Reaming in fabrication and forming

May 2014 - The process of reaming slightly enlarges an existing cylindrical or conical hole. It typically is used to produce a tightly toleranced ID. Once the hole is drilled, reaming is required to improve the hole’s inner surface quality and the accuracy of its shape and size. Reaming is often used to produce either a press fit or a snug fit, allowing cylindrical and conical dowels or pins to be seated exactly or machine screws and shoulder bolts to fit easily with very little play.

When a hole is to be reamed, it must be undersized to account for the material reaming removes. How much smaller depends on whether a fixed or adjustable reamer is to be used. For adjustable reamers, for example, the hole should be no more than 0.1 mm (0.00393 in.) undersized.

Common examples of reaming in fabrication and forming include manufacturing large size flanges or man holes, building foundations for engines and installing machines and equipment.

Reaming can be included in automated operations. However, because it is a delicate operation and very little material is removed, reaming in the shop is frequently done by hand.

Types of reamers

Unlike reamers intended for machine use, hand reamers have a square cross section at the end of a cylindrical shank that fits into a tap wrench. The shank is typically smaller than the basic diameter of the reamer so that the whole reamer can pass through the hole.

An adjustable reamer has a slotted cutter body with separate blades inserted into the slots. The reamer diameter can be adjusted to within 0.01 mm (0.000393 in.) of the rated diameter by moving a conical inner pin. The blades must be reset after sharpening.

A fixed reamer is made from a single piece of high-speed steel. The forward end of the shank is fluted to provide the reamer’s cutting edges. The flutes may be straight or curved to varying degrees (e.g., spiral, helical). A short chamfer, also called a starting taper, is provided at the leading part of the reamer to help when starting the operation.

Straight fluted reamers are used for through holes and blind holes whose depth is less than or equal to the diameter of the hole. Spiral fluted reamers are used for holes of greater depth. Reamers with clockwise fluting are used only for deep blind holes.

With spiral fluted reamers, the cutting force resolves into a vertical and a horizontal component. If the spiral is clockwise, the vertical component exerts a corkscrew effect pulling the reamer into the undersized drilled hole. If the spiral is counterclockwise, the direction of the fluting is opposite to the direction of rotation of the reamer. Therefore these reamers resist being drawn into the hole by the same corkscrew effect.

About the cut

The rake angle of the cutting edge is nearly 0 degrees, which results in a scraping effect. The actual cutting process creates small chips and is distributed over several cutting edges.

The chips produced break off and slight furrows may be produced on the wall at the break points. Reamers have an even number of teeth but unequal tooth spacing. If the reamer were to have an equal tooth spacing, the chips would always break off at the same point and the teeth would catch in the furrows, creating chatter marks. Those chatter marks impair the surface quality of the reamed hole. 

The use of thread cutting oil during the reaming process improves the quality of the reamed surface. To remove the reamer from a reamed hole, continue turning the reamer clockwise while at the same time pulling it out of the hole. If  the reamer is turned counterclockwise, the cutting edges of the reamer could break out and chips would bind up in front of the rake angle, possibly scraping the hole larger than required. FFJ

Udo O. J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.