DIY Stirplate -MagCraft NSN0592 Mounting Question

I have a pack of NSN0592 magnets laying around, and thought that they would be good candidates for a stir plate. However, the thing about these are that the magnetization direction makes it appear that I would have to mount these vertically.

The specs for these magnets can be found here: NSN0592

I've done some dry mockups with them and a pair of them mounted together do a good job of turning my star head stir bar from about two inches. Testing was done by putting the stir bar on a glass plate and manually testing the magnet strength from varying distances. With sixteen pounds of pull each, I am sure that these will be overkill once mounted.

The mounting is what I am trying to figure out, though. My two leading candidates are either mounting them on some sort of plastic square tubing centered on the case fan or by using some sort of structural plastic I-Beam with the top shaved off. From there, epoxying the structure to the fan and washers where the magnets will be placed.

Anyone else have recommendation, or worked with magnets similar that prevented flat mounting?

 

Those are the exact same magnets that I use on the stir plates I have built. IMO, they are nearly the ideal magnet to use. I say nearly as I think the 7/8" dia version of the same magnet might even work better. Vertical mounting is, indeed, the way to do it. The best way that I have found to mount them is to cut a 2.5" dia. wooden disc. I used a hole saw in my drill press to do this. This leaves you with basically a 3/4" thick wooden puck. I bore sockets into the disc to hold the magnets and mount them flush with the disc surface. To do this precisely it is best to bore the hole pockets with a Forstner bit. These cut a nice clean hole with a relatively flat bottom. Go slowly and check the depth by inserting one of the magnets. Once satisfied glue the magnets in place with super glue or most any kind of glue. You only need a couple of drops to hold them in the pockets. They should be a fairly snug fit. Take extreme care when drilling the hole pockets to be sure they are aligned and spaced accurately. This makes all the difference, so do it carefully. Lastly, use some spray adhesive to attach the disc to the face of the fan. You should have a 1/4" hole through the center of the disc left by the hole saw. Use a straight edge to position the disc on the fan. Spin the fan and lay the straight edge up against the disc. Slide the disc around as needed to center it. Once positioned well enough, put a few drops of Super Glue down the center hole. This will hold the disc in place where the spray adhesive alone may not, yet you can fairly easily separate the disc from the fan if need be down the road. Here's a pic of the disc and magnets. You'll get the idea. It's all about alignment, positioning and balancing the magnets which is easier said than done. When running the fan with the magnets and disc mounted, you should feel nearly no vibration. There will always be some vibration at high speed, but it should not be much at all.

The pic is looking down through the acrylic platform above the fan. I've arrived at about 5/16" clearance between the fan and the Plexiglass top for the gap. The fan is fully adjustable vertically, so some trial and error here is very easy. The top is about 1/4" thick IIRC. The magnets are positioned 1-1/2" center to center.

I've experimented with a lot of different magnets and methods of attaching them to the fan and this has worked the best and it was easy to do.

Oh yeah, I have the magnets spaced for a 2" sitr bar. Different lengths of stir bars will require different magnet spacing for optimum performance. ie, much less likely to throw the bar and it minimizes vibrations. This is important.

 

Thank you very much. That is very interesting. I was looking at orienting them differently (vertical, in my case, was perpendicular to the magnet on a central shaft). By orienting them the way that you have, I am assuming that the fields from the magnets overlap, basically providing opposing fields that "pull" the stir bar to the middle. You utilize more of the strength of the magnets, which is definitely more optimal than what I was looking at.

The puck design is very logical, as those two magnets would want to attract each other in that orientation. I definitely need to work on some additional mockups and determine how I can come up with something similar with a minimal amount of equipment - or find someone with more of a workshop (and skills) to help accomplish this.

Do you have any problems in throwing the stir bar at higher speeds with this approach?

 

No problems at all with it throwing the stir bar. It will throw the bar at wide open throttle, but you can easily run it considerably faster than necessary. I probably run it at about 300 rpm or so normally. I'm guess at the rpm, but that should be fairly close.

The cylindrical magnets shown have their poles on the top and bottom of the disc. The stir bar poles are at the ends. So the bar will have a north pole and a south pole. You want to mount one magnet with north up and one with south up to match the stir bar poles. The horizontal attraction when the magnets are flat and in the same plane is very minor relative to the vertical strength. Be careful when handling those magnets. They are scary strong and can pinch you big time. Wear safety glasses when working with them. Trust me on that. I'm not exactly sure what is going on with overlapping magnetic fields when magnets are mounted in close proximity like this, but it seems to not cause any kind of problem at all. I've built about 20 stir plates like this one over the past few years and all of them are still in use AFAIK.

 

Ah, I think what I am seeing is that the stir bar that I have currently is a disc arrangement, similar to this. I am guessing the polarity of this one is also oriented top to bottom. I have a couple of traditional bars en route, so I will hold off on jumping to conclusions until I get them.

I definitely have been wearing protective lenses when working with them already. Having used them before, I learned the hard way about how they can break blood vessels and leave wicked bruises. Luckily, that painful lesson is years behind me now.

 

That stir bar is a cross configuration. It is 12 mm (0.47") thick and 30 mm (1.2") across. It's one of a number of cross shaped bars available. The poles are on the ends of the arms. The stir bar has a small bar magnet embedded inside. Only two of the leg ends are magnetic. N & S are on opposing legs. The pole orientation is the same as with a straight stir bar, not on the top and bottom. You can probably get that bar to work with the magnets you have, but there may be a problem as the minimum center to center distance for the magnets will be greater than the distance between the poles on the stir bar. Not the best way to configure it. A 2" bar would be much better with those magnets.

 

Good to know. I had this one laying around since college - been sitting in a toolbox for almost fifteen years, and I couldn't even figure out how I came about having it when I went to thinking about it. Ordered a one inch and a two inch the other day though, so they should be here shortly. Definitely not an expert in magnetism, so any and all feedback is always welcome.

Thank you for all your help! Looking forward to get a start on this project next weekend once the remainder of the parts arrive!

 

You will probably not be able to use the 1 inch bar as it will be too short to span the magnets center to center. The 2 inch bar, however, should work very well. How you mount the magnets doesn't matter much so long as they are well aligned, spaced and balanced. The wooden puck makes this much easier and it also works as a spacer to move the magnets further away from the fan motor. Mounted too close to the fan, the magnets can interfere with operation of the fan.