Quote Originally Posted by Jonathan View Post
Not bad, but I bet you can do better...

Use the indicator as you have set it up in the video. Move the gantry so that it is measuring over one of the 4 supports (bearings). Now take readings every 20mm (or whatever distance you can be bothered with) and record them in a spreadsheet. Plot the graph and you'll find there will be some small variations due to the aluminium not being flat and a linear error due to the bearings not being at the same height, which is the bit we're interested in. Suppose you get something like the blue line on this graph:

Click image for larger version.  Name: X bed error example.png  Views: 495  Size: 24.7 KB  ID: 7033
As you can see, if a straight line is plotted using the blue curve, it shows the average gradient of the bed. If this function is subtracted from the readings, you end up with the orange line which shows the 'flatness' of the bed with these reading if the perfect size spacer was added. So once you've got the readings, use your favourite spreadsheet program to find the linear regression line and equation, look at the gradient of this line - in this example it is -0.0015, so if the distance between the blocks is 300mm you would need to add a spacer which is 0.0015*300=0.45mm thick. You should then repeat this test measuring parallel to the other axis, then over the other two bearings and you should end up with it being extremely close. The advantage with this method over measuring directly above the bearings is it will account for local variations that could otherwise skew the readings.

Just watched the video again and it looks lie the bed is bowed, can you take the readings and post them here then we can work out if it's worth proceeding?

From the video it looks like you're close enough, but if it was my machine I'd try this to see how much closer I can get it.
Thanks for the advice Jonathan but in all honesty I'm supprised I got so close and it was a pain in the arse to take everything apart, at some point I know I will want it perfect but for nowI want to move onto doing some actual CNC milling and as it mainly art type work 10th of a mill wont make stripping it again for a while worth it.. but like I said thanks for the advice