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March 25, 2006

mass producing joints

by sven at 11:59 pm

A week ago I showed off my first successful brass "open hole double ball" joint. Since then I've been working on figuring out how to mass produce these things.

tonight's work

The photo above shows where I got to tonight. This is my second attempt... And I expect there'll be a third. When the process is perfected, I want to do a tutorial.

forty sandwich plates

With my first attempt, I started by cutting forty 18mm-wide plates. That was a mistake.

the aborted first attempt

Problems with attempt #1: It was incredibly fiddly getting the parts in and out of the vise... The plates weren't always perfectly even with one another...

broken drill bits

...And I broke two drill bits -- I believe because the drill would knock against the head of that screw as it punched through the brass strips.

Drat. I had a batch of 20 joints getting close to completion. But then I discovered the design (or rather, the process) was fatally flawed.

making a template

I was in the middle of a conversation with Gretchin, and suddenly stared off into space... Ah-ha! A new idea for how to mass produce these joints!

The gist: Rather than do one joint at a time, bind two strips of brass together, drill all the holes, and then cut the strips into pieces.

drilling holes using the template

This time I made a template out of a thin piece of metal. It turns out that the template strip was too thin (it curled!) -- and I also drilled more holes than I actually need -- but the concept is sound.

When I got around to using the template to drill holes in the plates, I used lubricant for the first time. Just water -- but the difference was noticable nonetheless.

filing off metal burs

The K&S brand brass strips I'm using are .093" thick, 1/4" wide, 12" long. K&S has seven standardized displays that they send out to retail stores. The .093" thick stuff is only sold in their "jumbo" hobby metal display. I can name six stores in Portland that sell K&S -- but only one has the jumbo display.

I bought the last two strips in the store last Sunday. I used these up in the failed first attempt Monday. ...Waiting until Friday for the store to get new stock in was murder.

the two strips screwed together

On the first attempt I used a fine-point Sharpie to mark measurements, and a hammer and a brass screw to start holes. For my second attempt, I used a metal scribe and a center-punch to make the template. It's my first time with these tools, and they helped a lot.

In the picture above you see the two strips fully drilled, and I've added 1/2" 4-40 stainless steel socket cap screws. Adding them in after all the holes have been drilled, I didn't have the same problem with breaking bits this time.

cutting and rounding joints

For cutting the individual joints off of the strip, I used a Dremel with a reinforced cut-off wheel. I tested normal cut-off wheels, and a jeweler's saw first... The jeweler's saw gave a nice hair-line cut -- but I broke two blades on my first two cuts -- so I'll be reserving that tool for finer work. The regular cut-off wheel made a cut 1/32" wide, whereas the reinforced wheel made a cut 1/16" wide. Given the amount of cutting I'm doing, durability won out.

Even with it being reinforced, I've worn one blade down from the size of a quarter (larger, actually) to the size of a dime -- and another splintered and flew at my face. Thank goodness Safety Boy is fastidious about wearing his safety goggles!!

An unforseen benefit of cutting the joints off of a strip: immediately after making the cut, I can round off the joint on one side. ...You see, this is such a time-consuming process -- every motion that you can eliminate from the workflow makes a difference.

I think I'm going to need 17 joints to make an armature. If they all prove functional, it looks like I got 13 done tonight. I may just try to salvage the remaining four from attempt #1.

--But there will be an attempt #3 -- and soon. The process is so close to being right...

posted by sven | March 25, 2006 11:59 PM | categories: stopmo