Thursday, April 4, 2013

Building a blower

Pictured above is the old blower I was experimenting with for the foundry. It was clearly too strong for the application so I sold it and bought a motor of less power, 1/3 horsepower as opposed to 1HP.
Here is the shell of the new blower being welded.  There is a large diameter pipe section 4" long sandwiched in between the sides to maintain uniform depth.  The pipe clamp is being used to hold the outside piece for tack welding.  The top side pictured was welded all the way around, and the bottom was only tacked because it was later ground off.  The circular parts were cut with a Beverly shear.
Here is the outlet made of 3" tube and notched with aviation snips.  I made it round since the old blower's was rectangular and made it harder to work with.
Joint preparation was not a priority
Since I cut the hole with a cutting torch, I was left to weld a few gapes.
The right half was TIG welded and is noticeably much cleaner
I ended up TIG welding half of the air outlet since the oxy-acetylene torch would've taken forever to get it hot enough to do a fillet weld.
After grinding off the intake side and taking the pipe out I then punched the holes.  I later welded on 4 tabs (not pictured) to bolt the intake side on.
The large hole was made by punching several holes with the Whitney punch.  I didn't have to mark where the holes needed to go because I just inserted the piece in as far as it would go and punched all the way around making a (near) perfect circle.
Filing the hole smooth
 A bolt was welded on the intake side for the shutter which is held on with a wing nut.
The blower shell fully assembled.

PART 2: THE ROTOR
I originally planned to cut the rotor plate on the CNC waterjet so it would be perfectly symmetrical and I could just weld the fins into the slots and everything would be nice and balanced.  However, geometry came to my rescue when I couldn't get a chance to use the waterjet.
I'm not sure this is proper clamping etiquette
The fins came out of the shear with inconsistent dimensions, so I put them in the mill (all at once) and faced the sides that had the most variation.
This is where I started to have doubts.  I thought this thing was going to come out asymmetrical, probably because at work and school I've been programmed to think anything out of tolerance by .002" is a big deal.
Here's the geometry part.  Since there are 6 fins, they are 60 degrees apart, so I sheared some pieces with 30 degree angles on each side and spaced them in between the fins as I was tack welding everything together.  I think I liked this method better than having a circular back plate because it saved weight.
After welding everything together, I reamed the hole with a 1/2" reamer held in a vise as I spun it on by hand.  It didn't fit in the lathe and I was too sissy to hold it in a drill press.  The hole was drilled 31/64" prior to welding.  Also prior to welding, a set screw hole was drilled and threaded.
Amazingly, the rotor ran pretty true (don't ask me the runout because I didn't check it)  I'll rank it at "true enough to not ruin my spindle bearings any time soon"
Now for the placement, I positioned the rotor off center so as the air goes around there is an increasing distance between the edge of the fins and the blower shell.  I don't know if this helps, but the old blower looked like this so I did it too.
After testing it, the results were very satisfying.  Sure a little air leaks out the gap where the side bolts on, but it is a trivial amount.  The shutter does an excellent job at regulating the airflow so there is no need for a motor speed control. 
The mount to connect the blower to the motor is just a piece of plate with holes punched in it, bent 90 degrees and some ribs welded on for support.  Later I'm going to add a stand for it to make it level with the furnace.
And finally, the good looking picture.

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