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Expansion Reamers Explained

 Expansion Reamers:list5733
Precision holes require precision tools. When machinists need an exact size hole, they want the best tool available to achieve the desired diameter. That’s a job for a reamer. The appropriate amount of stock removal for carbide tipped reamers is typically 2-3 percent of the finished hole size.  This is so the reamer can remove the remaining material to that all important size, and leave a smooth clean finish.  Click here for more on Pre-Ream Hole Sizes. As with all precision tools, instruments, or components, increase costs are explicit in the name, and when costs go up businesses cringe. Enter the expansion reamer.

Expansion reamers are precision tools that can be reused multiple times and are generally used for reaming abrasive material. They are built with slots cut between the flutes and a tapered screw in the head. As the reamer is used and the cutting edges are worn down, the user simply turns the screw to expand the reamer, re-clears it, sharpens it back to its original size and goes right back to work.  Expansion reamers are handy tools that allow the user to keep working with the same reamer multiple times, without having to buy a new reamer.

When using an expansion reamer there are some important things to keep in mind:

  1. Expansion reamers are not meant to be used for multiple sizes. The reamer is set to a specific size and should only be used for that size.
  2. Expansion reamers are not adjustable reamers. The set screw acts as a wedge being turned into the shank of the reamer. As it is turned inward, the steel flutes are bent outward. Once this happens, it cannot be undone. (See chart below)
  3. Once the screw is turned it should never be backed out. Expanding the reamer is a one way street. To bring the reamer back to its intended size, the set screw should be turned, but very minimally (usually about 1/16 to an 1/8 of a turn, and not more than a ¼ of a turn). If the screw is backed out, the tool may not hold its size and the threads could be damaged, preventing further tool expansion.
  4. Expansion reamers are cost effective. Since there is more work involved in making expansion reamers, they are more expensive. However, this higher cost is offset by a longer life span. Using expansion reamers can cut down on the number of tools that will be needed because of the ability to expand the reamer multiple times for reuse.
  5. Expansion reamers are not able to be coated.

Expansion chart for Super Tool’s expansion reamers:

Tool Diameter

Guaranteed Minimum Expansion

5/16”  –  15/32”


½” –   31/32”


1” –  1-1/2”


1-9/16”  –  2-1/2”


For each of the given diameter ranges, Super Tool guarantees that the minimum available expansion of each tool is that specified in the chart. To maximize the number of tool uses, only turn the set screw small amounts and regrind to size. Once the screw is turned, it cannot be backed out. Expansion beyond this given amount is not recommended and may not be possible due to the taper of the screw and the length of the flutes.

Super Tool’s Carbide Tipped expansion reamers have carbide the full length of the flute. This allows for a better finish on the hole.

Super Tool’s standard tolerance on expansion reamers are:

  • Diameters up to 1-1/2” =  +0.0003/-0
  • Diameters over 1-1/2” = +0.0004/-0

Here is a link to Super Tool’s selection of Expansion Reamers.

If you have any questions about Expansion Reamers (or any other cutting tool questions) feel free to leave us a comment here on our blog or fill out the form on Super Tool’s Contact Us Page.

Chadd Brown
Super Tool, Inc.