Part 11: Rotatable Electromagnet Project

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By Timothy Raney…Bald Engineer Guy with Glasses

This week, we’ll continue with building the coil winder – yes, we’re almost done with the coil winder.

Are We there Yet? Well, here we are again. I bet you’re thinking, “Is this guy ever going to finish this freakin’ electromagnet?” Sure, that’s a valid question and it’s a good segue to some truly priceless wisdom. Though we often want to get a job finished to see the results, there are times when the journey has its own rewards. This project is a case in point – it is focused heavily on improving my knowledge of the processes involved and the science. We will get to the end of this project, but it will foster more work and various experiments. Along the way, we’ll have another detour or two. The details on the coil winder and its construction are an example of one such detour. In the end, we’ll have covered various topics useful for other projects and experiments. So, it’s all about learning and acquiring a deeper understanding of these particular fields, principally metalworking and magnetostatics. What a combination.

Locating the Holes in the Motor Mount Bracket
Let’s continue with the coil winder now. In the photograph (right), you will notice I’ve cut a slot for the 1.1875” wide motor bracket that will hold the gear motor. The dimensions are based on the material on-hand. I have several pieces of “surplus” aluminum alloy, all 0.340” thick. This was a good time to use it. Aluminum or steel of other dimensions would suffice, keeping in mind the bracket function. The slot was then milled with a 0.5” diameter 2-flute HSS end mill at 2580rpm. The intent was to create a slot to accommodate the bracket securely. The next task was to locate the motor’s mounting holes on the bracket. For this task, I removed the motor’s front end plate and used it as a template to locate the mounting holes. With the end plate clamped to the bracket and the gears fixed to temporary shafts, I used a transfer punch to spot the hole locations. Afterwards, the holes were drilled on the drill press. The hole for the shaft used an “F” drill (0.257”) to provide clearance for the shaft. The shaft rubbing on the bracket would add to any parasitic frictional loss mechanisms already present. The motor mount holes were drilled with a 3/16” HSS drill and the hole for attaching the bracket to the bearing block was a 1/4" hole. Each hole was counter-sinked with a 90o countersink for flathead machine screws.

A Few Notes on Fastener Selection
I used stainless steel flathead cap screws that use an Allen® hex wrench for tightening. I like these screws since you can apply a much greater clamping force without the deformation often seen with slot tip or phillips head screws when the screwdriver tip “cams-out” of the driving slot. The reason for using flathead screws was to eliminate any interference with the gears and the micro-switch. The flathead screws give an item a nice, clean look too. Once the bracket had its 1/4” mounting hole, I used it as a template to locate the corresponding hole in the bearing block. I just had to clamp the bracket in place and spot the hole location with a 1/4" diameter transfer punch. I like transfer punches – they are very handy. I then used a #7 drill (0.201”) preparatory to threading the hole with a 1/4-20 HSS tap. Tap hole depth was about 3/4" – deep enough for the mounting screw. Sometimes it’s a little challenging to adapt existing parts to materials on-hand, but this process is no different than designing a system in a professional setting.

Locating, Drilling and Tapped the Micro-Switch Mounting Holes
By this time, I’ve located, drilled and tapped the two holes for mounting the micro-switch on the coil winder’s bearing block. I did change the configuration somewhat – the switch was moved to the front, just under the drill chuck – shown in the photo below. With the drive gears in the back, there was just not enough clearance for the switch. After this task, I machined the cam that will actuate the switch. This is probably a good place to talk about the switch’s function again – it will close once for each revolution of the coil winder’s shaft, completing a circuit with the electromechanical counter in series. The counter does its thing – it counts each revolution. This is a 5A-125VAC switch – its rating provides an adequate margin for this low current/low voltage application. Once we assemble the coil winder, I’ll show you the schematic. I won’t show you the schematic yet since it now looks like a pencil drawing done by a trained monkey. No offense to trained monkeys or other primates is intended. Back to the cam.

Cam Dimensions and Machining
The first task was determining the cam’s overall dimensions. In this case, the thickness was determined by the shaft space between the drill chuck and bearing block. The cam’s diameter was based on depressing the switch’s roller lever or cam follower enough to close the circuit. Given these parameters, nominal cam dimensions were 5/16” thick and 0.860” in diameter. Notice I mixed fractions and decimals again. Why on earth does he do that? Well, I follow the convention – dimensions expressed as fractions generally have a greater plus or minus tolerance band, whereas dimensions shown in decimals denote a tighter tolerance. I just thought you’d like to know. The next task was determining the location for the cam’s 0.375” diameter off-axis hole to achieve the 1/8” displacement needed to depress the switch lever. Well, we can do this the hard way or the easy way. Being inherently lazy, I chose the easy way. With the cam starting as disc with the dimensions above, it was placed in the 3-jaw chuck on the lathe. But wait, there’s more. To create the 1/8” off-set, I merely placed a 1/8” thick copper spacer between one jaw and the disc. From there, it was a simple task of center drilling and drilling the off-set hole. An instant cam – pretty much.

Next, I drilled and tapped the cam’s 10-32 setscrew hole. At this point, I also milled a 0.030” deep flat on the coil winder’s shaft. With the setscrew bearing against the flat, it is less likely to rotate and score the shaft. Another option is using two setscrews at a 90 degree angle on the cam, but that’s more work. I believe the flatted shaft method will suffice in this instance. Lastly, the world of cams is fascinating – a basic and very versatile mechanical component with many variations used in countless applications. So, if you would like more information on this topic, type “cam design” into a search engine of your choice. Alternatively, basic mechanical engineering texts are an excellent resource. Another resource is Mechanisms and Mechanical Devices Sourcebook (3rd Ed.) by Neil Sclater and Nicholas P. Chironis, published by McGraw-Hill, 1996. This book is readily available, new or used. I like books too.

Next week, we’ll finish building the coil winder and we might go into more depth on calculating the flux we can expect from the electromagnet. Hope to see you then!

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One Response to Part 11: Rotatable Electromagnet Project

  1. Pingback: Build a Rotatable Electromagnet for Experimental Work | Citizen Scientists League

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