Revisiting those blasted divots...

[davek0974]

Ok so i have some scraps and some spare time so I thought it a good time to try and improve my divots

My test part is a 65mm square with a circle in the centre, 3mm thick, 45A, 4100mm/min.

Six cuts in the picture, details below, cut was from left-->right

All cuts were with 4mm perpendicular lead-in, no lead out.

From the front to back, details were...
1 - 1.5mm overburn with torch off 0.3mm before end
2 - 3.0mm overburn with torch off 1.0mm before end
3 - 5.0mm overburn with torch off 3.0mm before end
4 - 0.5mm overburn with torch off at end
5 - 1.0mm overburn with torch off at end
6 - 1.5mm overburn with torch off at end

Clearly 4-5-6 are, out as expected - yes it burns the pip off but the dead stop/torch off creates a bigger pip.
I think the answer lies in 1-2 somewhere, i will try some other variations here but is there a better direction??
I'm not using a lead-out as it most stuff tends to drop and leading out into nothing tends to bugger the nozzle i think.

[davek0974]

Just found a little issue with SheetCam - it treats the overburn as a lead-out and i did not have "include lead-outs" checked in my rule so it was turning the torch off before the overburn started.

Doing some more tests now.

[davek0974]

Ok, here we go, top edge and bottom of inner circle here, think i'm getting somewhere now?

Front-back parts 1,2,7,8

1-2 as before
7 - 5mm overburn, torch off 3mm before end, include lead-out
8 - 5mm overburn, torch off 3mm before end, include lead-out, arc lead-in.

I think 8 is pretty good, on part 8 i also slowed speed on the circle to 70% and it seems to have pulled the sides more vertical, my previous slow-down was only for smaller holes of 30mm or less, this hole is 50mm and clearly shows a good improvement in angularity.

Not sure if i can get better really, what do you reckon??

I'll set these parameters as default unless more info comes my way.

[WyoGreen]

Those cuts are looking pretty good, I'll have try those out myself, Thanks.

[davek0974]

Yeah, I think its a step forward.

The big issue was the fact i though that Sheetcam, when it had no lead-out, you left the "include lead-out" unchecked in the rules, clearly not so and once sorted it made the difference.

There is still a divot in the circle but that was where the centre dropped away and the arc was pulled to the work as it died out - no real easy fix for that I think.

[SeanP]

You getting there with that one Dave, yes you need that unchecked all right, you should be able to get the same result on a hole as well, your numbers are a bit different to what I use, but cant quite get my head around if it amounts to a similar thing or not

[davek0974]

Yes but I'm finding it works best with the include lead-out box checked and having no lead-out, just the overburn.

The figures seem table specific, was searching and found values all over the place, so just messed around and arrived here.

Might try some more tomorrow if i can shift this damn cold i've had for 4 weeks now

[davek0974]

Ok, thought I'd play with some 3mm Alu today, totally different animal due to the thermal only cut action. Tried the same settings as yesterday and as the torch was cut before the end, it did not finish the cut and left a small bridge. Moving the torch-off closer to the end improved matters and it seems ok on inside cuts but not on outsides, will have to play with these a little more I think.

I think overall the cut is really good, 45A @ 5500mm/min but why do I get top-dross on the downward strokes but not the upward ones?

This is the reverse as it comes, no dross at all, very nice.

Tried some letters, the small ones are about 35mm high, seems ok I think?

The finished sign, just something to hang on the shop wall, I later finished it with a fine disc then scotchbrite and a coat of varnish.

Then the PC died so ended play, I have a couple of spares luckily, pre installed with Mach, trouble is they are at work

[vmax549]

You alum cuts are very good but I still do not understand WHY you are getting divots in steel like you do. Something odd going on there.

(;-)TP

[davek0974]

I thought the last test was pretty good?

what sort of settings is everyone else using for start and end of cut????

[vmax549]

I use a straight lead in on start and no lead out AND I don't get the divots you are getting. Just looking at the divots it appears the machine is dwelling way to long at that point with the torch on for some reason.

[vmax549]

The only time I would see something like that is on a straight cut with no offsetting in artsy work used for detail and thin sheetmetal . AND then I use a flying start that pretty much eliminates it as you do NOT have to guess on the proper dwell time. No dwell time needed on thin sheetmetal it is light the fire and go.

[davek0974]

Just to be sure, we are talking about this divot ????

[davek0974]

Can't find a decent combination for 3mm alu, either get a big divot or not full cut at end.

Get a semi usable finish by adding sheetcam rules to slow to 40% 10mm before end and torch off 2mm before end but that seems very complicated and its still not too pretty.

Any suggestions here?

Maybe an arc lead-in is no good for alu?

[SeanP]

I haven't done so much alu but it does usually leave a tag on what I have done, just cleaned up in my case.
10mm & 12mm parts tend to have a little tag as well, the only way I can cure that is to have a slowdown to 60% on the last 2mm before leadout.
I still feel it's better having a leadout and having the torch moving away rather than keeping on the cut path while the torch off is happening, I don't really do that much tweaking and it seems to give a decent enough finish 2-6mm range.

[davek0974]

Hi Sean
Yes, i think it might be best to return to having a lead-out, i only stopped because i was trying to eliminate the reasons for my premature nozzle blow-outs, but as i have already had another, i now know its not the lead-outs and more likely poor pierce-height.

The cut is excellent on the Alu so i'm certain the speeds are good, its just the start and end, but there's a lot of combinations there

[SeanP]

You're right there is a lot of combinations,,,,,wrecks my head trying to think about those sometimes
Ah it's good that there is the possibility to change tho.
You probably right about the pierce height there, it doesn't take much!

[davek0974]

3mm Alu, basic settings...
5500mm/min
45A
70% slowdown on the circle
2mm Overburn
2mm Arc lead-out
5mm Before end slow to 40%
2mm Before end torch off

Ok lead-ins from front to back - Tangent, Perpendicular, Arc

Looks like Perpendicular is likely the best.

Going to try another with all the other rules turned off and a plain perp lead-in, arc lead out.

[davek0974]

Something not right here, turned off all sheet cam rules, 5500mm/min, 45A, 3mm Alu, 2mm over-burn, 2mm Arc lead-out, 4mm Perpendicular lead-in.

Why is it skipping the last 2mm when it comes round to the lead-in location?? It has only cut maybe 75% of the depth.

I have no idea what is going on here now, giving up for today unless someone has a specific test they want running. The sign I cut a couple of days ago would look terrible like this, the only difference is the material has changed, its all supposed to be the same stuff but I have known my supplier send out the wrong grade - these are all off-cuts from jobs. This stuff has lots of dross and a rougher cut.

Has anyone got a reason it is skipping the last part of the cut?

Anyone got a specific test they would like run?

Here's the g-code file.

65mm Square Circle 2 base.tap.zip

(1.34 KiB) Downloaded 101 times

[Old Iron]

Here's what I'm seeing when I view your file.
This is the bottom of the outer circle and all the rest look just like it.

[Old Iron]

Noticed the straight line cuts are good and the lead-ins and lead-outs are connected
The cuts are on the wrong side of the lines because of the lead-ins and lead-outs are not connected on the circle cuts.
I know the start points are wrong but, thought you'd take care of that
Hope the pics help.

[davek0974]

Hmm, I'm not totally up to speed here yet

Don't forget there is a 2mm over-cut in place here, that effectively makes the cut path an overlapping spiral I guess, pretty much like the first picture?

Maybe the problem IS the over-burn??

I will need to do a test run with no over-burn at all, no rules etc, just a bog-standard plain vanilla cut, see whatnot yields.

Begs the question - why don't the paths overlap perfectly when using an over-burn???

But then why does the cut fail before the over-burn starts??

[Old Iron]

With the lead-in cutting past the the kerf line, you'll end up with a divot because, it's cutting on the wrong side of the kerf line to begin with. If the torch slows down at the 90 degree lead-in it will magnify the problem.

[davek0974]

Yes, that makes sense now!

But why the alignment error?

What app did the first picture come from please??

[SeanP]

I din't realise you could load a tap file back into sheetcam, it doesn't look pretty on mine, but neither does my own files, maybe I'm using the wrong import settings.