The Lincoln SP-135T

I was in the midst of deciding what MIG machine to buy when a friend gave me this unit. It is a fine little machine that runs off of a 115 volt 20 amp circuit, and is pretty much as good as it gets for a machine of this class.

There are times when I might wish I had a 220 volt machine like a Miller 250, but that would require a lot of work to get a 220 outlet into my shop. A nice thing about a 115 volt machine is that you can use it almost anywhere.

If I win the lottery I will get a 220 volt circuit installed in my shop and get a nice TIG machine.

Here is the manual for the SP-135T:

My machine does have its limits. The manual does not recommend gas welding metal thicker than 14 gauge when using gas. 14 gauge is 5/64 inches thick (0.0747 inches). I have a couple of tanks of the CO2 - Argon mix. One is small and easily portable, the other is big and saves time and money. I have been advised that plain CO2 is a lot cheaper and works fine for non-critical work, but I have yet to try this out.

The manual says you can weld up to 1/4 inch when using flux core .035 wire. The trick with using flux core wire is to reverse the polarify (it wants electrode negative, whereas gas welding wants electrode positive). It makes a huge difference.

I set the gas flow rate to 20 scfh. The section in the manual on this is poorly written. When using gas, the flow should be set for 25-30 scfh (cubic feet per hour) with C02 - Argon mix. When using straight CO2, set the flow to 15-20 scfh. Set the flow rate with the trigger depressed.

The machine has a 20 percent duty cycle rating, and the manual says something vague about a "10 minute basis". This has never been a limitation for me. I am told that duty cycle limitations can be a big issue with cheaper machines.

Settings

You have two knobs: "Arc volts" and "Wire speed". They are marked with meaningless labels that boil down to less and more. There is a chart inside the welder (you knew I was going to say this, but keep reading) that gives suggested settings for different materials and wire.

The SP-135T has a tapped transformer that supplies 4 voltage settings. They are labelled A, B, C, D and I have not found any description of what the different settings are in volts. The manual says that the machine yields 29 volts "OCV" (open circuit voltage), and gives ratings at 19 volts, but there is not indication of whether you get 19 volts at A, B, C, or D. There is also some double talk about the current rating. The official rating is 90 amps, but the manual says you get 25 to 135 amps -- no clue what it is that causes you to get more or less.

Wire feed has a speed control knob marked 1 to 10. The manual says that feed rates range from 50 to 300 ipm (inches per minute). If I decided to waste some wire some day, the way to find out about this is to just hold the trigger down for 3 seconds, measure how much wire comes out, and multiply by 20.

Experiment and Practice

Welding is a skill (like playing a musical instrument). Anyone can quickly learn to set up and run a machine, after that it is getting "seat time". I find it amazingly hard to weld a straight line, even with my self darkening helmet. With experience you will know what to expect with a given thickness of material and what the cause is when things go wrong.

When starting out, I found it very useful to experiment with settings. There are lots of videos that show you somebody with years of experience and a powerful welder making great welds. Hardly anybody takes the time to show a beginner how to set up their machine. Basically what you do is to first select a voltage appropriate to the thickness of the material you intend to weld. Then for a given voltage there is a sweet spot for the wire speed where the machine "runs right" giving a smooth buzzing sound.

What I have learned is that for thick material (that you really shouldn't even be trying to weld with this little welder), you want the voltage setting on "D" (as high as it goes). The other settings are for thinner material that you are just going to blow holes in unless you use a lower setting (refer to the chart). Once you pick your voltage (based on the thickness of your material), you want to set the wire speed to get the machine running smoothly. I recommend experimenting. Too slow a speed and the wire just keeps melting off and the machine is running on and off. At the slowest speed where the machine runs smoothly you are effectively getting more heat (the same effect as cranking up the voltage some). Beyond this you get the nice "sizzling bacon" sound, which is what you want. Faster speeds the machine doesn't run right and you get blobs of weld that are just sitting on the material. Spend some time playing with different settings and getting an education with some scrap material.

Another variable is the speed and pattern you use to move the gun along.

When you try to weld thicker material than you really should you get welds that look like worms sitting on the material. There just isn't enough energy to get proper penetration. When you get porous welds with pits and bubbles, check that you have a decent ground and that you haven't run out of gas. Most of the time when this happens I have done something silly with the ground.

Practice making tacks or spot welds. These are amazingly easy to make and effective, even on thick materials.

Have any comments? Questions? Drop me a line!

Tom's Welding pages / tom@mmto.org