Another Constant Velocity Question

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nick7251
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Another Constant Velocity Question

Post by nick7251 » Tue Sep 15, 2020 5:56 pm

I have notice at higher speeds my machine is stopping and starting at every little node in the file. I am sure it pauses at lower speeds too, but it isn't noticeable because of the slow speed. I am also pretty sure I have CV set up correctly in Mach3 as I have been reading about this topic to death, but I won't rule out that I missed something.

My question pertains to the Post Processor. I am using sheetcam. Do I need to have G64 written in my post to enable CV mode? If so, where should I write it in? I have dug in my post and it isn't in there currently.

Thanks for the help.

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Re: Another Constant Velocity Question

Post by AREA » Wed Sep 16, 2020 10:15 am

I am just beginning to try and understand g code myself and am curious on this also.

I believe the G64 just turns on the smoothing but there are other setting somewhere that allow you to tune its sensitivity.

Anyone else have some insight on this?

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Re: Another Constant Velocity Question

Post by nick7251 » Wed Sep 16, 2020 10:48 am

I think from what I have read the G64 smoothing is what enables Constant Velocity. Then in Config->general config you can "tune" the CV control. I just don't know where to put the G64 in the post to enable this. Or if it is truly needed. I love this forum for stuff like this... A lot of very smart people here.

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Re: Another Constant Velocity Question

Post by robertspark » Wed Sep 16, 2020 10:49 am

general configuration tab.

Look at your lookahead, this needs to be about 200 lines so that the buffer remains full with a motion controller.

Also look at "CV Distance Tolerance"

here is a document for you to read https://www.machsupport.com/wp-content/ ... ngs_v2.pdf
2020-09-16 15_47_59-(1) Another Constant Velocity Question - PlasmaSpider.com.png

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Re: Another Constant Velocity Question

Post by nick7251 » Wed Sep 16, 2020 10:52 am

Thank you Robertspark. Lots of info on those two settings everywhere. I am more curious of the need of a G64 in the output code of the post processor to enable those settings?

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Re: Another Constant Velocity Question

Post by robertspark » Wed Sep 16, 2020 2:13 pm

all motion control software I have seen have a default setting for G64 or g61 (which mach3 has radio buttons on the general config tab)

hence once it is set there is no need to change it from G64.... unless of course you or your gcode flip between the two then good gcode would set and clear all modal commands for the task at hand ...eg you will see a lot of gcode / even the default mach3 initialisation string have g80 programmed (to clear any canned cycle.... that may still be active..... although few today probably use canned cycles in the hobbyist side of CNC)
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Re: Another Constant Velocity Question

Post by nick7251 » Wed Sep 16, 2020 2:22 pm

Okay good information. That clears up a lot, but now it raises another concern. I have CV set, but I am noticing it pause on nodes within an arc. That is what was leaving me to believe even though CV is set that it may not be active for some reason. Any ideas on what to check?

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Re: Another Constant Velocity Question

Post by robertspark » Wed Sep 16, 2020 3:08 pm

..... there are no nodes in an arc (G03/G02)..... except at the start and end that is

if your CV tolerance is small then the machine will decelerate between the junctions and accelerate at the other side of the junction.

there has been various long running discussions on this over time, I will try to find and link to the last one. there are differing points of view but constant velocity is simply not a velocity which is constant it is more just the opposite of exact stop.

if we can agree that exact stop requires motion to actually stop at the end of each gcode motion command in anticipation of the next.

then the opposite of this is simply a continuous motion through all of the nodes. not the maintaining of a constant velocity.

eg.... when you drive your car you generally drive it in constant velocity mode.... you don't stop at every junction or bend in the road. but constant velocity is not the maintaing of a specific velocity (such as setting the cruise control at 50mph and you just turning the steering wheel to navigate your way around a city).


here is something for you to read

viewtopic.php?f=4&t=30670&p=187054&hili ... al#p187054

viewtopic.php?f=4&t=30670&p=187054&hili ... al#p187017

we never did get to the bottom of rods math (or he did not disclose it).... but does it really matter....
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Re: Another Constant Velocity Question

Post by nick7251 » Wed Sep 16, 2020 3:51 pm

Thanks Robert. I've been reading some of your other posts over on the sheetcam forum as well. Good info. I guess I was calling the start and end points of the arcs nodes. I see now that is the wrong term. Even still the machine is pausing at those points even though CV is turned on. What would you say a good starting point would be for CV dist Tolerance? In the meantime I will dig through the links you provided and try to find the answer myself.

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Re: Another Constant Velocity Question

Post by robertspark » Wed Sep 16, 2020 5:09 pm

My advice [note, I own 1 plasma cutter, 1 mill, 1 lathe & 2 routers all cnc in a hobby environment so never classify myself as a has-been under pressure (expert)] is

for a minimum tolerance:
to adjust the CV to the radius of the cutter or 1/2 the kerf width.
(no point in less than this as the cutter ain't going to cut any smaller [in 2.5D or as a rough cut in 3D])

for a maximum tolerance:
to adjust the CV to the diameter of the cutter or kerf width. (this is what I use with plasma)

I don't make anything "high precision" {if I did / when I have needed to, I hire someone to do machine / laser cut it from a CAD file}

With plasma I start at the kerf width, and then think what is an acceptable number (I would accept for rounded corners). For artwork you probably want them nice and tight (eg: tree of life sort of stuff viewtopic.php?f=6&t=30254&p=182675&hili ... fe#p182675 ...... but then the corners are sort of rounded for the most part anyway but there are a few tight changes in direction)

For engine mounts, gussets and general transmission bracketry I've used a larger radius (1/8") [CV setting] as it will be welded up anyway and a nice radius is helpful sometimes.

Outside corners I tend to loop my corners so the velocity is kept up and you still get a nice tight corner (1/4")

note, I use UCCNC and G64 can be programmed with settings to adjust the tolerance from the gcode file from the default setting (which is also possible in LinuxCNC) but is not possible (from what I can see in manuals in Mach3 or Mach4)

find a number you are happy with... [trial and error]

kerf widths are given in the hypertherm manuals and are around these sort of numbers (depends on the torch and consumables etc etc.... check your specific operator manual for the charts)
Image

It would be nice to know what others do use (but everyone seems tight lipped on some settings / tolerances they use and why)
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Re: Another Constant Velocity Question

Post by Rodw » Thu Sep 17, 2020 6:47 am

robertspark wrote:
Wed Sep 16, 2020 3:08 pm

we never did get to the bottom of rods math (or he did not disclose it).... but does it really matter....
Neither did I. Time poor I cry! I have had a go at it myself but made an error with unbalanced units which I picked up later. I've asked 3 engineers and they all came up with a different answer....

Come to think of it, Robert, I am still waiting for your exam paper to be returned.

I think these are the two simultaneous equations that we should solve for.

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Re: Another Constant Velocity Question

Post by robertspark » Thu Sep 17, 2020 7:46 am

I've been busy too.... in the last month I've been made redundant (I saw it coming since the very start of covid19 in the UK), started tryng to setup on my own from scratch (website, photos, examples etc) + find alternative work. officially unemployed Monday and had 2 job offers by Tuesday PM, and I've accepted one .... so one less worry.... I'll have another look back at that post again

life.... never fails to amaze / confuse / change!
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Re: Another Constant Velocity Question

Post by Rodw » Thu Sep 17, 2020 8:01 am

robertspark wrote:
Thu Sep 17, 2020 7:46 am
I've been busy too.... in the last month I've been made redundant (I saw it coming since the very start of covid19 in the UK), started tryng to setup on my own from scratch (website, photos, examples etc) + find alternative work. officially unemployed Monday and had 2 job offers by Tuesday PM, and I've accepted one .... so one less worry.... I'll have another look back at that post again

life.... never fails to amaze / confuse / change!
Glad you were able to get off the redundancy scrap heap. Covid has been kind to us so record sales has cut into my plasma play time. But this month the government support for businesses cuts out so we'll see how it goes from here. I think we will continue with strong sales.

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Re: Another Constant Velocity Question

Post by robertspark » Thu Sep 17, 2020 8:28 am

yeah, the governments support started to be cut back from 1st August here, and ends 31st October (for employees).... for businesses I'm not sure, but its all done with loans I think.... which have to be paid back at some point and not just general taxation later on.

I think the worst (jobwise) is yet to come.... but hope as I ususally am... 100% wrong!

prepare for the worst, hope for the best.

back to plasma cutting / enough of problems / politics / points of view or opinion!

If you can't cut it, screw it, glue it, weld it or get paid for it. Its not important!
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Re: Another Constant Velocity Question

Post by nick7251 » Thu Sep 17, 2020 10:08 am

robertspark wrote:
Wed Sep 16, 2020 5:09 pm

for a minimum tolerance:
to adjust the CV to the radius of the cutter or 1/2 the kerf width.
(no point in less than this as the cutter ain't going to cut any smaller [in 2.5D or as a rough cut in 3D])
I have my CV distance set to my kerf width right now. I'm still getting the little pauses at the start and end points. What other settings should I look at? Again this is Mach 3 unfortunately. Don't have it in my budget to get a new controller to shift to Linux based software.

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Re: Another Constant Velocity Question

Post by nick7251 » Thu Sep 17, 2020 10:11 am

Robert, in your screen shot above you set CV distance to 25. Is this mm? So that would be 0.984 inches. That is quite a bit more than three radius of the kerf. I'm thinking I'm way of on something here.

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Re: Another Constant Velocity Question

Post by robertspark » Thu Sep 17, 2020 11:50 am

ignore those numbers that is just a demo install of mach3
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Re: Another Constant Velocity Question

Post by nick7251 » Thu Sep 17, 2020 11:55 am

robertspark wrote:
Thu Sep 17, 2020 11:50 am
ignore those numbers that is just a demo install of mach3
Okay. So I have 0.06 inches in my CV distance. I know that it was suggested to do half the kerf so I might update to 0.03". Sorry to need so much hand holding. Does that seem right? If so, I am back to square one on thinking that the machine isn't actually in CV mode even though I have it set. With those numbers correct it still pauses.

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Re: Another Constant Velocity Question

Post by robertspark » Thu Sep 17, 2020 5:22 pm

Try setting the number to a large number (like shown in the manual I listed above / earlier).

put the number at 25 and run it again and see if it slows for the nodes?

(I have probably asked this.... ) but I'll ask it again.... are you using a motion controller, if so, which one? Does it have a buffer adjustment? try increasing the buffer if it does. Increase the lookahead lines to 200.

I've just looked for the setting but I cannot see it.... but you can / could turn off the toolpath and see if that improves things.
Mach3 although being an older piece of software uses the toolpath quite a bit and if you have an older PC sometimes I've seen / heard of issues with graphics cards causing other issues (hence turning off the toolpath [which I cannot see where the checkbox is for turning it off] can improve things (or at least allow you a test run to try to see if its causing the issue.

As pointed out by Rod, acceleration is key with plasma cutting so if your acceleration is low, when there are changes in direction the node junctions can be more apparent (even in CV mode, with a small CV error setting)
Sorry this is a bit washing machine mechanicish..... try this / that and see if it works....

But you have an unusual problem (or others would be shouting from the rooftops) if it's not one of the issues listed above (low acceleration, small CV error / too small for the acceleration setting, small lookahead, graphcis card not up to scratch [old pc / wrong graphics driver] {unlikley these days as mach3 spec is soooo low.... but possible})
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