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# Reamer Speeds and Feeds + Hole Sizes for HSS and Carbide Reamers

## Reamer Speeds and Feeds

What are the best reamer speeds and feeds?

Saw this question come up on HMEM. A lot of machinists are taught a rule of thumb similar to this:

1/3 the SFM, 3x more feed than the same sized Twist Drill

(Click here to learn what is SFM and how to go from SFM to RPM)

Opinions vary on exactly what that rule of thumb ought to be for reamer speeds and feeds. Some say the ratio to a twist drill is 1/3, while others suggest it should be 1/2.

In looking over a number of different manufacturer’s recommendations, we’ve found 1/2 yields results reamer speeds and feeds that are much closer to their recommendations.  Moreover, when we say, “2x more feed”, we actually want “2x more chip load”.  2x the feedrate would be 4x the chip load since we’ve already doubled the spindle rpm relative to a twist drill.  This chip load is a synthetic number for G-Wizard, so don’t try to do too much math on it!

Here’s what G-Wizard would suggest for reamer speeds and feeds with a 3/16″ HSS Reamer in 300 series stainless, which was the question in the post:

A 3/16″ reamer going 1″ deep in 304 stainless should run 458 rpm and 2.75 ipm feed.  Chipload is 0.0015″

For comparison, here is the same cut with a 3/16″ HSS Twist Drill:

Twist drill is 703 rpm (a little less than 2x the reamer), and 1.863 ipm (reamer is feeding a little faster)…

Okay, those numbers are neither 1/2 nor 1/3, though the reamer is spinning close to 1/2 the twist drill, and the reamer feed is faster.  What gives?

Well, this is another reason to use a calculator instead of a rule of thumb.  There’s more at work here than a rule of thumb can account for.  For example, it gets harder for a twist drill to extract chips the deeper the hole.  Reamers don’t even try to extract chips–they have straight flutes and don’t produce many chips if a large enough pilot hole is provided.  Hence, the twist drill needs to behave more and more differently than the rule of thumb the deeper the hole goes.  In this case, we’re quite deep–1″ is 5.3 diameters deep.  I made the hole deep just to make the point.

## Reamer Hole Sizes

Reamers are a fast way to finish a hole and very convenient relative to other precision approaches like boring. Their purpose is not to change the location of the hole.  In fact, they often have long shanks so that they will deflect slightly to ensure the go into the hole.  Rather, they’re made to improve the surface finish, roundness, and hole size quickly and easily.

Reamers are certainly not the be all and end all of hole boring, but if you’ve never used one, give it a try. One thing to keep in mind when using reamers is hole size guidelines. If you make the hole too large (i.e. to close to the reamer’s finished bore size without going over), there isn’t enough meat for the reamer to do its job. Too small and you’re making the precision reamer work way too hard.

There was recently some back and forth on CNCZone about reamer hole sizes, and I felt like it would be a good time to throw out that G-Wizard tells you the recommended guidelines for how far undersized to make your holes before reaming:

G-Wizard says to make the hole 0.010 – 0.025″ undersized for a 1/2″ reamer…

Did you notice that down in the Tips area (right above the Tortoise and Hare), G-Wizard tells you the right twist drill size?

Give G-Wizard a try. It’s gives you the starting hole size and good feeds and speeds for both HSS and carbide reamers.  Best of all, it’s free during your 30 day trial.

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