Guest
Show Posts
1
General Mach Discussion / Parts are not correct size!
« on: April 05, 2009, 02:39:11 AM »
Hello All,
I've got a slight issue at the moment...
I'll use one of my parts as an example but I get the same results from all toolpaths.
Basically everything is machining beautifully with the exception that nothing is 'spot on' for size ... sounds like a bit of a contradiction I know - but what I mean is that the same part can be made over and over again and each time the job completes without issue .. no missed steps, no chattering, beautifull straight cuts with a mirror like finish, internal circular pockets are also mirror like and shaped correctly (and a I said above this is the same for multiple toolpaths).
I can make the same part 3 times over out of one piece of stock ... my toolpaths use offsets - so I just re-home and apply new offsets then pick up the sub file and begin the next part.
PC is a dedicated dual core AMD, 2gb RAM, etc etc - it is a dedicated PC with NOTHING except XP and CAM software on it.
The issue is that all outer edges (ie profiles) are approx 0.3mm too big and circular pockets are approx 0.3mm too small - so a part that has an external dimension of 70mm comes off the machine as 70.3mm ... the 26.25mm dia pocket comes out as 25.95mm.
Toolpaths are all fine - I've checked the positional data and allowing for tool diameter the toolpaths are spot on. I should mention that the same toolpaths run under TurboCNC3 cut the part exactly as it should be (ie correct sizing).
Tools are high quality TiCN and Zirconium bits (approx $50 & $70 each!!)
I have Mach3 set to Exact Stop mode. There are no tool offsets in the system at all - I never use them. Every toolpath has the same header info in it:
G61 (Set exact stop mode)
G49 (Cancel tool offsets)
G17 (Set X-Y plane)
G40 (Cancel cutter radius compensation)
G21 (Set units to metric)
G80 (Cancel motion and canned cycle mode)
G90 (Absolute mode)
G52 X0 Y0 (Remove any offsets applied to X & Y)
Backlash compensation is used but the system has very little backlash anyway (.04mm on Y and .03mm on X ... so lets say 1 thou" on each). Having backlash compensation on or off makes no difference to this issue anyway.
With digital calipers fixed to the table and head the measured movement is spot on ... tell it to move 100mm and it moves to 100.00 on the calipers - so motor setup/ratios are correct.
Have tried various cutting speeds, acceleration, top speed (rapids) etc - again no effect on the outcome.
I've had the CRO's on both step and direction outputs for each axis card - again they are all good.
It seems to me that Mach3 is applying a 0.15mm cutter compenstion on every toolpath or is somehow rounding off all movements.
I don't know if I'm missing something - or if there's a bug in 3.042.020.
Sorry if the answer to this is elsewhere on the forum ... searching for it came up with either no or unrelated results (everybody terms things differently).
Can anyone shed any light ??
Thanks
Marc
I've got a slight issue at the moment...
I'll use one of my parts as an example but I get the same results from all toolpaths.
Basically everything is machining beautifully with the exception that nothing is 'spot on' for size ... sounds like a bit of a contradiction I know - but what I mean is that the same part can be made over and over again and each time the job completes without issue .. no missed steps, no chattering, beautifull straight cuts with a mirror like finish, internal circular pockets are also mirror like and shaped correctly (and a I said above this is the same for multiple toolpaths).
I can make the same part 3 times over out of one piece of stock ... my toolpaths use offsets - so I just re-home and apply new offsets then pick up the sub file and begin the next part.
PC is a dedicated dual core AMD, 2gb RAM, etc etc - it is a dedicated PC with NOTHING except XP and CAM software on it.
The issue is that all outer edges (ie profiles) are approx 0.3mm too big and circular pockets are approx 0.3mm too small - so a part that has an external dimension of 70mm comes off the machine as 70.3mm ... the 26.25mm dia pocket comes out as 25.95mm.
Toolpaths are all fine - I've checked the positional data and allowing for tool diameter the toolpaths are spot on. I should mention that the same toolpaths run under TurboCNC3 cut the part exactly as it should be (ie correct sizing).
Tools are high quality TiCN and Zirconium bits (approx $50 & $70 each!!)
I have Mach3 set to Exact Stop mode. There are no tool offsets in the system at all - I never use them. Every toolpath has the same header info in it:
G61 (Set exact stop mode)
G49 (Cancel tool offsets)
G17 (Set X-Y plane)
G40 (Cancel cutter radius compensation)
G21 (Set units to metric)
G80 (Cancel motion and canned cycle mode)
G90 (Absolute mode)
G52 X0 Y0 (Remove any offsets applied to X & Y)
Backlash compensation is used but the system has very little backlash anyway (.04mm on Y and .03mm on X ... so lets say 1 thou" on each). Having backlash compensation on or off makes no difference to this issue anyway.
With digital calipers fixed to the table and head the measured movement is spot on ... tell it to move 100mm and it moves to 100.00 on the calipers - so motor setup/ratios are correct.
Have tried various cutting speeds, acceleration, top speed (rapids) etc - again no effect on the outcome.
I've had the CRO's on both step and direction outputs for each axis card - again they are all good.
It seems to me that Mach3 is applying a 0.15mm cutter compenstion on every toolpath or is somehow rounding off all movements.
I don't know if I'm missing something - or if there's a bug in 3.042.020.
Sorry if the answer to this is elsewhere on the forum ... searching for it came up with either no or unrelated results (everybody terms things differently).
Can anyone shed any light ??
Thanks
Marc