Milling a hardened micro mold
01/01/2007
By Lee Richmond, Makino
In an endless pursuit of the capabilities of micromachining, we often run across test cuts that are of interest. Recently, a medical staple mold demonstrated what is possible with current technologies.
A customer challenged a Makino user with a medical staple mold that would require cutting tools down to 0.20mm radius. The user was very capable and experienced in mold building, and was comfortable at 0.30mm, but anything smaller was uncharted territory.
This mold is a good example of a lot of current micromachining work. Innovative products need innovative molds that often have difficult feature details, which can be nearly impossible without the proper machinery and techniques. This is especially true when it comes to the medical market, where cost of production is typically secondary to protecting intellectual property, following stringent government guidelines, and making a superior product.
The mold steel is 420 stainless, hardened to 52 - 54 HRc (Rockwell hardness C scale). It has eight cavities and inside corner radii had to be held to 0.20mm. The part is so small that hand-polishing to finish any details or to clean up a rough finish is impossible, so the mold must be accurate and polish-free straight off the machine.
The original process was to CNC (computer numerical control) mill an electrode, EDM (electrical discharge machine) the cavity detail, finish mill and grind the rest of the details. This took a total of 67 hours. It’s an efficient method when direct milling is not possible, but there’s a lot of time tied up in prep work, and not actual production of the mold.
The new process was to hard mill the mold on a Makino V22 vertical machining center, skipping the EDM process. This alone saved the user a substantial amount of time. However, to challenge the production capabilities even further, the customer requested a unique test cut be preformed, cutting the corner radii in half to 0.10mm.
 Micromachining bears great resemblance to its macroscale counterpart. Photo courtesy of Makino |
The part required five tools with nine different programs to keep the cuts in different areas of the mold to match specifications. Tools from 2mm down to 0.10mm radius were used. Tool-to-tool blending is critical, so we used Makino’s hybrid Automatic Tool Length Measurement (ATLM) system, installed on the V22. This system ensures that tool growth and wear is controlled, since the margin of error is so minute and surface finish requirements are so critical.
The final result allowed us to complete the core in 21 hours and the cavity in 13 hours. The majority of the 21 hours on the cavity was to pick out the tiny corners. The total time to produce the mold was reduced from 67 to 34 hours, or approximately 50 percent.
After the success of the new hard milling process, we decided that it might be interesting to push the boundaries of this process even further, just to see what was possible.
We decided to tweak the part design a bit - shrinking the overall size of the part and tightening up the corner radiuses, now down to 0.076mm. To do this, we had to use a 0.05mm radius tool, as no standard tool in between 0.10mm and 0.05mm radius is available.
The smaller cuts increased the cut time from 13 hours to 32 hours, a 244 percent cycle time increase in the cavity. The core proved too long to continue the cut. Step-over in the core got down to 2.5 microns and step-down to 4 microns. Tool failures started to pile up.
In conclusion, we were able to get great results cutting corner radii to 0.20mm, then even better when we went down to a 0.10mm. We hit a wall when we attempted a 0.076mm corner radius with a 0.05mm radius cutter. Cycle times spiked dramatically and the tooling wore out quickly.
Without a tool between 0.10mm and 0.05mm radius, a 0.076mm corner radius is impractical in hardened 420 stainless steel.
Why does this matter? Because we learn what’s possible from our failures as much as we learn from our successes. Since we were able to hold 0.10mm corner radii while improving cycle times in a hardened steel mold, it shows that we’re going down the right path. All the ingredients aren’t there to go even smaller, but we’re getting very close.
The test cuts will continue, and we are confident we’ll get a 0.076mm corner radius, and even tighter, with some more tweaking. In the meantime, we need to examine our processes, tooling, and machine to make sure we’re doing everything we can to increase accuracies even further.
To get tolerances like these, there are a few essential components that must be considered. Typical machining applications allow for several microns or more of variation. When you’re dealing with micro-molds, a single micron of inaccuracy will often ruin your work piece.
First, the machine you choose must be stable, both thermally and for vibration. Any growth in the tool that isn’t addressed properly can devastate your accuracy, just as chatter can when milling hardened steel with a tiny tool.
The way Makino accomplishes thermal stability is with a core-cooled spindle and insulation of the work envelope. These advancements permit us to accurately control and predict how much the tool will grow, which allows the machine’s software to compensate, and therefore maintain our accuracies. Vibration is kept to an absolute minimum with heavy castings, a spindle designed to be utterly rigid, and software which compensates for the properties of metal being cut at high speeds.
Another aspect of the machine tool that must be considered is the servo control technology. Makino has implemented high resolution scale feedback that is able to recognize and achieve sub-micron movements, and therefore hold the tolerances of micro applications.
The second thing you’ll need to accomplish cuts this accurate and small is a programmer with a creative streak. Most programmers aren’t accustomed to working with step-overs and step-downs of only a few microns or less, or with tooling that will break if it rubs too long or hits a corner incorrectly, so it’s important to consult someone who’s done it before to program the cuts correctly.
To program a mold in hardened steel is hard enough, but when you factor in the accuracies needed and the unique tooling, it’s simply not something that just anybody can do. And that programmer needs the right tools - programming software that gives him the ability to run the tricks, like trichoidal roughing and arc fitting corners, needed to successfully mill hardened micro molds.
Finally, you need the right tooling. Everyone has their vendor preference, but without tools with predictable life and performance, that cut as they’re designed to, you’ll never hold the tolerances needed for micro work.
Most of the tooling needed in submicron work is so small you literally can’t touch it without breaking it - most machinists aren’t used to using it, much less buying it. And keep in mind that tooling is usually not an area where cutting corners saves you money. You’ll just end up paying on the back end with re-work, delayed deliveries, or even worse, outsourcing the part when you can’t produce it.
Nearly every micromachining process is a trial-and-error situation, especially when it comes to sub-micron cuts. This process can be shortened greatly with the aid of engineers experienced in micromachining, along with some fundamental ingredients like a capable machine tool, tooling, and programming tailored specifically for micromachining.
Lee Richmond is micro market manager at Makino (www.makino.com) in Auburn Heights, Mich. He can be reached at [email protected].