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Author Topic: How to slow down on rounded corners?  (Read 2388 times)

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How to slow down on rounded corners?
« on: November 08, 2017, 07:27:59 PM »
Hello all.  I am a fairly new user to mach3 and I have been successful in finding solutions for my issues by doing searches and I have seen many results for this topic but none that really show me how to do this.

I am cutting 0.045" thick coated aluminum with an onsrud 3/6" bit and it does a great job, it was recommended by the aluminum manufacturer and they shared their formula with me (100 IPM, 18,000 RPM, single pass) and I am doing 0.125" radius rounded corners as these are all rectangular panels.  About 15% of the corners come out imperfect.  The company suggests I make it slow down when doing the corners.  I read a lot about constant velocity but I am not sure what exactly I should change to have it slow down for doing corners.

I asked a CNC consultant, and he said to adjust the "acc dec" which is short for acceleration/deceleration.

Can anyone point me in the right direction on how to make my machine slow down on doing the corners, or share any other information to help get nice consistent rounded corners?  I have a vaccuum holddown table and it does a great job so I dont think its lack of power as the parts are not moving.

I posted picture of my issue, and my mach3 settings.

Offline Davek0974

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Re: How to slow down on rounded corners?
« Reply #1 on: December 24, 2017, 06:40:21 AM »
I think that 'wobble' is exactly that - mechanical wobble caused by machine flex. You do not need to slow down in corners, Mach will control that within the bounds set in motor tuning acceleration setup.

You could try just slowing the whole path down a bit but watch for cutter burning etc - to me it looks like the machine can not handle the speed requested accurately.

I had the same thing on my Plasma table as that has to run fast, much faster than milling, I had to redesign things a lot to get the wobble to an acceptable level, slowing down on Plasma causes more issues so its better to fix the problem.

Check your motion with a dial indicator for slop etc at the tool tip, it could just be loose pulleys, worn belts etc
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives.
Plasma table, Mach3 V062, Step motors, C&CNC THC.
Re: How to slow down on rounded corners?
« Reply #2 on: December 24, 2017, 11:28:42 AM »
I would try Exact Stop just to see what difference it makes.  In exact stop the machine will come to a complete stop before rounding the corner. So it will be starting from zero speed and accelerating through the corner, effectively slowing it way down on the corner only.  This doesn't work well on tool paths that have lots of little segments to a curve, but may work well here.
Re: How to slow down on rounded corners?
« Reply #3 on: December 24, 2017, 11:50:50 AM »
I don't think a lot of people understand how little rigidity many machines have.  There can be looseness in the linear bearings, looseness in ball screw nuts and end bearings, looseness and stretch in timing belts, flex in the machine frame, flex in the moving cross members, looseness in the spindle bearings, flex in the spindle, flex in the toolholder, and flex in the too itself.  This explains why industrial machines weigh thousands of pounds.

An interesting experiment that may help many of you get better results.  Place a dial indicator on the table and against the tool in the spindle.  Press on the tool with you finger and see how far the indicator moves.  If you take the torque rating of your spindle motor and convert it to force at the radius of the tool you want to use you can then push on the tool with that amount of force to see how much deflection would occur at maximum load.  Moving the dial indicator up onto the spindle will remove tool deflection from the measurement.  Moving it onto the spindle housing removes the spindle deflection. Moving it on to each axis shows the axis motion.

The axis themselves may see much larger forces as they accelerate and decelerate in each direction. You can estimate that force from the motor torque rating and the gear ratio applied to drive the axis.  Pressing against the axis with that amount of force will show how much deflection there can be in a rapid turn around a corner.  If you move the dial indicator to show deflection in one part to another you may find there is a real weak link worth fixing.

Years ago I built pneumatically actuated bagging machines and a thermal printer mounted on the machine printed very poorly due to vibration.  Everyone suggested changes to fix the problem.  Using the dial indicator as above I isolated the bulk of the problem to one frame member no one suspected.  Beefing that up solved the problem very simply.

Offline Chaoticone

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Re: How to slow down on rounded corners?
« Reply #4 on: December 24, 2017, 04:52:35 PM »
Yup, all very good advice so far.

If running servos could possibly be servo tuning too. You will want to read the manufacturers docs to help with that (if that is the case). Most drive manufacturers will remote in a tune them for you too.
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Offline beefy

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Re: How to slow down on rounded corners?
« Reply #5 on: December 25, 2017, 03:05:29 PM »
Not saying you need to do this for your problem (i.e. if your problem is due to lack of machine rigidity), but if you have Sheetcam you can use Path Rules to change feedrate on the fly. You can set exactly where on the cut path feedrate changes happen.

Acceleration / deceleration settings can only get you so far, especially as the linear distance around a corner increases. E.g. for a larger radius you can't just go decreasing accel/decel. That would mean it would be applied to any change of direction.
Re: How to slow down on rounded corners?
« Reply #6 on: December 25, 2017, 03:23:07 PM »
corner chatter exists in the commercial machine shop world too.  we deal with it by reducing the feed by as much as 90% in the corner itself, ie, the g02 and g03 moves, then back to full feed for the straight lines, like this:


commercially, it is female corners that give the most troubles, with chatter.

yes I copped out on my I's and J's with the R command, I used to be able to easily do it mostly in my head, but its been a long time.  Merry Christmas.