Author Topic: Erratic Feedrate When Cutting Spline Lines  (Read 4011 times)

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Offline dq828

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Erratic Feedrate When Cutting Spline Lines
« on: September 23, 2017, 07:34:32 AM »
Ivé had this issue both times I have cut something when the original Fusion Sketch was drawn with Spline Lines. The issue being, the Feedrate changes a lot during the cut. When the cutter is cutting the peaks of the knobs in the attached image it runs at the correct feedrate, when the cutter gets half way into the valleys, the feedrate slows to about 1/5 of the set feedrate and jerks forward until it is half way out of the valley and then it speeds up again, repeating this over and over as it goes around. Very annoying

Does anyone know what might cause this erratic behaviour, if anything I thought it might slow down on the tighter peaks?

At this stage I dont know much about code and rely on Fusion Cam to do it for me. I have just had a look at the code and there are sections throughout that look odd to me, see below, maybe you can tell me whats going on.
« Last Edit: September 23, 2017, 07:41:55 AM by dq828 »

Offline garyhlucas

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Re: Erratic Feedrate When Cutting Spline Lines
« Reply #1 on: September 23, 2017, 02:45:37 PM »
There are no splines in G- code so all moves get converted to arcs and short lines. So if you have exact stop programmed the machine will start and stop for every line segment making for rough motion.

Offline Mauri

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Re: Erratic Feedrate When Cutting Spline Lines
« Reply #2 on: September 23, 2017, 03:29:13 PM »
dq838,
You must use CV to reduce machine shake.
Selecting the correct CV will produce outcomes very close to your profile.
If your machining at between 400mm to 1000mm per min, set all the CV 180 angles from 0 to 100.
Regards,
Mauri.

Offline dq828

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Re: Erratic Feedrate When Cutting Spline Lines
« Reply #3 on: September 23, 2017, 07:58:39 PM »
Thanks for the reply's.

I set CV and the CV angles a while back (from your advice) to overcome the Wonky Cutting issue, so I dont think it could be causeing this issue.


Offline Mauri

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Re: Erratic Feedrate When Cutting Spline Lines
« Reply #4 on: September 24, 2017, 12:25:36 AM »
dq828,
Yes, you are correct you have CV running, I did not fully read all your notes until now.
The slow down is the angles and the CV in action.
As you do not show us the exact full F speed I cannot confirm weather the CV is set correctly.
Please advise the F speed and the CV setting, the cutter type and size and the material type being machined.
With this info, I may be able to provide a more optimal set of settings to perform your job.
Using Exact Stop will do the task correctly however will take longer.
Jerkiness is also caused by going too fast and your machine is not built to handle these speeds and directional movements.
Regards,
Mauri.

Offline dq828

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Re: Erratic Feedrate When Cutting Spline Lines
« Reply #5 on: September 24, 2017, 01:26:43 AM »
The Feerate is 1000,
The cutter is a 1/4' 2 flute carbide wood bit.
I am cutting plywood and only about 1/4"deep each pass
The CV angles are all set to 100

The bit I dont understand is the code that cuts the tops of the peaks, which has much tighter curves looks like this;

G1 X70.556 Y156.473
G2 X70.605 Y156.572 I2.863 J-1.372
G1 X70.944 Y157.221
G2 X70.979 Y157.286 I2.814 J-1.471
G1 X71.349 Y157.955
G2 X71.369 Y157.99 I2.779 J-1.536
G1 X71.76 Y158.678
When this type of code runs it all works fine and runs smoothly

And the code that cuts the shallower valleys looks like this, and it's where the jerkyness is, and the Spline line is all one continuos line;

X71.764 Y158.684
X72.566 Y160.086
X72.943 Y160.769
X73.298 Y161.445
X73.623 Y162.108
X73.919 Y162.772
X74.042 Y163.073
X74.16 Y163.385

Is there anyway to enter the CV angles in bulk, rather than one at a time!

Offline Mauri

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Re: Erratic Feedrate When Cutting Spline Lines
« Reply #6 on: September 24, 2017, 02:02:42 AM »
dq828,
It looks like you are using a Router Table and probably have single one direction rails.
If you had rails also in the other direction it would stop any kicking "jerkiness".
So on some angles it probably depends on the direction of movement more X then Y making it Jerk more on the X axis than the Y axis.
You can slow down the F speed to compensate or reduce the CV settings.
Or if you can affix a Roller under the Router Table to stop the X axis kicking that would eliminate your problem.

Two was of setting CV on all axis.
If you do not change the Mach 4 Settings you could make a set of machine.ini with different CV settings.
You can manually edit the machine.ini in notepad make a Set 100 then save it as machine100.ini.
Then make a CV 75 and CV 50 and save it as machine75.ini and machine50.ini etc.
When you want to use a specific one your copy one into another directory and rename it to machine.ini and put it back inti Mach4 directory replacing the existing one (make sure you copy it back and rename it back to machine*********.ini.
When you make the changes in notepad do a search for say = 75 if only the CV angles are 75 and you want 50 then do a replace all.
Hope this helps.
Regards,
Mauri.

The other method is if you know Lua you could code it up on a Mach4 Screen to do it on the fly automatically.
Regards,
Mauri.

Offline garyhlucas

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Re: Erratic Feedrate When Cutting Spline Lines
« Reply #7 on: September 24, 2017, 11:30:06 AM »
Your two code examples, the first the Cam program was able to generate lines and arcs so the motion is smooth. Then the spline curves could not be done using arcs so it made lots of tiny lines to approximate the curve and that motion is jerky as a result.

Offline dq828

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Re: Erratic Feedrate When Cutting Spline Lines
« Reply #8 on: September 25, 2017, 05:46:59 AM »
Yes, but it's all one Spline line, not an arc and then a Spline,  as you say the CAM just couldn't do it all as arcs, I'm sure there is some geometric reason for this but it is beyond my knowledge.

I did manage to speed it up by increasing the CV angle speeds and making the Smoothness setting in Fusion coarser.

I thought I'd out smart the program but using the Spline on the peaks and then an actual arc in the valley's but of course Fusion then refused to generate a toolpath !!!

All good fun and learnings

Thanks for the help

Offline dq828

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Re: Erratic Feedrate When Cutting Spline Lines
« Reply #9 on: September 25, 2017, 05:58:42 AM »
One last question, I'm assuming that with the CV angles, 1 degree is basically the tightest angle,  and 179 degrees is the shallowest angle, is this right.

I would have thought the best thing to do with the angle speeds is have the slowest speed required to cut the tighest angle, and then gradually increase the speeds as the angles get shallower.

I am amazed that one cannot copy and past into the CV Wizard table.