feed rate different - arcs and straights

feed rate different - arcs and straights

Postby sseale » Sun Jun 12, 2016 6:58 pm

I'm new to cnc and control software operations.
I was cutting a lettered sign in 1" thick pvc trim board and noticed how much faster the straight line cuts (like H and A)were being made compared to the curved letters (like O).
The feed rate was set to 100IPM in the tool setup, and based on the bit size/type, chipload, material being cut, cut depth/width etc...
The large feed speed reduction observed on the curved edges could be a problem for a less forgiving material where overheating and chip re-welding may occur.
The post processor I select in Aspire is LinuxCNC arcs (inch), and this is for output to a Probotix Nebula machine.
I'm trying to understand where the reduction is applied, why, and does the user have any control over this?
Thanks,
Scott
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Re: feed rate different - arcs and straights

Postby scottp55 » Sun Jun 12, 2016 7:13 pm

What's the Probotix OS? Mach 3?
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Re: feed rate different - arcs and straights

Postby sseale » Sun Jun 12, 2016 7:22 pm

It's LinuxCNC ver 2.5
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Re: feed rate different - arcs and straights

Postby JoeM » Sun Jun 12, 2016 7:26 pm

It's normal. If you google something like "cnc slows down on arcs" (without the quotes") you'll probably find a few thousand explanations they "why" of it, ranging in level from "CNC for Dummies" to "PHD in Quantum Physics Machining"
Basically you can get speed or accuracy but not always both. You can't expect the machine to make a turn where it's constantly adjusting x and y at the same time, and still go as fast as on a straight run, just like you can't expect a NASCAR driver to go 200mph on the straight runs, and 200mph on the curves and not end up driving off the track into the stands.

You can adjust the tolerance settings for your machine using G64 instead of G61 and setting the parameters as loose as you are comfortable with - knowing you might get a little/lot loss of accuracy if you go screaming into a curve at warp speed. G64 tells the machine how much accuracy you're willing to give up in order to maintain speed.

On the software side, if you use the "fit curves to vectors" option it makes it a bit more efficient too although it still won't make the curves cut as fast as the straight parts.
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Re: feed rate different - arcs and straights

Postby sseale » Sun Jun 12, 2016 7:51 pm

Thanks. You've pointed me to the trajectory control and tolerance settings where I can learn more about them - hopefully adjust and optimize each job depending on the accuracy required.
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Re: feed rate different - arcs and straights

Postby IslaWW » Sun Jun 12, 2016 7:52 pm

Like Joe says above you cannot expect the machine to go as fast around a corner as it could going straight. That said, there should be settings in the control software to adjust this to real world conditions. I.E., if set at 100 ipm you should easily be able to maintain that speed on a radius of a certain size. That size varies with the overall speed, but none the less, there are settings in the control software for user testing and adjustments.

Some higher end controls actually use an algorithm that factors in centrifugal force and have a single number that allows for easy adjustment. Think of this as a weight suspended on a string from a cars rear view mirror. From straight down (zero force) to straight out to the side (max force) as a scale of numbers. This allows the number to be adjusted to the machines capabilities (mass, rigidity) and then perform close to those abilities on speeds fast and slow on arcs or curves of small or large radii. This allows for speed without loss of quality or accuracy.

Find out what is available for your control, do your homework and test away!
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Re: feed rate different - arcs and straights

Postby sseale » Sun Jun 12, 2016 8:16 pm

Will be looking into the settings and options available, now I know! I understand the reduced speed round the curve concept, but it was the severe reduction I saw which really got me to bring this up. The arc radius of the letter O was quite large, and a smooth curve of low complexity. I would estimate a drop down to about 25IPM from 100IPM. Will dig into the machine settings next time at the shop and see what the current defaults are.
Thanks to all.
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Re: feed rate different - arcs and straights

Postby martin54 » Mon Jun 13, 2016 1:39 pm

sseale wrote:Will be looking into the settings and options available, now I know! I understand the reduced speed round the curve concept, but it was the severe reduction I saw which really got me to bring this up. The arc radius of the letter O was quite large, and a smooth curve of low complexity. I would estimate a drop down to about 25IPM from 100IPM. Will dig into the machine settings next time at the shop and see what the current defaults are.
Thanks to all.


Are you sure it was arcs & not a series of hundreds/thousands of straight line segments that looked like an arc? Don't automatically think that because you have a curved shape it is made up of arcs :lol: :lol: :lol:
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Re: feed rate different - arcs and straights

Postby IslaWW » Mon Jun 13, 2016 3:30 pm

Martin...

"Are you sure it was arcs & not a series of hundreds/thousands of straight line segments that looked like an arc?Don't automatically think that because you have a curved shape it is made up of arcs"

In a modern controller, it shouldn't make a difference. This does assume that the controller has the proper settings. All curved vectors are cut by CNC machines using hundreds/thousands of straight line segments. These segments are generated either by the design software or by the controller itself when G2/3 commands are present.
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Re: feed rate different - arcs and straights

Postby martin54 » Mon Jun 13, 2016 10:25 pm

Thanks for correcting me Gary, I run mach3 & I quite often convert stuff to bezier curves purely because I was told that it would run faster that way :lol: :lol:
Mind you it isn't the first thing this guy has told me that turned out not to be true :lol: :lol: :lol:
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Re: feed rate different - arcs and straights

Postby IslaWW » Tue Jun 14, 2016 3:42 am

Martin...
I am not saying that what your friend says is not true. What I am saying is that it doesn't have to be.

In most cases "jackhammer" motion on segmented curves and unneeded slowdown on arcs is a result of controller settings that are not as good as they could be.
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Re: feed rate different - arcs and straights

Postby sseale » Tue Jun 14, 2016 7:02 pm

The cutter motion I saw was quite smooth, just really slow. I've had a chance to check the active G codes and also found G64 P0.001 in the third line of the file.
On checking the vectors themselves, the curves are comprised of numerous nodes with straights between, might benefit from the 'curve fitting' as it did seem to reduce number of nodes in the drawing. I'll try as a test sometime, but have a feeling the higher tolerance G64 P parameter might be the main driver of the speed reduction on 'simple' cuts.
Just now wondering where this gets applied and how to edit- by user in LinuxCNC, or by Aspire postprocessor when saving toolpaths?
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Re: feed rate different - arcs and straights

Postby 4DThinker » Tue Jun 14, 2016 9:53 pm

If the G64 P.001 you found is in the cut file, then it was put there by the post processor you used. It tells the CNC to stay withing .001" of the nodes while otherwise trying to move (accelerate/decelerate) as fast as it can. The close nodes are indeed why your curves are cut slowly as the bit slows down when approaching each node and speeds up when leaving it. With such short distance between nodes it will never get up to the programmed feed speed. Converting the segmented curves to arcs will certainly speed up the travel around them.

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Re: feed rate different - arcs and straights

Postby sseale » Wed Jun 15, 2016 3:50 am

Thankfully I'm gaining some understanding of g-codes and post processor preambles as well as how geometry can affect cutter motion and speeds. It's easy to get up and running with cnc just using the defaults in CAD and CAM packages, but it looks like much more control and optimization can be gained from digging into the control logic - all part of learning.
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Re: feed rate different - arcs and straights

Postby martin54 » Thu Jun 16, 2016 12:45 am

I'm not familiar with linuxcnc Scott but know it is used by a wide range of people for home built & converted machines as well as commercial machines the same as mach3 is. There will be a lot you can do to fine tune your own machine within the control software before you even start looking at cad & cam software :lol: :lol:
I run mach3 & although I managed to get my machine up & running pretty fast getting the best out of it is an ongoing process, still really only scratched the surface with the software & know as I learn more I will be able to get more out of the machine but obviously that takes time.
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