Tramming With Taut Wires

[Kitwn]

Some results! Numbers are to 2dp to avoid going way off for calculations but in reality there is some interaction between the effects of different errors and I will need another set of measurements after the first round of shims go in. At least I have some ballpark figures for cutting the required shims.

There's a 1.78mm change of height between the two ends of the Y travel. Obviously skimming the base will hide this for some jobs but therewill still be a geometry error with the Z axis. A little over 2mm of shims will be required at one end of the gantry.

The measured diference between the front and back wire measurements varies between 0.44 and 2.23mm so the gantry is both leaning backwards and the rails are not parallel. 0.44mm over the 600mm between the wires equates to shims of about 0.1mm at the rear bolts of the gantry base.

The difference between the differences of the measured values calculates out to require shims totaling 0.42mm at one end of the lower Y rail. Since the whole rail will need some pretty thin shims at each fixing point I'll correct for the gantry leaning back here as well. This will be the longest and most tedious job but it will be possible to fit a shim, make a measurement, correct the shim, move the spindle, make a measurement, fit a shim..........

To put the twist in the Y rail plane into context, the measured error works out to cause the tool tip to make an unwanted move of about 1mm along the X axis when the Y axis is moved between it's two extremes. When squaring up my gantry using the draw-a-square-and-measure-the-diagonals method I have never been able to draw a square with the two diagonals equal in length to better than about .5mm. This is probably the reason.

Once the gantry is sorted out then the Z axis can be aligned as well.

Looking at the calculated thickness of shims required I'm actually quite impressed! The adjustments needed for my hand-built machine seem very small to me, especially as building this gantry was the first welding I ever tried and was done without the use of a proper welding table to jig all the bits together. I hope my experience will encourage other prospective first time builders to have a go. Just make sure that all your adjsutment points are designed in from the beginning and can be accessed easily once the machine is complete.

Kit