Why CNC?

[saxonhawthorn]

Gentlemen, you have converted me! I am now gathering the bits and pieces to make a CNC pick-and-place machine for populating PCBs. I couldn't even estimate the number of PCBs I have built by hand over the years, but now I'm looking forward to leaning back in my chair and watching a robot do all the work.

After that will come the various robotics for machining die-cast cases, fitting the PCBs in them, and testing the end product. Time scale to be in production is six months, but for the first time in my life there are no commercial pressures on me so if it takes longer, who cares?

By the way, surveying the CAD scene there seems to be plenty of 2D and 2.5D software around at sensible prices, but there doesn't seem to be so much 3D. How important / useful is 3D? Do people here use it? If you do, could you live without it?

My grateful thanks to all who have responded so magnificently to a stupid newbie's questions! I now understand your enthusiasm for the subject. :)

[Robin Hewitt]

I am now gathering the bits and pieces to make a CNC pick-and-place machine for populating PCBs.

Now that is a subject dear to my heart. Are you doing SMT or conventional? Do tell more

[saxonhawthorn]

Robin,

SMT. It's smaller, neater, and I won't have all those flippin' pig-tails to cut off.

I haven't made up my mind about applying the solder paste yet though because I have never worked with lead-free (sshhhh.....) and I don't know the preferred method: silk screen or a dab from a syringe. Silk screening would be quicker, but probably less controlable for the tiny dots. But I'll have to ask the paste manufacturers.

Otherwise I can't really see any insurmountable problems. I have bought a beautifully machined large worm and wheel for the robot arm major axis, and an equally well made tiny worm and wheel for the component pickup head horizontal rotation. (OK I'm cheating, and I should have machined them; but I'm a beginner, and hobbing those gears is something I'd need to get some practice with).

Ian

[Robin Hewitt]

I haven't made up my mind about applying the solder paste yet though because I have never worked with lead-free (sshhhh.....) and I don't know the preferred method: silk screen or a dab from a syringe.

The paste mask is usually thin brass stretched tight on a wood frame so you can place it slightly above the pcb. The paste dries out to unworkable pdq, at least the cheapies I have tried do. There was some bod lasering paste masks out of Mylar, I forget the details.

Problem with CNC is you either hand feed it or pick from the reel and then square it off while it's on the vacuum head. If you were in no particular rush, squaring might be easiest as a seperate put down.

Have you found plans for this gizmo?

A crummy worm will work because you can go for limited rotation and spring it. An elbow arm is fine if you set the position for every component manually, it's repeatability rather than precision.

[saxonhawthorn]

Plans? What are they? I've never had any plans for anything! In 25 years of writing commercial software I never even once got a specification from a customer. They always wanted me to tell them what they wanted.

I hadn't thought of brass as a screen. Not a bad idea. Not sure if I can afford a laser powerful enough to punch it though. I'd have to etch it.

I envision picking up components from the reel with a vacuum head on a robot arm, then holding them over a camera with some vision recognition software to square them up (that's where the tiny worm & wheel come in) before plonking down on the pasted-up PCB.

I bought the large worm & wheel (£18) because - like Mount Everest - it was there. I didn't have any particular use for it, but I had a hunch I'd find one. When it arrived I was really delighted with the quality, and I'm sure it will be adequate for the job.

As regards Tribbles point about volume: yes, and I can indeed rattle out boards manually at a rate of knots if I have to. But now that I'm liberated from the shackles of somebody else's factory I'm planning new products which I intend will go well, and anyway somebody's got to give the Chinese some competition! Once you've got a production line set up of course, it's easy to switch from one product to another. You just load the appropriate product code, and away you go.

Ian

[Robin Hewitt]

I don't think you have to laser your own paste stencil any more than your assembler would, there's a ready made industry out there. First hit on Google...

http://www.smtstencil.co.uk/?gclid=C...FU0A4wodF1p6rA

Don't think you need a worm for the rotation, a dinky stepper will easily microstep anywhere you want, with no load mirostepping really works!

Is an elbow arm really the best solution? As soon as you locate over a camera you can't "teach" the arm and you are back to positioning by co-ordinate :naughty:

[saxonhawthorn]

Robin,

Actually, I thought about this in bed last night, and grinned when I realised the answer. CNC! All I need to make holes in the solder mask is a CNC drill. I'm fairly sure the CAD outputs a Gerber file that will either do the job as it stands, or will do so if I just fiddle a bit with the hole sizes. But thanks for the link to that mask manufacturer. I've made a note of them, just in case. The price is certainly right. As a general rule I like to keep as much as possible in-house, for both cost and quality reasons. But sometimes it can make sense to contract out.

And yes, it's true that I don't really have to use a robot arm. I could just as well do it by 2D XY positioning. But that's just a tad boring, and I have to have a challenge in it somewhere to keep up my interest. Besides, a robot arm will impress the hell out of the customers; and although it's irrational, in real life that's an important part of getting contracts. :) Then I can make vids like the one Tom posted!

Ian

[saxonhawthorn]

Robin,

About micro-steppers.... when I mentioned using an arm I was working on the assumption that the minimum angular resolution of a bog standard stepper would be 1.8 degrees, and to get anything better I'd need a worm. Was that assumption wrong?

Ian

[Tom]

Sounds great! Here are some plans.... (tee hee)

[ame="http://www.youtube.com/watch?v=YZ_xyh4ehNo&feature=related"]YouTube- Gigabyte Mainboard Factory[/ame]

I've been in a board manufacturer before, and this is the method that they used:
Screenprint solder paste (stainless screens), 3D vision system to check for paste registration (and volume), pick and place machine(including pick, scan to check orientation, rotate to correct, and place)(as in the video above), then another vision system to confirm, and finally the reflow oven. You could obviously skip the pre and post paste vision systems (what I saw was pretty high volume).
If the boards are small, consider processing them joined together then do a final op (punch, or route) to separate into individual boards.

It sounds fun! I'm looking forward hearing all about it!

[saxonhawthorn]

Blimey Tom! That machine was going pdq. My only experience of automated board manufacture was at Sony forty years ago. We built a factory in Wales for making TV sets, and at £10k each the pick-and-place robots were the most expensive machines in the factory. But they went at a stately plodding pace compared with the clip you posted. Quite an eye-opener. Thank you very much for the benefit of your experience.

Yes, I'm not planning to manufacture TV sets! My boards will be quite small, so I'll place several on one sheet and route them out.

Ian