In my quest to build a stir plate using a computer cooling type fan, I found that the most difficult part of getting that to work well was mounting the magnets properly.  I settled on two magnets and in spite of several attempts I was unable to get an efficient (or even stable) mount for the magnets.  Magnet alignment is an important part of stir plate design.  Uneven or mis-spaced magnets mean easily thrown stir bars and poor performance.

I decided to design and print a 3D printed mounting aid to make magnet mounting easier and more precise.  I’ve since dubbed this the HBF Stir Plate Mounting-Thingy.  In my opinion, it turned out pretty amazing.

The Stir Plate Fan

I sourced this specific cooling fan for the project.  I like this model because: 1. It’s a good size (120 mm, 4.7″ square), 2. Is already wired for AC and, 3. Includes a variable speed function that can be used for controlling the stir plate speed.  Of course, you could use any fan that’s compatible with the size of the mounting thingy.

AC Infinity AI-120SCX Speed Control Fan Kit for Cabinet Cooling, Single 120mm – affiliate link, note that multiple variations of this product may be available, as such a different version may appear at this link

Printing the Magnet Mounting-Thingy

Here it is printing on my XYZprinting Da Vinci 1.0 3D Printer – check below for links to the STL file on thingiverse.

Here is the completed Magnet-mounting-thingy!

A close up of the finished piece.  I tried several designs.  Magnet alignment is an important part of stir plate design.  Uneven or mis-spaced magnets mean easily thrown stir bars. That’s not good.  I figured that out on my own, trying to place magnets isn’t fun.  They don’t want to cooperate.  This piece helps mount magnets easily and precisely.  I decided on the hole in the middle because the fan I purchased has a dimple in the exact center.  The magnets are centered and held in place on the mounting aid and alignment on the fan is as easy as lining up the dot in the center of the circle.

Magnets and Magnet Alignment

About the magnets… I tried three different sizes of magnets.  I was about to go with a smaller size that worked okay, but decided I wasn’t happy with their performance.  Yes they did work on a moderately sized starter.  But, I just wanted more holding power.  I didn’t want to have to struggle with thrown stir bars.  I decided on 1/2″ by 1/8″ Neodymimum Rare Earth Magnets.  If any magic exists in this design… certainly a good bit of it comes from these magnets.  The alignment and fan are both huge parts to the puzzle.  But the power of these magnets is important.

Note: Be careful with strong magnets.  Read the warning on the box and always read and follow manufacturer’s instructions.  Take special care when using these around each other and around metal in general.

Note: The original magnets I used are no longer available.  For whatever reason, magnet offerings seem come and go quickly.  The Mounting-Thingy is designed to use 1/2″ by 1/8″ Neodymium Rare Earth Magnets.  Search on eBay

You want to line your magnets up so that the polarity is in agreement.  I’ve done this a few times now, not always successfully I might add.  Now, I start by putting the magnet on a stir bar.  Get it placed so everything is nice and happy.  I put this on a tasting paddle.  I’m working on my deep freeze here and the magnet would prefer to make friends with the deep freeze vs the stir bar.  The tasting paddle helps.  Note that here and throughout these tests, I’m using this 6 x 25 mm Stir Bar.

Now take the stir bar and magnet combo and place it into the holder.

When you’re comfortable with positioning, glue the first magnet down.  I’ve been using Gorilla’s Super Glue.  Wait for that first magnet to set up.  It’s worth noting that the individual magnet holders are tight enough (at least for me) for this specific magnet to hold them, at least temporarily, in place while you play around with positioning.

Next, I take the other magnet and move it close to where it needs to go.  See how the stir bar has moved to the side?  That’s because the polarity was incorrect.  The stir bar was repelled.  Rotate the magnet around until the stir bar is attracted and you’ve found the correct position.

Here, I’ve found the correct orientation for both magnets and the stir bar is attaching perfectly.  Glue the second magnet into place and wait for it to set up.

Mounting the Mounting-Thingy On The Fan

Notice the dimple in the middle of this fan.  Disregard the scuffing on this.  I’ve tried a bunch of things with this fan.  The fact that it’s still working well after the paces I’ve put it through is a testament to it’s hardiness.  If your fan doesn’t have a dimple, you could measure (carefully) to find the exact center of the fan and add a mark.

Add some glue and line up the dimple in the center section and whammo you’re lined up and installed.

Installed and aligned correctly using the center circle as a guide

With a temporary 3D printed top installed to give proper spacing.   Note: I believe that it’s important to minimize distance between the magnets and your Erlenmeyer flask.  Minimizing this distance helps the magnets hold the stir bar more efficiently.

Mounted Stir Plate Magnet Tests

With some stir plates, it’s necessary to move the flask around a bit (with the stir bar in it) to train up the stir bar on the stir plates magnets.  Not necessary with this design.  Drop the stir bar in and the two immediately find each other.

Here is a test of 1.6 Liters of water in a 2 Liter Flask

Here’s another picture of of the same test.  This is vigorous and violent.  It’s worth noting that this magnet/spacing/stir bar combination doesn’t throw stir bars (at least in my tests) even at top speed.

Here’s a video of the 1.6 Liter Test

This setup is entirely capable of holding a 5 liter Erlenemeyer and stirring large starters.  Here is a beautiful vortex with 4 Liters of Water in a 5L Erlenmeyer.

A video of the 4L Test

How about a full 5 Liter starter?  Yup!

I’m very happy with these results.  Note that this is a project that requires additional materials to build a completed stir plate.  This HBF Stir Plate Mounting-Thingy is a guide designed to aid in the difficult (at least to me) task of mounting magnets accurately.  STL files for this guide are available for free.

• AC Infinity AI-120SCX Speed Control Fan Kit for Cabinet Cooling, Single 120mm – affiliate link, note that multiple variations of this product may be available, as such a different version may appear at this link
• SEOH Magnetic Stir Bar Plain PTFE 6 x 25mm – affiliate link, note that multiple variations of this product may be available, as such a different version may appear at this link
• Stir Plate Magnet Mounting-Thingy STL files – via thingiverse
• The original magnets I used are no longer available.  The Mounting-Thingy is designed to use 1/2″ by 1/8″ Neodymium Rare Earth Magnets.  Search on eBay

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