disc_harrow_grinder_standsI was working on a design for a work stand, you know the kind of thing that holds a vise or a bench grinder (or table top or whatever..) In my case I need a few of these.  I started looking at stuff online. I was quite impressed with these stands from Matt Walrath.  Apparently Matt used 24″ diameter harrow disc blades from an agriculture supply place.  Way nice.  I’m thinking wow, this might be a nice way to go.. get a little bit of welding practice in, make something cool and useful.  Did I mention that I need a whole lot of improvement in my TIG welding skills?

So I started to make some inquiries. Called / searched around, Ebay and what not.  New blades are definitely available at a reasonable price, but holy cow are those shipping costs expensive.  No Amazon Prime free shipping here, sigh.  Now I hate spending money on stuff like that.

 

 

collector_vise_stand

I’m thinking, hey now wait a minute, how about a different design?  And hey, while I’m at it, how about some more TIG practice?  So I came up with a design, inspired by some awesome welding work I’ve seen by guys building custom exhaust manifolds.  Here’s what I came up with.

Its a tripod stand built exhaust collector style.  I’m intending 3″ thin wall tube, with welded on feed flats, so I can fill the thing up with sand.  Everybody knows a tripod is the most stable base for something, right?  And how wide should the ‘stance’ be? Well that’s easy… I’ll make sure the end of the tripod feet fit into a 24″ diameter circle.  If that was good enough for Matt, that should work for me, too, right?

It turns out that’s not exactly the right answer.

 

 

 

 

 

Bicycle Repair Stand Tripod

I was pretty determined that a tripod was the best most stable base for what I was doing. I mentioned this to my brother, and he asked, hey, if that is the case, why is his bicycle repair stand so tippy?  He’s right, those things when loaded up with a bicycle up high are sort of tippy. So what’s going on? Yeah, 24″ isn’t 24″ isn’t 24″ or in other words, there is a reason virtually all wheeled office chairs have five legs on them as my buddy Dirk so graciously reminded me.
What really matters is the distance from center to the “tip over axis”, and that’s the shortest distance observed from center.  So I went ahead and did a little spreadsheet analysis.  Two notes:

  • Angle refers to the angle between two adjacent legs of any polygon in plan view.
  • For any polygon, the sum of all the angles = (no. of sides – 2) * 180 .
  • Leg length (Plan view) = Inscribed Circle Radius / sine( angle / 2 ) .

 

 

 

How many legs? How many sides to the polygon? Sum of all angles of that polygon degrees per corner degrees per corner divided by 2 Tip Over Distance from Center (Inscribed Circle Radius) Length of leg (Plan View)
3 180 60.0 30.0 12 24.00
4 360 90.0 45.0 12 16.97
5 540 108.0 54.0 12 14.83
6 720 120.0 60.0 12 13.86
7 900 128.6 64.3 12 13.32
8 1080 135.0 67.5 12 12.99
9 1260 140.0 70.0 12 12.77
10 1440 144.0 72.0 12 12.62
11 1620 147.3 73.6 12 12.51

 

Whoa… that means the three legged stand, may take up a whole lot of floor space if we want a 24″ stability inscribed circle diameter.  Here’s three shapes laid one over the other, to show what a dramatic difference the number of legs makes.  Does this make it clear?

BaseStabilityGeometrySketch

 

 

(bike stand image here from: http://nuxx.net)