Is this an example of what backlash can do?

[Minnican]

ok..... i'll adjust the X and Y to 800 and rerun the sphere. I presume I need to lower the Z from 700 as this is losing steps too.....?

[blackburn mark]

I presume I need to lower the Z from 700

yea, thats the one, dont run the sphere unless your sure youve cracked the problem (or you have a boat load of wood and time) might be wiser to cut a hole in something and measure how round it is

[Jonathan]

Hmm, that 'sphere' looks familiar! :whistling:

I think you should try running back and fourth with a greatly reduced feed rate. Say 50%. If it does go back to zero accurately you then know it's missed steps, not something more interesting happening.

You may find that it runs fine when jogging, but not in reality due to there being multiple motors running when cutting. So try jogging both X and Y by pressing multiple arrow keys at the same time.

If you really want to do it properly then get a piece of string, a pulley and a suitable weight (say 10N ish, much more if you're thinking of cutting aluminum) to simulate the cutting force. Set it up so that moving the X axis lifts the mass vertically and ideally have it so the string from the Z axis to the pulley is horizontal. Shouldn't be too hard ... Repeat the process for Y, and Z if you're keen!

Hope that helps...

Oh and to answer the original question - no that's not what backlash will do. Poor backlash would cause 4 lines to be visible on the surface of the sphere, each at the point where the X or Y axis changes direction.

[Minnican]

ok..... so I've just found a setting for axis calibration and adjusted as follows.....:

X set to move 80mm and moved only 77mm
y set to move 80mm and moved 80.5mm
z set to move -50mm and moved -48.5mm

lets see if this make a significant difference.

Also noticed this line in the GCode:

N80X0.000Y0.000F2400.0

Is the F for feed rate and is it too fast...?

Thanks in advance!!!

[Jonathan]

N80X0.000Y0.000F2400.0

Is the F for feed rate and is it too fast...?

Thanks in advance!!!

Yes that's right. 2400mm/min is much too fast for your machine seeing as the x/y will only do 1500. Mach3 will automatically limit the feedrate to 1500, but still if your machine only just manages to rapid at that speed it's no wonder it looses steps with the additional force due to cutting.

I'd set the feedrate to maybe 800mm/min to start with and see how it goes. Be careful though - too low a feedrate and too high a spindle speed will burn the cutter. So check it's not getting too hot...

[Minnican]

This forum is great........ thanks for all the advice...... I'll let you know how my next sphere gets on in the morning......!!

[Minnican]

Ok.... so had a day off yesterday but back today.....

I have changed and rechanged every setting I can find without any change in performance but finally I changed the "step pulse" to 10 and the "dir pulse" to 15 for each of the axises. The whole mill now sounds sweet without the "grinding" noises I had experienced from day 1. It actually sounds like other mills I have seen on Youtube etc.

I've run the top half of the sphere again and this is the result......



Note my cutter seems to get very close to the sphere on the roughing toolpath, this was something I didn't expect and a potential problem, I presume this is solved using longer cutters?

Thanks for all the advice...... I've learnt much following all your suggestions....... and above all found a cure for this problem.

[Jonathan]

Note my cutter seems to get very close to the sphere on the roughing toolpath, this was something I didn't expect and a potential problem, I presume this is solved using longer cutters?

I think you're referring to the chuck colliding with the sphere? If so then yes a longer cutter is the way to go. Be careful though - cutter rigidity is inversely proportional to the cube of the length of the cutter.

[pavlo]

That looks a whole lot better

[blackburn mark]

I've run the top half of the sphere again and this is the result......

Splendid!!..... i bet that felt a whole lot better

Note my cutter seems to get very close to the sphere on the roughing toolpath, this was something I didn't expect and a potential problem, I presume this is solved using longer cutters?

not sure if i get what you are saying...
i cant see any rough cut marks through the last cut... if your having problems though you will have to stipulate at the CAM stage of your G-code production how much clearence you want between your rough cut and finish cut