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Repair Center  Mortise Bit Buying Guide

Why Read This Article?

  • Mortise bit maintenance tips.
  • Bit installation video.
  • Compatibility measurements explained.

Article Breakdown

  • Mortise bits are used in mortisers to drill square holes.
How Mortise Bits and Mortisers Work
The two parts of a mortise bit are:

1. a circular drill bit, and

2. a surrounding square-shaped chisel

  • Mortises are used as one half of a joint called a mortise and tenon joint.
Mortise Bit Shanks and Measurements
  • Shank diameter is the major compatibility issue for mortise bits.
There are two common shank diameter sizes for mortise bits:

1. 5/8", and

2. 3/4"

  • Mortise bit length and bit diameter can also vary.
Mortise Bit Care and Use
Here is a list of things that help mortise bits last long and perform their best:
  • Keep chisels sharp.
  • Lubricate the bit.
  • Square the bit to the fence.
  • Adjust the center bit properly.
  • Use the right drilling technique.


Mortise Bit Buying Guide

This article explains what mortisers are used for, how to use them, how to shop for hollow chisel mortise bits, and how to make mortise bits last.
Mortisers are very useful woodworking tools that use special bits to drill square holes. The bits are called mortise bits or mortiser bits, and the holes they drill are called mortises. In most cases, a mortise is used as one half of a woodworking joint called a "mortise and tenon joint."
Mortisers are surprisingly easy to use considering that they are such specialized tools. Just a few pointers on mortise bits, bit installation, and proper operation will ensure that your bits last as long as they're meant to and that your tool continues to perform.

Mortiser In addition, we'll point out some tips about shopping for mortise bits and how to ensure that you get a compatible bit.    How Mortise Bits and Mortisers Work
Mortise bits are used in woodworking machines called mortisers, which work much like a heavy-duty drill press works.
Mortise bits are made up of two parts:

1. A circular drill bit (much like an auger bit), and

2. A four-sided square chisel that surrounds the drill bit. The center drill bit clears most of the material in the hole, but it leaves material behind in the corners because of its shape.

Mortise Bit That's where the square chisels comes in. By clearing out the center material, the drill bit makes it much easier for the outer chisels to cut the remaining material into the shape of a square. In fact, the chisels help to guide the extra material into the center where it will be ejected by the turning bit. When several of these mortises are cut in a row, the squared, flat-bottomed hole that it makes becomes the receiving end of a mortise and tenon joint. A tenon is an extending joint piece precisely cut to fit tightly inside the mortise.

Mortise and Tenon Joint [Back to top]
  Mortise Bit Shanks and Measurements
Shank diameter size is the biggest compatibility issue that shoppers have to worry about when looking for mortise bits, and it helps to know a little bit about other mortise bit measurements too. There are two common mortise bit shank diameter sizes:

1. 5/8" (most common), and

2. 3/4" (less common)
1/4" and 3/8" shank diameter mortiser bits are also available. Most mortisers most likely takes 5/8" shank diameter mortise bits, because 3/4" shank diameter mortisers are used for heavier drilling. It's best to just check your mortiser's owner's manual if you have a question about the what bit size(s) it can accept.  Mortise bits also vary in length and bit diameter. Bit length is usually determined by the depth of the mortise that is needed.
Most bits are available in diameters between 1/4" and 3/4", progressing in 1/16" increments. [Back to top]
  Mortise Bit Care and Use
There are several things that mortise bit users can do to make sure that their bits and mortisers operate correctly, and we've listed several of these tips below.
These suggestions will also help extend the life of your mortise bits:

  • Keep the chisels sharp. Dull mortise bit chisels will cause bits to heat up during use. Excessive heat can cause damage to the center bit, the chisels, the workpiece, and even the mortiser.

Drilling a Mortise

A conical grinding stone is one of the most efficient ways to sharpen a mortise bit, because it will sharpen all four sides at the same time.


  • Lubricate the center bit. Mortise bits can make a lot of noise if they're not lubricated, because their two-part design allows for moving metal parts to rub on other metal parts.

Oil- and silicon-based lubricants should not be used on mortise bits, because they are likely to stain the workpiece. Instead, a non-staining lubricant should be applied to the center portion of the mortise bit. Paste wax and dry aerosol lubricant are preferable.

Paste Wax

Proper bit lubrication will also prevent unnecessary bit wear.


  • Square the bit chisels to the mortiser fence. Use a small square to line-up the mortise bit chisel with the fence of the mortiser. This will make sure that your mortises turn out right when you cut them.
  • Adjust the center bit to hang below the chisels. Push the center bit all the way up with your finger after inserting it into the mortise chisel, then allow the bit to drop 1/32" from that position.

Adjusting the center bit to hang down an extra 1/32" inch like this prevents heat damage to the bit and prevents excess noise. See this article's video for a demonstration of this procedure.


  • Use proper drilling technique. Mortise holes should not be cut in a row without spaces between them, because the mortise bit will tend to slip toward the less-resistant, already-drilled part of the mortise.

Mortise Drilling Technique

The best way to cut a mortise (especially long mortises) is to first drill both ends. Then drill mortises in the area between the ends spaced about half a mortise bit width apart. The remaining material can then be drilled.

Mortise [Back to top]
Conclusion has everything you need to keep your mortiser and bits in great condition. Take a look at our inventory of JET Mortiser Parts and our Mortiser Bits Page. This great Mortising Attachment-Kit is also available for order on our website, complete with mortiser bit attachment and four mortiser bits and chisels.
Get the most out of your shop, your tools, and your work with the best tool repair information and service around.
  [Back to top]
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