We’re using fairly strong magnets and have not encountered this.
We attacked a bunch of dead hard drives this afternoon. We broke 5 of them by trying to shear them off by striking them. We broke another one by trying to shear it off by applying pressure with a pair of vice-grips.
We got two off intact when I found an industrial heat gun at work. It took about two minutes of heating on high. The magnet shifting with a gentle tap means that the glue has been sufficiently weakened. The magnet can then be removed when cool (as my son found out).
Hope that helps.
Make sure you don’t overheat the magnet.
Overheating a magnet makes it turn from a cool magic hunk of metal into a useless hunk of metal.
I am not exactly sure which type of magnets we use, but what i do know is that they are pretty strong, but probably not as strong as some teams use. When mounting them, we are incredibly close to the pole, less than 1/16 in. This has allowed us to have a relatively safe and light magnet and still have enough traction for a sub 2 second minibot. We used two total, however only one really applies traction, the other makes sure we track straight up the pole.
Hello Matt
Perhaps soaking in solvent is the way to go even though it would be much slower.
I just read a datasheet for some neodydmium magnets last night and they indicated the magnets would start to lose magnetic strength above a surprisingly low temperature. I don’t recall the exact number but it was something like 80C. I am sure the industrial heat gun gets hotter than that since we were charring the solid maple workbench with the blast. Unfortunately, I don’t recall which manufacturer posted this data (it was pretty late).
We also noticed that the pulling force of the magnets that we measured seem to decrease to a fraction after removal from the backing plates. This observation applied to the ones we heated as well as the ones we sheared off. We suspect that the backing plates are some sort of high-permeability steel and they concentrate the magnetic flux. They look rather thick for the mechanical stresses involved.
Another strange phenomenon we noticed is that the magnets seem to be magnetized in zones along the arc. The pair of magnets from one hard drive would stack nicely but magnets from another (different brand/model) would only stack with an offset. In the past, I had noticed this same effect with strips of flexible magnets.
Since we could not get the pulling force we needed with the magnets from the dead hard drives we now have some on order from an online supplier.
1519 is using two Neodymium magnets also…
As always, be careful with high-strength magnets! I do recommend using a smaller magnet at a closer distance (partly for weight, but primarily for safety). We didn’t have any trouble mounting ours ~.125" from the pole, and the two of them have only 23 pounds of force each!
Here’s the link for the ones we used:
http://www.kjmagnetics.com/proddetail.asp?prod=BX884DCS&cat=173
Some recommendations:
- Use ones with countersunk holes… they make mounting them a charm!
- Mount closer to the pole (3/16" or less), to minimize the weight of the magnet you have to lift and reduce the danger from having a higher-strength magnet.
- Not that important, but the narrower your magnets are the more of the magnet is at the minimum distance from the circular pole.
- Spread them out over the length of the minibot, if possible. This will help create a longer moment for your resistance to the minibot driving up the pole at an angle. We relied more on near-perfectly balanced weight to achieve a straight ascension.
Best of luck, the magnets really made our design possible!
And here are some links of our minibot climbing:
Climb Only: http://www.youtube.com/watch?v=3Ht0gHEOb14
Slowed Down Minibot Race at GSR: http://www.youtube.com/watch?v=Dl55U1PuDKM
Thanks to Joe Barra from Team 20 for posting (http://www.chiefdelphi.com/forums/showpost.php?p=1035959&postcount=1) the second video.