The Rundown on Runout

As covered in our 10 Things No One Tells You About Machining article, runout is everywhere! However with most tools we use this amount is negligible. Using, say, a 3/8” endmill with .0008” of TIR won’t have significant consequences (especially when roughing). Unless you’re chasing tenths, the runout is such a small percentage of the tool’s overall diameter (~0.21%) that it wont have any noticeable affects. As you small diameter tools, this percentage begins to increase. Using a 1/32” endmill with the same runout amounts to a TIR that is 2.56% (about a 10x increase!). This can result in uneven wear, chatter, bad surface finish, missing tolerances, and even broken tools.

1. Tools with a small corner Radius

Why are some end mills so much more expensive than others, and are they worth the cost? Every machinist must have wondered about this question at some point.  Many develop fierce loyalties to the brand that has worked for them.

What Is Spindle Runout?

Spindle runout, or tool runout, as it can also be called, is the inaccuracies that cause a tool (in a mill) or workpiece (in a lathe) to spin off the ideal axis. It’s very bad for tool life, so it’s good to know more about it.

Machinists are familiar with spot drills–they’re stub length and have few or no flutes.  These little drills are designed to be extremely rigid so that they can precisely spot a hole for a twist drill.  Maximum meat in the shank keeps them on target.  The goal is use the spot drill to make a little dimple in the work-piece that keeps the twist drill from walking so the hole winds up in the right place.