Constant Velocity - Don't do it! - CNCnutz Episode 248

[@egemens]

exact stop shakes the machine like hell especially if you have a heavy gantry. then you need to reduce acceleration to create a longer ease in/out move to dampen the energy. the time passing during acceleration moves adds up and drastically changes the working time. here are some of my tips that changed he way i work. -get closed loop steppers at least -constant velocity -max out acceleration, just enough to dampen the shake -max out feedrates -keeping rpm as low as you can, making fat chips and keep endmills cold. -evacuate as much chips possible. -full depth cuts, use most of the flutes to cut not only the tip - using roughing bits with adaptive clearing -spring/finishing cuts to bring everything to tolerance after roughcuts or slotting moves. -i would say avoid slotting and plunging but we all do those. Maybe everyone has a different flavor for using their machine. at least thats my experience so far.

[@giuliobuccini208]

Uh! By the way, there is one case where G64 (constant velocity) is very useful: when milling a big pocket. In the above case, I manually go into the code generated by my CAM software (FreeCAD, in my case) and I insert a G64 instruction before the cutting instruction. Then I insert a G61 instruction just before cutting the final layer, to enable the exac-stop mode again. In such way the surface of the final cut (i.e. the floor of the pocket) is smooth and precise. ----------------------------- The G64 instruction (i.e. CV-mode) can be also useful when you are going to cut shapes on thick material that cannot be cut in one pass. In such case you can just roughly (a quickly) outline your shape just to remove some material around of it. Then you apply the final, precise "spring-cut" after enabling the exact-stop mode by inserting a G61 instruction in the appropriate place.

[@cncdavenz]

I was under the impression that constant velocity was used for laser cutters but I could be wrong :-). Good to see you back Peter. Cheers Dave.

[@TReischl1]

Yup, you are wrong.

[@StafPypen]

I switched between ES and CV often and i came to that same conclusion a while ago. I stopped using the CV in Mach3, ticked it off. Thanks for the CI Peter, CI = Constant Inspiration ;-)

[@pwrouzaud]

Such contrasts in the comments! Understandable in philosophy, but in things mechanical...With people like me, who are often confusicated for months by some (which turns out to be a) small issue, the idea of having a machine perfectly set up and tuned, and my G code perfect, is only a dream. "How does one do all this?" I ask myself. For people like me who are incapable of anality, "turn it off," is the right answer. Thanks

[@cncnutz]

Very philosophical Peter. I think this is one of those things that some people like and others don't. All we can do is be happy and contented with our lot. 😀 Cheers Peter

[@leonsearle9440]

Good to see you back Peter. KZbin and life can get very busy.

[@cncnutz]

Thanks Leon, It sure can.

[@Man-go-Everywhere]

fantastic problem solving,

[@pixmooo1]

From iraq, i really enjoy your videos, thank you peter

[@cncnutz]

Thanks pixmooo1

[@sylvainforget2174]

Constant velocity seems like a great idea for roughing passes assuming the controller never gouges the part.

[@DaveGatton]

I'm a big fan of constant velocity IF you have it set up correctly.

[@cncnutz]

Thanks Dave. I know you are a great user of Mach3. I'm curious to know the settings you use on your machine. Lines, tolerance and angle. Your right that CV must be set up right to have a chance of working properly. Cheers Peter

[@imqqmi]

I've been playing with the CV settings in uccnc because when running fusion 360 trochiodal adaptive clearing (or whatever it's called) machining uses a lot of curves and ran very slowly and jerky at default settings. But after some tuning I got much higher velocities removing more material quicker. When v carving though the same settings caused small gaps between the stock and inlay. ie about half a millimeter, it just gets filled up with glue and shows up as a thin line between the two pieces. I'm still not completely for or against CV. I think I'll just set it a bit more conservatively and jobs that need accuracy I'll just slow it down and match spindle rpm accordingly. When machining aluminium and other metals or plastics it becomes important to have the velocity be kept constant as a low velosity may end up rubbing more than cutting, heating up the material and the bit. With acrylics you can end up with a completely melted mess and broken bits, with steel more tool wear shortening its life. Surface finish will also be affected. Wood is more forgiving, still, it's better to produce properly sized chips than ultra fine dust. In the end, it's a matter of understanding and when to use what strategy to get the intended results.

[@giuliobuccini208]

Is the CV-mode overriding the G64 or G61 instructions contained in the g-code program? I suppose not, I'm right?

[@pindari9]

Peter, I have a view different than yours. The problems you are having with your constant velocity is more a symptom of poor performance from your machine, it's inability to accelerate and decelerate fast enough. I am lucky enough to have made a machine with servo drives and constant velocity works great on it and properly set up steppers would show an improvement too with constant velocity enabled. Without constant velocity it would bang and clang at every sharp turn. With my set up I get smooth motion with less than 0.01mm deviation from the perfect path - you can't even measure the change to the path. Don't blame constant velocity for what you see here - you even admit that you haven't tried to set it up properly - have a go and set it up properly and it will work great. It's there to look after your machine. No offence to you and kind regards - I enjoy your work Steve

[@uzkanda]

Beat me to it, constant velocity is a game changer, but machines need to be tuned to use it.

[@cncnutz]

Hi Steve and thanks. I agree that higher velocity will greatly help with CV and servo's are great for this. Unfortunately most people don't have servos myself included so we don't have the high acceleration. I also did say I didn't set it up properly not to show that it is bad but to show the effect. As you say the effect of sharp angle bends is not pleasant to watch or hear and in CAM software a 90 degree corner is always output as a curve to give better motion around the corners rather than a stop and a sharp right angle bend. As you also said the poor performance is a symptom of the machine and each machine need to have different settings based on the machines physical limitations / characteristics. You clearly have yours set up well and have a nice machine with high acceleration but I suspect most users don't have machines with those specs and they will struggle keeping the accuracy with low acceleration though they are also running lower top speeds which will help even the score. Thanks for your comments. It's good to hear differing viewpoints. Cheers Peter

[@stripples2]

Looking forward to the next video.

[@Balsamancnc]

CV is a must for 3d machining.

[@Greolt]

If your having the demonstrated issues on a wood router then your machines performance is woeful. I always tell machine builders that acceleration is king. Forget chasing super fast speeds, priorities acceleration. Exact stop mode does just that. The machine will come to a FULL STOP at EVERY node. You can reduce nodes. For example, no need for extra nodes in straight lines or arcs. But what about curves? There is no Gcode to define a curve. So it must be broken down into a close enough representation with arc segments or short line segments. At EVERY arc or short line join there is a node. This is unavoidable. Exact stop mode demands it must STOP AT EVERY NODE.. Makes for very jerky movement. Unless movement is at snails pace. Vee carving is all short segments. I would shudder at the thought of doing a large vee carving in exact stop mode.

[@cncnutz]

Thanks Greolt. The demonstration was unfair to CV but it was done to show the effect and as i said I didn't set it up. If I had any machine that did that to cut I would throw it away. The consensus here from everyone is that Acceleration is the key to CV performance. You are dead right about there being no Gcode to describe curves so the trick is to get the CAM to do what CV is going to do anyway and change the toolpaths to lines and Arcs instead of outputting 1000 straight lines. Things that look like arcs can also be curves and you not realize. I've been caught before. I want to demonstrate that I can get smooth motion out of exact stop with the help of CAM. Cheers Peter

[@johnrevill9592]

Another great video Peter. I've not used my CNC for Lasering for many years, but I do recall having to use Constant Velocity to prevent the machine from slowing down and/or stopping at each change of the laser power setting. Not sure if this is still an issue these days. I personally don't bother changing the CV settings from Default, but I do have a fast acceleration setting with allows for pretty rapid direction changes anyway.

[@cncnutz]

Thanks John. I haven't played with the UCCNC Laser to see how it behaves myself. Cheers Peter

I have to disagree with you on this one, Peter. Constant velocity is a very nice feature if you tune it properly, taking into account the maximum acceleration your machine can pull off. If you are machining curves that are composed of hundreds of arcs and lines, constant velocity is a must, otherwise your machining will be extremely jerky and that will show in the finished part. Sometimes, my paths have hundreds of thousands of nodes. If I didn't have constant velocity on my CNC controller, the machine would have to stop at every node, which is absolutely ridiculous. What is important is to understand the pros and cons of constant velocity, know what maximum acceleration your machine can pull off, and adjust the feed rates accordingly. You can get extremely accurate cuts with constant velocity and a nice cut without burning or scratching the piece.

[@cncnutz]

Thanks Kresimir. And that's the trick. Where a file contains hundreds / thousands of little moves to reduce them in CAM to Arcs which is what they normally are anyway. It is also what CV is doing internally. This makes things run much smoother and reduces your Gcode from thousands of lines of code to hundreds. I have absolutely no problem with people disagreeing with me. Cheers Peter

[@louisfeyt4188]

Great channel thanks

[@cncnutz]

Thanks Louis

[@chrisadesigns]

CV in mach3 is horrendous. CV in UCCNC is fantastic. You should have done a comparison rather than just a shameless masso plug which just doesn't support it along with other features. Just my opinion.

[@Greolt]

CV is better controlled in UCCNC, no doubt. I now use it. But it is a bit dramatic to call CV in Mach3 horrendous. And yes I have used Mach3 for over 13 years on a variety of CNC machines. (I presently have 5) All very successfully with Mach3 and CV. All stepper driven. The problem with a large number of home built CNC machines is that they have terrible acceleration. Under powered, poor rigidity or just not tuned properly. Something like a router will handle Mach3's CV implementation like a breeze if it has nice crisp acceleration.

[@brisolar]

Mine has a g71 there... i'm here because my machine is doing inside corners of 12mm, using a bit of 4mm, i believe i found my problem

[@blindsagacity]

My machine is just about unusable in exact stop. I do a lot of organic splines and toolpaths generated through Fusion 360. I’d love to hear about your workflows for CAD/CAM that make smoother running Gcode.

[@cncnutz]

Hi Nick. One of the things I would say is if you are doing Art projects where dimensional accuracy is not important then CV will not really be a problem. CV is not as bad as I demonstrated. I purposely exaggerated for artistic reasons. The use of splines are the cause of your motion problems in exact stop and simply converting these to arcs in CAM will help immensely. Basically that is what CV is doing except you are at it's mercy rather than doing it yourself. I am not a user of Fusion so not sure of the process. Cheers Peter

[@blindsagacity]

@@cncnutz Thanks for your thought out response, Peter. I appreciate it!

[@TReischl1]

Peter, I am a fan of your usual common sense approach to things. However, on this one you are way off. I think you need to do a bit more work on the subject. It is a challenge to tune CV I will give you that. But if I ran my machine with CV off it would look like a dog chattering its teeth in -40 degrees. Strange too, I created your example and my results do not look anything like what you got. Simply put Peter, you are wrong. Please do not be offended.

[@TReischl1]

BTW, my machine runs on steppers, not servos. Corners are where you will see some differences if CV is used, unless of course you have the parameters set up really wonky, then you wind up with incredibly weird looking parts. When set up properly CV uses an "in position tolerance zone". As you enter that zone the machine is allowed to begin the next move. The parameters in Mach 3 are used to define that tolerance zone. It also has angle adjustments that control that in position tolerance zone.

[@MrBrettStar]

Is there a quick way of adjusting the drawing in vcarve other than checking every single corner?

[@WouldWorkforWoodWork]

I just checked the g code for a project... I don't have either listed. N110G70G91.1

[@jims11762]

Great video! Really like to learn more about this subject. Only think I ran into this problem once using a diamond engraving bit at really high speeds.

[@MACKdaddy1220]

Great info as usual.

[@cncnutz]

Thanks Bruce

[@stripples2]

Looking forward to the next video. I am not seeing g 64 or 61 in my g code but mach 3 is set to cv if I change to exact stop it moves jitters.I use g code inch tap.

[@simeoncaronan9438]

I love to hear that sound when I curving in the street..3:35 is better than 3:25 when it comes to turning what would you say?

[@jigsey.]

Where can I download door cam

[@cncnutz]

What is door cam?

[@jigsey.]

@@cncnutz your door 😂

[@tomahawkcnc5583]

Constant velocity doesn't work properly in mach3 , it works fine in UCCNC , sounds like you are using a poor cad cam software with lots of nodes

[@cncnutz]

Thanks Daniel. For years users say that Mach3 CV is horrible and UCCNC is Good. I see the same comments here. I have excellent CAD/CAM software but certain drawings are made up of a lot of nodes.The trick is to sort them out before you make the Gcode file to output the best most efficient code possible. It's too easy to think that software it will sort everything out by itself and blame the machine for doing what it is told. Cheers Peter

[@NeonblueIndustries]

Constant velocity Sounds like some 'brilliant' idea by an engineer with stuff all real-world experience...

[@cncnutz]

Not necessarily as I'm sure they didn't go to all that trouble for nothing. There is a lot of math goes into working out trajectories and I don't think it was done for fun. Not what I call fun anyway. Cheers Peter

[@Embeddedtronics]

Mach3 has very poor CV implementation and will deviate from path depending on your feed and acceleration settings. Better controller software such as Linuxcnc and UCCNC have much better CV and you can set the amount of deviation from true path. Mach3 does not have this kind of setting.

[@cncnutz]

You can actually set the tolerance in Mach3 . It's called CV distance Tolerance and is measured in units but the settings are very limited compared to others. The demonstration was very unfair to Mach 3 but I wanted to show the effect or how it works and having to get out a magnifying glass to see it somehow isn't quite the same effect. Cheers Peter

[@Embeddedtronics]

CNCnutz (Peter Passuello) Yes mach3 CV settings are very limited and many times it is ignored. It’s good that you have pointed out some of the issues with Mach3. I have a Mach3 license purchase way back when the program first came out and I still use it for a couple milling machines. I’ve since moved to UCCNC on my gantry router which has much better CV tolerance settings. Even better is Linuxcnc G64 path blending gcode.