cutting problems

[gaz221269]

Hi, any help would be appreciated

i have a problem when cutting material, i am using a 10mm 3flute down cut spiral, when cutting the bit plunges into the work at a given point, cutting what it is told to and when it reaches the end the bit is retracting and leaving a small blip on the work, i am sure it is something to do with tool offset as the tool enters at a point then finishes where the trailing edge of the cutter entered, thus giving me this annoying uncut material at the end of the work, i am using cut2d to generate g-code, below is a copy of the g-code at the start of the program, this is just cutting a circle out of the centre of the work


( Mach2/3 Postprocessor )
N20G00G21G17G90G40G49G80
N30G71
N40T7M06
N50G00G43Z6.001H7
N60S18000M03 M07
N70G94
N80X0.000Y0.000F8000.0
N90G00X866.307Y569.271Z6.000
N100G01Z-4.750F1200.0
N110G3X882.800Y516.552I36.672J-17.467F8000.0
N120G3X935.316Y533.140I17.175J37.048
N130G3X918.823Y585.860I-36.672J17.467
N140G3X866.307Y569.271I-17.175J-37.048
N150G01Z-9.500F1200.0
N160G3X882.800Y516.552I36.672J-17.467F8000.0
N170G3X935.316Y533.140I17.175J37.048
N180G3X918.823Y585.860I-36.672J17.467
N190G3X866.307Y569.271I-17.175J-37.048
N200G00Z6.000

any help please

[Hood]

Code seems to be fine.
Hood

[RICH]

Posted code is fine here also.

g-code at the start of the program

So that is not the whole story......so post all the code.

RICH

[alenz]

His Gcode doesn’t have any approach/depart curves. Might that account for what he is seeing?  I’m not familiar with cut2d so can’t offer help with that.
Al

[BR549]

Is the tool you specific in Cut2d the exact same diam as the tool you are cutting with?? That is the only reason for a spur left uncut that Ican think of.

(;-) TP

[Hood]

His Gcode doesn’t have any approach/depart curves. Might that account for what he is seeing?  I’m not familiar with cut2d so can’t offer help with that.
Al

Dont think it needs a approach as its plunging in.

Is the tool you specific in Cut2d the exact same diam as the tool you are cutting with?? That is the only reason for a spur left uncut that Ican think of.

(;-) TP

 I dont think it would really matter as the cut is starting and ending at the same  point so even if the cutter was smaller it should still enter and leave at the same point. What may possibly be the cause is play in the spindle or even the axis, dont know what material is being cut but at 8m/min its pretty fast so may be quite a bit of force on the spindle/axis which would possibly be enough to leave the "small blip"

Hood

[RICH]

Post a picture of what you are seeing.

RICH

[alenz]

I was thinking that a plunge and retract on the cut line will always result in a slight divot on the cut surface. On second reading I doubt that is what the OP is referring to.
However what he might be seeing is due to machine flex, especially if it is not an end cutting mill. On the final plunge cut the DRO may show the proper depth but the G03 move will begin before the bit is at full depth. It actually 'ramps' the last little bit of plunge thus resulting in an uncut blip such as he describes.
I used to run into this with my tabletop (Sherline) machine. The fix is to use approach/depart curves with overlap. The plunge is off the part and the approach allows the tool to ramp down (if needed). Should be standard procedure IMO.
Al

[gaz221269]

Hi folks
thanks for all your suggestions, i have slowed the feed rate right down to 3m/min, all i am cutting is 6mm poplar ply, slowing the feed rate seems to have helped a little but i have found the main cause is the vacuum bed is not holding the work tight enough and it is moving slightly, is there a setting in mach 3 to overlap approach and depart curves as you explain or does it need to be written into the g-code, if so could you please give an example of this as i new to this
many thanks all

[Tweakie.CNC]

Sounds like a larger vacuum pump or new suction cups would be the best solution. 

Tweakie.