cutting in air

Hi all again. Having got past the 1st thick stage, the machine is set up with limit/homing switches, the safe limits are set up, but rarely get used as the machine is a small cnc6040Z and to date every sign I have cut has activated the safe limit so I tend to leave that button inactivated, I have not understood offsets or set any up, although YouTube makes it look easy, it still goes over my head so I just zero the milling bit to my job everytime. I have settled on Inkscape and JScut and for the simple jobs I am doing at present it suits my needs, but I am going to look at Vcarve later on.
Anyway the problem I am having with my machine is that when I start a job and I am happy all is ok with the cutting path, the mill is not going to hit any clamps, screws or such like, I tend to leave it going and get on with other thing in the workshop, very often I find that the milling bit is cutting fresh air when it should be at least 3mm into the wood, the Mach 3 readout is exact and reading what or where the Z axis should be, I then have to return it to zero and restart the programme and watch it making manual adjustment on the stepper motor to keep it at approx. the correct depth, ok on the signs I turn out.
I do not have the answer as I have not looked at the mechanics but think it could be the coupling slipping each time the Z axis goes up and down each time it passes over a tab or to a new place on the job. Could this be the problem and how do I fix it? if not what else can it possibly be?. One way to stop this happening is to slow the feed rate down, but this makes the job really lengthy causing the motor to get hotter than it should, as I am not using the water cooling, as I have no heating in the workshop and there is a risk of freezing, to this point could I use antifreeze, will the small plastic/ neoprene tubes be ok?

First, check the mechanics of your machine, usually that is the main problem, Mach3 is the last place where loosing/gaining steps can occur.

Second, do not run your spindle without cooling. If you are afraid of freezing, just use antifreeze. You can leave it in the cooling system even in the summer. Also, I would advise to check if the antifreeze contains any chemicals that prevents algae/bacteria. If not, you can add a little of potassium permanganate in solution.

A friend of mine just rebuilt a 10 yr old Chinese laser and added a bit of permanganate in the cooling water.

Thank you, I did realise that it would be the machine, but wondered if it was likely to be the drive coupling slipping or something else, I have not taken anything apart yet, in case there are any pitfalls which is why I was asking, also I was told by another member that I would be ok without the cooling, but then he was probably not aware that I would have to do each job twice and on such a slow feed rate, Having said that the motor has never got too an excessive heat. just warm to touch.

Richard - I run mine with antifreeze, as do many others, not least for the antimicrobial nature of the stuff (stops algae grow if exposed to light). Yeah, if the motor is not under excessive load you can run them without coolant. It's heat, not absence of coolant that will kill the motor - until the motor gets hot you're not risking an awful lot.

You have limited reasons to lose Z positioning -

(from the sharp end, backwards...)

Collet not tighten (bit rides up into collet, been there, done that - marker pen, witness mark on the shank of the bit.
Spindle clamp not tight (spindle rides up into clamp - unlikely!, marker pen, witness marks)
Z-axis ball-nut mount - unlikely
Z-axis ball-screw mount - unlikely
Z-axis ball-screw to motor coupler - check the grub screws are tight. Use marker pen to add witness marks to spindle/coupler.
Z-axis stepper stalling - check stepper driver current settings. Harder to prove without risking damage.
Z-axis stepper signalling - dodgy wiring. Check by calibrating Z0, then run a few hundreds/thoasands G0Z20/G0Z0 cycles.
Z-axis stepper signalling - short pulse-widths - unlikely if your Mach3 kernel is lower that 100kHz.

Thanks Mike, the collet is and was tight, always use spanners and make sure I give it an extra tweak just to make sure, but I will put a mark on it on the next run,
Motor clamp is tight, two alan bolts, but again I will mark it, although if that was loose with the weight of the motor I would expect that to fall or at least for the milling bit to be on the wood.
Z axis ball mount nut, and mount look okay.
I have the step motor off now, the drive coupling are tight on the shafts
The last three items on your list I have no idea what you mean or how to check.
The step motor is very easy to turn and I did wonder if the speed had something to do with it, i.e. as the motor lifts to clear the wood the force pushes the motor a couple of clicks more than it should, which is why it does not happen if I slow the feed rate down, I did think that the motors were braked when the power was on to the machine, well that seems to be the case.
This is a dam nuisance as I am having a cabinet built from alloy and perplex so once its running and the suction is on I could leave it to run, especially as I have all the saftey switches on.

If your acceleration is too high, you will loose steps, resulting in incorrect positioning.
Even with your car you can not speed up to 60 if 1 sec :eek:
If your max speed is too high, the same as above, pushing motor above its limits.

The reason you will be losing position in this manner is 99% likely to be because your Step pulse is falling on the wrong side of the slope. This happens often and often goes unnoticed because it only shows on longer jobs or jobs with lots of direction changes. On Profile jobs or short jobs, etc it often goes unnoticed.

The solution to this is an easy fix.! In ports-n-pins / motor outputs toggle the "Step Low Active" to the opposite of whatever it is set to now.

If you want to test for this the write a G-code file which moves the axis back n forth a few 100 times then ends by coming back to zero, you don't need to move far 10mm will be enough and you can use G0 moves so travels at rapid speeds. Just make sure the last move is back zero. Something like this repeated 200 times will do.
G0 z-10
z0


Before you run the G-code Zero the axis and mark a point on the axis. When the code is finished it should return to the mark. If it doesn't then change the Step Low active and repeat the test. (don't forget to Zero and remark)

Do this test on each axis individually.

Thanks, I have been looking G0 Z10 Z0 at a you tube review of Todd Flemings JS Cut, and realise that there is an adjustment that can be made to the plunge rate, I had my profile programme set really fast, so I am going to reset that and try it. If that fails then I will try your suggestion, I am very reluctant to alter my Mach3 settings as you know how long it took me to get to where I am. While I am writing, even though this may not be the right place to ask, I watched a JScut review with Todd Fleming where the programme showed more options for setting the job up. like width and settings for different type of cutting on the one job, which is not on the version that I get on the web browser, am I missing something? as looking at that version it will be much more versatile
How would I write or add to G0 Z10 Z0 to repeat a few hundred times without doing it manually, I have not got into writing G code as I just let JS cut do it for me :-)

Quote:

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Originally Posted by RichardR

How would I write or add to G0 Z10 Z0 to repeat a few hundred times without doing it manually, I have not got into writing G code as I just let JS cut do it for me :-)

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Just use a text editor like Notepad and use copy n paste. To change for other axis just use the find n replace option ie: find Z and replace with X.

When writing the G-code you only need G0 Z-10 on the top line only because it's what's called a Modul command and the G0 which means travel at max velocity will stay in effect until another command comes along like G1, which is a commanded feedrate move.

So you can write it Like this, then just copy and paste the last 2 lines over and over, or if you're clever you'll do 10 or 20 lines then copy n paste those.
G0 Z-10
Z0
Z-10
Z0

I would like to say Thank You to JAZZCNC for his help, the test programme showed a problem, with some odd noises and the Z crashing into the limit switch.
A really BIG thank you to Mike (Doddy) who after chatting with and suggesting to swap the leads at the back of the control unit so X was operating the Z axis the problem was cured, however not satisfied with just leaving it at that after several texts he advised me to set the step pulse and Dir pulse to 5, they were set at 1 also to slow the acceleration as it was a tad too fast, all this in the motor tuning within Mach 3, I swapped the Z and X leads back to where they should be and did several tests using my Vernier to measure the start and finish positions of the Z axis, which was good.
Today I did a job that took approximately 1.5 hours and it was perfect, so it looks as if the problem has been solved.
All I need to sort out now is a strange sort of squeaking noise on the X axis as it travels along, just started during the job I have just completed. it has always been lubricated at regular intervals with a silicone lubricating spray, (not WD40)

You've got a Squeak now?, time to bin the machine mate :-)

not yet, mechanical I can do, IT I cant without help, getting warmer from Sunday so will have it sorted after the weekend, cant be anything much, maybe some foreign object in one of the dust seals.
I'd like to wear the machine out before I bin it :-) :-)

all sorted on the squeak and I never had to ask for help :-)