When purchasing an optical filter, most customers are primarily focused on the optical characteristics of the filter, and rightly so. But one thing that is sometimes overlooked is the manner in which that filter should be cut from the larger substrate after the coating process is complete. Customers may be able to save money by knowing what fabrication options are available to them and tailoring their quote requests accordingly.
Coring is the process of using a thin-walled diamond hole saw to extract a core, or a circular piece of glass, from a larger substrate. Coring is a very efficient method for producing round parts. The thin walls of the tool mean very little material is wasted, and engineers use circle packing equations to maximize how many cores can be extracted from a single substrate. Engineers will specify the physical properties of each tool based on the specific material to be cut, and customize each program to achieve the fastest cycle times and highest quality cuts. Costs are kept low for coring operations because operators only need to perform one setup per substrate, and depending on the final size, can generate many parts out of one plate with little material waste. The setup for these plates is relatively quick and easy when compared to cylindrical grinding or milling.
Diamond saw blades can be mounted in a CNC machine horizontally, in a standard collet with a special arbor adapter, or vertically, with the use of a right-angle head. They can be used in concert with a rotary table to produce numerous parts comprised of straight edges, or for chopping operations. The most common applications for sawing are creating square and rectangular filters, roughing a substrate into a workable shape and size, and chopping small sections off of a substrate, like cropping the corners from a square. As with coring, both the saw blade and the cutting program are customized to provide the fastest cycle times with the highest quality cuts. Also similar to coring, CNC sawing can produce multiple filters from a single setup, with low waste, and the setups are relatively simple compared to milling or dicing. These factors make sawing the most cost-effective method for producing square and rectangular filters.
Milling is used to machine filters into almost any custom shape beyond the standard circles, squares and rectangles. Using special diamond endmills at a high RPM, programs are designed that trace the profiles of the desired parts while plunging incrementally deeper into the material over a number of passes. The diameter of the tool is chosen based on what is more important: wasting less material, or speeding up the time of cut. Smaller diameter tools can be used to reduce the kerf of cut, but the speed at which they can be run, measured in surface feet per minute (SFM), is directly proportional to the diameter of the endmill. All of this means that larger tools will waste more material, but can be run at higher feed rates without sacrificing tool wear or quality of cut. Milling can be a more costly method of producing filters, especially if the quantity of parts being purchased is high. Cycle times are significantly longer than sawing and coring operations, and more material is wasted, which reduces yield. If at all possible, specifying a circular, rectangular, or square filter rather than an odd shape will help to keep costs down.
Chamfering & Beveling
Chamfers and bevels are applied to sharp edges of cut glass for a variety of reasons; sharp corners are structurally weak and prone to chipping, so a protective chamfer is often applied to dull that edge. They may also be applied to protect the people handling the filters from cutting themselves, to eliminate edge chips resulting from the machining process, or to fit the filter into a custom holder. For cored or milled parts, chamfers and bevels can be applied to the top edge of a part with special diamond tools manufactured at the specified angle. While a CNC chamfer at the end of a coring or milling cycle is very cost effective, the downside is that, if both sides require a chamfer, the backside has to be done by hand at a chamfer bowl. Hand chamfering is a labor-intensive operation that requires a fair amount of skill and experience, and tight tolerances on face width and angle can only be met on a best effort basis. There are also specialty machines devoted entirely to chamfering; auto-centering machines use micrometers to grind circular filters to an exact diameter, and then apply a precision chamfer to one or both edges. These machines are a very cost-effective way to apply chamfers, but come with their limitations as well. The effective final diameter range for these machines is between 7 and 80 millimeters, so they are not viable for final sizes that are very small or very large. Parts have to be cored oversize to ensure centricity, which means there is more waste, resulting in a lower yield per plate. And the centering mechanism is not accurate enough to allow laminated filters to be run, so only certain products can be run on auto-centering machines.
Dicing saws can be used to cut square and rectangular filters out of plates up to 12” in diameter. The dicing process uses an ultrathin blade composed of diamond particles suspended in a resin matrix. The blade kerf typically ranges from 0.010” to 0.015” depending on the thickness of the filter, and the size of the diamond particles in the blade varies with the chosen substrate. Dicing is typically chosen over traditional CNC sawing for square and rectangular filters that require a clean, sharp edge, but it offers other benefits as well, including faster cycle times, thinner blades that waste less material, and greater positional precision. The tradeoff is that dicing is much more effective at cutting softer, thinner materials, such as silicon and germanium wafers. Thicker, harder, and more abrasive materials can be diced, but not at the same speed or edge quality. Dicing is constrained by material thickness: up to 3 millimeters for softer materials like silicon and germanium, and up to 2 millimeters for harder materials like soda lime glass and quartz. Due to the more complicated setup and higher quality, dicing is more costly than CNC sawing, but is worth it if your application requires a truly clean, chip-free edge.
Cylindrical grinding is the process of using a cylindrical grinder to turn a substrate down to a certain diameter. Substrates are clamped between chucks, and can be stacked in may cases so that an operator can grind more than one filter at a time. Cylindrical grinding can be ideal for removing very small amounts of material; for instance, edging a filter down from 1 inch to 25 millimeters in diameter. The operator is able to very precisely center a filter within the chucks by using a dial indicator. The obvious downsides of this grinding method are that it is slow and labor intensive, both of which drive up costs. It is only used when absolutely necessary, most often as an intermediate step when constructing a laminated filter.
Andover Corporation has many years of experience with all of the methods and technologies discussed above. Our knowledgeable quote team will work with you to determine the fabrication method or methods that best meet your needs while keeping costs as low as possible, and our seasoned engineering team and optical fabrication technicians have all the skills necessary to apply even the most stringent set of requirements here.