Cutter accuracy

[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

[Neale]

Thanks, Nick - you got there first! I did some quick back of envelope trigonometry earlier and reckoned that any misalignment that gave that linear error would be so great that even with my eyesight I would see it. My ballscrews are about 1700mm which I reckon would put one end about 76mm out of line.

I'm also using Hiwin rails as well as ballscrews; you either use accurately machined mounting surfaces (just not possible without access to some pretty big kit) or you build in adjustment. I was surprised just how unforgiving of misalignment the Hiwin rails are. I have adjustments available at all key points and, not surprisingly, used the method described by Boyan (although I suspect that it has been described many times before). However, I have been using a vernier caliper (not digital, unfortunately) and although still in good nick and a reasonable brand, it's not really to be relied on at this level of resolution. I've no idea what the realistic accuracy of a vernier caliper is as I've only found mention of reading resolution which is not quite the same thing. Maybe the Easter bunny will bring me a decent digital caliper, by which time my garage temperature might even be approaching the nominal 20degC required

[Piccyman]

Neale, I have exactly the same problem, did you manage to resolve it?

I don't have stiffness problems on the machine, to test I am only cutting 3mm deep slots 1mm at a time, i have tried 0.5mm at a time.
I have set the steps in mach3 and can run the machine up and down the x and y axis all day at 3m/min and still go accurately back to the start point.
Not losing steps have dial gauges set on the x and y axis at the home, so i would see and changes.
I have measured the bits and they are spot on.
I am using aspire to generate the gcode

I have also setup Uccnc to see if its a Mach3 problem, still the same

2005mm ballscrews (not out of align)

I have tried cutting a few lines 25mm, 50mm, 75mm long using profile cut and 100mm, 50mm and 25mm square using pocket cut, all out by 0.2mm to 0.4mm

I tried 4 slots (for a guitar fretboard) that should have been
35.64 measured 35.98
33.64 33.86
31.75 32.00


Any ideas?

[Piccyman]

I have the same problem, the cut is always out between .2 and .4 mm

The steps have been set, there is no steps being dropped.
I set the steps with a 900mm rule, and checked the settings by moving the x axis about 10 times while holding it back with all my strength. still spot on.

I have checked the 6mm bit and I use aspire for the code.
I have made 25mm, 50mm and 100mm square pockets, 25mm 50mm 75mm slots, they are all out 0.2 to 0.4mm

I have tried Mach3 and Uccnc, both the same.

UC300-lpt5 motion controller and HG08 Breakout card
Stepper motors are direct drive, onto 2005 ballscrews (2 on x axis linked not slave), with dial gauges at the x and y homes (just to check)

Any ideas?

[Neale]

My apologies - you've reminded me that I never really described my conclusion. After a lot of testing, I came to believe that apart from a very small amount of backlash there were no significant errors in my machine (and bearing in mind that this is a primarily woodworking CNC router and not a toolroom machining centre). In other words, it was working as well as my cheap C7 ballscrews, etc, allowed.

What I did conclude was that for whatever reason, my cutters did not cut to their nominal size. What I do now with a new cutter before using it for anything critical is to use it to cut a test piece - two parallel cuts along each edge of a small block a known distance apart as defined in gcode. I then measure the actual width of the material between the cuts and compare it with what I expected. That gives me the effective cutter diameter. For instance, my 4mm single-flute cutter cuts as if it were 3.80mm or somesuch. I just plug this number into the tool library in Fusion 360 and just carry on. Recently I have been cutting 3.5mm ply panels which use a tab and slot arrangement to make up shallow trays. Once calibrated, I get very repeatable and predictable dimensions so that tabs fit slots very nicely whether slots are horizontal or vertical, even when dimensions are changed to make other size boxes.

So, conclusion - my cutters do not cut to their nominal size, but once the effective cutting diameter is measured (by actual cuts, not by measuring the tool) that dimension is very repeatable.

Maybe I should just buy better quality cutters!

Hope this helps - I've stopped worrying about it now.