Tolerances and Material Removal Processes

If you haven’t already, I recommend reading and understanding my previous post about tolerances before digging in here.

Material removal processes are often used to build tooling for other manufacturing processes such as plastic molds, dies and punches for metal stamping and forming, extrusion dies, EDM electrodes, and many others.  Understanding material-removal process capabilities will be invaluable in understanding capabilities of downstream processes.

Common material-removal processes include lapping and honing, grinding, boring, turning, broaching, reaming, milling, planing and shaping, and drilling.

Engineering Toolbox has a good summary of tolerance limits for different material-removal processes:

Tolerance Grades
4 5 6 7 8 9 10 11 12 13
Lapping and Honing
Cylindrical Grinding
Surface Grinding
Diamond Turning
Diamond Boring
Broaching
Reaming
Turning
Boring
Milling
Planing and Shaping
Drilling

ansi-standard-tolerances

As you can see, lapping and honing give the tightest tolerances while milling, planing and shaping, and drilling have wider tolerances.  Also notice that  the tolerances get bigger as the part size gets bigger, regardless of process.

This is great information, but it doesn’t tell you anything about cost.  In general, as tolerances get smaller the parts gets more expensive regardless of process.  That is, a milled part with a dimension of 1.00 +/-0.05mm will be more expensive than a part spec’d at 1.00 +/-0.20mm.  How much more?  It’s impossible to know for certain because there are so many other factors that affect cost.  The take-home message is simply that better parts are more expensive.

Another key piece of information you do not get from this table and chart is any indication of the applications for these processes.  If lapping gives me the tightest tolerances, why don’t I just make all of my parts by lapping them?  Well, lapping only works on flat surfaces.  I suggest clicking through the links above and checking out what wikipedia has to say about each process.  All of the overviews are pretty good.

We will discuss applications of these process as they relate to mechanical components in later posts.

Author: admin

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