By Stan Trout | Spontaneous Materials
We continue with the third blog in this series, describing the many types of mistakes made with permanent magnets. Again, my intent is to help engineers in the future avoid the mistakes made in the past, and not to embarrass anyone.
EveryoneÂs experience is different, so please feel free to add your insights in the comments section below.
The third group of mistakes on my list are:
- Believing that magnets do not require any special expertise to produce or use
- Designing a product with a non-existent material
- Assuming that cost is only from raw materials, or an easy to use $/kg figure, independent of quantity or complexityÂ
It would be easy to imagine that making permanent magnets is a simple and straightforward process. After all, it is easy to find a process flow chart with the various steps described in some detail in many textbooks and review articles. How difficult can it be to make such simple objects? The steps do not seem overly complicated. The truth is that some magnets probably are easy to make, but the vast majority of them take a fair bit of skill and practice to make on a consistent basis. This is something that one learns on the factory floor, where experience is the great teacher. Let me highlight two key examples.
Frequently magnets are specified in terms of their chemical composition. This is a basic reference point in our understanding of a material. While hitting the composition target is necessary, it is not sufficient to guarantee a good magnet. As many engineers learn as undergrads, there is a difference between getting the composition right and getting the microstructure correct. In order to make a good magnet, both composition and microstructure need to be on the money. So in the case of producing a magnet, being a little off on the particle size or the sintering or heat treating temperatures can yield an inferior microstructure and corresponding poor magnetic properties.
Just as they are not trivial to produce, magnets are equally not as trivial to use. As mentioned in our last blog, the process is a bit longwinded and contains a few traps, that either need to be addressed or avoided.
I guess everyone has their favorite example of designing with a non-existent material, mine happened early in my career. A very famous customer called and asked for an applications engineer to appear at their facility the following day for a meeting about a magnet they wanted us to make. Because of their importance, there was no turning down this invitation, so I was on the plane that night. What I discovered the following morning was a design containing a rare earth magnet with really poor magnetic properties. It wasnÂt spelled out, but my guess was that they had designed this device with MMCo5, a primitive form of SmCo5. At the time, some people thought MMCo5 would be the cheap rare earth magnet, before the advent of sintered NdFeB, but it never really caught on commercially. I found that the magnetÂs properties were not negotiable. It became a take it or leave it situation, so we left it for others to wrestle with. My guess is that someone read about the material and its properties in a research paper and assumed it was widely available. This wasnÂt true, nor was it possible to talk them out of this awkward material choice. In this case, cheap raw materials did not yield an inexpensive magnet.
Please be careful to design with materials that are actually in production.
In the last few years, I have picked up several consulting engagements from people who were convinced that they were spending entirely too much for the magnet they purchased. My solution for these clients is always the same. Much to their amazement, I tell my clients that every good magnet plant employs an industrial engineer, who spends his or her time trying to figure how to make various parts with the available equipment, or via outsourcing. The result of this exercise is a spreadsheet that lists the steps needed to make the part and an estimate of the cost to do it. So I create a similar spreadsheet my clients. However, it is without the benefit of knowing the exact cost structure any magnet producer might have. I suggest that clients share this information with their suppliers, and ask them to correct any information the supplier finds to be off the mark. This exercise usually works to alleviate the uncertainty my clients originally had. While they donÂt admit it to me, I imagine that many clients have done a much simpler back of the envelop cost estimate, which concentrates on the material cost and underestimates, or ignores the processing costs. Ultimately my approach is successful because we give our clients a more realistic and accurate picture of the cost structure.
Similarly, people expect pricing for small quantities to be almost as low as high volumes. To the uninitiated, this expectation might seem realistic. However, the previously mentioned cost analysis looks carefully at the quantity to be made. It is an important consideration. The production techniques will vary widely according to the quantity. For example, an order for just a few pieces might be made from cutting them from a large block, whereas the high volume production will no doubt employ special equipment and tooling. The costing will vary accordingly.
We donÂt help ourselves in the magnet industry by discussing prices in $/pound or $/kg for the various types of materials. Industry insiders appreciate the subtlety that there is a range of prices in play for each material, depending on the size, quantity and other manufacturing issues. People less familiar with our business often incorrectly assume these prices are fixed and apply universally. That is where the trouble begins.
If you want to estimate the cost of a magnet, create a spreadsheet with all the necessary steps. Do not multiply the weight of the magnet by the last published $/pound figure, youÂll be misled.
Seven down, ten to go.
About the AuthorÂ
Dr. Stan Trout has more than 35 yearsÂ experience in the permanent magnet and rare earth industries. Dr. Trout has a B.S. in Physics from Lafayette College and a Ph.D. in Metallurgy and Materials Science from the University of Pennsylvania. Stan is a contributing columnist for Magnetics Business & Technology magazine. Spontaneous Materials, his consultancy, provides practical solutions in magnetic materials, the rare earths, technical training and technical writing. He can be reached atÂ firstname.lastname@example.org.