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Z Axis slippage on CNC Sherline 2000

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j ferguson

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#1
I'll try to share enough of what I'm doing so that someone can pick off the mistakes.

The problem is that I'm losing depth of cut in pocketing operatoins in 6061-T6. For example, If I intend to cut a pocket to .750, I might instead get .662. I'm using a .375 4 flute endmill (I know it should be 2 flute for aluminum). I'm using misting coolant, running at 2800 rpm, Z-Axis travel 6 IPM and horizontal at 16 ipm. no chattering, but it isn't quiet either. I'm using 40% overlap and .020 depth of cut. I have not drilled a starter hole for the cuts nor am I ramping the cutter into the work. I suspect that this is where the problem might be. Or maybe the cutter is dull.

The stepper is Sherline's but I built the controller and it doesn't seem to have any problem with the other axes.

My plan is to buy some new cutters, try 45 dgree ramps into work, but keep same speed and feed and see if that cures the problem. I should add that I'm having similar problems drilling. the suggestion of the local guy who actually knows something was I wasn't advancing the drill fast enough and was either hardening the aluminum or dulling the bit or both. I've seen the calcculators for cooling up with feed rates depending on drill size and rpm and it'as true, I've been way timid. I do pecks at one drill diameter and the holes turn out nicely, but not as deep as specced.

Can any of you suggest anything else I should look at or identify where I'm screwing up.

thanks much,

john
 

DAT510

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#2
A couple questions come to mind:
1) Is it possible the bits slipping in the chuck or are you losing Steps?
2) Is the Z axis calibrated? (Is the Z axis is moving as far as your CNC program is calling?)
3) What CNC program are you using? Mach3? (If so, could your Z acceleration be too high, such you are loosing steps?)

I've personally experienced all three, at different times.

Hope this helps.
 
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j ferguson

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#3
I'm using LinuxCNC running through the parallel port and don't lose any data on x and y axes which are seeing a lot more code go by. I thought I was having loose bit problem so I switched to an ER 32 collet chuck and believe me, the bit's cannot slip in that. I tighten it off the machine so i can really torque it up.
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It's not impossible that there is a problem in the controller although I biult it all at once and all axes have same hardware configuration.

good thought there, thanks
 

DAT510

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#4
In my case, I had a work machine where the "steps per inch" were set incorrectly in Mach3, which resulted in the program displaying the "correct" distance, yet the actual distance travelled was incorrect.

With my new/used personal CNC, the z axis air spring had lost it's spring, and though the Z axis stepper was Stepping, it was not actually turning the screw with every step, which resulted in it "losing steps" since not every step resulted in the screw turning, and therefore with each Z axis movement the program got further out of sync.

Could your Z axis be out of calibration or your Z stepper be losing steps due to binding or something else?
 
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j ferguson

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#5
Dat510, checking the software for correct z-axis steps per inch is a good idea. I had some convulsions early in the fall with the pc I run with and it could be that the configuration got screwed up.. fortunately this is easy to check. Wow. I hadn't thought of that.

easy enough to check what's there and also see if it moves an inch when it's told to.

thanks again
 

j ferguson

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#6
Software config is ok. so I must be losing steps. I'm going to do some testing with different drill feeds and while I'm at it see if ramping endmill into pocket cut helps. it's good that I have a lot of scrap around.
 

j ferguson

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#7
I didn't know about ramping with a non-center cutting end mill. Duh. sorry to have wasted this space.
 
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