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August 24, 2009

armatures: limits of miniaturization

by sven at 1:30 pm

small ball and hinge

Within the field of armatures, I've decided to specialize in "small and precise." Working on The Whisperer in Darkness (new blog!) has given me a great opportunity to push my previous limits.

In the photo above, the ball is 1/8" in diameter and is brazed to a 1/16" dia. rod. The hinge joint is made from 3/32" dia. rods, and a #0-80 screw. For comparison, the screws in your glasses are likely #0.

These sizes represent certain practical limitations.

Stainless steel balls are made in two sizes smaller than the 1/8" ball I used: 3/32" and 1/16". However, you typically want to use rod that is one-half the diameter of the ball. The usual 1018 steel used for rods in armatures only goes down to 1/8" dia -- so you have to switch over to stainless steel rod, which only goes down to 1/16".

A 1/16" rod could go into a 3/32" ball -- but the range of movement would be rather limited.

Theoretically, you could also use a lathe to thin 1/16" rod down to 1/32", so you could put it into the tiniest 1/16" ball… But at that point, the rod is so fragile, I wouldn't trust it not to bend during use.

Even with the 1/16" rod, bending is an issue. It might be used on the fingers of a hand, or possibly for the outer arms of a very small puppet (~6" in height, or shorter)… But if you fantasize about using it for Jack Skellington style legs, you're bound for heartbreak when you accidentally bend the rods while animating.

So, if you want rods one-half the diameter of balls, and use standard-size rods, then a 1/8" ball and 1/16" rod combo is the limit.

* * *

As for hinge joints: the limiting factor is the screw.

I purchase nearly all my raw materials from either McMaster-Carr or Small Parts, Inc. For the hinge, I used a stainless steel 18-8, slotted, binding head machine screw.

Small Parts offers sizes of this type of screw below #0-80: #00-90 and #000-120. However, once you go past #0, prices are ten times as expensive. Whereas a box of a hundred #0 screws costs $7.25, a box of a hundred #00 screws costs $76.95!

[This is according to my invaluable Small Parts catalog from a few years back. Unfortunately, the company has stopped printing catalogs, and now does everything online. On the plus side, though, Small Parts' entire inventory can now be accessed via's "Industrial & Scientific" department… Although, frustratingly, it's turning out that items are out of stock far more often than I'd like.]

Furthermore, in order to drill holes to tap (make threaded), you'll need to go below the standard numbered drills, which run from #1-#60. To make a hole to tap for #00-90 screws, you need a #65 drill. For #000-120 screws, you need a #71 drill.

Personally, I have a set of #61-80 drills on hand… But they look like hairs -- so I haven't dared to try using them yet.

Oh, and in case you're wondering: another set of drills was created not so long ago that goes from #81 to #97. The #97 is .006" thick. For comparison, a piece of notepad paper is .002" thick. So try to imagine a drill that's only as thick as three sheets of paper!

At this point in history, #97 seems to be as small as drills go -- but I wouldn't bet that things stay that way forever.

Anyway, back to topic: making a hinge from 3/32" rod seems to be the practical limit. You could try to make a hinge from 1/16" rod… But the price of the screws skyrockets -- and you move into sizes of drills that are perilously delicate.

posted by sven | August 24, 2009 1:30 PM | categories: stopmo