Hey Auntjemimma,
this is very simple:
Checking for perpendicularity between milling motor axis and (different areas of) worksurface
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First, you make sure your Onefinity CNC is not twisted. That means all four feet must be in one plane. Since three points are always in one plane, it is enough to make sure that the fourth foot is also in this plane. Adjusting this can be done by placing a shim under one foot, where if the fourth foot is lower than the others, you place a shim under it, while if it is higher than the others, you place a shim on the next foot (in either direction). To perform the measurement to ensure a non-twisted machine, you can use one of the methods described here, the easiest way is the fishing line method, which is very reliable since it really measures on the rails (remember when using this method that in case the two fishing lines touch, you must first swap them over (the other one down) and there must not be a gap either, to finally be sure the four feet are in one plane). Another method used since ancient times to check for twisting are winding sticks.
If you have the QCW Frame which now includes the Any Surface Leveling System, or if you have height-adjustable feet or casters on your table, adjusting coplanarity can be done on the fly after having moved the table.
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Then you surface process the wasteboard with a surfacing bit before the router is trimmed. I repeat: You surface it before the router is trimmed. You may then as result have nice even patterns on the wasteboard. But that doesn’t bother you, because:
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You then place a glass plate on the wasteboard. A glass plate, not a block of granite. You take a block of granite if you want to bend your table top. Here you take a glass plate (the size of the glass plate should be twice the length of the swing arm you use in the next step). Since you made sure in steps 1 and 2 that your wasteboard is flat and not twisted, the glass pane lies flat everywhere without tilting or being twisted. The bumps that the untrimmed router left in the wasteboard are irrelevant, because the glass plate lies on the elevations of the pattern and they are the same height everywhere.
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Now take your swing arm with one dial indicator and clamp it into the router’s collet. I repeat: A swing arm with one dial indicator, not two. Having two dial indicators means that the axis of the instrument has to be perpendicular to the leg with high precision and that is expensive. Spending money without having to is nonsense. With only one dial indicator, however, it doesn’t matter if your swing arm is perpendicular to the axis. Not at all. This is very inexpensive and perfectly accurate (as long as it doesn’t bend or wobble when measuring). You also might use the swing arm method with no dial indicator at all, just bend a steel rod into shape, or you can use the swing arm method with touch probe.
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And now you place your glass plate anywhere you like on the wasteboard, you move the router (with CNC controller powered off) so that it is above the glass plate, and do: Swing dial indicator forwards, measure, backwards, measure, to the right, measure, to the left, measure, and then you have the four values you need to trim the router.
(How the trimming itself is performed is described in another posting.)
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After finally having trimmed the router and checked that now the four measurements on your dial indicator are equal everywhere, you then repeat surfacing the wasteboard, and voilà, the nice even patterns are gone.
Hope this helps!
In fact you are able to surface it correctly. As long as the machine is not twisted (checked in step 1 above) you can surface it – if the machine is not twisted, the wasteboard will not be twisted either and thus be planar. And as for the even patterns that the untrimmed router left behind, you resurface again after the router is trimmed (step 6) and they are gone.
Notes
I have a block of granite like this, and do you know what its purpose is? Dress japanese waterstones and cast-iron hand plane soles with clinging wet sandpaper. For that they are excellent.