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Author Topic: Great mystery.......  (Read 2675 times)

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Great mystery.......
« on: March 28, 2013, 11:18:27 AM »
I was getting some slippage when I went from balsa to mdf. So I decided to change the accel velocity for my k2cnc 2514.  I set the new velocity to 70 ipm and the acceleration to 80 in order to make the x and y axis slip.  For this machine, 110 ipm is the maximum speed. To my amazement, neither axis, even when I place a load on it, slips.
I reduced the acceleration to 30 out of fear, but I am still puzzled by the fact that I cannot make the motors slip at such a high acceleration number.  A long time ago, I place damping wheels on all three motors to reduce resonance. I remember that this helped a lot in reducing slippage.
Any ideas? My steppers put out some 280oz/in and my drivers are set at a half a step. My power supply is a healthy 80volts.
I had not trouble running the machine faster with Balsa, but with MDF I assumed that it had to run much, much slower  due to the fact that the parts were not cutting precisely.

Another thing I noticed is that when I set the pulse rate to a number other than 0, Mach 3 brings it back to 0. Is this normal?
thanks
tony
« Last Edit: March 28, 2013, 11:22:58 AM by screwhead »

Offline RICH

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Re: Great mystery.......
« Reply #1 on: March 28, 2013, 01:34:03 PM »
If the original velocity and accelerations preformed well with no skipping irrelevant of the material or programing then leave them alone.
For actual cutting you adjust the feedrate in the gcode or CAM for the material your working with.

RICH
Re: Great mystery.......
« Reply #2 on: March 28, 2013, 01:49:29 PM »
Actually, I had done that. My assumption was that the motors never had sufficient torque , but the balsa was very forgiving. I was doing production work and using multiple depth passes on 3/8 stock. I am assuming, therefore, that the load on the motors was minimal.
When I began to cut MDF, I tried cutting a simple pocket, .125 deep at 58ipm, with a .1875 tool, and it failed to cut evenly.  By evenly, I mean that the x y went off the normal circular pattern.  When I slowed down the machine, things improved, but I was still puzzled by the drastic errors. We're talking about the work being ruined due to the errors. When I reduced the feeds, things looked almost good, except for the z failing to drill all the way down.  The motors sounded fine; the work was not coming out right.  I purchased the machine for Balsa and had never worked with MDF. When I cut pine of similar thickness, things were fine. I have to assume that the mdf's high density places major strains on the motors.

All of this, however, does not explain why I cannot make the motors slip by increasing the acceleration.

If the motors do indeed have that much torque, then the original feed of 58ipm would not have created all the errors in cutting.

When these things happen, it's like going back to school to study math 101.

tony

Offline Chaoticone

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Re: Great mystery.......
« Reply #3 on: March 28, 2013, 03:21:24 PM »
The MDF is much denser than the balsa............. might show up flex a lot more too.  Backlash could show more.  What kind of bit do you use?  High speed steel will dull in about 10 seconds in MDF.

Brett
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Re: Great mystery.......
« Reply #4 on: March 28, 2013, 04:00:05 PM »
The MDF is much denser than the balsa............. might show up flex a lot more too.  Backlash could show more.  What kind of bit do you use?  High speed steel will dull in about 10 seconds in MDF.

Brett

.1875 carbide two flute downcut....

By increasing the acceleration, the machine feels tight and has cut some very nice pieces at 35 ipm.
tony
tony

Offline RICH

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Re: Great mystery.......
« Reply #5 on: March 28, 2013, 07:19:18 PM »
Like I replied. If experience has shown that there are no problems with the material then continue on as configured.
But, you changed the material and that required additional power to cut the material and by lowering the feedrate you increased the power.

Torque and power decrease as the motor speed increases.
Torque x motor speed = power ......... and power is what you want .....but where the power range  is requires a look at the motor curve.
If you change the material again then suggest some trials on scrap using different feedrates, cutter diameters and depth of cuts.

For G0 moves you are not cutting so the only difference is the weight of the mounted piece which could have an affect on acceleration and velocity
settings, but you should still be below max by say 30% or so from a reliability perspective.

RICH
« Last Edit: March 28, 2013, 07:24:48 PM by RICH »