With graphite, the magic words are high speed. High speed machining centers with 30,000- to 60,000-rpm spindles are capable of accelerated feeds that serve to reduce cycle times and improve surface finish, as well as edge quality.
The motors required to run these spindles are smaller and lighter and this results in lower cutting forces and reduced tool breakage. This is particularly critical because many electrodes are intricate and the production of them involves small, fragile micro tooling.
Spindle
The smaller the tools, the higher the spindle speed needed to efficiently machine quality parts and avoid tool breakage. High frequency spindles with speed ranges up to 60,000 rpm are ideal for milling, drilling and engraving graphite electrodes using micro tools. High speed machining technology uses high rpm rates, taking a smaller stepover, but with significantly increased feedrates.
Move your hand through the flame of a burning candle. If you move too slowly, there’s enough time for the flame to cause damage. But if you sweep your hand swiftly through the flame, there’s insufficient time for the fire to damage your skin. The same principle applies to high speed machining with micro tooling. Move fast, and there’s insufficient time for heat to feed back into the part and cause issues.
Using small micro tools for graphite or virtually any other material simply isn’t as easy as finding an adapter to hold a tiny tool in a 40-taper spindle on a conventional CNC machine. Because that spindle was designed for large tools like a three-inch fly cutter intended to hog out deep cuts in dense substrates. As such, it has so much torque and force that it just breaks small tools, which is both inefficient and very costly over the long haul.
The only option an operator has in this situation is to slow the rpm and feedrates down to a crawl, and this isn’t efficient either because it results in unacceptable cycle times. A vivid, and perhaps comical, analogy is the Hemi-powered pick up truck versus the sportscar. The reality is that you wouldn’t compare the two or even consider racing them against on another. Why? Because the truck was designed with the power and force to haul or tow enormous mass, while the sports car was designed for speed and maneuverability.
In essence, conventional CNC manufacturers who tout the ability to run micro tools are like an auto manufacturer putting a spoiler and racing stripes on a clunky SUV and claiming that it now possesses the same qualities as a Porsche. Well, just like you can’t put a spoiler and racing stripes on an SUV and expect it to perform like a sports car, you can’t retrofit a high speed spindle onto a clunky conventional machine and expect it to efficiently accomplish high speed machining with micro tooling.
Geometry
Just like conventional CNCs can’t be retrofitted to efficiently run microtools, conventional tooling geometry (for larger tools) can’t just be scaled down for micro tooling. The geometry of the tooling used to mill electrodes plays a key role in both quality and efficiency. Scaling down the tool geometry of larger diameter tools to a smaller format yields unacceptable feedrates and unsatisfactory finishes.
Tooling requirements change when tool diameter is decreased and spindle speed is increased. Conventional tooling using inserts is not appropriate for micro tooling applications. This is primarily due to the high rpm rates rather than the tool diameter.
Tool Wear
Increased rpm rates require properly balanced tools with significantly increased chip room to assure proper chip removal. While geometry can assist in both quality and efficiency when milling graphite, tool wear is still a concern due to the inherently abrasive nature of this material. Specialty tool coatings can be a weapon in battling excessive wear.
Some coatings to consider are diamond coating, titanium aluminum nitride coating (TiAIN), which is often used for applications in steel and, finally, aluminum chromium nitride coating (AlCrN)—generally used for applications in very hard aluminum (such as cast aluminum or sand cast aluminum), but which has shown effectiveness with other abrasive substrates like fiberglass.
The production of molds and dies in hardened steel is one of the most difficult machining tasks, mainly in cases where good dimensional precision and superficial quality are required. Normally, electrodischarge machining (EDM) and milling processes are used for mold and die manufacturing. Thus, a better understanding and analysis of the graphite electrode manufacturing process is of absolute importance for tool makers, where the machining of deep cavities and complex forms is commonly required. In this study, graphite machinability tests were carried out, followed by an analysis of tool wear and surface quality generated in down- and up-milling. Conclusions are drawn regarding the best machining strategies and parameters to use in the machining of this material. The technological information gained from this study will be of great interest to small and large tool making industries and will enable them, through a better understanding of the graphite cutting process, a greater productivity in electrode manufacturing.
Milling of 2-D and 3-D graphite electrodes has grown tremendously over the last several years. Standard tooling for our mills was exclusively carbide. High speed and cobalt tools were used only when a diameter, length, or special cutter shape make the cost of carbide prohibitive. To increase capabilities for tolerance and challenging inspection requirements, Tri-Gemini has invested in a number of Mikron High-Speed Mills. High-speed milling centers should be equipped with diamond tooling to meet the need for tooling tenacious enough handle that increased speed.
When milling graphite conventionally (non high-speed), most people tend to set both the spindle RPM and the feed rate too low. Get those RPM’s up there – you’ll see the difference! If you want to be a bit cautious, start by increasing the RPM’s by 10%, but remember to increase the feed rate as well. If the chip load per tooth is not sufficient, the graphite will act almost as a burnishing agent on the tool. Move the tool through the work, but not at a rate that produces excessive tool pressure. Chip loads of approximately .005”/tooth/rev. for roughing and .002”/tooth/rev. for finishing are effective starting points for milling applications.
Another area of consideration when milling is the chipping of the graphite on the exit side of the cut. Although reducing the feed rate at the end of the cut and making sure that the cutter is sharp helps to reduce chipping, try “climb milling”! “Climbing” significantly decreases the chip out. I’d recommend climb milling only when using CNC equipment or those machine tools that are equipped with “ball screws”. Climb milling can be accomplished safely and effectively because “ball screws” have no backlash as in manual machines equipped with lead screws.
Chip and dust removal need to be addressed! Be sure to keep the cut zone clean and clear of graphite chips and dust. Remove the dust/chips by using either vacuum or compressed air – or both. We have found that by keeping the cut zone clear of chips/dust we are able to take deeper cuts and still maintain an excellent finish. Keep the cut zone clear of lubricating and coolant fluids as well. We recommend machining dry at all times. Chips/dust that are mixed with fluid tend to collect and stick in the flutes of the cutter causing poor finish – not to mention the fact that a slurry of abrasive graphite can cause premature wear of all moving parts of your equipment. Given the fact that graphite is dirty to start with, who really needs to spend their time mucking out buckets of sticky black mud? Farr’s case study on their installation of dust removal equipment is an excellent read on the subject and is available here.
The advent of High Performance Milling has raised the bar in the milling arena. The high RPM of the spindle results in reduced tool pressure in the cut. The reduced tool pressure allows for the milling of very thin ribs and the ability to use micro end mills (less than .020” diameter) without tool breakage. Improved machine response, upgraded software with improved capabilities that produce better surface finishes, diamond tooling with ten to fifteen times the life of carbide, advanced work holding systems, and unattended operation, have all joined to move the milling area into a more prominent position within electrode manufacturing. Pallet changing devices on the mill table help to keep the mill in productive cut time, rather than fixture load time. Multiple set-up areas are used to run different jobs simultaneously with a little program linking. It’s all about productivity and cost effectiveness.
When milling graphite conventionally (non high-speed), most people tend to set both the spindle RPM and the feed rate too low. Get those RPM’s up there – you’ll see the difference! If you want to be a bit cautious, start by increasing the RPM’s by 10%, but remember to increase the feed rate as well. If the chip load per tooth is not sufficient, the graphite will act almost as a burnishing agent on the tool. Move the tool through the work, but not at a rate that produces excessive tool pressure. Chip loads of approximately .005”/tooth/rev. for roughing and .002”/tooth/rev. for finishing are effective starting points for milling applications.
Another area of consideration when milling is the chipping of the graphite on the exit side of the cut. Although reducing the feed rate at the end of the cut and making sure that the cutter is sharp helps to reduce chipping, try “climb milling”! “Climbing” significantly decreases the chip out. I’d recommend climb milling only when using CNC equipment or those machine tools that are equipped with “ball screws”. Climb milling can be accomplished safely and effectively because “ball screws” have no backlash as in manual machines equipped with lead screws.
Graphite has specific properties that make machining a challenge. It’s strong, but also brittle, and is susceptible to chipping if not handled properly. Graphite also is abrasive, making unusually high tool wear a common occurrence.
In reality, graphite is much easier to machine in terms of spindle load when compared to machining steels, EDM-grade graphite is much softer than steel, allowing for higher machining feed rates, but the material turns into a powder or dust when machined.
If you don’t know how to mill graphite, the rate of rejection is high.
I really appreciate your sharing, and milling graphite electrode really requires technology.
You can write this article to show that you are an expert in the mold industry.
It is true that graphite electrode processing requires technology and experience, otherwise the rejection rate will be very high. thanks for sharing
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