Gantry Squaring Problems

Assembly questions and answers for the Shapeoko 3 CNC Mill
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John_TX
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Gantry Squaring Problems

Post by John_TX » Mon Jul 25, 2016 9:23 pm

All,
I've had my SO3 all year. It's been tons of fun. I finally decided to rebuild it, somewhat following TWForeman's blog.

From the fist time I built it, I seem to have had this issue:
http://www.shapeoko.com/forum/viewtopic ... are#p59136
Something is out of square because one Y-plate touches an end plate, while the other is 0.1" away. I have ignored this because precision hasn't mattered up to this point.
However I now require some precision work - hence the rebuild.
I thought if I clamped the Y-plates to the end plate, measured distance from end plate to X rail at each end - as described in that thread - I would solve the problem.
Here's what happens: I do that, then release the clamps, and -- voila -- the right Y-plate springs back to its spot about 0.1" away!

One hypothesis I have is that the ends of the X rail are not cut square. I think that would explain it. The end plates being off square seemed more likely, but I have done both corner-to-corner checks and used my digital protractor (Y rails to end plate) and it seems to be spot on.

Suggestions?

*Update: I removed the X rail and checked both ends - it is cut perfectly square. So that explanation is out.

madhatter
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Re: Gantry Squaring Problems

Post by madhatter » Mon Jul 25, 2016 10:10 pm

Mine is the same way, When the right Y-Plate touches the end plate, the left has an approximately 0.040-0.060in gap. I cheat and just hold it in place and turn on the steppers and use it because I don't get a lot of time on the machine and I don't want to tear it down and open up the holes in the wasteboard and whatnot to allow a bit more adjustment and. . .(and this is honest) I'm kind of lazy, and doing what I do works. HOWEVER, my father is coming in a few weeks and I am selling my small SO3 to him and we are going to set up my XXL, and when we do, we're going to tear down my small SO3 and figure out what we need to do to get it square.

I have loosened all the screws that attach the wasteboard to the straps and angle brackets and used a large pipe clamp to try to pull it (gently) into square, then tighten the screws, but it always returned to as 'not square' condition. I read - here I believe - that opening the holes in the wasteboard and possibly Y-Axis motor plates (just a bit, nothing drastic) might give me that extra bit of give to get it square.
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WillAdams
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Re: Gantry Squaring Problems

Post by WillAdams » Tue Jul 26, 2016 12:36 am

I would shim the less acute corners w/ aluminum foil or thin sheet to bring the machine into square.

Other ideas :

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twforeman
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Re: Gantry Squaring Problems

Post by twforeman » Tue Jul 26, 2016 12:53 pm

Whether the base is square or not is less important than whether the X and Y axis are square to each other.

Get a square (carpenters square will do) and check to see if the X and Y rails are square to each other.

The important things to check are:
  • Y rails are parallel to each other
  • Y rails parallel to the base plate and the same height on all four corners
  • X rail is perpendicular to the Y rails
  • Both ends of the X rail are the same height above the base plate
  • Z rails are parallel
  • Z rails are perpendicular to the base plate
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cvoinescu
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Re: Gantry Squaring Problems

Post by cvoinescu » Tue Jul 26, 2016 1:12 pm

What @twforeman said.
twforeman wrote:Whether the base is square or not is less important than whether the X and Y axis are square to each other.
Given that the gantry can rack slightly, even with a perfectly shimmed X rail (which is why there are two Y motors, after all), you can get extra accuracy if you square the frame -- either the front or the back (can only do both if your Y rails are exactly the same length) -- and always start the machine with the gantry pushed against the stops in that direction. This is more precise and repeatable than relying on the squareness of the X rail alone, and can even overcome slightly imperfect shimming, because the Y motors can hold the gantry in square. This is especially helpful for larger machines.

Of course, do the basics first.
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John_TX
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Re: Gantry Squaring Problems

Post by John_TX » Tue Jul 26, 2016 10:12 pm

Update:
I re-checked the X rail to make sure the ends are cut square. Turns out, they are not, and that is the source of the problem. My protractor deceived me. I will shim exactly as Will suggested.

cvoinescu
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Re: Gantry Squaring Problems

Post by cvoinescu » Wed Jul 27, 2016 12:33 am

A protractor is not usually precise enough for that. You can stand the rail on its end on a flat surface, and use a square to check that it's perpendicular to that surface. You don't even need an accurate square, just something that's close to vertical. If turning the rail round* does not make any difference in its angle to the reference object (a square, another piece of rail stood on its end, the side of your computer monitor), then the bottom of the rail is cut squarely.

* 180 degrees around the vertical axis.
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