milled a larger structure with more depth (8mm) for more reliable measurements
used a dedicated caliper for the larger square for more reliable measurements
What I milled:
67x67mm outer square with 42x42 inner island in coarse MDF at 2 different positions on the workplace and 1 fine MDF (which I suppose to be harder). Followed by a finishing pass (taking 0mm). Then removed 0.5mm on the contour, followed by one or several finishing passes.
The second image is just for illustration of the coarse mdf.
[attachment file=99179]
[attachment file=99180]
The results (short version):
x/y results are comparable (x:y deviation after a finishing pass is around 0.1mm which I think is great!)
I have a persistent total deviation towards structures being smaller than intended (mill forced away from the workpiece due to elasticity?) of around 0.5mm in coarse mdf and 0.6 or even 0.7 in fine (harder) mdf, which in principle makes sense.
The deviation of the larger square is larger than for the smaller one - which is not clear to me
The results are reproducible at different positions on the working area (which is great)
I have the absolute deviations even after repeating the finishing pass 10x.
I measured the milling bit, it actually is 5.88mm instead of 6mm (due to wear?).
So we could correct reduce the absolute errors by 0.12mm.
What I do not get is that even 10 subsequent finishing passes leave an error of 0.6 for the larger square. The needle test - force free positioning gives me an incredible precision for that size of the machine which is better 0.1mm @ 540mm in y (almost 0 deviation for 70mm) and 0.8mm in x along 800mm (better than 0.1mm for 70mm).
Is this due to the fact that the bit is pushed away from the workpiece due to elasticity and can only cut away a negligible amount? In fact, the numbers from 1. to 10. finishing pass actually get slightly better.
I need to do some experiments in hard foam (XPS) which should result in much less force and therefore in better results; I’m not sure if I can measure the foam with the necessary precision; however it would be interesting to see if the numbers get even better in foam.
edit: I did use 'conventional milling because estlcam states it should be used in soft machines. However, the tutorial seems to prefer climb milling. do you think climb milling will help or be disastrous?
Why did I build that large machine: my main intent is to cut surfboard foam kernels - for which even 1mm ‘off’ is negligible - so the machine is great for that I think.
Every now and then I make some airfoil moulds. I could make them in XPS as positives and cast the actual mould from that (for which the machine should work ok). However, milling the mould directly in MDF would save me the positive to mould casting step.
Whats in there (all numbers corrected for real tool diameter):
left: last mdf numbers from yesterday for comparison
mid: switched to climb milling, cut away 0.2mm followed by 1 finishing pass.
Interpretation: even for the cut pass the numbers a better than after 10. pass using conventional milling. The x/y deviation is quite small -> i will use climb milling in the future
right: new square milled with gantry all up (80mm above the level of yesterday)= most stiffness in Z i can get
Interpretation:Numbers get a tiny bit better
In conclusion:
using climb milling and gantry all up, I get an accuracy 0f about 0.3mm and x/y deviation of almost 0
at z=-80, I get almost comparable numbers
which is a good result for that large machine - don't you think so?
I generally think of climb milling as dangerous because it pulls the cutter into the cut, but now thinking about it more carefully there are actually two actions. Yes it pulls the cutter into the cut in the direction of feed, but it pushes the cutter away from the part lateral to the feed. Conventional milling is reversed: it pushes the cutter away from the cut in the direction of feed, but also pushes the cutter into the part, lateral to the feed.
And this would also have big implications for slotting. You might expect one side of the slot, the climb milling side, to be accurate to dimension, while the other side, the conventional milling side, could be off due to the lateral force on the cutter.
Maybe this is something that veteran machinists know without thinking about it, but for me this is an Aha moment.
Try thinking of it in terms of cutting metal. Starting thin to thick can actually work harden the material by rubbing before it bites or even ramping the cutter away until it has enough lateral force to bite, to me I used to think of it as “easing in” that is wrong. Taking a good bite and easing out is a better cut. Slotting, too complicated o sum up in a few words.
