Is "Constant Speed" important?

[Hamadamj]

Hi all and thanks for the graet help and useful tips

I am not sure about the effect that "Constant Speed" would have on the total time it takes to make a part using the TurboCNC

I hear that Mach3 has this Contant Speed feature and TurboCNC does not

Can anybody here explain the difference and if it worths switching from the otherwise GREAT TCNC to Mach3??


Thanks

Hamadah

[tobyaxis]

Are you talking about Constant Surface Feet Per Minute or something else??

CSFM= is used on a Lathe. G96 call, G97 cancel (Fanuc/Yasnac).

Some Mills have this feature but I've never seen it used with the exception of Mazatrol Conversational Controls.

There is a feature in some CAD/CAM Softwares that decrease the Feed Rate around Corners, but other than that, it's a little confusing on this end.

[Hamadamj]

I should have said constant Velocity, not speed,.soory

I think that is the feature where the program will cut short joined straight segments at constant velocity and not deaccelerate and accelrate for each segment


I noticed when using TurboCNC to cut signs that the machine is not very fast (may be it does not have time to accelarte)when cutting small joined details but it accelerate and becomes fast when cutting one long straight line(segment) of the sign

The way I understand it is that if the program has "Constant Velocity" feature like Mach3, then it will accelerate when it starts cutting the fisrt segment but will not deaccelerate at the end if there is another "joined" segment

I am not sure though and need some "expert" input to tell me if am going to get real improvment if I switch to Mach3 because right now I like TurboCNC but the machine takes a relativly very long time to cut signs even though it is fairly fast during jogging

Any inout is highly appreciated





Thanks

Hamadah

[Al_The_Man]

Not sure what you mean by 'joined segments' but at the end of a programmed move the axis has to decelerate!
When the motor/drives are initially designed for a machine, one of the most important criteria is to take into account motor to load inertia, if the motors are undersized, the required accel/decel will not be able to be accomplished.
The final speed rate and cutting forces may be within the design parameters, but due to inertia the machine is not capable of any faster accel/decel rates.
This factor is often overlooked on home designed machines.
Al.

[ger21]

Originally Posted by Al_The_Man Not sure what you mean by 'joined segments' but at the end of a programmed move the axis has to decelerate!

What Mach3's CV mode does is blend the segments together to eliminate or minimize the deceleration. You'll end up with rounded corners, the severity of which depends on your machines acceleration capabilities.

[stirling]

Stopping at each segment intersection is called ES (Exact Stop) mode and is all TCNC can currently do - Mach3 has ES mode AND CV (Constant Velocity) modes and effectively everything in between the two.

In order to stop the machine slowing down and stopping at each segment intersection, Mach3's CV mode allows you to set a variety of tolerance parameters between segments where you can either allow or dissallow blending and the extent of the blending. This is hugely powerful and I'm not sure how anyone can get by without it - especially when machining "flowing" or non-prismatic 3D. Without it (I was using TCNC) the machine made a pretty good impression of a machine gun about to self destruct!

[samco]

I started with turbocnc also - The reason I looked at emc was it would do blending.

emc2's trajectory planner has been rewritten recently - you can read about it here
http://wiki.linuxcnc.org/cgi-bin/emc...jectoryControl

this is running the linux penguin (it is made up of short line segments)
http://www.youtube.com/watch?v=Hdhn_j6PrCw
you can see how smooth it is. turbocnc would be doing what stirling said - machine gun

[NC Cams]

When surface finish and/or tolerance control critical, constant surface speeds and feeds can be QUITE useful and/or critical.

Example: we cut semi-round cam grinding masters using a VMC. Since the shape of the part is changing as you go around the profile, you really need to keep the instantaneous feed rate to remain constant in order to NOT have a different finish and therefore a different "profile". Consider that we're looking at shapes that are critical to 0.0001" and rates of change even smaller.

Theoretically, it can't be done but when you know the limitations of the machine and account for them with speeds and feeds and G code tricks, it is amazing what you ultimately CAN do with a machine that "simply can't do that!!!!" - and CSS is/was one of the tricks we used to do just that.

[dkowalcz]

I think for a lot of machining it's overrated, but until I get the code for it working right you can do the following in TurboCNC:

Increase your start/minimum speed for each axis as much as you can. This is the bound below which acceleration is instantaneous. For a modern stepper motor and drive it can easily be 500-2000Hz or beyond.

Often this is not much slower than the cutting speed, so you get an effect that is pretty similar to exact stop CV.

[samco]

Good to see you around again

sam

Originally Posted by dkowalcz I think for a lot of machining it's overrated, but until I get the code for it working right you can do the following in TurboCNC: Increase your start/minimum speed for each axis as much as you can. This is the bound below which acceleration is instantaneous. For a modern stepper motor and drive it can easily be 500-2000Hz or beyond. Often this is not much slower than the cutting speed, so you get an effect that is pretty similar to exact stop CV.

[flyboy1015]

This sounds like the very problem that I'm having. I used CamBam to generate a heightmap from a picture. I converted to G-code and then opened in TCNC. I think the acceleration between each of the steps in the code has seriously slowed the machine. I'm not currently capable of running Mach3, but I can see the need for it. It looks like the acceleration/deceleration at each step adds more time than you would think when looking at the length of lines being cut and the feed rate. 1 line is 3 inches in length, not counting the contour, however, it is taking about 2-1/2 minutes per line at 10ipm feed rate. Any advice?

[tobyaxis]

Originally Posted by flyboy1015 This sounds like the very problem that I'm having. I used CamBam to generate a heightmap from a picture. I converted to G-code and then opened in TCNC. I think the acceleration between each of the steps in the code has seriously slowed the machine. I'm not currently capable of running Mach3, but I can see the need for it. It looks like the acceleration/deceleration at each step adds more time than you would think when looking at the length of lines being cut and the feed rate. 1 line is 3 inches in length, not counting the contour, however, it is taking about 2-1/2 minutes per line at 10ipm feed rate. Any advice?

Can you implement G2/G3 Arcs instead of G1 linear interpolations? This would greatly reduce file size and smoother corners.