[RX7_LS7]

Posted this over in the General Discussion section - and got a big yawn, so I'm hoping you CNC DIY Router table guys can help me out... Thanks!

Hi Everyone - relatively new to CNC, but not new to Mechanical Projects (Mechanical Engineer).

I need to build a 4ft by 8ft X&Y CNC driven table to test very large capacitive touch sensors - (Think about your new 84" TV with a giant iPad touch screen on it, and you'll be close to our product.)

I've seen some 4 x 8 router table CNC set-ups that look like they might be "adaptable" to what I need to do. The table will need to locate a "iPad stylus" within 0.2mm if possible - then gently drop the stylus down to the glass sensor surface - simulating a touch to the screen. Would like to "draw" lines on the sensor if possible (Stylus down then move). So obviously I need maybe 20mm of Z stylus travel as well as X & Y.

Is there anything out on the market that could be adapted for this purpose? Don't need hardly any force as I'm just moving a small stylus. Speed and accuracy are going to be more important - and the motor drive system needs to be as electrically quiet as possible. A bunch of EMI from the motors will screen up my touch sensor.

Does anyone have any ideas on where to get started? Would like to build it ourselves if possible. Driving the table with a Windows PC is preferable. Total budget: 3 to 5K USD for the table & drive system, not including the windows PC to drive it.

All help appreciated - in advance !

Jim

[nlancaster]

For such a light load at the contact point, maybe look at the openbuilds hardware and designs. 4x8 should be completly doable, and resolution should quiet good.

OpenBuilds Part Store

Budget should be totally doable, and nema 17 shouldn't be too noisy.

[dodger889]

I would use shielded motor cables to keep emi from happening.Using the shielded cable on one end only for a ground. This keeps ground loops from happening and reduces noise. Also use a spring loaded stylus to be able to keep the pressure to what is needed. That way it could "float" across the surface. And nothing gets broken.

[ger21]

The problem with the Openbuilds machine is that it's so lightweight, that it will likely sag enough in the center to make your stylus put a lot more pressure in the center.

Although maybe it won't be an issue, because the Z axis is only moving a stylus??

[fixtureman1]

I do some vinyl work with a drag knife that takes just a little pressure to cut without cutting all the way through on my shopbot. you could do the same thing with a stylus

[ger21]

But can you get a 4x8 Shopbot for his budget of $3K - $5K?

[fixtureman1]

Yes you can get a used one for that. Also it can be done on other cnc's

[RX7_LS7]

OK - haven't done a ton of reading yet - but maybe an OpenBuilds CNC Frame like this?

Beaver's OX 1500 | OpenBuilds

This is a 1500 x 1500mm table - I'd need something more like 1200 x 2400mm. It looks like Openbuilds doesn't make rails that long - so I'd need to join rails together to make something 2400mm long? Openbuilds has NEMA motors?

Forces will be very small (I hope). I need some kind of Z axis motor to pick up the stylus between test points - but the stylus only needs maybe 100 grams of downward force to insure it registers a touch - and probably a lot less (20grams).

Will try to do some reading on Shopbot - I assume they have a website?

Thanks for all the suggestions so far.

Jim

[fixtureman1]

Shopbot has a web site and a forum where you can see what used equipment is for sale

[nlancaster]

As far as gantry sag, For a stylus it shouldn't matter, all you have to do is set the stylus on the screen, as far as I am aware there is no particular reason to "press" it in to the screen. All the styluses (styli?) that i have only have to just touch the screen to work.

Yes to get your 8 feet of travel you would have to join 2 sections together, but these can be the fixed rails and supported from below.

[RX7_LS7]

Thanks nlancaster

Still trying to figure out what I need for the Z-axis. I need to pick up the stylus between test points and place it down on the sensor glass each time. It doesn't need a lot of downward force, but it needs maybe 100 grams to squish the foam finger on the ipad stylus. Doing a Z-axis stepper motor almost seems like overkill - maybe just an air cylinder or something to force it down and spring it back up?

[wizard]

Interesting project.

For a large machine such as this your budget will be a problem. Luckily you don't have machining forces to contend with. This means considering used equipment like plasma table is a possibility.

If not used equipment you can consider buying commercial linear slide assemblies and may be able to source something in the right price range. You need nothing fancy here, a belt driven solution ought to be good enough. Just make sure the slide assembly supports the resolution you need. By the way if you need to hit 0.2 mm consider at a minimum resolution 4 times greater. The gotcha here is if you need absolute accuracy of .2 mm over the entire working area. With low cost components that will likely require some calibration effort.

Some suggestions for slide assemblies: RACO, SMC come to mind initially. There are many more of course, in this case though RACO has a slide assembly that may be worth looking into that is belt driven.

I suspect you are grossly underestimating the expense here for a commercial project. No matter what you buy for linear motion you still need a frame to mount everything on. Without such you will have challenges holding the tolerances you want.

Much of the testing you want to do such as probing the surface can be done via I/O functions. However realize that integrating a CNC controller into a larger test apparatus isn't always easy. Sometimes you are better off taking another approach that avoids raw CNC programming and that allows tighter integration.