What is a Rotary Rasp and How Do I Use It?


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When it comes to a professional woodshop there are a ton of different tools and accessories around, but some of the strangest are probably rotary rasps. If you've never seen a rotary rasp before you might be a little confused. 

After all, what would you use a round grater for?

That's exactly what many rotary rasps look like, round graters. In fact, rotary rasps even work something like the cheese grater you use in your kitchen. Don't worry if you've never used one before, we'll talk about what they are, how you use them and give you some tips about the different shapes and even the right tools to use with a rasp. 

Don't worry, it's perfectly natural to want to get a set of rotary rasps for yourself after reading this article.

What Are Rotary Rasps?

At their core, rotary rasps are tools that are designed to remove a moderate to a large amount of material very quickly. Most rotary rasps work for a reasonably wide range of materials, including most woods and plastics. Some are also designed for stonework and shaping tile, but they tend to be more specialized. 

There are even a few rotary rasps that are hardened for working with metal!

It's important to make sure you're using the right kind of rasp for the materials you're working with. A metal rasp, for instance, will probably be too aggressive and hard to control if you try to use it to shape a piece of wooden furniture. 

Most rotary rasps are designed to work with hand-tools and help create detailed work. They remove more material than sandpaper and work for a wide variety of shapes that most grind wheels and other tools. 

Rotary rasps are also particularly helpful for small details and tight corners because a small rasp can be used to remove both small and large amounts of material. That means you can likely do shaping work with a rasp, and then do some detailed decorative work with the same rasp!

How Do Rotary Rasps Work?

We already mentioned that rotary rasps are similar in some ways to your cheese grater at home. That's because they work almost the same way. Rasps have many small bladed cuts in the base metal that catch and remove material while the rasp turns. 

Some rasps are more aggressive than others. More aggressive rasps have deeper sections in the metal, just like the largest setting on your grater. Detail rasps have smaller openings that remove less material at a time. 

However, the relative aggressiveness of your rasp also depends on how you use it. Skilled users can use an aggressive rasp for detailed work, though the opposite isn't true. That's because a detailed rasp used to remove too much material at once is likely to clog and eventually dull before the job is done. 

Advantages of the Uneven Pattern on Your Rasp

If you've looked at a rotary rasp you've probably noticed that the notches in the rasp don't line up exactly. While the pattern isn't exactly random, it looks like it at first glance. 

But why not have a perfectly even distribution of cutting elements all across the rasp?

Because if all of the cutting elements line up they'll create more drag on the tool, pulling the rasp off course while you're working. You could potentially compensate for the extra drag if you knew what direction it was going to pull, but that would add to the difficulty of using the rasp. 

Instead, the manufacturer creates a more scattered pattern of cutting elements across the rasp. The random pattern helps prevent the rasp from pulling in any specific direction and gives more control to the user. 

Of course, this isn't always perfect. Anytime you're using a new rasp you should pay attention to see if there is any drag trying to pull your tool in one direction. Just make note of it and keep working if there is. 

Fortunately, a lot of the hand tools rotary rasps are designed to be used with are heavy enough to help counteract any drag left in the rasp itself.

Why Are There So Many Different Shapes of Rasp?

A common question from first-time rasp users is 'why are there so many shapes and sizes?'. Like any tool, rotary rasps come in a lot of specialized shapes and sizes to make using them easier for specific jobs. 

Rotary rasps tend to come in shapes ranging from cylinders to spheres to cones. The exact dimensions of each shape, and variations within the shape, work to make different kinds of natural shapes from the rasp. 

For instance, a large cylinder rasp is good for removing a lot of material quickly, or for turning a 90-degree angle into a curve. But a spherical rasp is good for creating slots in your material, shaping round decorative elements, and other detail work. 

Cone shapes rasps are great for even more detailed work and for creating decorative triangle shapes in your material.

What Tools Do Rotary Rasps Work With?

Rotary rasps are designed to work with a relatively wide range of rotary tools, though not every rasp will work with every tool. 

Rasps also tend to prefer slower speeds than some tools, which means that tools like routers often aren't a good option. 

However, your regular drill is probably a solid choice. You want to maintain speeds below 3,000 RPM if possible, for the most efficient use of your rotary rasp. 

That's because above 3,000 RPM the rasp doesn't have enough time to 'grab' the material you're working with and sheer it off. Instead of working efficiently, it'll only grab very small amounts of material. 

It's a little counter-intuitive, but many rotary rasps will actually work more slowly once they're rotating above a certain speed. 

If you notice that your rasp seems to be losing power, try slowing your tool down slightly. 

Rotary rasps can also work with hand-held rotary tools and Dremels. However, those smaller tools aren't usually good for stripping away a lot of material. The motor inside the tool just isn't designed for the constant heavy load of removing a lot of material. The rotary rasps that come with these smaller tools are meant for detailed work and shaping small pieces. 

If you want to shape a larger piece, like the joints in furniture, you'll want to use a bigger and more powerful tool like a drill. 

Every rotary rasp has a different set of tool compatibility. You'll need to look at the individual rasps to know what your options are. Choosing the right tool from there is a matter of matching the tool to the project's needs.

How to Use a Rotary Rasp

Now that we've covered what a rotary rasp is and how it works, let's talk about how you use a rotary rasp. This guide is meant to be very general so that you can use these tips and skills with any tool and for any project. 

That also means that it's a good primer, but we aren't going to go into the details of how you achieve any specific shape or design with a rotary rasp. 

Supplies You'll Need in Addition to Your Rotary Rasp

Just like any power tool the rasp itself is only one of the things you'll need for a project. Here are the other tools and work supplies you should have ready before you get started. 

Tools:

  • Rotary Rasp (and replacements)

  • Drill, Dremel, or other compatible rotary tools

  • An old toothbrush or wire bristle brush for cleaning the rasp

  • Sandpaper (for finishing work)

Safety Equipment

Having proper safety equipment on hand is important for working with a rotary rasp, just like any power tool. While you can customize your safety equipment a little based on what you're comfortable with and what works best for you, it's better to have more safety equipment than you need. 

Here's the minimum we recommend:

  • A woodworker or metal worker's apron

  • Safety glasses/goggles

  • Woodworking gloves (or heavy-duty work gloves)

  • Ear protection 

Miscellaneous Equipment

This is stuff that's useful to have, but not necessarily required to work with a rotary rasp. 

  • A shop rag

  • A tarp (spread under your workspace to catch sawdust and other debris to make cleanup easier)

  • A clamp (to hold your project depending on what you're working on)

  • Finishing supplies

Prep Work

Before you start using your rotary tool it's important to do some prep work. That way you're less likely to make a mistake and more likely to be able to work quickly and efficiently. 

First, draw the shape you want to create onto your materials. You'll likely remove the marking as you're working, but having the reminder will help you create your preliminary shape and make It easier to remember what you're going for. 

You should also look for any cracks or weaknesses in your material that could catch on the rasp and cause problems. 

Lastly, you should make sure your workspace is clear and ready. Remove anything you don't want to get shavings on since rotary rasps can throw small debris in all directions. If you're going to use a tarp or canvas to make cleanup easier, you should lay it out before you get started. If you're going to use a clamp or any other tools to hold your workpiece in place, get that set up on top of the tarp or canvas. 

Step 1: Choose the Right Rasp for the Job

Once you've got all of your equipment ready and your workspace is prepared, the next step is choosing the right rasp for the job. 

Fortunately, rotary rasps are usually pretty easy to change out which means you can switch to a different rasp as needed for different parts of your project. 

Most of the time you're going to want to start with a slightly more aggressive rasp, which means one with either bigger or more cutting elements. The bigger the rasp itself the more material it will remove at once, so a slightly larger rasp is also often a good place to start. 

Cylinder rasps usually remove the most material and are also relatively easy to control and direct. They're a good choice for beginners, especially if you're looking to remove a relatively large amount of material or to change the shape of your material from a square or rectangle to something more curved. 

However, if you're doing detailed work, you'll want a smaller and less aggressive rasp, and may want to choose a different basic shape as well. 

Step 2: Choose the Right Tool for the Rasp

In addition to choosing the right rasp for the job you also need to work with the right kind of tool. We generally recommend using a drill or other heavy-duty power tool for removing a lot of material at once, and smaller tools like a Dremel for detail work. 

However, that also depends on the kind of Dremel you have and the kind of drills. 

You'll also have to make sure your rotary rasp is compatible with the tool you choose since that isn't a given. 

Ultimately this takes some trial and error. If you're just starting to learn how to use a rotary rasp it's a good idea to use some scrap wood and experiment with your different rasps and compatible tools until you have a sense of what each combination can do and what it's best for. 

Step 3: Start Slow and Steady

Once you've picked your tool and the rasp you want to start with, using the rasp is relatively simple. 

Start the tool before you touch the rasp to your materials. If you try to start the rasp rotating while it's in contact with your material, it's likely to get dragged off course and ruin your shaping. 

You want to start at a moderate speed, neither too fast or too slow. If you're using a tool that has adjustable speed settings like most rotary tools and Dremels the middle setting should be good to start. 

It can be more difficult to control the speed of a drill. But if you're using a drill with an adjustable speed motor you should depress the trigger slowly until you've hit a good working speed and hold there. Some drills that means you'll hold the trigger at its maximum setting. Other drills you'll only need to hold half-way to get the best results. 

Instead of shaping all at once, work slowly and take off layers of material smoothly. You can always go back to touch up and remove material that was accidentally left behind. It's much harder to replace material you've removed accidentally.

Finishing Work

When it comes to rotary rasps you aren't done when you've got the basic shape you need. Rotary rasps leave a rough, unfinished surface behind on most materials. 

Instead of leaving that unfinished surface and calling it good, you'll need to take a few more steps to finish your project. 

Sanding is the most important part of finishing work. You'll typically want to do at least one pass with medium-grit sandpaper to remove the worst of the roughness from the rotary rasp. This is important even for hidden sections of your project and areas that won't be seen or used because the rough surface isn't as stable as a smooth one. 

Wood especially can be prone to water damage and rot if you leave the surface of your project rough. 

For a finer finish continue using increasingly fine-grit sandpaper until you have the final surface you're looking for. 

Depending on the material you're working with you should also do something to seal the project once it's complete to help it last longer. 

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