Typically, one would be set up for base curves (BC) and the other set up for front curves (FC); the main difference being the type of collet. In the last decade, we have seen a reduction in the projected production numbers to 50 -100 lenses per day with even a few at 25 lenses per day. With this level of production, it is hard to justify the cost of two lathes. Of course, the types of lenses have grown to the point that a two axis lathe is required to turn both corneal and scleral lens designs.
This concern brings me to the subject of what blocking tooling should be used so that the lathe is a universal machine? Three blocking tool styles that offer the ability for a lathe to turn both base curves and front curves without changing the collet. I will discuss these and end up with a discussion about legacy blocking tools and where they work and why they have limitations in modern CL manufacturing.Photo 1: 1/4" (6.35 mm) diameter dowel pin with a Delrin head. This type of blocking tool lends itself to using a 1/4" lathe collet with a 1/2" (12.7 mm) step. The step is used for holding the lens blank to turn the base curve and lens diameter.
1. Allows one lathe to be used for both BC's and FC's.
2. Making use of the hardened* 1/4" dowel adds precision and longevity to the system. *One must specify hardened steel (which will rust) of hardened stainless steel.
3. The Delrin head is replaceable and can be cut into which prevents damage to the diamond.
4. The Delrin head does not need to be heated to adhere to the blocking compound.
5. Since the precision is in the dowel pin, moisture absorption is not a problem.
Cons: 1.The lathe collet needs to be adjusted to close down firmly on the dowel pin, and then be adjusted for a light hold on the lens blank. We have been asked to size the steps of collets to .002" (.05 mm) oversize to accommodate the lighter force on the lens blank. This doesn't work because one can not anticipate the diameter of the lens blank to with in .002" (.05 mm). Also, the oversize step will contact a small lens blank on three points causing mechanical distortion which causes poor optics.
2. The Delrin head must be smaller than the 1/2" (12.7 mm) step, by say .010" (.25 mm) to ensure that there will be no interference with the step. If the head is 1/2" (12.7 mm) the 1/4" (6.35 mm) dowel pin will "fight" the 1/2" (12.7 mm) head and cause run-out which will cause prism.
3. If soft stainless steel is used for the dowel pin, it will be damagedand cause prism.
4. Cost is relatively high.
Photo 2: Single piece Delrin blocking tools for both lens blanks and blocking the BC for front turning. These are usually used in transfer blocking. There are two variations on this approach:A. Hollow flat ended arbor to block the lens blank to.
Pros: 1. Allows the center thickness (CT) to be directly measured through the arbor.
2. The simple design allows for precision centerless grinding.
3. Low cost.
4. Double sided tape can be used with this type of arbor.
5. The center of the lens blank is clean (no wax residue) for front probing.
Cons: 1. The blocking compound needs to completely cover the end of the arbor to ensure an airtight seal when used in a vacuum autoloader.
2. Precision is dependent on the depth of the collet and cleanliness; it should be at least 1/2"(12.7 mm).
3. Delrin (and all plastics) absorb moisture. These arbors must not be left in water other than to remove the wax, then set out to dry. B. Solid flat ended arbor to block the lens blank to.
Pros: 1. More simple than the hollow arbor.
2. Works with a vacuum autoloader with either wax or tape.
3. Simple design allows for precision centerless grinding.
4. Lowest cost.
Cons: 1. The center of the lens blank needs to be cleaned for front probing.
2. Precision is dependent on the depth of the lathe collet and cleanliness; it should be at least 1/2" (12.7 mm).
3. These arbors absorb moisture; they should be in water only long enough to remove the blocking wax, then setout to dry.
4. A different CT monitoring system where the lathe cuts are recorded and used to control CT.C. Radiused solid arbor for blocking the BC for front turning.
Pros: 1. This is a stable design that is easy to make with either a flat end so that the lab can turn the radius or with a pre-turned radius.
2. The design allows for precision centerless grinding.
3. Lowest cost.
Cons: 1. The precision depends on the depth of the lathe collet; it should be at least 1/2" (12.7 mm).
2. Delrin absorbs moisture; the arbors should be in water only long enough to remove the blocking wax, then set out to dry.
We have been asked to supply front blocking tools with an internal taper. This so they can fit onto existing tapered spindle polishers. We understand the motivation. The lab doesn't want to change the polishing machine. You may well wonder if a hollow lens blank arbor is OK, what is wrong with a hollow front arbor? After all, hollow front arbors were used decades ago. The problem is that a front blocking tool with and internal taper usually means "O" Morse taper. It's all about stiffness.
The "O" Morse taper makes for a very thin walled arbor. If the arbor was made from metal, the stiffness is there. But Delrin is about 100 times less stiff than most metals. Among the results, are poor finish due to chatter, pulling the arbor out of the collet leading to CT errors, and poor optics. Another variation of using these tools is to de-block the BC for polishing and CT/ BC radius measurement. This can be done providing the outside of the lens blank is turned to 12.6 mm when the BC is turned. Then a special l2.6 mm collet must be used in the blocking machine for front blocking since the 12.6 mm diameter is concentric with the BC.
It is a bit cumbersome, but can yield good results.Photo 3: These are flanged blocking tools for both lens blanks and BC blocking to a radiused arbor. The main feature of these tools is a flange that is large enough in diameter to rest against the lathe spindle nose. This requires a lathe collet where the face of the collet is recessed just inside the spindle nose. This type of collet allows the flange of the arbor to be pulled against the spindle nose for an added control of run-out and the additional possibility of utilizing a dead length system. Again, stiffness comes into play, so where that tools are to be used for a dead length system, they are made of hardened metal and have a Delrin core to block the lens. Another use for the large flange is to allow a vacuum autoloader to operate with large scleral lens blanks. In this case, the blocking tools can be a single piece of Delrin.
Pros: 1. Excellent precision in terms of run-out induced prism.
2. The dead length system can be implemented to control CT precisely.
3. Precision is better than internal collet dead length systems.
4. Internal tapered blocking tools may work OK, this needs verification.
Cons: 1. High cost associated with manufacturing; they cannot be centerless ground, so the precision needs to be generated on a production lathe, possibly using roughing and finishing tool bits.
2. A new lathe collet may be required to implement this system.
Legacy blocking tools: Many of you are familiar with the George Nissel Company in the UK several decades ago. They used a modified "0" Mores taper to hold the blocking tool on their polishers. Their lathes used a clever collet/post system that could hold both a lens blank or a brass blocking tool. The lens blank sat in the collet against a post. The blocking tool slipped over the post and was held in the collet for a full 1/2" (12/7 mm). Thus, the lathe was "universal" in that it could be used for BC or FC without changing the collet. G. Nissel Co. also sold lathes with tapered spindles, but for small labs, the universal lathe was the choice. I have been asked why this system isn't an option today. The biggest reason is that less than 50% of lens designs have a spherical surface which can be polished with pitch. I'm sure that many of you remember re-setting the radius gage to what the radius read after polishing (you couldn't read a radius from the lathe turned surface). Another factor was that the lens diameter and peripheral curves were cut in at finishing.
The finisher had control of edge thickness, profile, and prism.Here is why things are different now:On the Nissel lathe, the operator would use the following procedure:1. The lathe operator tightened the collet drawbar to feel how tight the collet clamped on the lens blank. This sounds conscientious on the part of the operator, but in fact it didn't mater. The fact is that the pitch polishing process ground out all of the distortion that the collet could impart to the BC.2. When the operator inserted the brass blocking tool into the collet, the drawbar was again tightened with more force to secure the blocking tool. Again, pitch polishing was the determining factor in the actual radius.Utilizing the same methods on a modern CNC air bearing lathe will have this outcome:1. Since there is no operator control of the collet drawbar, there is no operator control of the collet force. This can yield to distortions in the lens surface. It is important to note that with soft tool (bladder) polishing, these distortions are not removed.2. To hold a blocking tool securely, the collet force needs to be increased over that used on the lens blank. This means loosening the collet lock screw and screwing in the collet.
Here is where the legacy comes in: around the late '80's, the Coburn Optical Company acquired the assets of the Geo Nissel Co. Nissel has been producing a CNC Air bearing lathe for a few years and Coburn took the concept and marketed the model 8001. The machine has good acceptance and continued to use the universal collet arrangement. At this time, pitch polishing was still a common practice. So, it was process of pitch polishing that made a good lens. A few labs stayed with the universal collet system, but with more and more non-spherical lens designs in the mix, labs moved to soft tool polishing. A fix for the need to set the collet force for lens blanks and blocking tools prompted a few labs to have oversize shank blocking tools made. Their idea being that the collet would hold the blocking tool tighter than the lens blank. This was true, but caused other problems such as damage to the collet and prism. Also, since the shanks of the blocking tools were soft metal (brass or stainless steel), the metal surface was easily damaged during cleaning which contributed to prism.Here is a summary of the three types of blocking tools that do work for a small lab with only one lathe:The 1/4"dowel pin and the metal flanged blocking tools both work well because of the hardened metal shanks. Although the 1/4"dowel pin type may require repeated adjustment of the collet force when switching from BC to FC turning. The flanged and straight Delrin blocking tools are used in transfer blocking which means the collet force can be high to hold the tools securely. Blocking the lens blank ensures no distortion of the lens material.