Boring on the Mill - 6: Spring back
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
All materials flex under load just like wood or rubber. Even steel flexes a little although it may not be apparent to the eye. This is why rigidity of setup is important and why spring back happens. Keep the concept of flexure in your mind at all times and you will be a better machinist. R.S.

When feeding a boring bar into a piece of material, the tool has a tendency to lightly bend towards the center of the hole as if it was made of some very flexible material. This explains why when you re-feed a boring bar into the hole at the same exact location - without changing the offset - you will still remove some more material.

This phenomenon is called spring back and you must be aware of it when you are using this tool on a milling machine. Also be aware that the longer and smaller diameter of the shank of the boring bar the more flexure or bend. Thus it may be required to take multiple spring passes to remove all flexure or bend in a deep hole.

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label: boring on the mill, rough cut, finish cut, spring back, cleanup, multiple stage boring, uses of boring bar, feed rate, coolant