Cutter accuracy

[magicniner]

If I now take cuts on opposite sides of a (roughly 50mm) block to produce two parallel faces and measure them, there is a consistent error of about 0.3mm. Even though I have checked machine movement and cutter diameter.

Try that with machinable wax or another material not strong enough to flex your machine's structure, there is a strong possibility that your machine is flexing under the forces applied by the test cuts.

[Neale]

I take your point, but I am taking light cuts off MDF (a 1mm pass, 4mm depth of cut, 6mm cutter). I'm not saying I'm right, but I can't see how I can take cuts much lighter than that! I'm only removing dust at this point.

[magicniner]

If your cut errors are consistent then you can use cutter compensation to correct for the mysterious error you are encountering.
If you want to locate the error and the cutters are not under size by the error you are seeing then something on your machine is flexing by close to the error amount, there is no magic by which a cutting edge can temporarily shift into another dimension to allow the edge to dodge the material ;-)

[Neale]

To draw a line under this one (at least, until something else makes me get out the worry beads)...

After doing a lot of checking, measuring everything I could, including duplicating measurements I had already done, I have concluded:

my X ballscrews (2005, C7) appear to have a pitch error of about 0.1mm in 100mm. This is somewhat greater than the nominal C7 accuracy of 50um in 300mm, and I would put it down to measuring error except that...

my Y ballscrew (1605, C7) appears to have a similar error but in the opposite direction. I would like to measure travel over a longer distance but I'm limited by the measuring tools I have available (Mitutoyo 200mm vernier caliper, in this case).

Both these errors could be compensated by tweaking "steps per" in Mach3 although I'm a little reluctant to do this without being able to check errors over a larger range.

All ballscrews seem to have about 50um backlash. I have not been able to find a spec for this for a C7 ballscrew, but it's a bit more than I would have hoped. However, for a machine intended for mainly woodworking, this is perfectly adequate, and even for making things like profiled aluminium plates isn't an issue. However, I might consider carefully doing things like machining a bearing housing and that might still have to be done on the milling machine with a boring head or suchlike.

Rechecking cutter diameters, I think that my original problem was that my single-flute cutters are cutting a tad undersize and I shall need to go through and carefully check and record actual cutting sizes so that I can include these in my CAM tool library. I note comments about tool tables, but on the whole I prefer to work with a "calibrated" tool library in CAM and not bother with tool tables in Mach3. I might change my preference in time (I have used tool tables with LinuxCNC in the past - I'm happy with the principle) but the net effect is the same. I have control of the whole CAD/CAM/CNC cycle and I'm not in a production environment where a machine operator will update tool tables locally due to wear/tool change or whatever without needing to re-CAM the part.

Thanks for all the comments, guys, but I have been suffering from my usual position that I don't quite trust anything that I have built while placing too much confidence in bought-in components!

[magicniner]

I would like to measure travel over a longer distance but I'm limited by the measuring tools I have available (Mitutoyo 200mm vernier caliper, in this case).

Get a good brand and quality steel rule, use your digital caliper to make a little brass vernier and you can read to a 0.1mm or better over the length of the rule.
Measure at 20 degrees C though ;-)

- Nick

[Boyan Silyavski]

I dont believe the ball screws have errors. More likely it will be that they are not parallel to rails a bit.

Hence when i designed my machines and in general my practice is to align as much as i can one rail in all directions using straight edge and so on. Then next in same axis. Using mounted-bolted plate what makes me sure all the way they are parallel to each other. Then i mount the ball screws or whatever and move axis to one end. Tighten ball screw by hand , move another end, tighten by hand, then repeat that a couple of times till all is tightened. And as the ball screw was guided by the rail, that means they always have same travel and are perfectly parallel. So no Mach3 compensation is needed. In short i dont see any other way to do it properly, except if all is machined to fit, even then doing it so will be better and easier.

Same is valid for belts, where i make perfectly sure all possible is parallel and square.

[magicniner]

For a 1mm linear travel error in an axis over 1000mm you'd have to have the ball screw misaligned by around 2.5 degrees, a misalignment with the axis of movement of around 44mm end to end.

With the misalignment with the axis of movement at 10mm the total travel error over 1000mm would be 0.050mm

- Nick