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increased speed equals bad corners
« on: March 14, 2007, 03:13:51 PM »
I have been asking around about feed rates and the info I have seen pretty much says to cut at the highest feed rate possible before breaking the router bit. Well I have been increasing my feed rates little by little.  I am making guitar bodies.  Shaping like tummy cuts and carve tops can probably get away with a little inaccuracy but neck pockets and cavity covers need to fit exactly.  I have cut many guitar bodies now at feed rates around 50 to 75 IPM.  These have been cutting properly, no problems.  But as I have increased feed rates up to 100 and 150 IPM,  I am now getting sloppy corners. 

Example:  In a square neck pocket with a .5" bit.  The first corner where the bit plunges in looks exactly as it should with a .25" radius corner.  The other corners that the bit travels through look more like .3" to .35" radius corner.

What I am wondering about is under Motor tuning - Acceleration.  I had this set at 8.  If I bump it up to 20, it seems that I get less deceleration going into a corner.  I am thinking that with the lower acceleration one axis is starting its move before the other axis is reaching its full position. 

I am going to run some tests on this but any thoughts on acceleration or anything else that may be affecting this? And are there any drawbacks to using too high of an acceleration?  I will say that the movement seems a lot more jerky and maybe could be harder on the motors and parts?...

Offline Chaoticone

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Re: increased speed equals bad corners
« Reply #1 on: March 14, 2007, 04:18:42 PM »
Hey Chris,
    Have you tried running in exact stop mode. It sounds to me like you are running in constant velocity mode.

Brett
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Re: increased speed equals bad corners
« Reply #2 on: March 14, 2007, 04:28:18 PM »
Brett,  I  saw that option but have not tried it yet.  Does this mean that it will go to and stop at every location?  Sounds like this may be more accurate but time consuming.  I'll try this as well and see what happens.
Re: increased speed equals bad corners
« Reply #3 on: March 14, 2007, 04:39:28 PM »
Brett,  I just switched to Exact Stop mode and ran the perimeter of a guitar body.  The thing is that going around the wide curves of the body, it is moving in very tiny increments.  I can see how this may be more accurate for pockets but it seems to be very slow and jerky type of movement.  Would there be any way to program this type of movement into the G code for the pockets and constant velocity for other portions of the guitar like the perimeter?

Offline Chaoticone

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Re: increased speed equals bad corners
« Reply #4 on: March 14, 2007, 04:44:32 PM »
Hey Chris, It will be slower, and jerk a lot more. Also, your gcode can change its modes as well. G 61 places it in exact stop and g 64 places it in cv. There are settings you can play with for CV mode that may help as well.

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!
Re: increased speed equals bad corners
« Reply #5 on: March 15, 2007, 04:18:11 AM »
Well after increasing the acceleration from 8 to 25,  I am now getting accurate cuts at 100 IPM.  Though I may experiment with switching to exact stop mode by editing the G-code for some parts of the process.  But i have tons of files and don't really want to edit all of them.  I'll try that on one of my most popular guitar bodies and see how it works out.

Offline chad

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Re: increased speed equals bad corners
« Reply #6 on: March 16, 2007, 12:48:56 AM »
Hey go read up on quantum.  It might be worth giving it a shot.


Chad
Re: increased speed equals bad corners
« Reply #7 on: March 16, 2007, 03:01:10 PM »
Quantum sounds really cool.  Probably just what the doctor ordered for my applications.  And lots of others.  But I think I'll wait for a final release.  It would cost me too much time and $ if I use something that still has glitches.

I am pretty much using Mach3 with its default settings.  I got it up and running easy enough and my focus has been on learning the CAM software.  So are there any other suggested changes to setting in Mach3 that I should consider?  Do most people run in CV or Exact Stop mode?  Are there any other settings that could be tweaked to make performance or accuracy better?  I think I need to go back and watch the setup videos again now that I have a better understanding of the software and machine.

Offline chad

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Re: increased speed equals bad corners
« Reply #8 on: March 16, 2007, 03:22:43 PM »
I on my big mill i pretty much only use quantium at this point it works verry well for being an alpha.

As for the cv yes you want to run with cv on.

As for the settings. try this:

Go to the settings screen:

Click cv feed rate turn it on enter a number that gives you a good cut on corners like 50

Now enter the angular limit. This tells mach whenever you are doing a cut above this number to use the slower feed rate for that part for the cut. So if you use 45 degrees any direction change greater than 45 will slow down to feed of 50. Play with this and you should be able to smooth out the corners and reduce the rounding.

You want your accel at the fastest stable speed you can get.

Chad