The required precision in radial run-out, but more importantly in tilt, necessary to manufacture scleral lenses demands that the work holding system to be finely tuned and maintained. For those labs that are looking to improve their precision and productivity, it is common to look to improvements in the work holding process.
This article looks at the limitations of current tooling styles and styles that you can expect to become more common in the future.
It has been about 15 years since Delrin blocking tools have been in general use in the contact lens industry. Delrin blocking tools offer many advantages, but come with some concerns that lab managers need to be aware of. The advantages of Delrin tools include: low cost, they will not damage the lathe diamond, they are forgiving to poor condition collets, and they don't require pre-heating. One concern is plastics absorb water. The water absorbsion rate for Delrin is from .12%-.25%. Depending on your process, you may find that your blocking tools are larger than 12.7 mm. We can re-grind these blocking tools to bring them back to the 12.7 mm diameter. Another concern is that Delrin is roughly 100 times less stiff than steel. If you are using solid Delrin tools, this difference in stiffness is not a factor. In fact, it is a benefit where imperfect collets are in use since the collet defects will sink into the Delrin. If the Delrin tools are made with internal tapers to fit front polishing machines, these blocking tools are much less stable than a metal tool of the same design. Delrin tools with internal tapers are much softer in the collet and may move during the cutting process resulting in center thickness (CT) errors.
The precision flange type blocking tool is the answer for many labs. Typically, lathe collets control both the radial and tilt alignment of the blocking tool. The flanged blocking tool can rely on the face of the spindle for the tilt alignment requiring the collet to control radial alignment only.
Using the spindle nose to register the angle of the tool greatly increases the accuracy of parallel alignment of the axes of the spindle and tool. Flanged blocking tools can be implemented in two ways:
1. A large flanged tool (25 mm diameter) that rests directly against the spindle face.
2. A smaller flange (19 mm diameter) that rests against a precision cover over the spindle nose. In option 1, the collet face must be recessed behind the spindle face to ensure that the flange of the blocking tool contacts the face of the spindle.
In option 2, the spindle nose cover must be made of a stiff material (steel, bronze, aluminum) and precisely machined. Using a non-ferrous metal allows you to face off the cover in place with the lathe diamond, ensuring a precise location surface. This process ensures that surface of the cover is perpendicular to the spindle. Some advantages of the flanged blocking tools are:
1. Precision location that can be used for dead length processes.
2. Flanges provide much better chucking stability resulting in better surface finish and CT control.
3. The flange controls tilt better than the collet alone which is a big help on large diameter lenses.
4. The flange can double as a surface for a lathe auto loader to grasp. Some labs are adopting precision flanged blocking tools.
So, what do these blocking tools look like? What features are required that are different from a 12.7 mm Delrin blocking tool? These tools are hardened or coated metal with a plastic core. [picture-490]. It may be OK for these tools to be made out of solid Delrin. This could be OK for applications where the lathe probes every lens blank, provided that the collet pulls evenly and the flange flexes evenly. But for increased precision, metal blocking tools are better. Metal choices include anodized aluminum, tool steel, and hardened stainless steel. However, these hardened materials are not friendly to diamond tools. To solve this problem, we make a hybrid tool with a Delrin core. This makes a durable, stiff, base with a wax and diamond friendly blocking surface. Another benefit of this arrangement is that the core can be replaced. We have recently found a supply of colored Delrin. Lab personnel like this because the colors can be used to indicate the radius to more closely match the base curve (BC) profile. It has been shown that minimizing the blocking wax thickness provides better support of the lens. This is achieved by turning a similar curve as the BC on the blocking tool.[picture 2].
Recently, I was in a lab that had the Gfeller lathess, chuck, and blocking system for both spherical and fly-cut toric lenses. It had been adapted to a 4 axis CNC fly cutter system using 12.7 mm collets in a 20 year-old CNC lathe. They were upgrading to fast-tool servo CNC lathes, but wanted to be able to use the Gfeller system as a back-up. This involved making threaded adaptors with 12.7 mm shanks to fit into our Auto-blocker. The Gfeller adaptor with the 12.7 mm diameter shank has a 30 mm diameter flange that is drawn up tight to the lathe spindle nose when fitted into a collet that is recessed into the spindle. During validation of the new adaptors and the calibration procedure, I checked the runout of the lathe collet. It was .010-.015 mm total indicated run-out (TIR) at various distances from the spindle nose. This is an excessive amount, but not bad for a 20 year-old collet.
The new adaptors were made with an 18 mm diameter flange; the same diameter as the Gfeller chuck. Inserting the new adaptors in the lathe collet resulted in similar run-outs of .010 mm TIR, which was expected. I decided to insert the original Gfeller adaptors with 30 mm flange and measure the run-out. The dominate locating surface was the flange, rather than the shank. The run-out on the locating surfaces (diameter and face) was up to .006 mm TIR. I tried all of the Gfeller adaptors for both toric and spherical production and the worse run-out was .006 mm TIR. This data proves the value of the metal flanged arbor/chuck.
Over the last few years we have supplied blocking machines into legacy processes and worked with labs to improve their precision. On one project, the lathe collet and blocking tools were based on a proprietary approach. We found that in adapting modern lathe collets to work with the legacy tooling, there was tilt in the system that was very difficult to correct. This error was discovered during the blocker alignment procedure. We knew that the blocker was aligned at our factory, but we couldn't get repeating results in the lab.
We proposed a blocking tool design with a large flange that rested against the spindle face to solve the problem. The design used the same special collet diameter of the legacy tools, so they didn't need to replace the lathe collets. The lab manager agreed to a pilot set of flanged blocking tools of aluminum with Delrin cores. These were configured for both lens blank blocking and front curve (FC) blocking. In this case, we made the flanges 30 mm in diameter to ensure good contact with the face of the spindle. The initial tests were good and provided a good basis to calibrate the system. The production blocking tools are anodized aluminum rings with different cores for both lens blanks and FC's. Along with solving many production and quality problems in their corneal and soft lens (SL) line, these blocking tools fit right into their scleral production. The improvement in the SL surface quality was such that all of the SL's could be produced polish free.
Another project involved a proprietary chucking system that was showing some limitations in wear and replacement cost. We were asked to look into a blocking system using hardened tools with replaceable cores. Among the choices was an off-the-shelf hardened and ground steel component with a 12.7 mm diameter shank and a 19 mm diameter flange. This part had a through hole that could be used for a core. The lathe chucking system, blocking machines, and blocking tools were converted to 12.7 mm diameter collets. Since the flanges on these blocking tools were 19 mm, collet covers were installed over the spindle noses. These covers provided a surface for the flange to be drawn against as the closed.
Again, these collet covers are made of metal for its stiffness. The conversion to flanged 12.7 mm X 19 mm diameter mm flanged blocking tools was a success in terms of the cost, availability, precision, and production.
When solid Delrin blocking tools became accepted in the CL industry, I thought that the industry may conform to a standardized set of tooling. That has happened to a certain degree. About 50% of our customers use this type of blocking tool. But, with legacy processes and the growth of scleral lens production, a better type of blocking tool may be needed. The hard coated flanged hybrid blocking tool with a Delrin core fulfills the precision, production, and cost requirements of these specialty lenses.