Alternative to pinch sticks for squaring up a machine

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WillAdams
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Alternative to pinch sticks for squaring up a machine

Post by WillAdams » Fri Oct 09, 2015 1:43 pm

Got this in my e-mail today, thought it was an interesting option:

http://www.woodsmithtips.com/2015/10/08 ... ssemblies/

Though of course, there's the whole chicken-egg problem of making one if the Shapeoko is one's first tool.

Here's a question --- could we design a special-purpose tool which would aid in the assembly and squaring up of a machine which could be affordably included w/ a machine, and then be useful afterwards?
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twforeman
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Re: Alternative to pinch sticks for squaring up a machine

Post by twforeman » Fri Oct 09, 2015 4:11 pm

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SmokyRick
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Re: Alternative to pinch sticks for squaring up a machine

Post by SmokyRick » Sat Oct 10, 2015 12:09 pm

Now THAT is really cool and looks like it would work great! And cheap too! Just need to find enough to buy from them to make the shipping worthwhile. (Not a problem!)
Smoky Rick Crawford in the middle of Northern Illinois, USA.
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JuKu
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Re: Alternative to pinch sticks for squaring up a machine

Post by JuKu » Fri Feb 05, 2016 9:39 am

Very cool idea! Unfortunately (for them) the shipping makes buying it non-sensical ($35 for shipping a 10$ item), so I'm going to steal the idea and 3D print myself a couple of those. Sorry.

Yet another method: If you have a web cam or even better, an USB microscope, mount it to your machine. Run a software that can draw a cross on the video image. Get a precise square reference (even accurately cut paper sheets will do, and it is easy to check the squareness of a sheet). Place the reference on the table so, that when you jog the machine in X direction, the cross precisely follows the reference edge. takes a couple of tries, but easy to do. Take the camera to the corner, and jog a known amount in Y. If the machine is not perfectly square, the edge drifts off center. You can trivially measure off-squareness with 0.0x% (<0.1mm/300mm) accuracy by noting, how much you need to jog in X to get the edge back at the cross.

The software I wrote for my pick and place machine has a correction for this. I wonder why that is not a standard feature for CNC machines? It is trivial to measure 0.1mm error per 300mm (about the size of A4 sheet long side) and far easier to input a number to a software box than trying to rebuild the machine to correct that.

WillAdams
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Re: Alternative to pinch sticks for squaring up a machine

Post by WillAdams » Fri Feb 05, 2016 11:22 am

It doesn’t happen for Grbl due to limited memory space on an Arduino. One could pre-process the file though.

It would also require homing switches, and that one use them always.

My concern would be that due to the forces involved when cutting, one would get stair-stepping along long straight lines when the machine tried to move some small distance along an axis, but the distance couldn’t be moved due to it being less than a partial step, which would accumulate until suddenly the machine snapped to the desired path. Naturally, a pick and place machine wouldn’t have this problem.

As I’ve noted elsewhere, precision has to start somewhere. Moreover, the simpler one keeps things, the fewer things can go wrong.
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cvoinescu
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Re: Alternative to pinch sticks for squaring up a machine

Post by cvoinescu » Fri Feb 05, 2016 12:30 pm

The solution to stair-stepping is to make the machine actually square. Not the frame, necessarily, but the gantry. The optical measurement, as described, is very easy and accurate. You can do the same thing with a square and an indicator.

I'm working on a way to self-square the gantry during homing, by driving the two Y motors independently, but still using stock GRBL. I just cheat a little bit in hardware: if you turn mist coolant on, the machine drives the second Y motor as the X axis (and switches the homing switch on that side of the gantry to the X limit input too). You can use a screw to adjust the trigger point of one of the Y limit switches to make it square, or you can do it in software: home normally, turn coolant on, home differentially, command the calculated move that brings the gantry into square, turn coolant off, and home normally again. Second and third homing cycles only take a second or so. The machine is accurately homed and squared after the second cycle; the third one is just to reset GRBL's idea of what the coordinates are, after the adjustment move. It's not needed if adjustment is done in hardware, with a screw.
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JuKu
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Re: Alternative to pinch sticks for squaring up a machine

Post by JuKu » Fri Feb 05, 2016 1:21 pm

@cvoinescu: Clever trick!

@WillAdams:
> It would also require homing switches, and that one use them always.
I don't see why.

> one would get stair-stepping ...
Didn't thought of that. On the other hand, the severity depends on the step size. GT2 belts on a 200steps/rev motor is 0.1mm/step, 1/8 microsteps is 0.0125mm stair size, which might be noticeable when start align just right.

> The solution to stair-stepping is to make the machine actually square.
True, just very difficult without your trick. Maybe I want double Y motors on my machine after all.

WillAdams
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Re: Alternative to pinch sticks for squaring up a machine

Post by WillAdams » Fri Feb 05, 2016 1:29 pm

You need to drive both sides of the gantry.

If you don't do dual-motors, at least implement a drive shaft: http://www.shapeoko.com/wiki/index.php/Drive_Shaft
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JuKu
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Re: Alternative to pinch sticks for squaring up a machine

Post by JuKu » Fri Feb 05, 2016 5:44 pm

WillAdams wrote:You need to drive both sides of the gantry.
Of course. My plan is/was to use an abandoned prototype of my pick and place machine. It has a driveshaft, but maybe I want to use a dual motor setup. Need to do my homework on how that would work and how difficult would it be to modify the proto for that.

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