Know How Notes: Cutting And Grinding Metal

If you work on cars long enough, there will come a time when you need to cut or grind on metal. If you are restoring a car, that time will come sooner rather than later. There are multiple ways to get the job done, and which method you choose often depends on the purpose at hand. Cutting and grinding metal requires special tools and cutters. We have put together a guide to help you sort through it all. Before we get into the specifics, it needs to be said- safety is paramount when you are working on these tools. Not only are these sparks flying, but these tools can break and cause serious personal injury or even death, so be careful and NEVER use these tools without the guard in place. A good face shield is always a good idea, even if you are already wearing safety glasses. 

Tools

There are lots of ways to cut and grind metal, but this article is going to focus on abrasive cutting. To accomplish the task at hand, we need to look at the tools themselves. There are air-powered and electric versions of most of these tools. The most common tools used for these tasks are the right-angle grinder (electric) and the die-grinder (air)

Electric Tools

The advantages of electric tools are full power at all times. You get instant torque and high speed. When it comes to heavy grinding, electric is the way to go. For sheet metal cutting, a cut-off wheel on an electric grinder makes quick work of the metal without bogging down. When it comes to wire-brush clean-up work, electric is absolutely king.

Drawbacks of electric grinders are size and power. Yes, sometimes there is such a thing as too much power. When you need to finesse trimming or grinding, an electric tool is not going to be the best bet. They are pretty much all on or all of, there is very little variation in speed. Some tools may have a speed switch, but that is rare.

And then there is the size. Electric grinders are large. They have heavy motors, long handles, and the head of the tool is big. A 4” wheel is about as small as you can go on an electric grinder. That makes them great for big jobs and straight-line cutting, but curves and tight areas create a lot of problems. That is when you need to switch to air-power.

Air (Pneumatic) Tools

Air tools come in about every imaginable flavor, far beyond what can be done with electrics. The small form factor gives air tools an advantage that you cannot get with electric tools- you can use very small wheels and grinding stones to get into the small areas and tight corners. Finesse is the air-powered tools domain. Another benefit of air tools is that they spin very fast. Where an electric is limited to sub-10,000 RPM speeds, air tools can reach 12,000+ RPM. Speed means less bounce and faster cutting and grinding.

Where the air tool fails is in power. Cutting wheels bog down and snag, grinding and finishing wheels slow down as you work. Air tools take more time and patience because you have to speed them up to get the job done. That is not a deal breaker by any means, but you need to match your tool to the job in front of you.

Another drawback of air tools is that you have to have an air compressor. The larger your compressor, better your tools will perform for longer periods. A die-grinder will empty a typical 5-gallon tank in 10 minutes.

The tools themselves are varied; there are a lot more options in air tools. Die grinders come in many flavors- straight, 90-degree, long-reach, and 120-degree. Then there is the dedicated cut-off tool that has a built-in guard for cut-off wheels. Different collets (the part that holds the wheel or cutting tool) are available as well, typically ¼” and 1/8” for a wide variety of tooling options.

Wheels

There are all kinds of options when it comes to these consumables. Keeping a selection on hand will make your life much easier as the tools destroy themselves as they work. The smaller the tool, the faster it will wear out.

Grinding

There two types of grinding tools- wheels and stones. Stones are used in air tools almost exclusively for air tools, and uses in tight areas when used for metal. Wheels are used with both air and electric tools and are by far the most commonly used consumable for grinding. Grinding wheels are great for big jobs- cleaning up welds and removing large amounts of material. The biggest advantage of a grinding wheel is that they are fast, aggressive, and cheap. You can buy good quality grinding wheels for a dollar or two. While the grinding wheel is the staple for material removal, there are some major drawbacks.

These tools cut heavy and fast. They put a lot of heat into the metal, which is fine is you are grinding on a big casting, but if you are working with sheet metal, that means warpage. For weld clean up on sheet metal, there are much better tools available. Another issue with grinding wheels is that they are fragile. Made of compressed composites, if they get in a bind or crack from heat, they can shatter, sending brutal shrapnel all over the place, including your face. 

Flap

The best tool for weld finishing is the flap disc. These are essentially layered squares of sandpaper bonded onto a plastic or composite disc. This allows them to vary in grit, which gives you the ability to control the finish. Because they are layered in a stacked fashion, there is a cooling effect that comes with it. This means much less heat is put into the metal, which greatly reduces warpage and discoloring. With flap discs, the final finish needs very little work afterwards.

Another benefit is the size. These are available in 8-inch versions and 1-inch, with all sizes in between, so you can really get after whatever the task is at hand.

The drawback of these discs is that they don’t last long and they are expensive. These discs can cost 4-10 bucks each, and they don’t last that long. The question becomes cost or efficiency? For sheet metal weld clean-up, flap discs are the best choice, for thicker welds like on a frame, you might want to do the big work with a grinding disc and then switch to a flap disc to do the finish work.

Finishing

Finishing discs are mostly used with air tools, as they are smaller and usually need more finesse than the broad stroke of a large grinder. Finishing discs are either sanding paper or Scotch-Brite™ type pads that mount on a twist-on backup pad. As big as 24-grit, on up to super-fine, the finishing disc gets the job done after the grinding is done. These are used for cleaning up cut edges, removing grind marks, paint, undercoating, all manner of processes.

Cutting

When it comes to cutting with a circular cutter, there are two options- electric or air. Air tools are the most commonly used for cut-off wheels, but the work is slow. Because cutting wheels are thin, they break easily, and they catch on edges very easily. This means a lot of starts and stops when using air-powered cut-off wheels. The benefit is that you can work in tight quarters with a small air-powered tool.

If you need to make long straight cuts, then electric is the best option. The high torque of an electric tool means fewer stops and much faster cutting. The drawback is that the tools are heavier and bulky, so it is easy to get off your line and you fatigue faster as well.

If you practice safe grinding and use the right tool, you can get the job done quickly without incident. As with most projects, use the right tool for the job and all will go well.

Check out all the tools & equipment available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on cutting and grinding metal, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.