Wednesday, December 18, 2013

Perfect Tumbler Aggregate

Right now the collection of aggregate in my tumbler is close to what I find is perfect.  I thought about this as I was writing about the nubbins which made up the majority of the aggregate when I was starting out with the new tumbler.  Later I added all the carbon steel balls I was storing elsewhere and, later still, a lot of punch drops collected from the Piranha punch machine drop bucket at Kan Fab.

I have found that it is a good idea to think specifically about the aggregate from time to time.  I have an intuition for how much aggregate material work best.  Too little and there won't be enough contact with the work pieces.  Too much and the tumbler is over loaded.  It seems to be as important as getting the tumbler rpm right.

By experimenting I decided to cull out all the pieces larger than approximately 1” square and all pieces smaller that about 3/8” square.  Anything much smaller than 3/8” can escape from the tumbler and irregular pieces larger than an inch square can cause unwanted surface gouging.

I remove the drop nubbins after they have been polished and deburred.  The main reason for that is it makes it easier to find small work pieces I have tumbled.  With only punch drops and carbon steel balls as the vast majority of the aggregate mix there is less visual confusion when searching for small parts.

I’ve also developed some intuition about how the pieces to be tumbled will fare and what the selection can and cannot include for me to get the result I want.  For instance, tumbling table legs forged from 1” square bar 34” long will get an even surface treatment and no end upset because that can’t rotate end-to-end in the 18” inside diameter of the tumbler.  Some blanks made in 14 gauge will deform unacceptably while the same pattern cut in 12 gauge will do well.  Short pieces of bar with sharp edges will upset and also get an edge cold shut.  In the image “Perfect aggregate closeup” careful study will reveal the edge fold-over cold shut phenomenon on the larger punch drops which had tumbled through many cycles.

Most of the pieces going into the tumbler are covered with fire scale and that scale also acts as part of the aggregate mix until it escaped through cracks around the doors.

I will guess there is about 50-75 pounds of aggregate in the tumbler now.  I usually only tumble one load of forging each day and the weight will vary from perhaps 10 pounds to 200 pounds.  The tumbler is powerful enough to carry more but I must manually rotate the cylinder into the position where the doors are on top to unload it and the heavier the load the harder it is to turn.

The current mix of aggregate shown in the images does an excellent job of removing coal fire scale, mill scale, paint, and rust and leaves a cleaned surface well suited for welding.

One feature which I did not anticipate is a characteristic surface appearance which seems to have a lot of “tooth” to aid adhesion of finishes.  I use several types but the most common finish starts with a light spray of Minwax fast-drying satin polyurethane applied immediately upon removal from the 

Optimal size, shape and amount of steel aggregate for me.
Detail of aggregate.

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