Stressed over thin ribs
When it comes to electrode fabrication one area that can create a lot of headaches and stress is the fabrication of thin, small ribs. Over the years I have observed many of my colleagues very frustrat...
When it comes to electrode fabrication one area that can create a lot of headaches and stress is the fabrication of thin, small ribs. Over the years I have observed many of my colleagues very frustrated and stressed over these types of electrodes. Many times I have witnessed them making their final pass on a 0.030-inch rib, only to have it snap off, resulting in rework and lost time. A few simple principles can make this process as straightforward as squaring up a block of material.
There are many pitfalls because of the properties of graphite. Graphite is both brittle and abrasive, so trying to machine small ribs without a supporting method, using improper cutting feeds and speeds, and vibration can create excessive pressures that cause the electrode to fracture. Because of the complex design of parts today, simply avoiding small ribs is impossible. In my professional career I have been faced with many challenging molds, tools, and parts that have required a vast variety of small ribs to be creatively designed and manufactured.
What are some practical methods that can be applied to this process? First, whenever possible incorporate some degree of draft on the rib. Draft, as little as one half of a degree, while only slight, will provide two important values. If it is a deep rib, it will prevent the cutter from rubbing, which could lead to breakage, and it will increase the base of the rib, providing increased support. I recommend using one degree or more of draft. I have witnessed many example of companies wasting a lot of time and money on cutting an electrode that continues to chip during the manufacturing process when a little draft would have easily solved their problem and wouldn’t have affected the part or the customer specification at all.
There may be some cases where draft cannot be used. However, most molds require draft in order to effectively eject the finished part.
Does it matter how you cut the rib? Yes. Along with the draft, the proper roughing and finishing techniques for graphite should be employed. The following recommendations are only guidelines to help provide a starting point. Roughing chip load for graphite is 0.003 in. is to 0.005 in. per tooth, and the finishing chip load is 0.001 in. to 0.003 in. Graphite can be machined at about 600 to 1000 surface feet per minute (sfm) for carbide, and at higher surface feet per minute for diamond.
SFM = RPM x TOOL DIA. x 3.142
RPM = SFM x 3.82
FEED RATE = RPM x CHIP LOAD x NUMBER OF TEETH
Of equal importance to the proper feed and speed for the cutter, is the machining process.
I recommend the following parameters as a guideline for those wrestling with this issue. Using only good grades of solid carbide d-bits, indexable carbide, or solid endmills, here are a few process steps:
1) Rough the top contour profile of the rib leaving 0.030 in. to 0.050 in. for finishing.
2) Finish the top contour profile
3) Rough ribs to the values in Table 1.
Next, finish the ribs to size. Start from the top of the electrode and proceed down. The process may vary slightly depending on the draft angle on the electrode. Height or length of the electrode, and width of the electrode can vary technique slightly as well.
Note: When selecting a cutter diameter to finish the rib with, make sure the cutter diameter is at least 0.125 in. larger than the remaining stock. For example, if we left 0.250 in. stock/side we would want to use at least a 3/8 in. diameter cutter to finish the rib.
In the case of a multi-ribbed electrode, rough between the ribs first if possible to remove a lot of that material before finishing.
Before you start to machine here are a few items to question:
How many straight walls are there and can I add draft to them?
What is the height of the rib?
How much overburn should I have on the electrode?
What type of graphite should I use for this rib?
What is the smallest size of the rib?
In a multi-rib situation, what is the spacing between ribs?
Are there sharp edges required?
What is the draft?
What is the percentage of material that must be removed?
I would like to close with one more tip. One of the most effective techniques to employ is a process I refer to as “cocooning”. Cocooning is a method in which the electrode is set up and just before the machining process the entire electrode is enclosed or cocooned in plastercine. Then the electrode is machined, cutting the plastercine away with the graphite. The plastercine becomes a very important element in the process, absorbing the vibration during machining. The plastercine does not interfere with the machining process. When the machining process is complete the electrode emerges from the plastercine, as a butterfly emerges from a cocoon. I have used this technique hundreds of times with great success, where others have failed in the production of the same electrode.
Mark Benoit is the chair of manufacturing and transportation at St. Clair College.
Table 1. Values for roughing ribs
|Electrode size||Rough size / Down step||Finish / Down step|
|(overburn included) (in.)||(in.)||(in.)|
|0.010 – 0.040||0.200/safe/side – 0.010||to size / 0.002 -0.005|
|0.045 – 0.085||0.250/safe/side – 0.010 -0.025||to size / 0.002 -0.005|
|0.090 – 0.125||0.300/safe/side – 0.010 -0.025||to size / 0.005 – 0.010|
|0.150 – up||0.350/safe/side – 0.025 – 0.050||to size / 0.010 -0.020|