Don't Overlook EDM Tapping

In many difficult hole making applications, the best way (and often the only way) to form internal threads is with electrical discharge machining.

Article From: 6/1/1997 Modern Machine Shop, ,

There was a time when, for most shops, EDM (electrical discharge machining) was synonymous with broken tap removal. Any mention of the "spark erosion process" brought to mind vivid pictures of the fountain of miniature fireworks that characterized tap removal operations. In those days, it probably never occurred to many people that EDM would emerge as an excellent way to form threaded holes, bypassing conventional tapping altogether.

But so it has. EDM tapping is widely used, but it attracts relatively little attention. Most die-sinking and wire-cut applications are far more glamorous and, in the past few years, the pace of advances in these fields has been breathtaking. No wonder you hear so much about them. Tapping with EDM, however, is highly practical and can be a real problem solver, even if it doesn't always get the notice it really deserves.

EDM tapping overcomes the same difficulties that recommend the use of EDM in other situations. Very hard materials. Tight tolerances. High productivity demands. The need for flexibility. Cost-effectiveness. Another important advantage is being able to drill and tap a hole in one operation—the electrode forms the threads as it sinks the hole from the solid. All that should make EDM tapping a very exciting prospect for potential users.

A Case In Point

Meyer Tool is one shop that is excited about what it can do with EDM tapping. A job shop, Meyer Tool's most challenging work is for aerospace but it serves demanding customers in a variety of industries.

"It's not unusual for us to have several jobs in the shop at one time that involve holes that couldn't be drilled or tapped any other way," says Paul Rowland, manager of Meyer Tools' Cox Road plant in Erlanger, Kentucky. "We constantly need to make threaded holes in hardened materials or in tough aerospace super alloys." With EDM, the shop has tapped holes as large as

5/8 inch in diameter and as small as 6/32 inch.

However, Mr. Rowland points out that diameter usually isn't the issue in the tapped holes it EDMs. It's hole depth.

"Many of our workpieces have thin walls or ribs requiring threaded blind holes that leave a thin wall at the bottom. Conventional tapping couldn't cut threads the full depth of the hole without distorting the thin wall at the bottom or causing the material to crack," he says. A typical case would call for a wall at the bottom as thin as 0.02 inch with no cracking or "push-out" on the other side of the workpiece wall. Exotic alloys would be especially prone to cracking if conventional drilling or tapping were attempted.

Meyer Tool uses a tapping attachment that it designed and built in-house. According to Mr. Rowland, the shop was looking for very precise and easy-to-set depth control. Operators simply key in the desired depth on the control unit (also designed and built in-house) and when the tap reaches the preset depth, the unit automatically retracts the tap. A glass scale provides highly accurate feedback. Mr. Rowland says the unit reliably achieves accuracies ±0.001 inch on hole depth.

The power supply is likewise an in-house creation, which is a modified version of the control units the shop developed for its extensive EDM small-hole production facility at its sister plant in Cincinnati. The cross-over from small-hole EDM to EDM tapping was a natural move because the operations are very similar. One important difference with tapping is the use of a threaded guide bushing that orients the tap as it enters the workpiece. The bushing's internal threads maintain the feed of the tap so that infeed matches the pitch of the threads to be formed. It follows that every hole diameter and thread pitch require their own guide bushing.

"Building our own control and power supply gave us the opportunity to fine-tune our EDM tapping operations and gives us greater versatility," Mr. Rowland explains. "EDM tapping is an integral part of the `tools' we need to have at our disposal to do the work we do. It's not so much that we do a lot of EDM tapping, but what we do is essential."

Tapping Units Off The Shelf

Although Meyer Tool opted to create its own EDM tapping attachment, EDM tappers are readily available commercially. Perhaps the most widely used units are produced by Custom E.D.M., Inc. (Holly, Michigan). These units are designed to be mounted on the ram of almost any manual or CNC EDM and interfaced to the existing power supply of that machine. A separate interface/control box, supplied with the attachment, allows the user to match the different gap voltages found on different models of EDM machines and to make adjustments to the machining parameters for EDM tapping.

In operation, the electrode feeds through a threaded bushing or lead nut, which keeps the electrode straight and on center. The lead nut also sets the pitch. However, because pitch diameter is variable, Custom EDM uses a split nut design that can be adjusted to accommodate threads of different diameter. With the electrode in place, a collapsible collar around the lower end of the nut is tightened until the electrode cannot move. This automatically sets the orientation of the lead nut's internal threads to match those of the electrode. Then the nut is loosened ever so slightly so the electrode will thread through as it feeds.

Depth can be controlled by an adjustable mechanical stop on some models; others feature a programmable servo unit.

Brian Stelter, president of Custom E.D.M., points out that some basic insights into the nature of the EDM process will help users understand the peculiarities of EDM tapping. For example, it is important to "burn aggressively" when EDM tapping. As he explains, the idea is to keep the electrical energy concentrated at the tip of the electrode and away from the sidewalls. When the tip of the electrode does most of the sparking, the sides will show less wear and therefore cut more accurate thread forms. Although the leading threads will round off, the threads that follow will have retained more of their full shape and will clean out shallow threads ahead that result from tip wear, yielding a high quality thread form. The worn tip of the electrode can be filed or cut off to sharpen it and extend its life, Mr. Stelter points out.

He also notes that the tendency of EDM sparks to concentrate on sharp corners explains why EDM tapping works best cutting from the solid. A pre-drilled hole would require the threads of the electrode to do most of the cutting and overall wear would be significantly greater. Thread forms would be inferior and the hole would show greater overall taper.

"The proof of this concept is in threads of a fine pitch," Mr. Stelter says. "The smaller the thread on the electrode, the greater the effect of wear on the threads. So the best tradeoff is to increase the voltage setting in the gap, which limits current amperage and decreases wear. Although burn time increases, accurate thread forms will be the result." The anti-arcing circuitry of newer EDMs is a real plus—for EDM tapping of holes of any size, but particularly with fine threads.

Mr. Stelter recommends that shops experiment with test cuts to find the optimal settings for any tapping attachment they use. The first consideration is the quality of the tapped hole that is needed, he says. "You can expect that cutting a threaded hole with a Class I fit will take longer and require more carefully set parameters than a Class III fit, but EDM tapping will be economical and capable either way."

Orbiting

One other approach to EDM tapping is worthy of note. Most of today's CNC ram machines have circular interpolation as a control feature. Therefore, it is possible to program a circular motion that coincides with the downward motion of the Z axis. The resulting helical path allows the tapping electrode to create thread forms on the sidewalls of a pre-drilled hole. The process is akin to thread milling on a CNC machining center. No special EDM tapping head or attachment is needed.

However, this process is generally slower than tapping from the solid because the electrical energy is dispersed across the entire length of the electrode where it is engaged with the workpiece. That is, threads the full depth of the hole are being burned at one time. Because an electrode for orbiting is smaller than a comparable electrode for "straight" EDM tapping, the depth of the thread form it creates is proportionately greater in relation to the electrode diameter. In short, you have more EDM work to do and you're doing this work with a smaller electrode "tool."

Nevertheless, providing for the pre-drilled hole may have the biggest impact on overall production time. In many cases, the hole can be wire cut and if other features are also being wire cut, the extra time for hole cutting may be insignificant. And for the occasional user, the expense of a tapping head may not be justified—orbital tapping is an economical and practical option.

Electrodes For Tapping

EDM tapping electrodes are supplied by a number of sources. Tapping electrodes for common hole sizes and thread pitches are usually available as catalog items, but several suppliers will fabricate special electrodes on order. Typically, standard electrodes are six inches long with five inches threaded. Except for the smaller sizes of graphite, most feature a center hole through which dielectric fluid can be pumped for efficient flushing.

All EDM tapping electrodes are manufactured undersized to allow for the spark gap or overburn, as it is called. Pitch diameters are undersized from 0.002 to 0.006 inch, depending on the hole size. Orbiting electrodes are likewise undersized but to fit the tap-drilled hole.

One of the most extensive lines of EDM tapping electrodes is offered by Saturn Industries, Inc. (Hudson, New York). The line includes tapping electrodes in graphite, copper graphite, brass, copper tungsten, and copper. They come in metric and English sizes, in regular and orbiting styles. Tapered taps for pipe threads are also available.

What's the right electrode material to choose? According to John Lee, president of Saturn Industries, the choice of electrode material in many cases is determined by the preference of the EDM operator. "Operators used to working with copper or brass generally choose those tapping electrodes, and the same goes for operators more familiar with graphite. On newer equipment, either one will cut equally well, but on some older machines, the power supplies were optimized for one or the other," he explains.

However, in smaller sizes, 6-32 and under, metals are stronger and sharp edges on threads are easier to maintain because they resist chipping. Copper tungsten is generally recommended for tapping tungsten carbide and refractory metals in holes of all sizes. Most recently, the company introduced tapping electrodes in copper, which has shown superior surface finishes and increased throughput in many materials. But for a broad range of applications, graphite electrodes are gaining popularity. Fine-grained grades of graphite such as POCO-3 perform very well and have excellent strength, and are vying with metal electrodes in many demanding applications, Mr. Lee says. Unless special conditions must be considered, graphite is the most economical choice.

"The easiest thing for a shop to do is consult with the dealer or manufacturer of its EDM machines about the choice of tapping electrode material," Mr. Lee observes. "From experience, suppliers know which products have performed well on their machines in certain applications, even which settings tend to give the best results."

Mr. Lee notes that there is clearly a trend to smaller and smaller electrode sizes. In standard styles, 0-80 and comparable metric sizes are now available in graphite. In orbiting styles, the small end is 2-56. Miniature workpieces in the computer and medical fields are driving this trend, but the fact that designers and engineers are becoming more aware of EDM tapping for small holes has also contributed.

EDM—Extra Dimensions In Machining

EDM tapping is one more example of how versatile and flexible the "spark erosion process" is today. "Can we do this with EDM?" ought to be a question asked by every shop owner, job planner, or manufacturing engineer.

As more and more shops add EDM to their lineup of basic machining capabilities, they will discover it to be a very valuable resource, and when it comes to hole making applications, they will find it a resource not be left "untapped."

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