DIY stir plate questions

I'm getting together everything I need to build a stir plate and I have a few questions that hopefully someone can answer. First, I see some people stating to test it before it is epoxied to get everything lined up... But I don't see where anyone says how to do that. How do I keep it all together before I epoxy to try to get it balanced?

Also, I have two really thick heavy washers that I was going to use as spacers/magnet mounts. My fear is that they will be too heavy and cause stress to the fan. Is there any need to worry about that? Should I get a thinner washer?

Thanks!

 

I never epoxied my magnets, my magnets are strong enough to stay in place without, you might want to try this with the rare earth magnets.

As for the washer, I would agree with you in worrying over the fan. My stir plate has a low speed limit which I think may be because of this, I would use only 1 and do of course glue that one to the base. If it's unbalance it will add stress to the fan, so be careful centering it!

Also check there is just a bit of clearance between the magnet and the box, you don't want a box that doesn't close!

 

I can explain what I did that worked.

I;
Cut a round steel disk from thin sheet metal, though a steel can lid of the right size should work fine.
Found the center of the disk and drilled a hole.
Found the center of the fan hub and marked it.
Epoxied the disk to the fan aligning the hole to the mark.
Spun the fan and made concentric circles on the disk with a marker.
Applied two rare earth magnets, with opposing poles, at equal distance from the center using the circles as reference.
Adjusted the magnet's spin diameter while adjusting fan height to maximize stir bar attraction and movement.

IMO, the order of preference for stir plate speed control is, PWM based, LM317 based, rheostat based. I used LM317 with adjustments to fixed resistors to get the widest usable speed band and it works just fine.

 

That looks great... Thanks for the pic!

What size are those magnets?

I am using a Rosewill fan which comes with a control. I'm assuming that the control comes with and LM317... I know it has some circuitry associated with it. It's this one: http://www.newegg.com/Product/Product.aspx?Item=N82E16835200020. I was all out of old computer fans and my local shop charges $9 for one. This was less and came with adjustment.... I'm hoping that works.

 

I've gotten everything together except the magnets. I will have them installed tomorrow... My stir bars probably won't be in until Monday, though. I'm anxious to test. I'll post pictures of what I built. It's pretty ghetto, but I'm pretty sure it will work well.

I used a spacer cut from a piece of pvc coupler which should work to separate the magnets from those operating the fan... It doesn't seem to have a lot of torque, but I hope that's not important. I guess I'll find out soon.

Thanks for the input!

 

Ok... Got the stir bars and it works great! I'll make a video and post when I get a chance. I used an old test tube rack instead of a box... It's what I had laying around.

I got 1x5/16 stir bars from Scientific Equipment of Houston and 1/2x1/4 magnets from totalElement, both via amazon. I recommend both for anyone trying to buy stir bars and magnets respectively.

 

So, here's my stirrer (I hesitate to call it a "plate") operating during its first starter...

[ame]http://youtu.be/rxZfyBMsRJ8[/ame]

and during the initial test with water...
[ame]http://youtu.be/2jUgP1it39s[/ame]

The support structure is built with an old test tube rack which I didn't use at all. I cut out the center portion to fit the fan, but left the bottom, which is now the top of the stirrer. The fan, speed control, magnets and stir bar are what I listed earlier in the thread. The switch is an automotive switch that Radio Shack had on clearance for $1. The power supply is 7.5 volts. I hacksawed a 1" threaded PVC coupler in half for a spacer and used JB-Weld WaterWeld to secure everything.

This combination seems perfect to me. Using water, I can turn the speed all the way up without throwing the bar (I ran it that way for 30 minutes... Maybe it would eventually throw at that speed, but I kind of doubt it.) I was able to throw the bar running at really slow speed (as seen in the test video,) but that's not something I'll be doing ever.

Thanks to all who added to this thread...