Ford 500 Cradle Robot Weld Report.

Date 23. July 2004.
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This report is a review of the newly installed, MIG robot weld line used to weld the Ford 500 steel cradles and sub assemblies. The purpose of this report is to evaluate the robot weld practices being established and examine the potential robot weld issues that will impact your future robot weld quality and production.

In evaluating the weld issues at the Chicago location it's important that the unique application requirements of this plant location be understood. With robot MIG welds, the primary weld concerns with parts over 2 mm is the attainment of acceptable, consistent weld fusion. On parts > 2 mm, as long as the part dimensional and TCP tolerances are maintained the robot, weld process and consumable choices are simple and weld risks are minimal. In contrast when welding parts under 2 mm, the primary weld concerns are the avoidance of "weld burn-through".

The Chicago plant provides many welded steel parts < 1.8 mm. On these parts MIG weld burn through potential is at it's highest level and part dimensional plus TCP deviations will compound the potential for weld burn through issues As this plant ramps up for it's robot weld production, its unlikely that the part dimensions will improve and the lack of automated TCP controls will ensure that the weld issues, unless addressed will increase in proportion to the weld production attained. To minimize the potential robot weld issues that will occur, a logical engineered approach is required in the best weld practices established.

 

In the automotive industry the selection of inappropriate MIG weld consumables and inferior weld process modes has for two decades created major weld issues. As the robot steel parts decrease in thickness < 2 mm, the consequences of poor weld consumable / process selection really become evident.

In many auto / truck manufacturing plants, weld consumable selection may have more to do with a purchasing decision cost reduction decision rather than a logical weld engineering decision. With the majority of the robot lines installed in the last five years, the selection of the weld equipment typically has had more too do with salesmanship than with practical weld process considerations. The following weld data will assist the plant in establishing best weld practices for parts < 1.8 mm.

Present Weld Practices. The Chicago location is utilizing Fanuc robots, E70S-6, 0.045 (1.2 mm) weld wire, and the new Pulsed, Lincoln Power Wave F355 i weld equipment. The Fanuc robots at this plant do not have automated TCP controls. The weld wire size selected is not appropriate, and the power source used is a poor choice which has added at least $250,000,00 unnecessary cost to the cradle robot line.

BEST WELD PRACTICES. THE IMPORTANCE OF PART DIMENSIONAL DEVIATIONS AND AUTOMATED TCP CONTROL WHEN WELDING THIN GAGE WELDS.

Of course part dimensions and TCP control is important when welding thicker gage applications, however on those parts we would be able to place oversized welds, use weld weaves and use through arc tracking. Typically on these parts we would deal with weld fusion / weld profiles concerns. In contrast with the thin gage welds <1.8 mm, the weld burn through potential will be high and the burn through will disrupt production as the weld arc is lost during the burn-through.

It's imperative at this facility that an automated TCP control if available from Fanuc be added to the robot cells and the TCP be checked daily at the commencement of the shift or when weld changes are made. All personnel involved with the weld decisions at this plant must be aware that in contrast to plants welding thicker parts, dimensional deviations in the thin gage parts will have greater weld production consequences.

BEST WELD PRACTICES AND THE GMAW POWER SOURCE. For almost two decades the MIG equipment manufactures have been developing and promoting pulsed MIG equipment for "steel applications". The Lincoln pulsed equipment purchased for your robot weld lines has undergone years of development yet still provides "poor arc stability" along with arc length sensitivity. The pulsed arc length sensitivity makes the pulsed mode unsuitable for most of your high weld speed steel applications. It's ironic that the weld equipment purchased also has cost 50% more than the available superior, stable, traditional CV equipment. The pulsed weld equipment purchased will likely cost three times as much to repair and you will need to keep more spare equipment than normally would have been required with traditional, more durable CV equipment.

When using an 0.045 MIG wire and pulsed welding the cross members and similar seam lap welds, to attain the desired minimum weld travel rate of 50 ipm, on the 1.6 mm lap welds, the MIG weld wire has to virtually make contact with the weld surface. This "wire to work contact" not only causes extensive weld spatter which will impact the tip life, the weld spatter also gets on to the fixtures causing part assembly / fit issues. When establishing the pulsed trim voltage (arc length) with the high speed pulsed applications, if the pulsed weld voltage (arc length) is set to a none weld spatter condition, the weld transfer and instability at 50 ipm will cause "skip" welds (missed welds / weld blobs).

Typically when pulsed MIG welding we need a sufficient arc length to enable the pulsed weld drops to form and transfer without making a short circuit contact with the work and wire tip. The desired minimum arc length required for optimum pulsed welds is detrimental when pulsed welding thin gage parts using 0.045 wire at high weld speeds. The thin gage parts limit the allowed pulsed wire feed rate which limits the pulsed frequency. The low pulsed frequency with the large wire diameters and a weld which for 50% of its time is at a background current of typically less than 100 amps results in a weld transfer unsuited to high weld speeds.

The robot, high speed skip weld issues you have at the plant are the same pulsed weld issues every wheel manufacturer and torque converter manufacture has had to deal with through for the last two decades.

Another ironic fact is when you use the Lincoln pulsed equipment in the traditional spray transfer modes which provides improved arc stability is the weld decision maker may not be aware that the slope influenced performance of the traditional spray mode from the pulsed MIG equipment is typically inferior to the lower cost traditional CV equipment.

When considering weld best practices one would simply wonder after two decades of poor pulsed arc characteristics for high speed MIG welds how the selection and purchase of this sensitive, electronic, costly inconsistent weld equipment is ever justified.

WELD BEST PRACTICES AND WIRE DIAMETER SELECTION. As I write this report one of your competitors is utilizing 052 1.4 mm weld wires on Ford frames and creating excessive globular transfer causing over 80% weld rework., The choice of an 0.045 wire for many welds that will be made on parts less than 1.8 mm is also an inappropriate choice.

The Chicago facility will benefit from stable spray transfer and a smaller wire diameter than 0.045 (1.2mm) Lets take a moment to look at the MIG weld current compatibility with standard gage thickness used at Chicago.

With robot, single pass "butt" welds on 1.6 mm parts we typically would be in the short circuit mode, welding at 170 - 200 amps, the lap welds do allow higher weld current, however the butt weld data shows the part compatibility with the weld current utilized. With 1.8 mm parts the butt weld current would be increased to approx. 220 amps. For any application that uses a weld current over 200 amps we would want to use the high deposition, stable spray transfer mode. Butt welds on 2 mm parts could be robot welded using 240 - 260 amps.

An 0.045 wire requires a minimum of 250 - 260 amps to attain stable spray transfer. As many of the welds you produce require less than 260 amps it makes no sense to use a consumable that will simply cause weld burn through on many of your parts. The bottom line it's imperative the plant uses an 0.035 or 0.040 wire. The common 0.035 wire will go into spray around 200 amps.

The negative attribute of the 035 wire is
[a] feed ability,
[b] a smaller weld width influenced by the smaller plasma.

The wire feed ability issues can be addressed from a maintenance perspective. Also keep in mind that the 0.040 wire is likely suited to many of your applications and this wire has less potential feed issues than the 0.035 wire. From my perspective the 0.040 wire should be the wire of choice for the auto parts industry for all applications under 4 mm.

By the way as you use about 1.1 lbs of weld wire per cradle, changing the 045 wire to a smaller wire will dramatically improve your weld productivity. Greatly minimize you weld rework yet only add 20 to 30 cents wire costs of a weld cradle.

The plant should consider when required the use of weld weaves. The weld weaves should comprise of an amplitude that creates a slight yet high speed oscillation in the middle of the weld. This oscillation will not impact the potential weld speeds it will however cause the weld to thin out and provide slightly wider weld beads. Two important weld benefits are attained from the weld weaves;

[1] Reduces weld burn through,
[2] Compensates for dimensional deviations.

 

I would also like to comment that when evaluating your NAMS weld cross sections, that most of the welds your personnel are rejecting are in reality perfectly acceptable. I suggest you provide training so your weld quality personnel understand the real meaning of weld quality.

 

 

 

Regards Ed Craig.