reamers

An Introduction to Reamers & CNC Reaming

Most machinists are familiar with CNC drilling, but did you know that the common practice for holemaking is to always use a reamer? When done correctly, reaming can be a fast and highly accurate operation that results in precision holes.

Critical Reamer Geometries

reamers

By examining the above image of a Harvey Tool Miniature Reamer and its critical dimensions, we can better understand the functionality of this useful tool.

D1 is the reamer diameter. This is the diameter of the specific size intended for your hole.

D2 is the shank diameter. These oversized, but common shank sizes help to maintain tool strength, stiffness and accuracy. It is also important to note that these shanks have an h6 tolerance, crucial for high precision tool holders such as heat shrink collets.

L1 is the overall length of the tool. This dimension is important to consider when touching the tool off or estimating its hang out. It’s always best practice to minimize tool hang out as much as possible.

L2 is the margin length of the tool. As defined in the Machinist’s Handbook, the margin is the unrelieved part of the periphery of the land adjacent to the cutting edge.

L3 is the overall reach of the reamer. This lets the machinist know how deep the reamer can get into a hole.

L4 is the chamfer length.The chamfer is the cutting portion of the reamer – the part of the tool that is actually removing material.

The Functions of Miniature Reamers

Reamers Provide Precision – As mentioned earlier, reamers are great for machining precision hole diameters. To use a reamer properly, you must first have a pre-drilled hole that’s between 90% and 94% of the final hole diameter. For example, if you need a finished a hole of .220, your predrilled hole should be somewhere between .1980 and .2068. This allows the tool to take enough material off to leave a great finish, but does not overwork it, potentially causing damage. The tolerance for uncoated reamers is +.0000/-.0002, while the tolerance for AlTiN coating is +.0002 / -.0000. These tolerances provide you the peace of mind of knowing that your hole will meet exact specifications.

Achieve a Quality CNC Finish – When a high surface finish is required of a hole, reamers should always be used to reach the desired tolerance. Both the pre-drilled hole and the tool’s margin help to keep the reamer centered while cutting, leading to a better finish.

Minimize Machining Production Runs – For machine shops, consistency is a priority. This is especially true in production runs. The last thing a machinist wants to see is an oversized hole on a part they have already preformed many operations on. Remember, reamers have the benefit of offering consistent hole size, preventing an out of tolerance finish. These consistent holes lead to valuable time savings and reduced scrap costs.

CNC Machining Exotic Alloys: When machining Inconel, titanium, and other high-cost materials, reaming your hole is important to ensure that the desired finish specification is met. With reamers, a machinists can better predict tool life, leading to a better finished product and less scrap ratios. It is important to note that Harvey Tool reamers are offered AlTiN coated and fully stocked in every .0005” increment from .0080” to .0640”.

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