The job is a prototype cellular phone. The cavity side is burned to a specified RMAX finish and requires no polishing, but the core side is not so lucky. It is a single block with a molding surface of about 1.5" wide by 4" long - all with more tiny ribs, bosses of different sizes and depths, stock savers and interconnecting intricate pads of different depths than can be counted. Normally a job with this much detail would quote out at about 20 hours of polishing time. Unfortunately, it's already 1:30 in the afternoon and the toolmaker has just informed the polisher that he has to stay late to assemble/water test the tool - all before an 8:00 a.m. sampling the next day. Fortunately, this company provided the polishing veteran with an ultrasonic profiler. While not applicable to all jobs, the ultrasound is perfect for this application and is the shop's only real chance at making an otherwise impossible delivery.
The first thing the polisher has to do is decide what type of cutting tool he is going to use. The options primarily boil down to three choices: a steel or brass thin blade, a super hard 320 grit EDM specialty type stone or a 320 grit RA type stone. All three options will have to be dipped in natural diamond compound and lubricated with mineral spirits. The veteran has to decide which avenue to take. With the right choice the job has a slim chance of making the deadline and allowing the toolmaker to do all of his work afterwards. however, the wrong choice means certain disaster.
Carefully inspecting the roughness of the EDM and the depth of the pit marks with a high intensity light and an eye loop, the polisher considers his choices. A steel or brass lap - while stronger than a stone - has three drawbacks that make it a poor choice for this particular job. First, metal laps this thin tend to destroy the bottoms of the ribs when used in ultrasonic profilers. It's not uncommon for the unwary to dig .020" or more grooves in the corners of the ribs and sidewalls in just the same time that it takes to clean the sidewalls of the rib. Most polishers cannot tell that this damage is happening by the way the handpiece feels because it always feels like you're just barely touching the bottom - even when you're accidentally .020" deeper than the print.
The second drawback is that the steel and brass laps also tend to destroy the brittle diamond particles very quickly, resulting in increased usage. Third, along with increased usage in diamond, there is a 20 percent increase in time as the effectiveness of the cutting drops.
As the RA stone is simply too fine a bonding agent for the coarseness of this job's EDM finish, the choice becomes obvious - a 320 grit super hard EDM specialty stone dipped in either 15 or 45 micron natural diamond - as the area demands - and lubricated with mineral spirits.
The ribs average .035" wide and between .250" to .500" deep. After sharpening a stone into the proper balance and thinness for the ultrasound, the polisher then dips the stone into a tiny bit of 15 micron natural diamond and begins to cut. The stone is clamped into the holder at a 90 degree angle to the handpiece, which allows for the maximum cutting power.
The first thing that the polisher looks for as he starts to cut is how much sideways pressure he can put on the stone without breaking it, how fast the combination is cutting the EDM and how loud the squealing from the concentrated cutting is.
Combined, these three things tell the veteran instinctively how much time the job is going to take and if he has a chance to make the deadline. Once the cutting starts, the polisher can begin to set a pace - cutting, reloading the diamond, cutting again until it's used up, sharpening the stone, reloading and cutting some more - all the while guided by the feel of the pressure against the walls of the ribs and the different intensities of squeal coming from the rib. Once an area is roughed out, the polisher then goes over it again - this time with no diamond compound on the stone. He then massages the walls with a lighter hand pressure that he has learned from experience will produce the softest scratch possible - at the same time tilting the profiler so the new, lighter scratch marks cross out the deeper, rougher marks in order to save time on the next step.
Once everything is set up properly, the roughing step goes surprisingly quickly - all of the ribs, bosses and pads are stripped of EDM and replaced with the finest scratches possible in about five hours.
After all of the details and rib bottoms have been roughed out, the second step begins. The work of the ultrasonic profiler is finished and now conventional profilers are used. The polisher knows the depth of the scratch still in the sidewall, the hardness of the steel and exactly how to get from the existing scratch to where he wants the finish to be (ready for an easy diamond polish). This will only take the polisher one step - putting a 320 grit RA stone in a conventional profiler after dipping the stone again in the 15 micron diamond. He can then begin to draw up all the sidewalls.
Now everything is falling into place. The rougher, sideways scratches of the ultrasound are easily seen when looked at with the high-intensity light and eye loop. The presence of these scratches tells the polisher to keep RA stoning because they're not out yet. Once the polisher gets a feel for how long a section of wall takes to draw up cleanly, then he can begin draw stoning in earnest. Here is where the consistency of the previous scratches pays off in time saved - every area should take exactly as long to draw stone as every other area since same professional created the same consistency of scratch throughout the entire job.
The RA stone is a truly unique stone in that it possesses two completely separate properties at the same time. It is surprisingly resilient - allowing for good downwards pressure and faster cutting time without breaking - and yet, due to it's unique resin bonding agents, it consistently leaves a wonderfully fine satin finish when gone over with a lighter hand pressure and a slightly slower stroke speed. It took two hours to draw stone all the ribs. It is interesting to note that the combination of a 320 grit RA stone dipped in 15 micron diamond has left a wonderful 600 grit + draw finish which will actually speed up the third step - diamond draw.
The final step from EDM to diamond finish is to diamond the ribs. For this task boxwood is chosen. Boxwood is unique in that it can cut large amounts of steel very fast when pressed, and yet it will leave a nice gloss finish when used under relaxed pressure. Even more remarkable is the amount of punishment boxwood can take without having to be redressed. So for this job - where time is of the essence and punishment is expected - boxwood is the perfect choice.
To dress the wood down to the proper thinness, either nine or 15 micron diamond can be used. This time the polisher goes with the stronger 15 micron, mainly for the speed at which 15 will cut. Since this is strictly a functional and not an appearance part, the nod goes to the faster cutting of the two.
Draw polishing with wood and diamond consists of applying the proper downwards pressure for fast shining and tilting the direction of the profiler back and forth 15 degrees to either side of the draw, finishing with straight draw alone. This crossing technique guarantees a better, smoother and cleaner draw finish than a straight draw alone would, and it takes no additional time. The bottom of the rib gets diamond polished automatically with the sidewalls.
The time to complete the third step takes approximately one hour. This job is a success because each step went faster than the step before it. Each step prepared the steel for the next step because each step uses a finer cutting vehicle. Every veteran polisher knows that you cannot leave rough scratches for a finer vehicle and expect to make time on the job. By using an ultrasonic profiler and the proven three-step method, the time spent on this difficult project was eight hours start to finish.
There are two important concepts to be learned from this job. The first is that a 15 micron diamond particle will fit very nicely into the pores of a 320 grit stone, turning an ordinary stone into a faster-cutting unlimited diamond file. This is due to the fact that whenever the diamond wears out you can simply recharge it with fresh diamond - you now have a brand new diamond file which cuts 10 times faster than a stone, and practically for free.
How much diamond compound was used to stone this job? Practically none. The proper way to put diamond compound on a stone is to ask yourself how much can the cutting surface of the stone hold without simply pushing it all around and wasting it. In this case, the answer is practically nothing. So, for almost no cost, this veteran has reduced polishing time on an intricate job substantially. A job that should have taken 20 hours or more has taken only eight hours from start to finish. In addition, the diamond draw provided slickness and release and allowed the toolmaker to finish his work that night and the company to keep a demanding customer.
The second important concept - though unmentioned until now - is that a diamond disk can be made from diamond sheet material glued onto a steel disk or purchased finished from many polishing suppliers. Used in a flexible shaft, this sharpening disk is essential to keeping the polisher at his bench and able to sharpen stones without breaking the pace of the polishing, which is hard to restart once broken. One hundred grit diamond is perfect for sharpening.
An additional point to pass along is that had this been an appearance part instead of purely functional, then after the wood draw lapping was completed the polisher could have used either basswood or balsa wood in the ultrasonic profiler along with three micron diamond and "melted" one of the glossiest, fastest diamond finishes you've ever seen in about another 30 minutes or so.
If you have intricate parts, try these three steps to a diamond finish - and even add the glossy polishing with the two different kinds of wood mentioned - and you will certainly find that the timesavings over conventional polishing methods will be approximately 60 percent for you, too.