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Oct 15, 2021, 01:26 AM
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Discussion

What could be the cause of Ripples on curved foam cut surfce ?


I'm using Keith Howlette's GRBL modified software and his drawer slide 4 axis foam cutter to cut a Clark Y straight wing . The wing was designed using DevFoam Wing . Everything cuts beautifully : The lower surface , the leading edge , trailing edge and spar cutouts , are very nice . Its very impressive . I have one small problem to which I have not found a solution. When the oblique surfaces on the upper wing are being cut small low frequency waves appear . All vertical and horizontal cuts are smooth and sharp. I have experimented with three things : kerf setting , wire temperature , and wire tension but there is no improvement . What should I try next ? Thank you.
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Oct 15, 2021, 01:33 AM
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A couple more photos one of the very smooth underside and a more oblique view of the top side looking towards the trailing edge. (The deeper inverted indentation behind the parallel ripples is my doing & nothing to do with the foam cutting.
Oct 15, 2021, 02:23 AM
Registered User
Try reducing the cutting speed. I have found very low speeds work best. I use a lab bench power supply in constant current mode to keep the wire at a stable temperature.
Oct 15, 2021, 02:47 AM
I hate propellors
emufingers's Avatar
Slight increase in temperature of wire and slightly lowering speed as advised above. I have been able to fine tune the effect to create corrugated wings. It appears that if the foam is not fuly melted that is lifts with wire on a roll of higher density melted foam eventually the tension in the wire increase and it is pulled back downto the correct height and it all starts over again. The problem is greatest on xps.

I tend to run the wire a little hot and correct with the kerf adjustment in the program.

This avoids finding out on a cold day that you have ruined a cut.
Oct 15, 2021, 03:50 AM
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Thank you gentlemen . As you can imagine there are a number of suggestions within these forums about how to cut foam . Some of them are conflicting . I've tried several things including running the temperature of the wire such that it just cuts the foam . But I'm coming round to the view you've recommended i.e I'm getting better results running hotter , definitely. I've also cut the speed from the recommended 2mm/s to 1.5mm/sec and Bartplus4 I will go lower still . I've changed the microstepping value from 8 to 16 . Things are definitely improving. I'm at the point now where I can see the ripples but they are only just perceptible to the touch .
I'm forming an assumption that because the rate of change of vertical movement compared to horizontal movement is so low during these oblique cuts that it tests the resolution of the system that governs the behaviour of the vertical stepper motors. I'll post further . Sometime over the weekend , I'm also going to change the controller board and try DevCnc in place of GRBL .
Oct 15, 2021, 04:26 AM
Registered User
Are you cutting by contact or by irradiation?
You must cut by irradiation to have accurate cuts.
Are you using a belt driven Cnc? (often this machines have a too low steps/mm value)
Oct 15, 2021, 04:50 PM
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It's driven by a hardware store threaded rod . Its hard to tell if its contact or radiation melting . I suspect its contact . Which is cosistent with the better results I'm getting with hotter temperature , i.e getting closer to the radiation temperature.

However what puzzles me most is why are straight cuts smooth Curved cuts cause ripples ?

Why is one set of settings producing excellent results for the straight cuts but imperfect for the curves on the same piece of foam during the same cut ?
Oct 16, 2021, 03:33 AM
Registered User
The problem is not in quality of applications, but probably in the way you use it.
If you don't know if you are cutting by contact or by irradiation, I suppose you are new in this field.

Try to cut by irradation.
You can cut a lot of info about in the web.

If the problem is still there, probably your carriages don't move in a smooth and accurate way, or your material has inside humidity in some parts, or (a long list of possible causes can be written here...)
You can read of many users with problems like your one in the web.
Cutting by hotwire is an art, if you ask for big accuracy.
Oct 16, 2021, 09:34 AM
The Junk Man
Quote:
Originally Posted by durone
If the problem is still there, probably your carriages don't move in a smooth and accurate way...
Yep. If the ripples were closer together then wire drag or vibration may cause them, but those look like the slides themselves might be the cause.

And you must always cut with radiated, not contact, heat with these machines. I thought everyone already knew that.

Tom
Oct 16, 2021, 02:02 PM
Registered User
Try cutting horizontal through a sheet that is laying on the table, with no weight on it. You should be able to cut it, without dragging it along.


Quote:
Originally Posted by onaboat
It's driven by a hardware store threaded rod . Its hard to tell if its contact or radiation melting . I suspect its contact .
Oct 16, 2021, 08:16 PM
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Thread OP
Thank you all for taking the time to analyze the problem . I can now confirm that I'm cutting with radiated heat , however the waves keep on appearing.

I'm pretty sure now that its a mechanical problem as T_om and Durone have suggested . The waves come closer together as the amount of vertical movement increases. And having cut close to 50 test pieces the waves appear in exactly the same place of the Clark Y profile . This suggests to me that the stepper motors in spite of 1/32 microstepping , slow cutting , radiation cutting don't transition smoothly at some constant point during the vertical part of a curved cut .

Whereas the predominantly horizontal cuts on the underside of the wing ( which produce very smooth results ) don't require as much change in velocity or direction as the vertical cut . That is the horizontal cutting vector doesn't change as much as the vertical one and so doesn't show the same imprecision as the parts of the system that control vertical movement. My money is on the behaviour of the stepper motor system. But I could be , and probably am , wrong.

My experiments will continue. I'm about to substitute the controller board. I'm now using an MKS Gen L v1.0 , with dvr8825 drivers and GRBL . I'll swap to Arduino One with CNC shield v3 and DevCad Foam CNC and see how it goes.
Oct 16, 2021, 09:47 PM
Registered User
What is the vertical displacement per turn of your leadscrew? I had a similar issue with my system. If the vertical displacement per turn is short, say a few mm, then if there is any wobble in the y-axis you will see it as a slight wave in the cut. The period of the wave will seem to decrease where the change in vertical displacement occurs over a short x distance, like near the leading edge. Where the change in vertical distance occurs over a long x distance the anomaly is not evident. Basically, for every change in vertical height equal to one revolution of the leadscrew you will see the anomaly. The fix is to track down and fix the wobble or go to a leadscrew that gives a larger vertical displacement per turn. I went from 2mm per turn to 8 mm per turn and the waves disappeared...most wings I cut have a thickness of 14 to 16mm, so the vertical has to displace 8mm to see the anomaly and basically you don't see it because the anomaly occurs over a very large x distance. Microstepping wont fix the issue as you've seen. 8mm per revolution is about 31 micron per step. I guess we don't need better than that for our wing cutting!
Hope this helps.
Oct 17, 2021, 03:23 AM
Registered User
Quote:
Originally Posted by onaboat
...

My experiments will continue. I'm about to substitute the controller board. I'm now using an MKS Gen L v1.0 , with dvr8825 drivers and GRBL . I'll swap to Arduino One with CNC shield v3 and DevCad Foam CNC and see how it goes.
Arduino Uno is not a good solution, as the memory is very low, so the buffered movements are low as well.
I suggest Arduino Mega 2560 + Ramps 1.4 or Arduino Due (but you must take care of the 3,3 v issue, for the drivers).

About your controller, of course I cannot know a lot about the used FW. May be it uses a poor approach with the steps management for very low speed movements (like the case of the Y axis there).
Just a thought, as I wrote the FW used by devCnc Foam, so I know rather well about FW internals.

A possible problem could be also the empty cache/buffer of movements in the firmware. Where the curvature is high, is higher the number of movements for each second as well, and the Usb connection used by Arduino has a severe speed limit. If the buffer is low or empty, the FW feature of look ahead (the feature that analyzes next movements, to try to keep the speed as costant as possible) cannot work, so many speed variations can happens, and this causes problems.
I added in devCnc Foam an optimization in that case, based on an accepted tolerance value on the final cut. And the buffer level is displayed in real time while cutting, so you can check its health.

I also used check and -correction of sent commands, as with some Chinese Arduno clones sometime happens that the Serial port of Arduino loss digits, causing the wire go to wrong positions. This is not your case, but just to say how many problems can be inside FW.
Oct 17, 2021, 04:13 AM
I hate propellors
emufingers's Avatar
The vertical axis is more sensitive to slop in the threads. I used the hardware store thread and improved this by using a longer nut, about 20mm long. that is available for that thread. You can also use a second nut with a spring between the two nuts. Look up backlash. The are a number of diy and commercial fixes.
Oct 17, 2021, 01:46 PM
The Junk Man
Quote:
Originally Posted by emufingers
The vertical axis is more sensitive to slop in the threads. I used the hardware store thread and improved this by using a longer nut, about 20mm long. that is available for that thread. You can also use a second nut with a spring between the two nuts. Look up backlash. The are a number of diy and commercial fixes.
I never had any problem with slop, even in hardware store threaded rod, on the vertical axis. The vertical axis cars weight alone was enough to keep the rod pressed down against the threads.

For the horizontal axis I 3d printed a long threaded split nut that could be tightened as it wears. Printed out of PETG it is slick as grease and has hardly any wear after hours of cutting.

Tom


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