cnc seems to lose depth of cut

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sentrum
Posts: 13
Joined: Wed Mar 12, 2014 10:04 am
Model of CNC Machine: Datacut

cnc seems to lose depth of cut

Post by sentrum »

I really need help, as I am at my wit's end and have tried all possible solutions I can think of.
My PROBLEM: Cutting a 420 x 330 mm photograph, my depth of cut (initially set at 1.5 mm but then changed to 2 mm in an effort to remedy the problem) fades from my start position at bottom right, progressively to where it is suppose to end at top left.
I have tried:
1. Re-levelling my sacrificial table top (work surface) twice with the same router that must cut the photo, so that it will absolutely conform with machine at any given position.
2. Changed my starting point from bottom left to top left (which is the area towards which the fading away of depth happens)
3. Double checked the thickness of my material with a digital vernier/caliper to make sure it is of even thickness.
4. Double checked with a dumpy level that all four corners of the work surface of my machine table is perfectly level.
5. According to the computer monitor the values of my co-ordinates of the cutting toolpath is perfect.
6. My router is definitely fixed properly and cannot move on the mounting bracket of the overhead Y-axis bar.

If anybody out there has experienced the same headache or have an answer to this problem, please assist.
Thanks

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martin54
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Re: cnc seems to lose depth of cut

Post by martin54 »

Have you checked the z axis is all tight with no backlash, also check the z height at the finish of the job & compare to starting hight, should be the same but a difference could indicate lost steps. There is a lot of z axis movement when cutting stuff like this so that is the first place to look for problems. Other thing could be to check the traming of the router to ensure it is sitting square to the table.

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zeeway
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Re: cnc seems to lose depth of cut

Post by zeeway »

Presuming you have ruled out mechanical issues, my guess is that you are losing z steps. The job you are cutting would have a lot of z movement, and if you are losing steps, that is where it would show up the most. It should also be showing up in other jobs with considerable z movements. When your machine has malfunctioned in this job for a while, please recheck the z axis zero, and see if it has changed.

In the early days my machine had the same issue...and after many failed attempts, I found out that the output signal from my computer was not consistently the five volts or above needed as input for my controller (Gecko540). Installed a SmoothStepper, and grounded computer to control, and problem was solved.

Don't know what kind of control your machine has...if it is proprietary, you will have to go back to manufacturer.

Good luck.

Angie

sentrum
Posts: 13
Joined: Wed Mar 12, 2014 10:04 am
Model of CNC Machine: Datacut

Re: cnc seems to lose depth of cut

Post by sentrum »

Thank you Martin54 and Angie, I really appreciate your advice and think that you may both be correct in your diagnosis. However, I do not think that I actually preside over the required know-how about how to go about fixing my problem. The only thing I could scratch up in my rather limited problem solving brain in this regard, was to "tweak" my router lower a little bit at a time as the program continued - but of course, at best that would be described as guess work.

When you refer to rechecking the z axis zero; do you mean a physical check with a measuring instrument? The reason is that the program display still shows the correct values, but there is no way that the actual depth of cut could be correct. I am sure that right towards the end, it may differ probably as much as about 1.8 to 2 mm. The x and y values are spot-on though...

Where and how can I check to remedy the slip/ loss of steps?

Mike

P.S. Flash Cut post processor. By the way, why did the signal strength not influence the y and x axis during the time you experienced the same/similar problem, or was it not only the z axis in your case?

rsetina
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Re: cnc seems to lose depth of cut

Post by rsetina »

When I had this problem I set my 0,0,0 position at the point where the project was cutting normally, in your case the bottom right. Then I moved the bit/router to the end where it was fading and reset my X and Y to 0 again, leaving the Z alone. I clicked the move to 0,0,0 and checked to see of there was a gap between the wood and the bit. If there was, I assumed my table wasn't flat. You should be able to do that anywhere on a flat table and the bit should be positioned on the material, assuming the material is the same thickness across its width and length. Use a set of calipers to check. You could do this on your spoil board also.

Hope this helps.
Rick

Shark HD 2.0

rungemach
Posts: 13
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Re: cnc seems to lose depth of cut

Post by rungemach »

Usually lost steps in the z axis usually evidences itself by cuts going gradually lower. Steps are lost in the fast retract of the z , as it must lift the router weight as well as overcoming the inertia of the assembly. Gravity helps the z in plunges. Your issue seems to be the opposite, in that you may be losing steps in the plunge direction.

Does the problem exist if you try and run the job at half the speed you currently are trying to cut?
If the problem goes away by slowing down, something in your system may not be capable of providing the correctly timed signals to the drivers at higher pulse rates. This can be not only in the steps signal but also in the direction signal, where a change in direction may not take place quickly enough and a step is lost at each transition, giving you that slowly sinking or rising result. slowing a job down is an easy first thing to check after you have determined there is nothing mechanically wrong. Some of the Chinese driver boards were notorious for losing timing at faster step rates. The slow test does not eliminate the possibility of bad timing completely, but it is an easy first test to start with.

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zeeway
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Re: cnc seems to lose depth of cut

Post by zeeway »

In my case, my machine also had lost steps on the downward movement. After your machine completes its cycle, the control will indicate 0,0,0 because the control does not know that you have lost steps. The control sends pulses to the stepper motors, but there is not feedback (unlike synchro motors) to tell the control if the desired motion has been achieved. So, if you are loosing z steps and let's say you had zeroed the z axis to the top of the material...when your z axis is as 0,0,0 you should be able to move it horizontally to a part of uncut material and see a physical gap between the tip of your cutter and the material, based on your description.
Angie

sentrum
Posts: 13
Joined: Wed Mar 12, 2014 10:04 am
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Re: cnc seems to lose depth of cut

Post by sentrum »

Angie (Zeeway) and Rungemach,
After a lot more of different checks and trial runs, I think that the explanation of Rungemach seems to be correct. Amongst some of the efforts to remedy the problem, I "shimmed" the one end up by about 2 mm and received a result which was at least of such a nature that one could see the whole of the photo all the way to the end, in stead of the scenario where it faded into no detail being cut or the majority being lost.
I then proceeded to do a rather long "V" groove engraving where 20 letters are in one line, and this time the Z remained the same as it was only all at the same depth, but then, to my absolute horror, saw that it was now losing steps on the X and Y axis as well! My heart really was in my shoes and if aluminium could burn, I suppose my CNC table would have ended up in the fireplace.
I then remembered what Rungemach advised, and re-did the thing again, this time at 25% speed. It worked perfect, but slow. At least I can still use it all now, only taking more time, but I can live with that as I do not have to achieve any production targets.
May I ask if the load (referring to the depth of cut and therefor the resistance offered to moving the tool) will have an influence on whether I can maybe get away with a faster speed, but at a shallower cut, or is it like "what you gain on the swings, you lose on the roundabout"?
Thanks for helping, guys, it is truly appreciated.
Mike

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