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General Mach Discussion / Re: Grinding of eccentrics on a lathe (cam lobes) using Mach
« on: April 04, 2020, 05:44:39 PM »
Hi,
No, the critical phrase here is 'as the A axis'. Its the fact that it is an AXIS, ie capable of positional control and can therefore
in incorperated into a coordinated interpolated movement. Because the A axis can be interpolated your grinding program would
work fine.
If you had a spindle that had positional control and thereby become a C AXIS, it to could be used in a coordinated interpolated
movement.
Mach is not suitable as a feedback controller by virtue of the motion buffer imposed by Windows OS's. If however you
required a very low bandwidth feedback loop, then if your spindle were fitted with an encoder you might achieve your aim.
Lets say you have an encoder fitted to the spindle, and at the moment the angle is 45.89 degrees from some nominal
home or index position. You could use Mach to calculate the required X axis position to grind the surface of the lobe from
your previously calculated lobe profile. Mach would issue an X axis movement. The motion buffer would mean that the
movement would ocurr but delayed by 200ms or so, the length of the motion buffer.
Now the spindle is rotating, and as you will see, rotating very slowly. Lets say that is has rotated by one degree, now
46.89 degrees. Mach could again calculate the required X axis position to grind the lobe and send the required movement
instruction via the buffer to the machine.
Provided the spindle rotated VERY slowly then the X axis would follow the proflie you want. If however you allow the
spindle to rotate to fast the Mach movement instructions to the X axis will be wrong by virtue of the motion buffer delay.
Having a motion buffer, with its associated delay, is an inescapable fact of using a Windows based PC.
You could achieve a sufficiently accurate X axis following PROVIDED you rotate the spindle slow enough OR have
a genuine C AXIS.
Craig
Quote
Would the same angular velocity issue apply to a rotary table configured as the A axis into which I put the cam lobe and then attach a grinding wheel to the mill spindle?
No, the critical phrase here is 'as the A axis'. Its the fact that it is an AXIS, ie capable of positional control and can therefore
in incorperated into a coordinated interpolated movement. Because the A axis can be interpolated your grinding program would
work fine.
If you had a spindle that had positional control and thereby become a C AXIS, it to could be used in a coordinated interpolated
movement.
Mach is not suitable as a feedback controller by virtue of the motion buffer imposed by Windows OS's. If however you
required a very low bandwidth feedback loop, then if your spindle were fitted with an encoder you might achieve your aim.
Lets say you have an encoder fitted to the spindle, and at the moment the angle is 45.89 degrees from some nominal
home or index position. You could use Mach to calculate the required X axis position to grind the surface of the lobe from
your previously calculated lobe profile. Mach would issue an X axis movement. The motion buffer would mean that the
movement would ocurr but delayed by 200ms or so, the length of the motion buffer.
Now the spindle is rotating, and as you will see, rotating very slowly. Lets say that is has rotated by one degree, now
46.89 degrees. Mach could again calculate the required X axis position to grind the lobe and send the required movement
instruction via the buffer to the machine.
Provided the spindle rotated VERY slowly then the X axis would follow the proflie you want. If however you allow the
spindle to rotate to fast the Mach movement instructions to the X axis will be wrong by virtue of the motion buffer delay.
Having a motion buffer, with its associated delay, is an inescapable fact of using a Windows based PC.
You could achieve a sufficiently accurate X axis following PROVIDED you rotate the spindle slow enough OR have
a genuine C AXIS.
Craig