Messages

31

General Mach Discussion / Re: First Post - Help with setup issue?

« on: June 08, 2009, 03:23:03 AM »

I will agree that behaps the backlash could be seamlessly integrated into the move, I don't know how it "senses" the changes that may be needed, nor do I understand if the number of "pulses" for a given line are calculated all at once, or whatever, to allow the requisite "backlash pulses" to be tacked on.

However it is done, if you have backlash, the axis will appear to stop moving, until the necessary number of pulses to reverse the gear train have gone through the motor.

The only other thing that comes to mind is that, where two axis are moving together, either diagonally or in an arc, one axis (usually the one with less distance to move) takes it's pulses at a slower rate to keep the line (or arc) true. In this case, backlash MUST be applied first otherwise the movement of one axis relevant to the other(s) will not be accurate .

My machine is a combined lathe and mill, a medium sized Warco, and although new(ish) it suffered badly from backlash. I think I have that contained now, and I can turn out accurate stull. The next problem is 140 steel railway wheels to re-turn to a better profile (My CNC man did them wrong)

32

General Mach Discussion / Re: Rough Transitions

« on: June 08, 2009, 03:05:33 AM »

I'm sorry I misunderstood, but your post said that  "it moves CW and every transition from a line to an arc makes a noticable jolt that shows up on anything being cut." Have you tried this with any material in - if so how much of a "jolt" is this. Does the cutter dig n, or come away from the material.

Have you handwritten this program, or is it generated by CAD/CAM.

Is "tool diameter" part of your program, or is it just a straight cut.

33

General Mach Discussion / Re: Major/ minor arcs in turn

« on: June 07, 2009, 04:19:07 AM »

I've come very late to this post, but the answer is to use +R or -R depending on which way you want the arc to go.

If somebody hase already come up with this, I apologise, I haven't read right through.

34

General Mach Discussion / Re: Rough Transitions

« on: June 07, 2009, 04:09:32 AM »

What I take it you mean is that, if the cutter is cutting on the outside of the piece, it is fine. Cutting on the inside, it digs in.

When you are cutting the outside of the piece, the cutter is rotating clockwise and moving anticlockwise. When it comes to the rounded corner, the natural movement of the cutter is AWAY from the workpiece, i.e. there is no pressure on it to DIG IN. Also (if you have backlash on the machine), there is no pressure on the tool to pull it into the work. It depends on the drive on the motors to move it round the profile of the job

When you are cutting an inside profile, as the cutter comes to the corner, more of the cutter comes into contact with the work, and it pulls in.If you have any backlash on the machine, the available movement will allow the cutter to move towards the workpiece, as the pressure on the cutter changes, with the change in direction. Say you have cut up one side then the backlash has been taken up in a downwards direction.You run along the top (no backlash on that) then you start down the other side, the pressure on the backlash changes to upwards and allows the cutter to move into the work. The same applies on the left and right sides (but with the other axis)

I can only suggest you check your machine for backlash, and if there is any try and get rid of as much as possible mechanically ( I recently changed my machine from ACME screws to ball screws, with a great decrease in backlash)  ,and then  try to compensate with backlash compensation, which, although tedious, does work.

35

General Mach Discussion / Re: First Post - Help with setup issue?

« on: June 02, 2009, 02:46:15 PM »

Yes - I can understand that at the speeds you are talking about, Mach2 implementation of backlash could be troublesome. I can see that,however, the axis must come to a complete stop before backlash can be applied, since, as I said , whether the steps are backlash steps or movement steps, the first shall we say 600 of these (1/1000th of an inch on my old lathe) will not produce any movement on the axis, they merely take up the backlash.

There is a speed box on the backlash page, and you could only shove this up to 100%, but yes, on reflection, I think the backlash pulses will be applied seperately. It may not be the backlash speed that is the problem.At 7000 rpm and a 0.006 per rev rate, your feedrate is 42 inches per minute. I don't know how many pulses per inch your motors are tuned to , so I can't calculate the pulse rate, but it might be that your setting of feed per rev (whilst it gives a good cutting speed) may be giving you the slow backlash rate, since, even set at 100%, it will still keep to the speed at which it is set (i.e. it doesn't revert to your maximum G0 rate).

I don't know what you are cutting, or what the reason is for choosing a feed rate per rev, but perhaps if you used feed rate per min, and backlash at 100% the results might be more acceptable.

36

General Mach Discussion / Re: Poor Axis Calibration

« on: June 02, 2009, 02:14:14 PM »

It is not a good idea to use a measuring system to calibrate your axis. The result can only be as good as your measuring. I cannot get down below 1/1000 of an inch with mine, which would leave Mach totally inaccurate for my purpose - which is turning steel. At 1/1000th of an inch over, a steel axle will not fit a steel wheel.

The only method of calibrating is to calculate your steps per unit.
The first thing to do is to decide on which units you are going to settle on and set the machine accordingly on Config/Select Native Units. This does not mean that you can only use what you select (G20 [inches] or G21[millimeters] takes care of that when running programs) but it is the base system on which you set up your machine and all the subsequent tool and program offsets.

It is normally tied to your leadscrew. If you have an imperial leadscrew  e.g. 10 turns to the inch, or a metric leadscrew e.g. 5mm pitch generally determines which units you must use. You now calculate and enter enter this in Config/Motor Tuning/Steps per unit.

The calculation then goes like this - AND IT IS EASY, AND ACCURATE

Your motors, if standard steppers, will use 200 pulses for 1 rev. (200). Your drives will have microsteps 8 or ten are usual - my Gheckos have 10 (200 x 10 = 2000). You may have step down gearing to your leadscrew, mine is 3 to 1 (2000 x 3 = 6000) and lastly your leadscrew. I had an imperial leadscrew at first - 10 turns to the inch (6000 x 10 = 60,000) I have just changed to a metric ball screw one turn to 5 mm (6000 / 5 = 1,200).

So my answers for steps per unit were 60,000 steps per inch, or latterly 1,200 steps per mm. The two are NOT the same since changing leadscrew altered the whole dynamics of the lathe table and having changed, I had to set up my offsets and tooling again.

This number DOES NOT CHANGE. You cannot alter it to improve accuracy, it is a matter of fact. If your calculation is clearly way out, you can use a measuring system to check it, but the calculated system is the one that is accurate. i.e. in my case 1,200 steps per mm, not 1,195 or 1,204 or any near number - 1,200 is the correct answer.

Once you have caluculated and entered the figure, then you can check if your table is moving correctly. If it is not, then some other factor is affecting it, but not steps per unit. This may be backlash, you may be missing steps for some reason, or whatever, but deal with that when you have your steps per unit set accurately.

37

General Mach Discussion / Re: Non linear axis measurement...

« on: May 27, 2009, 11:58:03 AM »

Steinar has a simple solution (and it might be easier) - but if you are set up with stepper motors, and no feedback, then you need to write a simple Macro and instead of using the simple G1 or G0 command, use the Macro instead.

If you are familiar with visual basic, then the logic would be (something) like this.
We will assume that the bracket starts in the vertical position (0). Two UserDRO's should be set up, one showing the current position and one in which to enter the desired position. The macro can be then placed on a button at the side of the second DRO. The axis should, as far as Mach is concened, still be in linear dimensions.

Start:
Enter desired position in 2nd DRO.
Press button
get desired position from DRO2
position = armlength*tan(new angle)
code "G0 x"&position
set DRO1 to equal DRO2

That is fairly simple, since Mach 3 will keep the present position of the pinion in linear form in it's DRO so you don't jave to recalculate it all the time.
The only problem is I have just remembered that I think vis basic uses radiams and not degrees in it's calculations.
You will have to work in some logic to reverse the arm to bring it the other way.

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General Mach Discussion / Re: First Post - Help with setup issue?

« on: May 27, 2009, 11:25:58 AM »

Simpson - Yes, but how else could it take up backlash. It senses the change of direction,stops and applies the backlash compensation, then sets off from the correct start point. The cutting tool does not move during the backlash application - it cannot do, it is not moving the cutter, it is taking up the slack in the drive train.

39

General Mach Discussion / Re: Settings problem

« on: May 27, 2009, 04:12:50 AM »

Config/Motor Tuning/Steps per unit has nothing to do with the speed of your motors. This is the number of pulses the motor requires to move a set distance (either 1 mm or 1 inch) on your system. Unless you have this correct your machine will not be accurate. Depending on your set up this will be a figure between 800 and 1500 for steps per mm, or 20,000 to 60,000 steps per inch. If you do not know how to set this, come back and we will explain.

Once you have accurately set the steps per inch or mm, then you can set the maximum motor speed, which again will vary depending on your set up. Basically you move the motors slowly, and then gradually speed up until you have the maximum speed combined with accuracy. You can then do the same with the acceleration.

What are you trying to do exactly.

40

General Mach Discussion / Re: First Post - Help with setup issue?

« on: May 27, 2009, 03:58:08 AM »

Dan - I admire your tenacity - but rest assured, if all else fails (and it will - you must have SOME backlash in the system otherwise it would seize up) Mach copes with backlash compensation admirably. I recently changed my lathe to ball screws, and was pleased with the results, but I still use the Mach backlash compensation.

If you think about your manual work, you always automatically compensated for backlash by always coming up to a cut in the same direction. CNC does not do this, it cuts in any direction, hence the need for backlash compensation. I must admit I still, if writing my own GCode, tend to run past the job on the return, and still always approach the cut from the same direction. I have no problems and my work is accurate and repeatable.