Tippy gantry

[Thom Dudley]

We have a 4'x8' PRS standard that has been getting moderate use for the last 6 months. I noticed yesterday that while cutting pockets with the .5" straight bit, it leaves little tool marks at the ends of paths - little round divots that are very shallow, where I assumed the bit might be diving a little (or...just a "bit"? lolz) when the motion stops. I watched the machine while it ran a test file, and thought I did indeed notice the 0,0 corner of the gantry diving a little so I pushed down on it and found that it's up a little more than 1/16 from the track when the Z car is at the 0 end of the gantry. When at the 48" end, the lift/twist is not as severe. But in general, the whole gantry seems to be able to rock on opposite corner wheels.
Would this be a matter of the entire table being out of level? When first installed & built, I had the entire table square and level. The tracks still measure equal distance from each other, but the table has twisted so the 0,0 corner is low. Re-levelling the table from the legs hasn't changed the tippy-ness of the gantry, so my GUESS is no, but I'm new to this. I could very definitely be wrong.
I'm also curious about the possibility that the gantry its self might be twisted - when we first got the machine I was totally unfamilliar, but now I can hear when something is off (compared to years & years with laser engravers, it's becoming as comfortable as a big laser). I might simply have not noticed the tipping, now it's got me obsessed. I got a lot of help from someone who said they knew what they were doing....while setting up, I realized they didn't know much more than I do.
Any ideas/suggestions? Is there a preferred way of starting over & re-levelling?

[richards]

Check to verify that all V-"wheels" are firmly contacting all V-"rails" at all points. If a V-"wheel" lifts at any point, find out what caused that "lift" and correct it. Also, check that the spur gear attached to the motor or gear box ALWAYS fully engages the rack as it travels the entire axis.

If an axis "floats", you'll never know how that axis will perform. On the other hand, if each wheel and rail are always in firm contact, you'll know exactly where that axis will be at any point in the cut.

I've never been very good at things mechanical (compared to things electrical), but I always tried to have clean V-wheels and V-rails, and clean spur gears and racks so that everything meshed properly. When the imperfections in the cut could be traced 1:1 to imperfections in the rails, I knew that I had obtained the best cut possible with the machine as it had been manufactured.

[garyc]

Thomas...
In most cases this condition is a result of the X rails being "out of level" or "non coplanar". With the wheel covers off and the motors disengaged from the rack, find which roller is loose and where. If it is loose over the whole X length, then the rails are surely out of level. Usually which ever wheel is loose, is also the corner of the table that is low.

In some cases the gantry could have twisted during shipping or a collision. In those cases there is much more involved process to get the tracking proper.

When I set them (X rails) I use a digital level and a verified 10' straightedge. Level and straight in 2 directions is sometimes hard to get without it.

More info here: http://www.shopbotblog.com/index.php/2008/12/tuning-up-the-prs-gantry/

[Thom Dudley]

Extremely helpful! Thanks!!