Getting divots on 90 degree inside corners

[ryanamurphy]

Hey all, new to this community. Im working with an Hypertherm XPR 300 with Pronest and their 2dCad trying to cut some different lettering out for signs and what not. Im currently cutting on 10 ga nitronic 60 using a 60 amp head with f5/n2 gas, and on every corner of every letter I get a divot instead of a sharp 90 degree corner. Ive read through some of the threads here and have tried several things including turning up and down my feed rates, turning on and off my thc as well as turning on and off my sample voltage.

All four corners of the top of a T

T Letter.jpeg (72.74 KiB) Viewed 1257 times

heres an idea of what i am working with. If anybody has ran into this problem or has an idea of what I could be doing wrong the help would be much appreciated

[weldguy]

Looks like your torch is slowing down too much in the corners allowing the arc to burn away more material. There should be a setting to reduce this slowdown to a point where you can maintain a sharp corners without the direction change being too abrupt for your gantry.

[Rodw]

To turn a 90 degree corner, your torch basically has to slow to a stop to change direction so its burning a hole as it does this. I always try to add a fillet in my designs so it does not have to do this. I know the XPR does not have many choices for tips but 60 amps is still pretty high for the thickness you are cutting. Can you go lower so its not as hot? You may be able to let it round the corner by relaxing your G64 tolerance settings in your gcode which is I think is what Weldguy is referring to.

[ryanamurphy]

Thanks for the input, on most the boards here everybody says to go off of what Hypertherm has as far as cut speed and amps since they have done all the testing. For our material it was suggested to use 60 amp but ill go ahead and try the 40 amp which is the next and lowest option I have. When adding a fillet to your designs how big/small are you making them. I had read in a post about doing that and I tried making one with a radius at .005", hoping that having a "radius" in the program would help it not stop. It did not work for me but maybe there has to be an actual curve for the head to not stop on the corners. If you have a size you set on yours that gives you cleaner corners Id appreciate the input.

[Rodw]

I don't really have the experience but the guy in the shop next door is a sheetmetal guy and has a Hypertherm Maxpro200 with Pronest. He has given me a few pointers and one of his rules of thumb was to use 10 amps per mm. 10 Ga is 2.6mm
When I look at the specs for your machine, https://www.hypertherm.com/hypertherm/xpr/xpr300/ it seems they suggest 40 amps.

I had this pic of a piece I cut out of 2mm steel at 40 amps on My Thermal Dynamics A120 air plasma. Pretty sure I can see a similar effect on the points of the 3 that you are experiencing when I zoom in, just not as pronounced.

I wondered also if the use of gas was making it worse by creating a "hotter" arc. I did find a Hypertherm Document talking about how to cut with F5 and it lists nothing under 1/4". I wonder if its worth trying with just air?

[ryanamurphy]

Thanks for the input, I hadn't read that it makes it hotter but I will definitely try cutting with different gasses to see if that helps any. In the book they gave me it listed F5 as the "best cut quality" therefore I did not even try to rule out F5 as my issue since I was trying to get my best cut possible. Both the 40 and the 60 amp give 10 ga a cut quality of 1, before I ordered 40 amps I wanted to post here to get other ideas of what could be wrong, 40 amps are now on order.

[jimcolt]

It clearly appears to me that you need to increase the acceleration rates on your machine. It is de-accelerating into corners and accelerating out too slowly. The plasma continues burning at full set power while the machine dwells due to low acceleration. You will need to work with your cnc machine and cnc control to increase acceleration. Jim Colt

[plasmanewbie]

Thanks for the input, I hadn't read that it makes it hotter but I will definitely try cutting with different gasses to see if that helps any. In the book they gave me it listed F5 as the "best cut quality" therefore I did not even try to rule out F5 as my issue since I was trying to get my best cut possible. Both the 40 and the 60 amp give 10 ga a cut quality of 1, before I ordered 40 amps I wanted to post here to get other ideas of what could be wrong, 40 amps are now on order.

Don't overthink this and try a bunch of different crap. Compressed air is just fine and your right to follow the Hypertherm charts for speeds. Those speeds are good but like mentioned above the machine has to make a 90 degree direction change and therefore must decelerate into the corner and accerate out of the corner back up to the Hypertherm recommended cut speed. When the machine slows down it cuts away more material, its that simple. You need to tweak your settings so the machine gets through those corners faster but go to fast and your machine won't like the abuse. This is where lighter gantrys are nice. Find that sweet spot!

[ryanamurphy]

Thanks for the last two responses, Ill go ahead and work on the corner speeds and see if that helps out. I Appreciate all the input

[motoguy]

Agreed it's an acceleration issue. However, you may be able to "cheat" it a little bit by putting small fillets on all your inside corners. Give it an arc to change direction, instead of a 90 degree corner.

[Castlen]

Couldn't ask for a better set up.