Plasma - cleanness of cut question -4% slope

[ADucci]

hi, i was wondering if anyone can explain the inner workings of the concept behind the angle that occurs down the face of the cut from my plasma cutter. When i cut large steel thicknesses (20mm).

i had some plate cut at a profile cutter and his product came back with less of an angled cut face. however there were little perferations down the cut face that makes me think that he diddnt use plasma to cut.

Can someone please explain to me, how to minimise this cut angle on my machine, and what other techniques i can use on my CNC table to cut metal plate (2mm - 25mm)

Thanks

[Torchhead]

All plasma spreads as the thickness of the metal increases. It's a function of several things. Take water and spray it through a small opening and it "cones" out the further you get from the tip. So does air and plamsa. The bigger the machine (arc current) and nozzle the further out the cone develops. High definition plasma concentrates the plasma "beam" more than conventional and gives straighter side cuts. It comes with a steep price.

You can effect the angle somewhat by changing cut speeds (feedrate) and air pressure but never eliminate it on thicker material. Bigger plasma cutters will cut straighter sides but when you approach 25mm you have to step up into commercial machines that have commercial prices. A 200A machine will cut 20mm quite well but still not give you perfectly straight sides.

Most likely your plate you had cut was done with oxy-acet not plasma. It works better in thicker material and gives straight sides. The perferations may be from an inconsistant feedrate for just rough bearings.

Conventional Plasma works good up to about 12mm for parts that are not critical (have to mate up tightly with other parts on the edges). It's not a precision cutting operation (blowing vaporized metal out of kerf from a 30,000 deg flame) and if the part calls for tighter tolerances either abrasive waterjet or laser needs to be considered.

A lot of shops use a combo table that has a plasma head and an Oxy-Acet head to cut different types of material.

[ADucci]

TorchHead, thanks. that info helps a lot.

so then technically, if you angled the head, you would effectively get a straight cut, but i see this will not work in curved profiles.


thanks.

[xjdubber]

buy an Hypertherm HPR-260 and you get verry nice cuts

try speeding up your travel a little

[ADucci]

what i intend to do, is get a 220amp, 380v machine, however i was told to lower my amps and feed rate in order to get a cleaner cut.

maybe i just need to play around with it when i get it.

[Dale Heart]

Originally Posted by xjdubber buy an Hypertherm HPR-260 and you get verry nice cuts try speeding up your travel a little

Hi.

Originally Posted by ADucci what i intend to do, is get a 220amp, 380v machine, however i was told to lower my amps and feed rate in order to get a cleaner cut. maybe i just need to play around with it when i get it.

I have a lot of old timers tell me how cut clean... especially with setting oxy-fuel. They generally say things like, 'slow it down' and 'add more heat.' They also say the same thing about plasma.

Don't listen to them. Play around with it and you'll see what works best.

[InspirationTool]

Originally Posted by ADucci TorchHead, thanks. that info helps a lot. so then technically, if you angled the head, you would effectively get a straight cut, but i see this will not work in curved profiles. thanks.

It would work fine, but you would need a computer controlled 5 axis head.

This is what they do with waterjets to put the "taper" into the waste side of the cut.

I've never heard of a 5 axis plasma, but maybe they are out there.

-Jeff

[ADucci]

yes, that is what i meant by that statement. i guess one could still use the two axis - type machine with the bed and gantry and just put a rotating head as the third axis, then just set the head at a slight angle and ensure that the head rotation is a function of the travel vector. i.e set up an imaginary north for the head (with its built in angle) and make sure that the head rotates at the same vector of the travel ..

this probably doesnt make any sense!!! sorry... just thinking out loud i guess...

anyway, i will just have to live with my slight cut angle.

[Dale Heart]

Originally Posted by InspirationTool It would work fine, but you would need a computer controlled 5 axis head. This is what they do with waterjets to put the "taper" into the waste side of the cut. I've never heard of a 5 axis plasma, but maybe they are out there. -Jeff

There are many machines that are 5-axis... and by many I really mean 'a few.' Most that work well are not original equipment (developed and engineered by someone other than the OEM and integrated). Most are used in shipyards to save time in weld preparation.

The biggest problem with such machines is the control system and usability. Because they are used mostly for weld preparation there can be multiple passes over a single piece and due to material movement and expansion with heat input on the first run, subsequent bevel runs can be ‘off course’ making the piece garbage or requiring rework. There is ways of getting around it like stitching the object to the sheet and clamping the sheet to the bed.

Using a bevel head to overcome the deficiency of plasmas lack of a square cut is not a real option. Bevel attachments range from $100k-300k (just for the head and control software/hardware and nothing else). For that cost to be spent on standard definition plasma to get a straight cut is not a good use of money… you would just by high definition plasma.

[xjdubber]

Originally Posted by ADucci what i intend to do, is get a 220amp, 380v machine, however i was told to lower my amps and feed rate in order to get a cleaner cut. maybe i just need to play around with it when i get it.

If you lower your amperage and feedrate at the same time your creating the same problem at a lower speed and a lower amperate. Never change two things at the same time. the faster you go the smaller your kerf will be, however bevel is something that you cant really change, its the nature of the "beast", but playing with speed and amperage might help out. What kind of plasma unit are you working with? dont be fooled, a higher input voltage machine is not going to change the cut quality at all, if the machine is hi-definition that i'd say go for it.