So I have the start of an inkling of a thought of what is going to happen. It's mainly driven by the 'goodenoughCNC', a strangely simple designed CNC device. It uses bog standard bearings and steel frames to do the job.
The main units are a set of three carriages. Two for left and right Y movement for a gantry, and a third for X movement. Well, I should say there are four carriages as one is required for Z movement, but that's the next subject.
The carriages are currently 100 x 100 blocks, that will run on metal bearings keeping it fixed on a 40x40 frame. Metal bearings, simple frame, square blocks, all adds up to nice and cheap!!
To keep things still inside the carriage, there are three bearings per plane (six in total). Two on one surface, one on another. Typically in a triangle pattern. All bearings act on the edge of the 40 x 40 metal, I'm guessing if they were driven on the middle surface the whole carriage would be able to rotate which isn't a good thing. The bearings are fixed in the carriage on threaded rod, a total of six lengths going through the carriage.
The NEMA motors sit on the side of the carriages, and are belt driven. I was initially going to avoid belt driven as I thought they were inaccurate. Technically they are... they're 0.2mm tolerance vs 0.1mm tolerance of a linear bearing!! The belt is kept on the drive by four further shouldered bearings, and fixed on the end by a screw driven tensioning device.
What's more, a quick price up of the relevant parts suggest each bearing is about 50p, the toothed drive for the motor is £2 and the belt itself is another £2. Threaded rod is about £5 for a length, with nuts, washers and stuff maybe making up another £3. Given the steel is effectively free, that makes each carriage;
Carriage bearings 6 x 50p = £3
Belt bearings 4 x 50p = £2
Toothed Drive = £2
Belt = £2
Rod = £5
Hardware = £3
That's £17 per carriage. Maybe the same again for a decent torque stepper (one per carriage), that's about £100 to get from nothing all the way to a working transport that just needs a Z movement. That's a nice budget for me.
The only thing is the size, 100 x 100 is quite large considering the entire device is only 500 x 500. One of those either side would leave a cutting bed of maybe 250 x 350. Is that enough? I'm not sure. Reducing everything by 50% would be better, that would leave me with 350 x 450. It would also mean the frame could go to 25mm or even 19mm, which I have a lot of. I'd have to figure out how much to shrink the bearing and threaded rod, and unfortunately the belt and pulley couldn't shrink.
Hmm, prototype time I think!!
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