RSW parameters?

By 803056

Date 10-05-2008 22:44

Did you try the Resistance Welding Handbook from the AWS?

Best regards - Al

By MDG Custom Weld (***)

Date 10-06-2008 16:39

I have done a fair amount of spot welding development both manual and robotic, and I'll try to help you with squeeze and hold.
First, RSW has sooooo many variables, but only 3 of them are absolutely critical to have right.

Current (k amps), Time (cycles), and Force (tip force in pounds). For Time 1 cycle= 1/60 second. Once these are right one can move on to all the other bells and whistles like up slope, pulse, multi-pulse, off time, down slope, hold, ect ect ect.

Squeeze is nothing more than a pre weld current timer. This is used to allow the tips to generate the required force before passing current. As we know, force is one of the 3 main variables in RSW. Let's say your weld stack up requires 1,000# force, and you use a force gage and close the tips and it reads 1,000#. The size of the cylinder and the stroke of the cylinder dictate how long it takes to reach the 1,000# required force. Once the tips hit the surface they begin to build force. We are not talking in seconds and you may not even see the delay, but there is one. You do not want to pass welding current before the required force is attained. The tighter you hold something together, the less electrical resistance it has. Think of loose electrical connection, what happens....it gets hot...Why...resistance. So if we pass current as the tips are generating force, the resistance will be increased, possibly causing expulsion or inconsistent welds. That is where squeeze plays a factor. If it is set too low, the resulting welds will be inconsistent at best. If it is set too high, no big deal except that it takes away cycle time. The right setting is based on the equipment, but usually can range from 25 cycles up to 90 cycles. The best thing during development is to set it high until you have your complete schedule worked out, then go back and reduce it by 10 until your weld starts to vary, then go up 10. Another way is to set everything (I mean EVERYTHING) except squeeze at 0 and place the system in "no weld". Then cycle the gun with the force gage between the tips and watch that the reading is. Go up or down as needed to generate the proper force. This is the best way to minimize your overall weld time.

Hold time is at the end of the welding cycle, and only holds the tips closed for the set amount of time. Since the tips are water cooled, excessive hold time acts like a quench and starts pulling heat out of the weld nugget. This can be a problem on some materials like ss and aluminum. Some controllers use "off time" as well, but it is the same as hold. Don't confuse these with cool time when pulse welding. Cool time is the time in between the pulses.

Send me a PM if you want more information on RSW, I'll be glad to help.
Mark

By Metarinka (****)

Date 11-01-2008 01:10

thanks for all the help for those who have chimned in. I've spent the last several weeks reading and learning about the RSW process of which they don't teach for my WE degree.

Upon further investigation it looks as if I'm battling an uncalibrated/ faulty machine as well.
On the very good advice of MDG I purchased a weld force gauge. Thank you again Mark for all your help. Be careful, if you keep on giving out such good advice you might just get more calls from me!
I went to calibrate the machine such that measured tip-force corresponds to machine settings. However there seems to be a variance of about 5 psi between what the controller *thinks* the pressure is at and what the line pressure reads. This is an older add-on controller (circa 95) In which the machine uses a solid state pressure sensor that reads the PSI in the cylinder and uses the given area of the cylinder to calculate tip force. It spits out a PSI reading for the operator to manually set line pressure at, based on a tip force value stored in the weld schedule.

Due to the variation I can alter variables so that the if the schedule calls say 1000# of tip force the controller will accurately display the correct PSI that the line setting should be set at by the operator. However internally it monitors tip force based off of an innacurate calculation and believes the line pressure is 4-6 PSI too low.

This is causing issues as the controller starts welding cycles when it believe tip force has reached the proper level. Since it is not accurately calculating tip force (although proper tip force is being reached) I believe it is skewing the welding schedules not to mention tripping low or high force alarms. The difference between line pressure as set by the operator and cylinder pressure as measured by the controller could be as simple as an old or faulty pressure gauge.

Also the current monitors on the primary and secondary loops don't seem to be functioning. I'm quickly changing from a welding engineer to machine repair technician! until I can accurately calibrate the tip force and welding current as computed and used by the controller I will have trouble maintaining welding schedules within acceptable parameters.

Back to the head scratching chamber... I mean Cubicle.

By DaveBoyer (*****)

Date 11-01-2008 03:33

A quote from an old friend about gauges: It is a "GAUGE not an INSTRUMENT" Bourdon tube pressure gauges are not always accurate, smaller ones can be set for the "0" point only, large ones can sometimes be set for rate of rise as well. If You remove the case and look at the linkage, You will understand what I mean.

By MDG Custom Weld (***)

Date 11-04-2008 14:20

I think it's odd that your education has no real focus on RSW. Being from Michigan and all of the automotive and office furniture manufactures, RSW is very prevalent in our local manufacturing environment.

The challenges that you face with tip force and machine calibration are going to be hard to resolve. Do you have a way to adjust the machine calibration settings so you are not faced with the over/ under force alarms? Most of those pressure switches have a calibration screw or analog voltage adjuster so you should be able to bring it back in line with the actual readings. Also, it is odd that it has a current monitor loop on both the primary (at or near the SCR) and the secondary (on the gun arm). I would think that may be a problem since the controller will try to compensate the current based on the feedback from the loop, but how does it differentiate between the primary and the secondary?

It is a good thing that you are learning how to be a machine repair technician along with your WE education :) You will become a much better WE if you have a good background in how things actually work on the production floor. As we all know, things on paper work much better than things in the real world! As a WE you will gain a lot of respect when you go to the shop floor to get dirty and work these problems out with the technicians that have to deal with these machines every day. That will also limit the 2am phone calls because they can't figure out the machine that you set up and left for them.

There is no book study substitute for real plant floor experience on everyday production equipment.

Good luck, keep us posted on your progress.
MDG

By Metarinka (****)

Date 11-07-2008 18:37

I can't speak for other welding schools but to my knowledge Ferris State has one of the most indepth RSW programs and does research in conjunction with all the RSW that takes place in the state of michigan. PCT not having ties with the automotive industry does not have a focus on RSW. I would be interested to know if other schools have any curriculum on RSW. To my knowledge only Ferris State and PCT offer Welding Engineering Technology degrees.

Back to the machine. I haven't figured out this difference in pressure yet It really could be a matter of say changing the manual air pressure gauge to reflect what the machine is displaying. Or calibrating the machine. I will be exploring that today. This machine has no record of calibration or maintainence the controller and machine are about 12 years old now but the

AS far as the current monitors, after reading through the manual carefully there is a monitor on the transformer input side. This measures incoming current and also operates the automatic voltage compensation system which compensates for power fluctuation from the wall. It can be used to infer the secondary current through a formula. The secondary coil measures welding current on the return side of the loop, it can be used to more accurately monitor welding current due to the natural drops. The controller has options in terms of which monitor is used to control alarms. Ideally the primary coil monitor would be used to run the AVC and the secondary coil would monitor weld current.

After selecting the current monitor, alarms can be set if welding current is too low or too high. However these alarms AND the monitors have been turned off or are not functioning. I.E right now the machine in now way monitors or reads current. Unfortunately I only work one day a week and my time is split between other tasks but I will be exploring this today and trying to determine the solution for this machine and get the controller working back at 100%

By MDG Custom Weld (***)

Date 11-07-2008 21:35

LeTourneau, Ohio State, Ferris State and your Penn COT are some of the schools that offer WE programs.

By Metarinka (****)

Date 11-15-2008 01:31

indeed, however only PCT and Ferris State have "welding engineering technology" programs. It mostly has to do with subtle differences in acreditation and curriculum.

I'm still curious as to whether other schools have RSW in their curriculum.

By ross

Date 11-15-2008 13:42

There will be a three-day "Resistance Welding School" at the AWS show in November in Chicago. Seems like a good program.

Ross

By OBEWAN

Date 11-17-2008 13:35

LeTourneau did not have RSW in the welding methods class when I was there. There were 50 other processes to study. We did study ultrasonic welding though. However, we did have ample theory in the metallurgy and electrical engineering courses to more than qualify for work with the RSW process. When I worked in the automotive industry, I picked it up very quickly process wise. The biggest thing I had to learn was all the hardware - ie. tips, shanks, copper tooling, etc...The Resistance Welding Manual was valuable, and I would have been cousulting it anyway even if the process were covered. LeTourneau has a heavy focus on the Oil & Shipbuilding industries and is a long way from Detroit. I can see why Ferris State has a heavy focus on automotive, but I used to joke with them by saying the only thing they learn to weld are ferrous alloys. (I know that is not true though.)

By OBEWAN

Date 11-04-2008 20:01

I worked automotive as a robotic welding engineer for 8 years. I was even part of a huge multi company + university research consortium which had the goal of developing a neural network control with artificial intelligence to sense "good" and "bad" welds on the fly. What I learned is that all the bells and whistles most of the time only serve to confound and clog the system. They can bring misleading data to the process, false alarms, a false sense of security, and little value added for all the effort that goes into developing and maintaining the technology.

At the end of the day, there are only 3 major parameters with RSW. Force, Time, and Current. The most important thing to realize is that the contact tips WEAR OUT!!!!!! This cannot be overcome with feedback loops or electronics, and is the variable that is almost always overlooked by any control/feedback loop schemes. When the tips are worn, the best process goes to the dogs and stops welding. You MUST understand EXACTLY HOW MANY welds may be made before the tips are replaced, dressed, OR the current is stepped.

It is really a matter of knowing the contact area. You can even calculate the current density by dividing welding current by contact area. This will establish a set of lower and upper limits for current density. When the contact tips wear to a large enough dia, the process stops welding. The only way to set up limits is to make hundreds of welds on test coupons unfortunately.

For all the $millions that were poured into the research project, I was very disappointed because it ignored tip wear, and as such had an unsuccessful conclusion. If a person monitors current with a good constant current control, maintains force with a good simple pressure regulator, and maintains control over tip wear, the RSW process is actually very simple and will yield excellent results under a repeatable bell curve for successful quality control.

By Metarinka (****)

Date 11-07-2008 18:43 Edited 11-07-2008 18:49

Interesting lesson. I believe due to the nature of the RSW process and the fact that it is hard to determine weld capability via visual and NDT means, has led to much research and industry interest in developing sophisticated controls and monitors for systems. These controls can be useful for quality control and automation applications where weld strength is critical and needs to be controlled. As you mentioned, without regards to the basics such a system will never produce useable results.

In the similar regard no amount of fancy power supply circuitry such as pulse on pulse and advanced filler metallurgy will compensate for poor welding application such as excessive long arcing or incorrect gun angle.

Since I've been called in to look at this problem I've learned several things
A:none of the machine operators have been properly trained on the operation of RSW machines
B: Consumables (electrodes) have not been maintained to acceptable limits and there is little regard for proper tip selection
C: The Machine and controller have not been properly calibrated or maintained in a long time. This is big part of my struggles right now
D: Welding schedules were developed many years ago and since then no calibration or modification has been made for new or modified welding variables (new metal thicknesses etc)

By Larry D Morey Date 10-09-2008 01:27

C1.1-2000 Recommended Practices for Resistance Welding

By OBEWAN

Date 10-09-2008 11:58

Try to get a copy of the Resistance Welding Manual by RWMA. It has set up tables for time and current, but not a lot on squeeze or hold. Squeeze is less important than hold. Hold can effect cooling rate, squeeze does not have a lot of metallurgical impact.

By Metarinka (****)

Date 10-10-2008 18:35

Thanks for all your help, I'll take a look at acquiring C1.1 and the Resistance welding manual, unfortunately while I have all the duties of a welding engineer, I do not have the purchasing power of a real engineer.

The specific task I have was given regarded welding dissimilar thickness C.R.S in the form of 18 ga L.C.S to 12 Ga. There were problems of weld strength or even having superficial bonds that would fail in assembly. When I reviewed all the RSW schedules I've found large variance in similar welds and material.

MDG I'll definately be taking you up on that offer

By Weldconsultant (*)

Date 10-22-2008 08:34

Metarinka,

When welding dissimilar thicknesses of the same base material, make sure you are using electrode tips diameter of approximately the same proportion; that will help focus the weld heat at the weld interface.

As for squeeze and hold, usually more the better. squeeze can be of the order of 200 msec to 1 sec; more for thicker parts. Unless you are running very high volume production, use ample squeeze. having sufficient squeeze reduces variation, especially on parts that are not flat to begin with.

Hold is the time when the weld develops strength; just like a solder bond develops strength as the solder cools from molten to room temperature. Again more the merrier, 200 msec to 1 second, more for thicker parts. The only time you have to worry about hold time if that leads to excessively rapid cooling leading to hardening of the steel. If welding hardenable steels, use a long cooling pulse to slow cooling rate or tempering pulse to reduce hardness. Energy (current) in tempering pulse may have to be as high as 70% of the welding pulse.

Hope that helps.

Girish

http://www.welding-consultant.com