Part 5: Rotatable Electromagnet Project

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By Timothy Raney…Bald Engineer Guy with Glasses

Last week, we covered fabricating the electromagnet’s coil form discs. Now we’ll make the two vertical yoke pieces – part of the magnetic circuit and structural frame.

Machining the Two Vertical Yoke Pieces
As we continue the electromagnet project, the next task was machining the two vertical yoke pieces. The yoke is part of the magnetic circuit along with the associated poles and base. I first placed the two steel yoke pieces in the milling vise after rough-cutting them to their approximate length. Next, they were aligned with a machinist’s square clamped to a planer gauge as shown in the photo (below). This arrangement ensured the ends were perpendicular to the long axis before milling. The next step was milling these work-pieces with a 0.625” diameter 4-flute high speed steel (HSS) end mill. Spindle speed was 2580rpm. Some might say this speed is too high for milling mild steel given the end mill’s diameter according to the standard equation, rpm = CS x 4/D, where CS is the cutting speed for the material (in surface feet/minute) and D is the diameter of the cutting tool – an end mill in this case. Now, it’s important to remember CS varies with the material. As one might expect, “Machinery’s Handbook” [1] and many metalworking texts list the CS values for most metals and alloy families.

End Mill Cutting Speeds
These cutting speeds are based on empirical data collected by industry over the years. The data is a guideline used for optimizing tool life in a production environment. As such, these CS values are good starting points and not necessarily the “last word” given our circumstances. In this case, my preference is running at high spindle speeds, taking light cuts (<0.005”) that result in a better surface finish. To make this approach work well, one must remove most of the metal via other means – sawing, for example - it’s much quicker. The situation you want to avoid is trying to remove a half-inch of metal by making these shallow 0.005” deep cuts. This approach has another advantage too - it places less stress on the tooling and machine. This aspect is particularly important for the smaller and less rigid machines many of us use in the home shop environment. Incidentally, the cutting speed equation applies to turning and drilling operations too. For lathe work, D becomes the work-piece’s diameter or the drill diameter for drilling operations.

Locating the Yoke’s mounting holes and Fastener Selection
Locating the mounting holes was the next step after ensuring the yoke pieces were square. Since I was working with ½” thick by 1.25” wide steel bar stock for the yokes, these dimensions allowed me to use ¼-20 bolts for eventual mounting on the steel base. As in many projects, selecting the correct fastener was a case of maximizing the holding force and overall rigidity. In an electromagnet of this design, the attractive force between the poles will exert a force perpendicular to the vertical yoke piece’s long axis. Though largely conjectural at this point, it is reasonable to conclude the cyclic operation of the electromagnet in normal use could bend the yoke mounting bolts if they were not sized properly. Remember, the magnetic circuit comprised of the yoke, base and poles are a structural framework too. Therefore, we want to avoid any failures where the bolted joint loses its clamp load or bends the bolt. In this application, we can essentially ignore vibration.

However, we can imagine some dynamic loading will occur from turning the magnet on and off. In time, this effect could loosen the bolts by reducing the friction between the threads, resulting in relative movement of the yokes. The result would draw the poles closer together with a corresponding increase in magnetic flux. The effect could then contribute to other ever-present experimental errors. Thus, it is a good idea to have a working knowledge of fasteners, to include their capabilities and limitations for a particular application.

So, after considering fastener selection and completing the layout work, I drilled the four (4) mounting holes in the two vertical yokes with the appropriate drill: a #7 drill (0.201” diameter) for tapping the ¼-20 bolts holes. These holes were 3/4” deep nominal. This process started with aligning the holes under the drill chuck with a wiggler, following by center drilling and then drilling with the #7 twist drill. Afterwards, the holes were tapped sequentially -  a given hole was drilled and tapped before moving to the next hole.

Counterboring the Yokes
Now we move on to a little counterboring. A relatively simple task, unless you make a mistake. A counterbored hole allows the fastener’s head to remain at or below the yoke’s surface. I finished the yokes for now by counterboring the two (2) through-holes in the yoke’s outboard side for the pole piece mounting bolts. I used a tungsten carbide 2-flute end mill for this task since I did not have a counterbore for a ¼-20 socket head cap screw (SHCS). This expedient scheme worked fine – I’ve done it often. An important point to remember is keeping the spindle speed low to avoid chatter and the resulting nasty tool marks. In this case, 110rpm worked very well. The result is a smooth counterbore adequate for the purpose.

However, I did forget to tighten the R8 end mill holder at first. The resulting vibration (chatter) was enough to tell me something was wrong. It just goes to show that paying attention is essential. No harm done since I caught things in time and completed counterbored holes satisfactorily.

Next week, we’ll continue this astonishing series with fabricating the electromagnet’s base –  its most complex part yet. See you then!


[1] Machinery’s Handbook (26th Edition), Industrial Press, Inc., New York, 2000.

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