Newbie to industrial robots needs some help deciding.

What kind of welding are you doing? That is, what does the path look like?

If you're doing circumferential welding, you will probably need a 7th axis system, to rotate the boiler through 360+ deg while the robot welds. Robot arms are going to have a hard time making a complete circle like that on a fixed cylinder.

Those robots you linked to are very good robots, but they're so old that support and parts are no longer available from KUKA, and they're much harder to find on the used market. Trying to upgrade them to support 7+ axes and add the ArcTech welding software is going to be very difficult. Basically, by buying a robot that old, you're more likely to be buying yourself a long-term DIY R&D project than something you can get into production reasonably quickly.

For KUKA, I can't really recommend buying anything older than a KRC4 series these days, and in fact I would recommend not buying a KRC4 built on anything older than KSS 8.3.

Quote from Christos Tsiligiannis

I already have made a mechanism that turns the boiler all the time 360 degrees. I only need this robot to weld vertically these two sides, first the one on the left then the other on the right while also doing slight zig zag moves.

The boiler is always rotating, while the robot welds? That's a potential problem. In order to get good welding results, the best approach is to have whatever mechanism rotates the boiler be part of the robot, so that the motion of all the axes can be synchronized. If the rotator and the robot are separate mechanisms, it's easy for them to get out of synch. If your rotator has very smooth and constant-speed motion, and you have some way to ensure the robot starts and ends at the correct point on the rotation, it can work, but it'll be tricky to get everything in synch.

"Slight zig zag" -- the term used is "weaving", FYI. If the robot has the arc-welding software installed, there's usually a utility for generating this weave pattern automatically by simply entering the parameters. But the arc-welding software usually relies on the rotator being a robot axis, rather than an independent mechanism. So if your rotator stays independent, you might be better off without the arc-welding software.

Quote from Christos Tsiligiannis

I now use argon welding, brand is SAF-FRO 3phase 400amp.

That will be another thing. Integration. You will need to wire the I/O of the robot to the controls of the welder. If the robot comes without any I/O hardware (which is often the case, people selling used robots often strip them down), you'll need to add some. KRC2s only support DeviceNet organically. Fortunately, DeviceNet, while archaic, still has a healthy market for I/O devices. What I/O hardware you buy will depend on what signals the welder sends/receives -- analog, digital, what voltage, etc.

i would say forget about the second robot.... that is a system with Denso arm and not good to be stuck with a unicorn. also no brake on A1 and forget about external axis.

the first robot looks ok, it is a proper robot with full size cabinet. this can be useful, specially if it has everything already (drive for E1, ArcTech package, recent KSS like 5.5 or 5.6). because as was mentioned, line is discontinued so getting software is next to impossible. getting hardware is not too bad as this was a workhorse for many years and there is tons of second hand units out there that get cannibalized for parts.

if it does not have ArcTech, you should still be able to make it work but you would have to do it yourself. all KRC2 controllers do have DeviceNet on MFC. Note that MFC is only DeviceNet master and most welders are slaves. I think the only welder that wants to be a master is Miller... In that case you would need to have LPDN card (this is an add-on)

If you don't mind me asking... what's your price-point and does it have to be a KUKA?

Quote from Christos Tsiligiannis

I didn’t know that the pulse for the welding machine from the robot could be a hassle. So I need to find a manual for my welding machine and check the protocol/pulse it needs. About KRC2 now, is there any documentation or quick start guide to see how is the configuration for welding machine communication is happening.

Depends entirely on the welder. Is it a handheld weld gun? You might have to break open the welder to hotwire a signal to take the place of the hand trigger, or wire a relay across the lines of the foot pedal (or whatever else your welder uses).

As Panic said, all KRC2s support DeviceNet natively on the MFC card. So adding a DeviceNet I/O device, and some relays, and wiring all that up, isn't too hard. Attention to detail is required, though. And bad wiring could easily fry the MFC card, turning the entire robot into an expensive paperweight. So... be careful.

I agree with Panic -- get the one with the full-size controller cabinet. The compact controller is going to be harder to get parts for or work inside, and if it's one of the old Denso robots that was re-labelled as KUKA (a short-lived, very odd partnership), then stay away.

Before buying any used robot, demand a complete accounting of what the robot comes with (Operating system version, software options, hardware options), and whether or not it is in fully runnable condition. Also demand to know what power source it requires -- that KRC2 has no transformer, so you will probably have to provide it 400VAC 3-phase.

Also, for one of these robots: before anything else, clone the hard drive. That is the biggest failure point on these controllers, and it's no longer possible to get re-install media from KUKA. If the hard drive fails and you don't have a full clone backup, you are going to have great difficulty getting it operational again.

Quote from Christos Tsiligiannis

Yes I have 400v 3 phase. I know about clone-ing of the drive as it’s just a super micro pc running however I hope I will be able to find a free clone software to get all the img.

CloneZilla always worked well for me in the past. You'll most likely have to dismount the HDD from the KRC2 and get a IDE-to-USB adapter or something similar, as I strongly doubt that the floppy or CD-ROM drive on the KRC2 work any longer.

Quote from Christos Tsiligiannis

so i understand that i can enable or disable an external motor using kuka commands but i would like some help on how to actually do it.

that is what I/Os are for - to allow robot to interact with external world.

so to add some I/O mouled to your robot you need to install them somewhere, connect them to KRC, configure fieldbus and map the I/O..

first thing is wiring,... and it better be right,, if it is not right, no amount of poking in the software will get you something that works. DeviceNet also uses additional 2 conductors for 24VDC. You need to provide that 24VDC - this is NOT something that one of the nodes on the bus provides (some assume MFC would do that). also like any bus, devicenet needs proper termination.

next you need to add the slave I/O modules to DevNet.ini. Each must have unique macid (node address).

then you need to edit IOSYS.INI and make sure DeviceNet driver is uncommented.

finally you need to map the I/O (also in IOSYS.INI)

Quote from Christos Tsiligiannis

ok so according to the manual here i see that a devicenet card has manu outputs. each output gives a digital 24v signal which you can forward it to a solid state relay and enable or disable an external motor or you can even enable the welding. so i can now understand how this all works.

No, not quite.

DeviceNet is a protocol. Sort of like TCP/IP. DN uses CanBus as the physical layer, the same way TCP/IP uses Ethernet. CB/DN is multidrop, so the cable daisy-chains from one device to another (no hubs/switchs), and it relies on correct termination at the ends. The bus cable is 5 wires: 24VDC, 0VDC, Shield, CAN+ and CAN-. This is where you need to be very careful about the wiring. There's also a second "aux power" 4-pin cable that carries 24VDC on two pins (one switched, one not), and 2 0VDC pins (to allow power isolation).

There are many different devices that can be connected to a DN bus. The most typical ones are simple Input and/or Output devices, and the most typical voltage they deal with is 24VDC, which is the de-factor standard for most industrial I/O.
It's also possible that your welder has provisions to connect directly to DN, or the manufacturer makes an add-on to make that possible. You'll want to check carefully.

However, those devices come in PNP, NPN, and dry-contact varieties, just to name a few. Some high-current, some low-current. Then there's modules made to handle 5VDC, or analog devices designed to work by voltage (0-10V, -10 - +10V, 4-20mA, etc).

So, to replace the manual trigger button on your welder, you probably don't want to wire an Output module directly in -- the welder might use a different voltage internally, or it might need a high current through the contact (buttons are direct shorts, after all), and then you have to make sure to use common grounds and... ugh. The best bet is probably to wire the contacts of a relay in parallel with the manual button, and wire the relay coil to an output module controled over DN by the robot. You'll have to make sure your relay is compatible with both devices for voltage and current. Don't skip the flyback diodes, either. Or, just buy a dry-contact style Output module (again, make sure the specs match) and skip wiring up extra intermediary relays.

The same will hold true for every signal you might need to move between the welder and robot. If the welder has a fault, you probably want the robot to know that and stop, but unless the welder has contact outputs, you might have to wire a relay across the fault light, or do something else creative.

Panic and the PDF you referenced cover most of the KRC-side configuration -- basically, the robot acts as the bus Master, and you describe the bus devices to it -- address (MAC ID), number of bytes produced/consumed, and bus speed are about it. Then you have to connect the KRC's internal "virtual" signals to the hardware signals in IOSYS.INI.