Why plug weld is used?

Plug welds are a round weld that is made inside of an existing hole most commonly in one piece of metal, welding that piece to another member. The plug weld symbol is a rectangle with a diameter symbol placed to the left of the symbol as well as the number associated with that diameter.

Some drawings will not indicate the hole in the print so the use of dimensions come in to play when locating where a plug weld will be executed. The location will be indicated by a centerline through the part.

Above is indicating a ½” plug weld offset 1” from the edge to the center of the weld.

Some plug welds may include a countersink of the hole of the plug weld. This is called the included angle of countersink. This angle is shown below the rectangle of the symbol itself or if the plug weld is to be on the other side it will be placed above the weld symbol. When figuring sizing of the hole remember that the diameter will be the narrow of the hole at the base of the weld.

Without a countersink included it will be necessary to follow shop standards and procedures to dictate what this needs to be, if any angle. Most shops have a procedure in place for tasks that will be done often. If it is needed it may be listed on a welding procedure for the plug welds that are being completed.

If a number of plug welds are needed there will be yet another element added to the symbol. This will be a number that is surrounded in parentheses, such as (6) for example.

When applying a plug weld it is important to know the depth of fill that is required. If the plug weld should fill the hole provided then the symbol will be left empty. This means there will be no dimension inside of the rectangle. If the hole should be filled only so much then this will be placed inside of the rectangle. This dimension will be in a fraction and indicates the amount in inches the hole will be filled, not the necessarily how much the hole will be filled.

Another element that can be added to this weld symbol may be the pitch (spacing) for multiple welds. This is located to the right of the symbol and is a number representing the center to center spacing for weld location.

Plug welds may have a contour symbol which will be added below the symbol or countersink angle if on the arrow side and above if it is on the other side of the reference line. There are many types of contours and finishing designations, these are covered in supplementary welding symbols.

This symbol represents:

Plug Weld

Arrow Side

½ inch in diameter

1/8” amount of fill

45 degree included angle of countersink

Flat contour

Finished by Machining

Slot Weld Symbol

The slot weld symbol is the same that is used for plug welds. The symbol will not show a diameter symbol before the size however. The size of the weld will be the slot width instead. This is shown to the left of the symbol just as it is shown in plug welds.

The length of the slot weld will be presented to the right of the symbol. This may also include a pitch showing the center to center spacing of the slot welds. If there is a pitch there will be a number of slot welds provided in parenthesis under the symbol on the arrow side or above the symbol on an other side weld.

The drawing must show the orientation of the slot welds as to not confuse direction along the part. The above image shows the slots with a vertical orientation to the part versus a horizontal layout as shown below.

A slot weld can include any number of elements, these are very similar to the plug weld symbol that was just explained.

These can include:

Arrow or other side

Size (width)

Length of slot

Pitch

Depth of fill

Number of welds required

Contour

Finish

Make no mistake on the fill of a plug or slot weld fill. There is a possibility of having a fillet weld inside of a hole versus actually filling the hole for a plug weld. This could also be mistakenly done on a slot weld.

Plug and Slot Quiz

Write down all information regarding the below Welding Symbols.

Position the welding torch with the wire in the center of the hole contacting the back sheet of metal. It is important to arc against this back sheet rather than on the edge of the hole, otherwise the weld might not penetrate into the back sheet. The torch should ideally be pointing directly into the hole rather than at the angle in the photograph.

Start welding in this position and don't move the welder until the hole is almost full of weld. Then move the welder outwards in ever increasing circles until the weld is completed.

A plug weld is a type of weld that is used to join two metal plates together. It is a very simple weld to make and can be done with a few basic welding tools.

In this blog post, we will discuss the four easy steps that are involved in making a plug weld. We will also provide some tips on metal preparation and clamping.

So, if you're new to welding and want to learn how to make a plug weld, read on!

A plug weld is a type of welding that bonds two pieces of metal together by fusing them in a small, round hole. The outer metal will have the hole while the back metal will be solid.

This type of weld is typically used to join thinner pieces of metal together and is often used in automotive and construction applications.

If you're a new welder, don't worry - making a plug weld is quite easy. Here are four easy steps to follow:

Steps to perform plug welds-

1. Clean the metal to be welded.

This is an important step because it ensures that there is no dirt, grease, or other contaminants that could prevent the weld from taking hold.

• Use a flap disc on an angle grinder to clean the metal and then wipe it down with a rag. I use acetone for this.

• If you're plug welding outdoors, blowing the area with compressed air can also help to remove any gross contaminants.

2. Drill the holes to be welded.

The plug welds will be holding the two pieces of metal together, so you need to drill holes for the plugs.

• Start with a small pilot hole and then slowly increase the size of the hole until it's drilled big enough for the plug weld.
• I like to use a step bit for this because it's quick and easy, but you can also use a regular drill bit.

• You'll want to deburr the backside of your drilled holes to allow for a better fit up.
• You can use a file or flap disk to get it knocked down if you have enough room on the backside.

• For this example, I used a file. This is similar to building control arms or other tubed projects, like roll cages.

• Here is a picture of it cleaned up from the inside.

You need to clamp the two pieces of metal together so that they don't move while you're welding.

• I like to use C-clamps, vise grips, but you can also use welding magnets if you have them.
• Just make sure that the pieces are held together tightly so that they don't move.
• You can also tack weld the pieces together before you start plug welding if you want to be extra safe.

Now it's time to weld the two pieces of metal together. Start by welding the plug in the first hole and then welding the plug in the second hole.

You want to make sure that you are welding around the entire circumference of each plug so that it's fully fused to the metal. If it's a thicker material, you'll want to fill the holes up to slightly above the original thickness of the metal.

• Once you're done, let the welds cool and then remove the clamp. You'll want to clean the weld just like other weld types.

• You can leave the welds as they are or sand them down depending on your application and aesthetic standards.

There you have it! You've now successfully completed a plug weld. Practice the plug welding technique a few times and soon you'll be a pro.

FAQ's

What is the difference between a spot weld and a plug weld?

Plug welding is drilling a hole in one piece of metal so that it can be laid over the top of another piece. The welder then fills the hole with filler rod or wire to join the two pieces.

Spot welds are where two pieces of sheet metal are pressed together really tight. After clamping the two, a spot welder is used to join the two metals together by running a current through  both metals. This current melts the two metals together without the use of filler material.

When would you use a plug weld?

A plug weld is often used when joining two pieces of metal together that are too thin to be butt welded. It is also a good way to join two pieces of metal together when you don't have access to both sides of the joint.

Welding threaded bungs into tube ends is an excellent example of when I use plug welding. I drill a small hole in the tube and then insert the bung.

You can then weld the plug. Once you've finished the plug welding, you can then come back and weld the joint all around the tube.

Plug welds can be used when installing thin floor panels when restoring a vehicle. You essentially trim the floor panels to fit.

Clamp the panel in place using magnets. You can also drill small holes through both pieces of sheet metal and use blind hole panel holders for a precise fit-up.

A frame stretch is where you cut the frame in half. Add a section of frame in between the cut to lengthen the wheelbase. This is a common Jeep modification.

You'll have but welds along the frame, but a common practice to strengthen the joint will be to weld an insert into the frame. This is where plug welds come in to play. Some builders will then add fish plates on the sides for even more strength.

What are the benefits of plug welding?

Plug welding is a strong way to join two pieces of metal together. It is also quick and easy to do, which makes it a good choice for many welding applications.

Final Thoughts-

 That's it! Those are the four easy steps to making a plug weld. Plug welds have a purpose and are some of the easiest welds that you will make.

If you're new to welding, we recommend practicing on some scrap metal before trying this technique on your project pieces. 

We also suggest reading our other welding tutorials for stitch welding, safety tips, tools, and more tips. Thanks for reading and good luck with your welding project.