DeskCNC Cutting Arc Problems

[HobbyCAD]

I’m using a DeskCNC 2nd generation controller card to interpret my cutfiles, and drive my CNC router. I’m having problems with it interpreting the cutting of arcs. Using the DeskCNC software to generate the contouring cutfile, or VCarvePro to generate the same cutfile, the result is the same, so I’m thinking the problem is with the program setup that controls the G-Code file interpretation.

If I set my X and Y axis steps/mm values to the same, all is fine. Loading a G-code cutfile containing straight lines and arcs shows the linear moves in blue, and the arcs in green, all OK. The card and machine executes the cutfile effortlessly, with the arc cutting moves just as fast as the linear cut move. The steppers sound fine, all humming along.

The problem is my machine has a slight accuracy problem with the Y-axis, I don’t know why, but I have to add a miniscule value to the Y-axis steps/mm value, to make it cut spot-on. I have no backlash to compensate for. With this updated Y-axis value, the cuts are spot-on. Problem is, now the interpreter does not cut arcs any more, it changes the arcs into linear lines. Loading a cutfile displays all the cuts as blue linear cuts. When I run the cutfile, the steppers kick up a racket, the arc moves are extremely slow, and the finish real rough.

I’m having problems interpreting the software settings that influence this arc cutting. Can anyone help with the following, and explain their effect?

1. Under the Setup, Options, Tolerance tab, what is the effect of the values entered under Rough Passes, namely the Min Arc Length, the Min Arc Radius, and the Tan Deviance? Are these values a length, a scale, how do I interpret the difference between entering 0.0100, and 0.0001?
2. Under the Setup, Machine Setup, DeskCNC Setup tab, what is the effect of ticking the Read Absolute Arc Centers selection?
3. Under the Setup, Machine tab, what is the effect of ticking or unticking the Break Arcs into Lines selection? What is the interpretation of the value entered into the data entry box, ie the difference between entering 0.10000, and 0.00001?

Any assistance in this matter will be highly appreciated.

[Dylwad]

1. Min Arc Length ~ Enter the minimum length of an Arc segment. Arcs less than this length will be converted to line segments. The length of an Arc should be less than the length of a few 'steps' as defined by your CNC machine. The software cannot interpolate an arc if the arc is only a few steps long.

Min Arc Radius~ Enter the minimum radius of an Arc segment. Arcs less with a radius less than this value will be converted to line segments.

Tangential Deviation~ Enter the minimum deviance that adjoining linear segments can have. Linear segments that are less than this value are joined into one linear segment. This value can reduce the size of the Machine Code file.


All values are Lengths.

2. I have no idea....try it?


3. with this command ticked any arcs in your .dxf file will be turned into line segments, the value sets how long each segment is, the lower the value, the smoother the "arc" will be. I have had to do this on 1 file, but did it in turbocad by exploding the arc many times, turning it into line segments.

Have you tried installing the latest version of software, or updated the firmware in your controller yet?


Have you tried swapping the x and y motor cables at the drivers to determine if your steps/mm problem exists in the software or hardware?

What difference in steps/mm do you have to make to get the machine to cut exact?

[HobbyCAD]

Have you tried installing the latest version of software, or updated the firmware in your controller yet?

Have you tried swapping the x and y motor cables at the drivers to determine if your steps/mm problem exists in the software or hardware?

What difference in steps/mm do you have to make to get the machine to cut exact?

I have software v2.0.2.66, with firmware v2.36, these are the latest releases.

Yes, the problem seems to be in the belt reduction between stepper and rack+pinion. Changing it makes no difference, Y-axis mechanics are out.

My X is 50.909443 steps per mm, my Y is 50,285414 steps per mm to be spot-on accurate.

[Dylwad]

This may shed some light DeskCNC

What its doing is Elliptical interpolation, which is software implemented, Arc interpolation is done in the controller.

Have you changed the contouring setting between 0 and 1 to see if that smooths out the Elliptical interpolation being used to cut arcs due to the Y axis being out?

Contouring setting is under machine velocities

Contouring Turns Continuous Motion Contouring (CMC) On/Off and sets the threshold. When Contouring = 0, CMC is off and all moves will decelerate to the Machine's Start Velocity. When Contouring = 1, CMC is set to Automatic. In Automatic CMC, each move will decelerate only the amount necessary to make a smooth transition into the next move. If Contouring is set to a value greater than 1, CMC is turned on with a threshold equal to this value. In this mode, if any axis has a change in Steps per Second that is greater than this value it will decelerate to the Start Velocity. If all axis' have a change in velocity that is less than this value, then no deceleration takes place.

Changing this setting might help with the racket cutting arcs, which are now ellipses(as far as the software is concearned)

[HobbyCAD]

Interesting reply...

My start velocity under the Machine Velocity tab is 40, the max just over 3000. What do you suggest I set my contouring number to? Currently it is 1. Maybe check with results with 0, then go 5, then 45, then 500?

Thanks, in any case, at least I'm getting advice from someone.

[Dylwad]

Try 0 first, although I dont think its going to help if your already at 1. the threshold value is really dependent on your machine setup, and the feedrates your using on that file.

Interesting your max is around 3000, with 50 steps/mm. Whats your rapid speeds/normal cutting feedrate? are you using microstepping? do your steppers howl if you set the max any higher than 3000?

[HobbyCAD]

Hi There,

My stepper controllers are set to 800 steps per rev. Steppers are standard 1.8 degree type. My rack+pinion and reduction ratio produces a lead of 0.019628mm/step.

I cut my sheetmetal at 0.5 IPS, the DeskCNC speed setting is around 762 SPS, to achieve that. My slow is 762 SPS, my medium is 1524 SPS, my fast is 3048 SPS. It can go faster, but I don't want to accidently slam the gantry into the stops at a higher speed. I have the DeskCNC Manual Pendant connected, the plug goes into the controller board's Input I/O connnector, so I cannot use the Home and Limit functions any more. Trying to find out from Fred if I can Y-lead the 2 sets of wires, to make it work together.

A bummer, for when you decide to spend the $250 to buy the DeskCNC Ready Pendant, to add to a nice functional installation, they don't warn you, you are about to loose the functions associated with the pendant connection now required!

[HobbyCAD]

I'm thinking of upping the steps per rev setting on the stepper controllers, to say 2000. I'll then recalculate the steps/mm values of the axis, and new speed values. Maybe the higher step pulse rate will smooth it out.

[Dylwad]

Interesting, The instructions for the pendant state that the limit switches dont function with the MPG hooked up. I dont really see why you couldnt wire your limit switches in series with the E-stop on the MPG, and I would bet the home switch terminal wires dont go anywhere inside the pendant.

Looking at the instructions for the pendant, I dont think its supposed to be hooked up all of the time, just for when you want to use your mill as a psuedo non-CNC.

The IMsrv SYS3 servo system is interesting in that it doesnt use the E-stop terminal on the DeskCNC board, Instead they have a SSR that powers up the driver power supply, which is controlled by an E-stop switch, and a 9 volt battery. It kills power to everything completely when you hit E-stop.

I wire my E-stop to kill power to the stepper driver's supply (on the A/C line!!!!) and hit the fault on the Geckos to bring the current to 0, while at the same time opening the E-stop connection on the DeskCNC board to let the software know whats up. Its set so that if the E-stop switch fails, it fails stopped.

[HobbyCAD]

Dylwad,

The pendant connection to the DeskCNC card uses all the I/O Input connections, to get data into the controller card. The x1, x10 and x100 scales are individually wired to the Pause, Limit and Cycle Start Connections, the X, Y, Z and A outputs go to the 4 Home input connections. The Pendant Enable switch works via the E-stop connection, so does the Pendant E-stop. The power comes from the Encoder connections, and the Pendant Encoder A and B signals go in via the Encoder Connections. So it's very difficult to have the original full functions available. I'm thinking of testing how it works if the pendant is not connected, but Pendant Enable selection is still active. Maybe I can use the pendant's enable switch to, when selecting it, have a relay bank switch in the 9 wires, and the pendant function works. If I release the side switch, the relay drops out, reconnecting the Home, E-stop, Limit, Pause and Cycle Start functions.

If I knew how it connected, I would Never have bought it. I would have simply piggybacked step/direction pulses to my stepper controllers, jogging the axis that way. OK, the position counters will not update, but I only use the pendant to jog to a zero position. I then 0 the position counters, and start the job.

I was a bit disappointed at how the position indicator feedback works. The encoder has a 100 clicks per rotation, and is marked 0 to 100. These are then processed as steps. If you have the setting on 100, 1 click is 100 steps, for my machine, with a step/mm setting of just over 50, it will move 2mm. If you turn the wheel a lot, say a full rotation, that is 100 x 100 steps going into the buffer. The machine will then go off and do that full jog. It does not jog while you turn the wheel, and stop if you cease turning the pendant wheel. If you have it on x100, and you turn the wheel a couple of times, you think it is being moved as you turn, no, you are entering a VERY LONG jog command. You have to wait until the movement buffer is emptied, before it stops. It is easy to run the machine into the stops, for remember, the home or limits don't work !! Only good thing, the counters follow exactly as you jog.

It would have been good if the software had somewhere where you enter your axis step/mm settings, and the indivitual wheel turn clicks on the wheel is then processed as say, x1 is 0.1mm, x10 is 1mm, x100 is 10mm jogging. Currently, you cannot move an exact predetermined amount with the manual pendant.

I'm seriously thinking or using the pendant hardware, making a control card for the pendant, that outputs step/direction signals. I will then couple it to the controller card Output connection.

[Dylwad]

Interesting, so selecting enable pendant in the software totally reconfigures the controller I/O ports, not the way I would have done it, but maybe its the only way they could get the DRO to update position when using the MPG.
I had looked at the pendant, its not for me! Home switches are a huge time saver, especially when using fixtures for multiples of the same part.

Sounds like a control card for the pendant, and going direct drive with the steppers or more presice pulleys would cure your ailments.