Plasma cut parts - big divot normal?

[M. Moore]

Here is a pic of some parts I just had cut at a local major steel supplier. They have a very expensive plasma setup.
When I complained that the large divot meant I had to weld and grind they said, “Sorry that is normal at the start and finish, order oversize and machine to size”.
This does not happen at all with torch cut parts.
Material is mild steel, 3/8” thick and 8” in diameter.

So is this normal or can it be done better?

 

[DDoug]

I've seen that many times before.
Having said that, the draft angle and taper looks like they are running very fast.
Can they doo better ? I think so, but getting them to slow down from their normal speed, is probably very hard, even if you offer to pay more.

 

[Scruffy887]

Looks like they do not use a lead in at all. Pierce and go.

 

[Milland]

Looks like they do not use a lead in at all. Pierce and go.

On a round that's just laziness.

Maybe send them profiles with a little hump and tell them to start the cut there?

 

[M. Moore]

My guess was that it could be improved. I was just not sure why these were so bad. With such an expensive machine I was also expecting on 3/8 thick material that the cut would be very close to square, which it is not.
On some larger rectangles cut at the same time they started at the corner and this was ok for those parts. No corners on round discs…..

 

[kustomizer]

My guess was that it could be improved. I was just not sure why these were so bad. With such an expensive machine I was also expecting on 3/8 thick material that the cut would be very close to square, which it is not.
On some larger rectangles cut at the same time they started at the corner and this was ok for those parts. No corners on round discs…..

It is not the machine, it is the "it's good enough for who it's for" guy running it, with a little practice you could make your parts that way too.

That is how we ended up buying a waterjet machine, outsourcing sounds a lot easier than it is.

 

[RC Mech]

Pierce mark is totally unacceptable. It’s operator inexperience or ineptitude.

There’s no reason for that taper on such thin parts either. Expensive plasma set up and a $3/hr “operator”.

 

[gbent]

Sloppy programming, worn out consumables, get it out the door asap attitude. This is what we do, that is what you get.

 

[dana gear]

Looks like they do not use a lead in at all. Pierce and go.

Absolutely no lead in, also torch direction, always cut rounds clockwise.
Watch consumables, they can run all morning long or 20 minutes. Monitoring cut quality during cutting is important.

 

[LKeithR]

...but getting them to slow down from their normal speed, is probably very hard, even if you offer to pay more...

Why? If you're willing to pay they should make the parts exactly the way you want them. Most of the
suppliers around here will burn parts to your specs. If all you want is quick, dirty and cheap that's what
they'll give you but if you want a better finish with no divots they'll do that instead.

If you can't get what you want I'd be looking for another supplier...

 

[Strostkovy]

You should at most have a slight bump from the leadin transition that can be taken down with an orbital sander.

The programmer of that machine is incompetent.

 

[DDoug]

Why? If you're willing to pay they should make the parts exactly the way you want them. Most of the
suppliers around here will burn parts to your specs. If all you want is quick, dirty and cheap that's what
they'll give you but if you want a better finish with no divots they'll do that instead.

If you can't get what you want I'd be looking for another supplier...

The quote you get is for their normal process.
Asking them to reduce speed from ex. 120 ipm, down to 40 ipm (what I would recommend)
Would cost your supplier more, would it not ?

 

[lucky7]

My supplier locally does much better on round plasma burnouts. Last batch of 7” round by 1” thick had a nearly vertical pierce and less than 1/16” indent. Had usual haz. Unfortunately I’m several provinces over from you so my supplier likely won’t work for you.

L7

 

[RC Mech]

The quote you get is for their normal process.
Asking them to reduce speed from ex. 120 ipm, down to 40 ipm (what I would recommend)
Would cost your supplier more, would it not ?

Right but if that’s their normal process then their process sucks.

My plasma guy gets about a 1/16” taper on 4” flame cut plate, because he cares about the quality. He’s pretty cheap too. If the shop above can’t be competitive AND produce decent quality parts they have no business being IN that business.

 

[DDoug]

Right but if that’s their normal process then their process sucks.

My plasma guy gets about a 1/16” taper on 4” flame cut plate, because he cares about the quality. He’s pretty cheap too. If the shop above can’t be competitive AND produce decent quality parts they have no business being IN that business.

Yes, I agree.
However if they get more jobs, because they are running faster, making them cheaper, and all you need is that poor quality, then they get the business.

To many customers, that part quality pictured is acceptable.

 

[Scruffy887]

Pierce mark is totally unacceptable. It’s operator inexperience or ineptitude.

There’s no reason for that taper on such thin parts either. Expensive plasma set up and a $3/hr “operator”.

Nailed it. Size 42 jacket, size 2 hat.

 

[true temper]

Absolutely no lead in, also torch direction, always cut rounds clockwise.
Watch consumables, they can run all morning long or 20 minutes. Monitoring cut quality during cutting is important.

Educated me, why would it matter which direction you programmed to cut?

 

[memphisjed]

That is not a pierce mark. That is small part falling out of the plate at the end of cut. Large tables have slats far apart, the small parts suck to cut because of this exact scar. The operator can stop it somewhat by holding part with magnet before it falls, then dodging the gantry as machine move to next part. It is literal definition of back breaking work running a plasma that size. you can not just reach over to grab anything- you stand on the plate as it is being cut.

 

[Kalispel]

Educated me, why would it matter which direction you programmed to cut?

The plasma gas is swirling internally to stabilize the plasma arc and protect the orifice in the nozzle. Imagine a tiny cyclone - the tangential gas velocity is greatest on the outside where the arc impinges on the sides of the orifice. The gas velocity is zero by definition on the centerline of the vortex. This is the purpose of the “swirl ring” in the torch.

The cut is directional due to the direction of the swirl, analogous to climb vs conventional milling. OD and ID contours are cut in opposite directions for the best kerf.

I don’t know if the marks are a pierce or the part falling forward as the cut is completed. Either way it can be eliminated with better lead-in strategy or by supporting the parts.

 

[Strostkovy]

That is not a pierce mark. That is small part falling out of the plate at the end of cut. Large tables have slats far apart, the small parts suck to cut because of this exact scar. The operator can stop it somewhat by holding part with magnet before it falls, then dodging the gantry as machine move to next part. It is literal definition of back breaking work running a plasma that size. you can not just reach over to grab anything- you stand on the plate as it is being cut.

At 8" in diameter they shouldn't be dropping.