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feedrate in feedbackloop
« on: August 03, 2015, 07:41:39 AM »
Hi,
I have just started to experiment milling steel with my homemade mill. It is equipped with a low torque high speed spindle
which is far from ideal for high torque requirements of steel milling. On two occasions over the weekend I broke two carbide
tools because I overloaded the spindle, it stalled and the axis overdrove the tool and 'ping'.

I wondered if it might be possible to have some sort of feedrate override so that when the spindle is near stalling (sensed either
speed lower than critical or high current) the federate could be automatically reduced in sufficient time to save the tool.

Some of the imediate questions that occur to me are:
1)When a line of code is in the instruction buffer is it too late to alter the federate at which it occurs?
2)If the tool load reduces and the feedrate is reset to the programed federate would that constitute instant acceleration and stall an axis?
3)If some sort of feedrate control in a feedback loop is not possible could a 'federate halt' be signalled and would it be quick enuf to
prevent a 'ping'

I'm trying to avoid an Estop event as it seems that several instructions get lost and have had difficulty in restarting at the right point,
this maybe my own inexpertise but suspect that a software triggered Estop may still be too slow to prevent the 'ping'.

Craig
My wife left with my best friend...
     and I miss him!

Offline BR549

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Re: feedrate in feedbackloop
« Reply #1 on: August 03, 2015, 10:31:32 AM »
Yes you can . IF you search the forum you should find a reference to it somewhere. It can run from the Macropump .

There is a setting that you can select FRO on buffer or not.

NOW to use it you MUST have a spindle index to show TRUE rpm. Without it you can't see the true rpm so there is nothing to monitor spindle rpm drop.

(;-) TP
Re: feedrate in feedbackloop
« Reply #2 on: August 04, 2015, 03:31:06 AM »
Hi BR549,
thanks for your reply.
The spindle has no indexer built in but several ideas occurred to me that might work.
For the rpm involved, less than 5000 say, it might be possible to drive a small motor by belt from the shaft above the clamp nut
as a crude tacho. Might also be feasible to use an optical counter such as RC guys use.
The last idea I had was that as I recall it was possible for the VFD to measure slip as a near to linear approximation to demand torque.
Either way I think it possible to reasonably determine rpm and or torque which would be sufficient for my purpose.

After I first posted I started looking at the OEM DRO's and buttons and noted quite a few which could offer some control over federate
under software control. As an aside it is a bad habit of mine to start thinking after I've already opened my mouth.
Your reply suggests that it may to reduce/stop feed live rather than delayed by the buffer and that interests me. My guess is that it
would be necessary to reduce the federate within milliseconds to avoid a stall with the rotational inertia of the spindle being the only
(time) saving grace.

Thanks again, I will RTFM. If anyone has any ideas to focus my search please post...

Craig
My wife left with my best friend...
     and I miss him!