January 29, 2008
Welding around any kind of circular piece can be a challenge. To make a fillet weld in the horizontal position, you will have to maneuver around the workbench, maintaining a constant arc if you wish to make a continuous weld. This type of work may be acceptable if you need only a few pieces, but can become tedious on production runs. One solution is to rotate the workpiece past the welding arc with a rotating welding positioner, such as a small- to medium-size benchtop machine.
Many fabricating shops must weld tube and pipe assemblies for both short-run and production-length jobs. Tube to tube, pipe to pipe, and pipe to flange welding all are common operations in today's fabricating environment.
Welding around any kind of circular piece can be a challenge. For instance, assume you want to weld a piece of pipe to a flange. First you tack the two pieces together and clamp the assembly to your workbench. To make a fillet weld in the horizontal position, you have to maneuver around the workbench, maintaining a constant arc if you want to make a continuous weld. This type of work may be acceptable if you need only a few pieces, but it can become tedious on production runs.
One solution to this dilemma is to rotate the workpiece past the welding arc with a rotating welding positioner, such as a small to medium-size benchtop machine.
A built-in tilting base allows the positioner to tilt at least 90 degrees. Of all the defined welding positions, the flat position (sometimes called downhand) is the easiest to perform and provides the fastest travel speed, highest deposition rate, and best penetration.
If you fixture your pipe-to-flange assembly to the table of a welding positioner tilted to 45 degrees, you are presenting the joint in the flat welding position. When joining two pieces of pipe end to end, tilt the positioner to 90 degrees to attain that same flat welding position.
Fixed-base machines, more accurately called welding turntables, generally are less expensive than the tilting variety. They are acceptable if the user of the machine wants to build an angled base plate for a dedicated job.
Benchtop welding positioners typically are rated by the weight they can handle. The most common sizes are 50, 100, 200, and possibly 250 pounds. These load ratings are based on the workpiece being centered and balanced on the positioner table with the table in the horizontal position. Work load is the weight of the weldment plus the weight of the fixture used to secure the weldment to the positioner table.
Out-of-balance loads don't pose much of a problem when the table is in the horizontal position. However, an unbalanced load can cause a tilted positioner to speed up or slow down as the overhung load goes through the 12 o'clock and 6 o'clock position. Unbalanced loads often are caused by a workholding clamp that is hanging off to one side of the fixture. Securing a like weight directly opposite the offending clamp can eliminate this out-of-balance condition.
Tall loads (positioner table horizontal), even though perfectly balanced, can cause a problem when the positioner is tilted. This creates an overhung load that can place a severe radial load on the positioner's shaft bearings. Try using some sort of steady rest to support the outboard end of the workpiece, such as a pipe stand with a dual roller head.
If you are considering a small positioner, be sure it has an adequate grounding system for return welding current to your welding machine. A capacity of 200 amps or more is a good rule of thumb.
Before you start welding, be sure to attach the ground lead of your welding machine to the stud provided on the positioner. Welding on a positioner that is simply bolted to a grounded steel workbench will cause high-amperage welding current to flow through the machine. If it flows through the bearings, it eventually will cause their failure. If it gets into the circuitry, a major burnout will result.
Benchtop positioners are powered by electric gearhead motors that usually drive the table shaft via a secondary geartrain or sometimes a drive belt. AC motors offer good performance; however, DC gearhead motors can provide smoother operation and more precise speed control and deliver higher torque at low speeds.
All commercial benchtop welding positioners feature forward and reverse rotation, plus variable speed control. A foot switch that initiates table rotation usually is standard equipment.
The foot switch may be an on/off device only, or it may be a variable-speed type. A variable-speed foot switch is suitable for gas tungsten arc welding (GTAW) of aluminum, which requires a slow speed at start-up and very fast speed after the weld puddle is established.
To figure out the best speed range for your application, you'll need to know the circumference in inches of the parts you intend to weld and the travel speed, expressed in inches per minute (IPM), of the arc across the workpiece.
You can measure part circumference directly or calculate it by multiplying its diameter by 3.14. Arc travel speed can be a little trickier. Most operators know their production rate per hour or per shift, but not the travel speed of their arc.
Set up your welding parameters and run a test bead for 10 or maybe 15 seconds. Measure the length of the bead and multiply by 6 or 4, and the result is arc speed in IPM. (Wire feed speed is not the same as arc travel speed.) Now apply this simple formula:
Arc speed (IPM) ÷ Circumference (inches) = Revolutions per minute
Try to find a machine that allows you to weld somewhere in the middle of its speed range so you can increase or decrease part sizes as different welding jobs arise.
The best-quality welding positioner and most talented welder cannot produce parts until the workpiece is secured to the positioner table. Make sure workholding fixtures are designed to minimize the time required to load and unload parts. Maximum arc time and minimum handling time are the goals.
Try securing very small-diameter parts in a keyless chuck of the type found on drill motors or drill presses. If you intend to weld a variety of ongoing jobs with parts with different diameters, you may want to consider building a fixture for each part on a base plate that can be quickly attached and removed quickly from the positioner table.
One simple fixture may consist of a short piece of angle iron secured vertically to a base plate. A vertically mounted, tapped steel post with a threaded thumbscrew extending through it and facing toward the open side of the angle will provide a three-point contact fixture. Instead of the thumbscrew device, you may choose to use a quick-acting toggle clamp to provide the clamping force. Light-duty lathe chucks also work well, especially if they have both OD and ID jaws.
Whatever fixture you choose, try to keep it simple. Apply a light film of high-quality antispatter compound to your fixtures to keep them free of spatter and soot.
A fatigued welder is not a productive one, so making your job easier always is an improvement.
Consider using some sort of steady rest that will allow you to support the gooseneck or handle of a gas metal arc welding (GMAW) gun during welding. After all, you don't have to manipulate your gun; the positioner brings the work to you.
Some shop operators have devised ways to automate entire welding processes using a benchtop welding positioner as the centerpiece of their system. When engineered and assembled properly, rack-mounted GMAW guns, actuating cams mounted to the turntable, and microswitches or timing devices to time the arc can lead to a system that requires only loading of the part and pressing a start switch.
LED readout of table speed is of value when you weld different parts and want to record table speeds to set up the same job later without a lot of experimentation. Simply dial in the table rotation speed that was used the last time you welded that specific part.
Some critical welding jobs require inert gas backpurging. Inert gas purging from the back side of the arc prevents contamination of the root bead during welding. This requirement usually is encountered in GTAW of stainless steel. Some positioner manufacturers offer the option of a hollow table shaft with a means of connecting it to an inert gas supply to facilitate purging.
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