WPS: WFS vs. Amperage

Date 07-20-2010 17:43

We I am running PQR tests, I use record the amperage (along with volts travel. etc) because the heat input has to be calculated. I also

But the welders like to know the wire feed speed because they can control that more easily.
The ammeter tends to "dance around" quite a bit when welding with constant voltage due to electrical stick-out and other variables.

By Duke

Date 07-21-2010 00:56

chet, how do you record the amperage on a CV machine? Hi-lo and then average?

By 803056

Date 07-21-2010 03:34

I record arc voltage, current, wire feed speed, and electrode extension when documenting the variables used on the PQR for any semiautomatic welding process such as GMAW or FCAW.

There are three constants when working with GMAW and FCAW; arc voltage, wire feed speed, and electrode extension. Amperage is a variable that changes as the welder varies the electrode extension, so it is not a good parameter to use when setting up the machine.

The welder dials in the voltage, sets the wire feed speed, and holds the gun so the proper electrode extension is attained. All is good until the welder varies the electrode extension, then the amperage drifts. The next thing you know the welder is kicking the machine and cursing that it is no good. The fact of the matter is the machine is fine providing the welder holds a consistent electrode extension.

Best regards - Al

By Magilla (*)

Date 07-21-2010 12:02

Thanks guys, i appreciate the help!

By eekpod

Date 07-21-2010 14:42

so Al, if the gun is on an automated track, I would summise that the amps variation would be very little since the travel speed is constant and the stick out is constant as well. Basically the amps woulndt dance around too much right?
Chris

By jwright650 (*****)

Date 07-21-2010 15:25

nah, it still fluctuates around some, maybe not as bad.

By CHGuilford (****)

Date 07-21-2010 17:01

Duke,
What I do is to use the clampmeter while the welder sets up the machine. I look to see that the amperage is in the range of the WPS. Once everything is good, we just note the WFS numbers. That is the easiest way to repeat good results. I also know about where the WFS should be from the PQR tests.

For PQR tests, I leave the meter on the ground cable and record "average" amperage for each pass. The PQR sheet will use the numerical average of those readings. However, I also record the WFS once at the beginning of testing- I usually check that by holding the trigger for 15 seconds and measuring the length of wire that came out (so far the meters have been right on but this makes some 3rd party inspector happy - but don't tell them that my stopwatch is not calibrated.)

By jwright650 (*****)

Date 07-21-2010 18:36

I was going to mention that if you use "WFS" on your WPS as one of the controlling parameters, that some TPIs will freak out if you don't have the WFS calibrated to a national standard....I found that the amp clamp is fairly reasonably priced to get calibrated and it can be used on all of my machines, rather than getting all of the WFS meters calibrated on every machine in the plant....but Chet beat me to it.

By Magilla (*)

Date 07-21-2010 22:22

OK, new twist to the question, when the code states Wire Feed Speed (if not amperage controlled), does that mean if the wire speed is amperage controlled it will vary automatically to maintain the correct amperage? On CV wouldn't the amperage be controlled by the wire speed, therefore requiring the speed to be listed on the WPS? Or am I over thinking it? Being a field ironworker, all the WPS's stated wire feed speed, except those for CC machines. My co worker, being from the rail car industry and a boiler maker, has never seen wire speed listed on a WPS. Any insight on this would be appreciated, and thank all of you for your help.

By 803056

Date 07-22-2010 04:27

I tried to respond earlier this afternoon, but I was interupted and had to shut down.

By definition a constant voltage machine will vary the current to maintain the proper arc length. In other words, if the arc length increases, there is an increase in arc voltage. With an increase in arc voltage, there is a corresponding drop in amperage. The change in current per change in voltages is a function of the slope of the power supply. The drop in current will decrease the melt-off rate. Since the wire feed speed is a constant, the arc length will decrease thereby reducing the arc length and the arc voltage.

With the drop in arc voltage, the current will increase thereby increasing the melt-off rate. With the increase in melt-off rate, the arc length will increase. The changes are dynamic, i.e., constantly changing (arc length and current). The system is dampened so that the machine doesn't respond instantaniously to each change in arc length, arc voltage, and current. The inductance of the power supply dictates how fast the machine responds to a change in voltage. The goal is to stabilize the welding parameters so the welder can have a "smooth" consistant arc.

Some power supplies have variable inductance that permitted the welder to tweak the system to produce the optimum arc characteristics for different base metals. That is, one inductance lends itself to a stable arc when welding carbon steels and a different inductance is preferrable when welding stainless steell or nickel alloys.

Most welders have little appreciation for the mechanics of how the welding power supply functions. They learned to weld using SMAW where the variable "set" at the machine is current and voltage is controlled by controling the arc length. They use the same thought used for setting a constant current machine to set the parameters of a constant potential power supply. Imagine setting up for SMAW using arc voltage instead of current. That is essential what many welders are doing when they try to set up for GMAW or FCAW using current.

You set the current when using a constant current power supply, i.e., SMAW. You would be better served to set arc voltage and wire feed speed when setting up a constant voltage power supply. The current is a function of electrode extension once arc voltage and WFS is set.

The fact that WFS isn't listed on the WPS is a serious oversight by the individual writting the WPS. If you review the essential varibles listed in D1.1, you will note that WFS is an essential varible for semi-automatic welding processes, i.e., FCAW and GMAW. The WPS is required to address all essential variables when qualified in accordance with clause 4 and WFS is required to be addressed in a prequalified procedures per clause 3.6 (D1.1-2006 and 2008).

Best regards - Al

By ziggy

Date 07-22-2010 10:53

In reference to D1.1:2008 3.6 it seems that the code is seeking amperage OR wire feed speed.

Likewise, on the non-mandatory forms in the annex, the WPS shows it as amperage OR wire feed speed.

It could be understood that either amperage OR wire feed speed can be listed.

The majority of WPS that I see list one or the other. Rarely do I see a WPS with both listed.

Just an observation,

ziggy

By js55

Date 07-22-2010 12:45

My opinion would be that it really makes no difference if you establish your procedure for amps or wire speed.
Utilize the one that the machine displays.
Ask yourself what it is you are trying to accomplish.
Al is correct of course in that with CV's the amps jump around. But IMO its a so what except for overzealous inspectors who might charge you with violation of your WPS if you have a momentary excursion beyond that which is written on paper. Which is likely if you set the machine towards the edge of the acceptable range and then watch the dynamic adjustment of the machine. Especially with analogues.
However, though an absolute reading of the code may prohibit such excursions, though I am not sure this is true, in the end from a practical standpoint these momentary excursions mean nothing to the viability of the weld, other than in the case of extreme uncoordination by a welder.
Personnally I prefer to control wire feed speed as Al does, but I see no weld viability reasoning for insisting upon such.
The thinking is not unlike that which controls much of micrometallurgy in that we can view things at a very small scale but does that mean that it is necesary to impose such on the shop floor. For example, do we really want to utilize SEM to test weld viability on the shop floor?
Or, if you interpolate (interpolation is not a code requirment by the way-though some are arguing such) code requirements for heat treat heating and cooling rates and we get better and better at seeing the momentary excursions of heat in the furnace do we want to insist upon a violation of code if a particular furnace zone has split second heat excursion of 5deg that happens faster than that listed by code?
Of course not and nobody is suggesting such(well, I have heard close to this on heat treat).
So, is a split second excursion of amperage significant?
Just because we can see it does not necessarily mean that it matters.
IMO it is perfectly acceptable to set your machine for amps or wire speed and utilize either, though I agree with Al in that I prefer wire speed myself. It is good welds youa re after.

By 803056

Date 07-22-2010 20:54

Hello Zig;

Read clause 3.6 again. Note that the requirement includes the verb "shall”, however the word "or" does not occur in the list of variable that are to be listed on the WPS.

While contractors regularly practice "selective reading habits,” inspectors should be very careful not to follow the same practices. If the variable applies, and in the case of semi-automatic welding it does, wire feed speed is one of the essential variables that are required. Clause 3.6 takes precedence over the Annex. We are dealing with AWS D1.1, not ASME Section IX.

There is an exercise I use in my courses, read the following:

"I like Paris in
in the springtime"

Read it again and write what you just read without looking back at the quote in your reply.

Best regards - Al

By Magilla (*)

Date 07-22-2010 21:59

Thanks again guys, great info!

By jarsanb (***)

Date 07-23-2010 17:21

I have welding equipment set up with the exact same welding machine type and wire feeders. None of these units have the same wire feed rate displayed to produce the same amperage output. Using .035" wire, same classification, some of the feeders are set 40 ipm's lower to produce the same welding amperage. Now, if I measured the actual wire feed speed from the end of the contact tip I'm sure there would be less difference. Tesion, drag, drive roll condition are all at work here. If my wire feeder is set at a certain ipm, and over time conditions slowly affect the actual wire feed rate, which can significantly change amperage output (20-30 amps is significant with short-arc for example), we're going to have a problem if amperage isn't also monitored. If you are going to list WFS then this should be checked frequently to make sure you are getting accurate results.

By 803056

Date 07-23-2010 20:23

If your wire feed speed is the same, check the electrode extension used by each welders to verify they are using the same extension. If the welding machines are the same model and if they are using the same arc voltage, and the same electrode extension, the variation in amperge is going to be minimal.

I usually find that the welders do not use the same electrode extension unless they are made aware of the importance of controlling it and holding it constant.

Best regards - Al