WHY CAN A CUT NOT HAVE ZERO BEVEL

[beefy]

With 2 or 3 exceptions I vaguely remember, the perfect cut is supposed to have a slight amount of bevel on the good side of the cut.

I've got the "Plasma Cutting Handbook" by Eddie Paul in front of me and on page 83 he says you will get ZERO bevel on the cut when the torch is at the right height. His drawings show the torch "flame" shape as bellowing out from the nozzle and coming back to a point lower down. So he's saying if the torch is high then the lower part of the "bellowed flame" causes a positive bevel. Likewise if the torch is too low he says you get a negative bevel from the upper part of the "bellowed flame". And when the torch is at just the right height you have zero bevel. My imagination would be that you'd have a slightly concave edge due to the "bellowed flame" but with the top and bottom being in line.

I've often thought that this makes sense so why can't we just adjust the torch height to give zero bevel. Is is that adjusting the torch height to get zero bevel negatively affects some other aspects of the cut, like dross for example.

A plasma table manufacturer here in Australia even teaches you to lower the cut height when cutting circles, in an attempt to give more "flare out" at the bottom of the cut, to try and counteract the positive bevel you often get when cutting holes.

Keith.

[jimcolt]

You will not get zero bevel with even a high definition industrial plasma. For that matter you will not get zero bevel with a laser as well. There are some high end laser and high def plasma cutters that can get close, and some even have complex bevel mechanisms on the torch carriage to compensate for bevel.

Height affects bevel , cut speed affects bevel. design of the plasma torch affects bevel. Wear on consumables affects bevel. Incorrect gas flows, impurities in the plasma gas (air) will also affect bevel.

To get the best (least) bevel with an air plasma, choose the set of lowest amperage consumables for your torch that the manufacturer suggests will cut the material, then follow the cut charts specs.....especially cut height and optimum speed. Lower speed will sometimes improve angularity, but also increases dross.

So....with my Powermax85 (Hypertherm) I have a choice of fine cut consumables (rated for 10 gauge and thinner), 45amp shielded (gauge through 3/8"), 65 amp shielded (10 gauge through 5/8") and 85 amp shielded (3/16" through 1-1/4"). If I want to cut 1/4" steel.....I can choose any set of consumables except the Finecuts. If I use 85 amps, I will cut very fast, the cut edge will have zero dross, the edge will have top edge rounding and fairly severe bevel. If I choose the 45 amp set.....the 1/4" will have approximately 2 to 2.5 degrees bevel, clean sharp top and bottom edges and slower cut speeds.

Many try to use the high powered consumables....and simply reduce the amperage and the speed. In simple terms....larger cutting nozzles have a larger orifice to shape the arc. By turning down the amperage on a large nozzle....you are dramatically reducing arc energy density....which softens the arc. This low energy density arc often creates a concave or convex cut edge, virtually eliminates the dross free speed range.....producing a drossy, ugly cut. Using a smaller nozzle at its max rated amperage gives you lower power with high energy density, The energy density of a Hypertherm 45 amp nozzle running at 45 amps is the same as an 85 amp nozzle running at 85 amps, roughly 12,000 amps per square inch.

Here is a pic of 3/8" steel cut at 45 amps with a 45 amp nozzle. The edge angularity numbers are written on the top for each side......degrees from 90.

Powermax45 edge angle 004.jpg (36.83 KiB) Viewed 2703 times

Powermax45 edge angle 004.jpg (36.83 KiB) Viewed 2703 times

Powermax45 edge angle 003.jpg (28.45 KiB) Viewed 2703 times

Powermax45 edge angle 003.jpg (28.45 KiB) Viewed 2703 times

Jim Colt

[beefy]

Thanks Jim,

I always get jealous when I see the pictures you post of cuts

I understand most of what you said but what I'm trying to understand is why we cannot get that zero bevel simply by lowering the torch heigh for example. From literature I have read, some authors say if torch too high then the cut is widened at the top. If cut is too low cut is widened at the bottom.

They CLAIM that somewhere in between these heights is the right level to get zero bevel. The THEORY makes sense, I mean as you alter the height there's a transition point from positive to negative bevel (zero bevel) so I'm wondering why it cannot happen.

Keith.

[BTA Plasma]

Plasma beams come in various strengths based on amperage and air flow dynamics. Plasma also has a wimpy trailing edge whem compared to a laser. The trick is to keep the cyclone of air as round as possible througout the material as its pushing the metal down and the machine moving forward. High density plasma units use higher amps and much higher cut heights. The beam is bigger as well. So beam power, air flow and speed. The plasma doesnt know what the material is doing it is just follwing the settings.

[Largemouthlou]

My humble opinion is that there is zero bevel at the middle of the plasma arc!! But after the middle there will be bevel. Whether top or bottom bevel there will be some.. So the thicker the material the more the bevel at correct cutting heights.

Hope to be cutting soon, again!!!!!!!!!!!!!!!!!!!!!!!!!!!

[Shane Warnick]

The only way it would cut with absolutely zero bevel would be if the plasma arc was flat and straight like a cylinder all the way from the tip through the bottom of the material. However, since it's not (I'm told anyways) you are chasing the middle. If the so called "flat spot" on the arc was the same thickness or length as the material is thick, then yes you could get zero bevel, provided you could keep the arc EXACTLY the same height above the material +or- say 0.0005 inches. The other factor here is the speed, as the arc moves through the metal and produces the cut, the leading edge is doing most of the work and oxidizing and washing away the steel, this is why the arc trails the nozzles as it is literally being drug along for the ride. This in itself would create some distortion in the arc simply from one side hitting cold steel, the other side being exposed only to the kerf behind it would have less distortion. Then it is confined in a swirling vortex of hot gas that is prone to some disruption from the material being ejected from the cut stuff on the plate pressure changes in gas supply etc. Like Jim says, you can get real close, but too many variables for perfect smooth as glass edges.


Shane

[beefy]

Thanks for all the replies lads.

When I said zero bevel I was not expecting the cut to be flat, because as was mentioned in my first post I described the arc as bellowed out (see the attached picture from the book). I was more meaning the top edge being in line with the bottom edge.

This is the pictures from the said book. The drawings in the picture are a bit funny really. They show the torch jet with a candle flame shape yet the side of the cut perfectly flat.

Shane, I wonder if what you said is the main reason, that the torch would have to be too close to have the middle of the "candle flame" shape in the middle of the cut edge. And so the torch height has to be a bit higher to prevent the nozzle shorting out on the plate or even getting contaminated. Probably a bunch of the other factors mentioned too.

I certainly won't be straying from the book settings, was just curious about this one.

Keith

[jimcolt]

The appearance in the pictures of a "candle flame" is highly exaggerated to make a point, which is to show the effect of raising and lower the torch ...and the way you should expect the cut to look. In reality, if you read my earlier reply....the angularity has more to do with loss of energy near the bottom of the cut as well as with mechanical (call it friction) of the molten particles dragging down inside the cut. I know our engineers have studied this many times...often watching the arc and the molten metal removal with 100 frame per second cameras. The same phenomenon occurs with a laser cutting system as well.

The high end oxygen plasma systems (Hypertherm's HPRXD series) overcome the taper using oxygen as the plasma gas and a special vented nozzle design.....which can produce 4 times the energy density as that of an air plasma torch. The angularity numbers for these high def plasma systems are much closer to zero bevel, however there still are laws of physics that inhibit the chance of reaching perfection!

Jim Colt

[Brand X]

Jim has posted that picture many times. I would prefer to see same cuts made with 90 degree corners. Overall bevel figures would probably change a bit.. Although software can help in that area some..

Here is a air plasma inside cut one inch sq..MS from a Esab at 90 amps Newer torch designs help a whole lot with bevel. Getting better, and better all the time..

[beefy]

Sorry Jim,

I did read your post (and BTA seems to have said similar), regarding the loss of arc density near the bottom. A bit tired I think after getting up at 4am for work then fixing some equipment when I get home, then reading the posts LOL.

Keith.

[jimcolt]

Sharp corners with plasma requires 3 things.....certainly a good torch with good energy density, a good height control that is intimately communicating with the cnc control, and properly adjusted motion control with the correct levels of de-acceleration, acceleration to allow the bottom of the trailing plasma jet "catch up" to the top. I have many pics of sharp plasma corners in my files...they look like the one posted by Brand X, so no real need to show more!

Jim

Jim has posted that picture many times. I would prefer to see same cuts made with 90 degree corners. Overall bevel figures would probably change a bit.. Although software can help in that area some..

Here is a air plasma inside cut one inch sq..MS from a Esab at 90 amps Newer torch designs help a whole lot with bevel. Getting better, and better all the time..