What's it about?

In the manufacturing business, there is one product that matters, money. Whether making shoelaces or aircraft carriers a business that doesn’t also make a profit doesn’t stay around long. Being able to predict operational expenses is critical to determining a product’s sale price. Priced too high a product won’t sell, too low profit goes away. In the business of precision optics manufacturing, predictability has been often impossible or had large error bars. Manufacturing unpredictability made setting price a challenge.

What if predictability could improve by changing the polishing process? Would a predictable, deterministic process lead to profit? Optimax Systems has experienced exactly that. Incorporating Magnetorheological Finishing (MRF) into its finishing process, Optimax saw parts categorized financially as “high risk” become a routine product of higher quality, delivered on time and within budget.

Using actual production figures, this presentation will show how much incorporating MRF reduced costs, improved output and increased quality all at the same time.

1. INTRODUCTION

Traditional optical manufacturing methods are often more art than science. Preston and Twyman quantified some things, but a process that worked yesterday may not work today for some not readily identifiable reason. If control of a process is lost form recovery proves time consuming at best, fatal to the lens at worst. Either way, labor time that could be spent on other things is lost.

This hinders output and growth, neither good things. Form has to be closely controlled, placing demands on labor. This produced an expensive labor base that required time to develop at the company’s risk. In some cases if an employee left so did the shop’s capability. Optimax needed a way to yield parts to the same quality level, the same way every time.

2. OPTIMAX SYSTEMS

Optimax Systems started in 1991 as a precision optics manufacturing shop. Over time Optimax expanded in both capability and capacity, and Optimax now manufacture custom spherical, aspherical, cylindrical and plano lenses. Optimax’s market niche is speed, often delivering custom fabricated, coated components with one week lead times. Given Optimax’s focus it’s vital the parts be processed predictably. For promises to be made there must be a reasonably high expectation it can be kept.

The traditional process shortcomings detailed earlier existed at Optimax. In no area did this manifest itself more than in Asphere manufacturing. The time and money commitment needed to build talent along with large error bars relative to spherical manufacture in each processing step often clouded predictability. Optimax decided in order to honor its commitment to speed a more deterministic process was needed.

3. MAGNETORHEOLOGICAL FINISHING

Optimax contacted several companies that offered deterministic aspheric polishing solutions. Eventually, Optimax chose QED Technologies and their Magnetorheological Finishing (MRF) process. MRF offered the possibility of producing surfaces corrected to a diffraction limit, and the technology had matured in the preceding years. Additionally, QED and Optimax were located near each other, offering the possibility of a high level of interaction and easier service and parts if needed.

MRF works through viscosity manipulation to turn on and off polishing action. Magnetorheological fluid increases viscosity in the presence of a magnetic field. The fluid “stiffens” and produces a polishing action over the area in the magnetic field. Aim the magnetic field and target the removal.

Determining removal rate is simple. Immerse a surface with a known starting form into the fluid to a known depth for a known time period. Measure the part afterward and subtract it from the original. There will be a spot in which material was removed. The volume removed from the spot is calculated. Since the time to remove that volume is known so is volumetric removal rate. The resulting “spot” is like a conformal small aperture lap with a known, constant removal rate.

Match the removal rate of the “spot” polishing tool with a map of the form error of the optic. Knowing the volume of material to be removed and the removal rate a toolpath can be determined. Make the map transmitted wavefront error, the same process works for TWE correction. This process offers the predictability Optimax desired.

4. A PART WITH SOME HISTORY

Optimax wanted to know if the decision was justified. By examining labor data for a given part process both before and after the introduction of MRF at Optimax the effect could be seen. Optimax looked at one specific case, the details of which are listed below. The run data collected is summarized after, with an explanation following that.

• Fused Silica Asphere
• Pure Parabola (k = -1, no higher orders)
• Diameter: 12.7mm
• Departure: 0.039mm
• Edge Thickness: 0.3mm

For the first two orders, Optimax did not have MRF involved in the process. The process results show it. Nonetheless, Optimax still made money, but as seen in the yield predictability suffered.

MRF was used during the third order, and the data showed a stark improvement in yield, timer per part and profitability. There were two other unplanned but fortuitous outcomes seen during this third order. The MRF process inherently yielded a part with lower form error, at no additional cost in time or money. The customer was willing to pay more for parts with smaller form error, something that did not cost Optimax anything to produce. The other outcome is Optimax did not need to run any parts to satisfy the fourth order. Optimax had started a quantity in line with the pre-MRF yield percentages, so there was a large number of good but unsold parts left after the third order. The prior run had yielded enough overage, an unexpected outcome in light of past yields.

The fifth and sixth order shows the effects of learning and yield improvement. The process got more profitable and more predictable. All of this could be done by someone with basic metrology and data entry skills, not a master optician.

5. SUMMARY

Optimax Systems is an optics manufacturer with a commitment to accuracy and speed. In an effort to reduce variation and improve predictability, Optimax integrated Magnetorheological Finishing into its aspheric lens manufacturing process. Comparing pre- and post- MRF processing results, Optimax found MRF could save the formerly unsavable, and what was formerly a challenge soon wasn’t. This can be repeatably done with a less expensive, lower skilled labor force. Yields rose, time per part shrank and profits piled with MRF in the process.  Optimax is doing this today.

Proc. of SPIE Vol. TD04 TD041A-3
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