Building a better Colour Changer – Part 1 – Designing hardware

“Can we make an automatic colour changer?  And can we do it cheaper than $500.”

I think Sarah posed this to me as a bit of a challenge, coupled with a desire not to spend $500+ on a yet another add-on for her knitting machine.

Like a lot of things, I think the answer is “Maybe…”  I mean, I’m fairly confident, but as this is an ongoing project I’m still a little unsure.  Still, the render looks good, so that’s something!

Seriously, I’ve seen Kickstarter projects that don’t look this slick.

Step 1 – What the hell am I replacing?

This thing.  I’m replacing this thing.

The KRC-900 colour changer Sarah uses has surprisingly tight tolerances and some fairly weird geometry.  It’s made using a combination of injection molded plastic, steel plate and heavy gauge wire.  Added to that, it’s not entirely obvious which dimensions are the important ones.

Fortunately, it’s not too hard to unbolt it from the arm to get to it.

In this case I used my cheap vernier gauge to get fairly accurate measurements.  This one is made of plastic and will feature in a future post about hacking a vernier to make a cheap Digital Read Out for a lathe, but for the moment it’s on my desk in a surprisingly un-butchered state.

Under these conditions it’s almost impossible to be 100% accurate and I suspected there was enough play that a few mm between friends wasn’t going to hurt.

Not to scale, obviously.

Step 2 – Sketchup.  You don’t have to like it, but it does work.

From here, I pretty much inferred the rest of the dimensions and ended up with a flat version.

Trust me, this will work….

This was a decision point.  Because the little catches are going to be moving in plane the obvious electro-mechanical driver would be a solenoid or a linear servo.  Unfortunately, I don’t have any of those to hand and they’re not the cheapest thing to source.  Much more common are the 9G rotational servos found in every basic Arduino kit.

Also, I have a gear generator that will help me turn that rotational force into linear force and the dimensions already in Sketchup, so that’s cool.

Wheels on the bus go round and round…

Abracadabra, I’ve got all I need to make a fully functional colour changer.  Let me just tweak this corner here and move this bit there…….

Ugh…wait…what…That’s a total dogs breakfast….

While I was originally doing this design it never looked this bad.  Unfortunately I didn’t take captures as i went so I did a bit of re-creation work for this blog.  In reality the process involves lots of minute change and takes a few hours (I have a tendency to space out and watch Star Trek while I work, that slows me down.)

The whole point of this step is to work out where parts are in relation to each other.  Each part of the machine obviously relates to other parts on that layer, but they also relate to parts on the layers above and below so they have to be built up in relation to the whole.

 

Step 3 – These things are made of layers, like an ogre.

After all the playing around and tweaking I’ve ended up with a design made up of 4 layers.

Layer 1: The thread catcher top plate.  5mm Acrylic.

This baby is made of 5mm acrylic and is transparent to show off all those naughty racks and gears!

Layer 2: Gears, racks and mobile thread holders. 3mm Acrylic.

Nice solid colours show up the gearing.

Layer 3: Base Plate. 5mm Acrylic

These fancy shapes are keyed to the top of the 9G servos. The tab to side holds the arm sensor.

Layer 4: Static Thread holders. 3mm Acrylic.

The bottom plate holds the servos in place while the top piece helps hold the threads.

All together now!

So similar to the teaser at the top….

Step 4: Dressing up for a date with a laser cutter.

Laser cutters obviously like stuff to be flat.  I use a plugin for Sketchup to turn these vectors in to DXF format so I can load them in my preferred CAM tool.

Speaking of CAM, stay tuned for “Building a better Colour Changer – Part 2 – Firing the Lasers” where I cover making my DXF into something physical.

This post will be updated with plans once I’ve got a solid, working version so add it to your watch list.

$500 is too much!  How much have we spent?

Well, so far I spent less time doing this design than I spent writing this blog post.  I’ve got some plans for servos and plastic, but nothing spent yet.

Dollars down is $0, not a bad start!

John Spencer

I’m the hardware geek. I lean towards laser cutting, 3D printing, CNC routing and designing circuit boards.

You can find me on Github and Twitter.

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