CUT-OUT TEXT CORNERS ROUNDED

[enzed]

I have cut out text in thin (1.6 mm) mild steel sheet on several occasions and have noted that some corners are rounded, though in the original drawing which was converted to G-code, the same corners are sharp. Please see the illustrations in the attached document.
Cutting is done by a Powermax45 machine torch, operated by Mach3. The CV settings of the latter are also shown in the attached document. Perhaps there are additional settings which are relevant. Sheetcam tool settings are as follows:
30 amps &111 volts. Pierce delay 0.2 secs, pierce height 3.8 mm, cutting height 1.5 mm, cutting speed 3.5 m per sec, pause at end 0 secs.
What am I doing wrong, or what have I set incorrectly? I will be very grateful for any advice.

Poor cut quality.pdf

(523.19 KiB) Downloaded 92 times

Poor cut quality.pdf

(523.19 KiB) Downloaded 92 times

[acourtjester]

Well I think 6 MM stroke width is your problem I use close to a .2 the differences is like a ball point pen and a sharpie when writing.
A smaller stroke makes the g-code follow the line much closer.

[robertspark]

low acceleration or CV too high (25 units..... 25 inches or 25mm)

[enzed]

Acourtjester, by stroke width of 6 mm I mean the width of the letter, not the width of the line, if I understand you correctly; what you say makes sense.
Robertspark, I will check this but as far as I am aware, all the settings are metric

[robertspark]

here is an explanation of CV for Mach 3
https://www.google.com/url?sa=t&source= ... ef9mRIyb8V

[robertspark]

I would suggest setting the CV distance at 1 unit (1mm) and give it a shot.

I suspect your acceleration is not very good for your machine, but your text is also quite small.

in CV mode the machine will try to maintain Constant Velocity (the set feedrate) , so try to image a car, you accelerate to 100km/hr or mph. and you set the curise control on and you come to a corner...... but you have the cruise control on..... how far in advance of the corner do you need to turn the corner to maintain the velocity (speed or feedrate) ..... if you set the CV distance at 25, and your units are km / miles. this means that as long as you can turn the corner within 25 km or 25 miles, you have no need to slow down, just make the turn earlier, anywhere up to 25km or miles earlier.

now we change the CV distance to say 0.2 (0.2km or 200meters or 0.2 miles)..... same scenario. 100km/hr or 100mph.... same cruise control.... same corner..... you are probably not going to make the turn..... so the planner looks ahead and says I need to slow to 60km/hr or 60 mph to make the turn allowing for deceleration before the turn and acceleration after the turn, as the acceleration and deceleration numbers are fixed at what you set them at.

it you have really good acceleration and deceleration then no problem you can still make the turn, no need to slow down.

[enzed]

Hello robertspark. Thank you very much for the detailed explanation and the link to Mach3 CV explanation, much appreciated. As to whether the acceleration of my machine is good or bad, I do not know, because I simply have no experience of any other machine! But I will experiment a bit and set the CV distance to 1 and see what happens. Thanks again.

[robertspark]

what is your acceleration presently at.

The guidance (note: guidance!) once listed by Jim Colt (ex Hypertherm, Plasma Guru / Yoda retired) was that acceleration should be about 0.3G (i.e. 0.3 x 9.81m/s^2 = 2943mm/s^2 or 115.9 in/s^2). Again guidance.... take it or leave it. Some designs can achieve it and some struggle..... Some achieve it no problem.

It's much more important to have acceleration than rapid motion.

Air Plasma has a maximum feedrate of 600"/min or 400"/min (depends on the plasma cutter / manufacturers test cut data).

This means that you don't really have much of a point of having a rapid feedrate (G0 motion) of over 600"/min (15240 mm/min) or 400" / min (10,160 mm/min) given that will be your highest cutting feedrate that your plasma cutter will ever use.

Sure it's great to see a machine doing 20m/min...... and it may save you some time if you are in a production environment making 10,000 of the same part..... but as a hobby user, try for 400"/min and increase your acceleration as high as it will go before the steppers stall, and then back off the acceleration to about 80% below the stall point.

You should not need to go above 0.3G for acceleration (I know some can get more, but it does place additional stress on your system that is probably unnecessary (IMO), but your mileage may vary.

Test your machine motion with simple motion to test for stall such as
G0 X0 Y0
G0 X100 Y0
G0 X100 Y100
G0 X0 Y100
G0 X0 Y0

AND

G0 X0 Y0
G0 X100 Y100
G0 X0 Y100
G0 X100 Y0
G0 X0 Y0
G0 X100 Y0
G0 X0 Y100
G0 X100 Y100
G0 X0 Y0

[enzed]

Hi robertspark. Again, your detailed guidance is much appreciated, thank you very much. Yes, I am a hobby user but unfortunately, G-code illiterate, so I will have to get guidance from someone local who is G-code literate, and as a minimum, find out how I can change the acceleration in the Mach3 program.