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TGTool
12-16-2013, 09:03 AM
So. The local model engineer group visited a tech school on Saturday that had a water jet cutting machine. They had examples of things they'd cut from wood, stone, and metal. Big tank, probably 5' x 10' and gantry design. All interesting enough.

I was looking at the drive and ways on the rail to see what they'd done and it suddenly got a lot more interesting. The rail was a big extrusion maybe 4" by 10" with a ground rod inset top and bottom near one edge and rollers for movement. I was looking for leadscrew and drive but no openings in the rail anywhere and nothing outside. Finally peeking in behind the carriage I spot a sensor that's adjacent to a thin stainless steel band like on a printer or plotter though I didn't find any slits, holes or other indications so it must be magnetic unless they're using something like an optical mouse sensor.

The instructor says that movement resolves down to .001 and maybe they don't need anything closer than that. But back to the drive system. It must have a friction drive and a closed loop servo then for movement. Haven't seen that before but it's probably a logical design for something like this. Longitudinal movement was also on ground rods with similar positioning. I didn't think to go round the other side but I presume they've got twin drives on each side synchronized for the long movement. All in all very interesting and educational, exclusive of the various shapes they were making.

dfw5914
12-16-2013, 06:24 PM
No cutting forces to deal with, so I guess they can build 'em like giant plotters.

J Tiers
12-16-2013, 08:24 PM
They don't seem to be that accurate... in an absolute sense... Our waterjet folks have a big machine and it holds around 10 or 15 thou over the size pieces we get from them, which have been up to about 1.5 meters long.

Still pretty good if you take it as a percentage.

kf2qd
12-16-2013, 08:35 PM
Interesting - Used to build and install plasma/oxy fuel machines and we went full servos and rack and pinion for the drives. $75,000US for the bare machine. A couple plasma units and the total came to $150,000. Water jet machines used stepper drives and ran open loop. Machine started at $250,000. We had one customer with a waterjet machine that had DC servos and rack and pinion drive that we retofitted to brushless servos, they cut some stainless disks 20" in diameter and they measured them to within 0.002. The reason the the not to close accuracy is not a fault of the process, its the fault of the motion design. Most of the time the need is not for close tolerances, but rather for getting pieces cut in otherwise hard cut materials, either because of material thickness, or because the materials are otherwise considerred non-machineable.

LKeithR
12-16-2013, 09:53 PM
I think you guys are out to lunch when you talk about the lack of accuracy delivered by a water jet. It's a "cutting" process, akin to laser, plasma and oxy-acetylene. It's not machining and it was never intended to be. For the fabrication industry--where the majority of the products get used--it's very accurate; far more than is needed for the average welding job. If you want better accuracy get your parts machined; but watch your costs go up, too...

TGTool
12-16-2013, 11:36 PM
I didn't get into detail with the guys on control, but IIRC they thought they could hold one or two thousandths and there was cutter comp so you could do a test with actual conditions on material, check what happened and compensate as necessary.

I don't know specifics about Jerry's experience but it might have been a machine that was looser or they left extra material as a matter or course. If it was a part I planned to machine later and the surface finish needed to be better or size closer I might want .015 left on for a finish cut.

DATo
12-17-2013, 03:54 AM
I think you guys are out to lunch when you talk about the lack of accuracy delivered by a water jet. It's a "cutting" process, akin to laser, plasma and oxy-acetylene. It's not machining and it was never intended to be. For the fabrication industry--where the majority of the products get used--it's very accurate; far more than is needed for the average welding job. If you want better accuracy get your parts machined; but watch your costs go up, too...

Totally agree! Having the metal supplier water cut our material has become de rigueur where I work. Last year we had a project which required large (for us) pieces of stainless steel to be cut from plate. The delivered shapes were donuts with a 24" O.D. and a 13" I.D. .... and .... 2" thick! The parts came in very close to size and without the burn and slag of plasma torch cutting. It was as close to "a pleasure" to handle and to work with them as any precut material I have ever had to deal with. The water cutting machine is computer controlled so you get exceptionally good dimensional accuracy when compared to band sawing or torch cutting though this is still meant to be considered a "roughing cut". We've also had aluminum pieces cut this way and the results are equally impressive. Also, totally agree with the OP that where welding fabrications are concerned involving parts of even semi-intricate shapes this method of ordering material is a no brainer.

jimcolt
12-17-2013, 03:49 PM
Just to clarify a few things:

-Most water jet machines use precision helical rack and pinions as the drive method on x and y axis, the reason they are hard to see is because the machines are built so that all of the way surfaces are sealed to protect them from the abrasives used in the (Abrasive water jet") cutting process.
-Water jet cutting processes on most hard materials are relatively slow (compared to plasma and laser), but more accurate. Typically steel parts can hold tolerances in the .003" to .005" range.
-Industrial high definition plasma cutters can hold tolerances typically in the .015" to .020" range. Cut speeds are dramatically higher than water jet on metals, and higher than laser on metals over 3/16" thick.
-Lasers typically hold tolerances in the .010" to .015" range.
-If you need a piece of steel cut and the plasma tolerances are acceptabel...it is by far the least expensive process.
-If you need a piece of steel cut and cannot accept any heat induced metalurgy (heat affected zone)...then abrasive water jet is the best choice.
-since water jets cut relatively slowly, their drive systems can be somewhat simpler as compared to laser and plasma.......with the exception of the fact that moving parts need to be sealed from the abrasive slurry.

Jim Colt Hypertherm Hypertherm manufactures plasma cutters, fiber laser cutters, and abrasive water jet cutters.

macona
12-17-2013, 05:51 PM
We easily hold less than .005" tolerances on our lasers, even the ancient Mitsubishi HC we have. In fact we dumped one vendor who was doing tube-laser work because they couldn't keep .005, we ended up setting up a fixture on the HC to do the part.

jimcolt
12-17-2013, 06:15 PM
You can always do a little better than the machine specs if you have a decent machine operator. I have a low cost entry level cnc machine with an air plasma in my home shop....and I can do short runs of parts in steel and hold tolerances of better than .010", however if I do production cutting and need a lot of parts fast and at the lowest cost, I would advertise +- .020". I know some of our waterjet system users can hold .002" on certain jobs as well, but a lot slower than plasma.

Most of the motion control components on laser machines are much more precise than plasma machines.....for the simple fact that people expect better tolerances. A typical laser that has a 5x10 cutting bed and the ability to cut 1/2" steel....will cost $400k to $500k today. The same size plasma (High Definition) will be in the $80k range, and the same size plasma (air plasma) will be in the $25k range. Water jet systems are priced between High Definition plasma and lasers.

Put a high definition plasma on a machine with Laser Quality motion......and you get better performance than the same plasma on a lower cost machine. You kind of get what you pay for!

Jim Colt



We easily hold less than .005" tolerances on our lasers, even the ancient Mitsubishi HC we have. In fact we dumped one vendor who was doing tube-laser work because they couldn't keep .005, we ended up setting up a fixture on the HC to do the part.

J Tiers
12-18-2013, 08:48 AM
I didn't get into detail with the guys on control, but IIRC they thought they could hold one or two thousandths and there was cutter comp so you could do a test with actual conditions on material, check what happened and compensate as necessary.

I don't know specifics about Jerry's experience but it might have been a machine that was looser or they left extra material as a matter or course. If it was a part I planned to machine later and the surface finish needed to be better or size closer I might want .015 left on for a finish cut.

I designed the parts, they got a file to put in the machine. They were cut to net shape, no machining allowances, because they would not GET machined.

I'm not complaining.... the PERCENTAGE accuracy is good. And for a machine that cuts BIG parts, those tolerances of 15 thou over 5 feet would indeed be considered very good.

The "local" accuracy is much better.... low thou on details and relations between details in the sub 1 foot area.

My comment isn't that they are bad, just that the sort of accuracy one thinks about off the lathe or even the mill may not be what you get in an absolute sense on a large part.

Just remember... a "master precision" level can be "out" 10 thou in 20 feet..... and it is still 0.0005"/foot accurate, which is very good.

What waterjet does well:
*************************************

Cuts any reasonable thickness.

Cuts nearly anything

Is usually cheaper than any other machining, including laser, for big parts.

Gives a better edge than plasma, and sometimes better than laser, due to no heat.

Cuts any shape you can give it data for.



What it may not do well:
*****************************

Give very precise lines.... the jet has some width and even some dispersion.

give high accuracy over long parts (most large machines do not)

give a smooth cut like a saw, the edge is even, but "abraded"

Cut a good small hole to precise size. It's not a drill, the "piercing" process can make a mark, and the jet has width.

usually will not do chamfers etc, although there are machines that apparently will.