Today I reset my z axis with a lot of trammeling, and it was a long way out.
In the end I got it pretty accurate, about .5mm over 120 cm diameter.
Then I did a number of tests including cutting two grooves 6mm deep into MDF with a brand spanking new 5mm bit.
The first groove I cut in 2 passes each 3mm deep, the second in a single 6mm pass.
I checked the groove width with the tip of my calipers,which has a precise 5mm width.
The single pass fitted very very snug, perfect even.
The one with two passes had a little play, not enough to insert a 0.1mm feeler guage in, but almost enough.
Now to my question.
In the ShopBot specs for my machine, the latest PRT96, they quote a positioning accuracy of +/- 0.127mm (my conversion from their imperial), and a cutting accuracy of +/- 0.381mm.
My 'error' in the two grooves is well inside even the first of these.
Finally my question.
Does this mean that if I continually cut the same groove, many times, it could end up 0.762mm wider than planned?
And do I need also to add the positioning error to this?
In fact, is it theoretically possible to cut just twice, and end up with +.127 and +.381 (.508mm or 0.02") error?
In all this I am assuming that there is no physical error, that in fact the accuracy of the way the Shopbot is controlled might lead to these type of errors?
Although this question is a theoretical one, and not something I have noticed whilst operating the shopbot, nevertheless I am interested in what all these specification figures mean.
My tramelling today gave me an error, at the bit, well inside the ShopBot specs.
I recognise that the set up of the ShopBot, and any bit run out can be even greater than this.
However, I am interested in the true theoretical repeatable accuracy of a perfectly set up ShopBot.

.........Mike