Gage Blocks For Precison Machining

Like a lot of precision measurement tools, gage blocks are quite simple, but very accurate. In fact, a good gage block set is calibrated within millionths of an inch! You can't get much smaller than that and still have something that is useful on a day to day basis.

These are also some of the most abused precision tools around. I have seen some cases that would make anyone cringe at the sight. I have seen them forced into openings to check a fit, and a hammer was required to remove them! They are also frequently used to block up parts on the magnetic chuck of a surface grinder, not a good idea!

Proper care of gage blocks

These tools must be treated with respect, if they are to remain useful for precision measurement. Once they are damaged, their practical usefulness is gone.

Keep them dry. Rust is the enemy of steel and will pit your gage block set very quickly. By keeping them in a wooden box with a sheet of rust resistant paper on top is normally all you need. If your shop is not climate controlled, you should coat them with oil after each use, taking care to wipe off any fingerprints. The skin PH level of certain individuals is such that it leaves fingerprints wherever they touch steel.

Put them back in the box after each use. Leaving them wrung together overnight is not a good idea, unless it cannot be helped because of the job. If they are left lying about, this only increases their chance to become damaged.

Never force gage blocks. It is contrary to common sense, if you are attempting to check the size of a slot, for example, you are not going to get an accurate reading by forcing it anyway. Take your time and do it right the first time and you will be rewarded later.

If you can avoid it, never use them as parallels. In reality, there are times you can and should use them as parallels, but care should be taken to handle them with the respect they deserve.

Do not ever use them as blocking material on the surface grinder. This is usually just a matter of laziness and poor toolmaking practice. There is always another parallel that can be used to keep your part on the magnet while grinding.

Gage block grades

For general tool room daily use the recommended grade is B. This is an old designation that is still in use. The new grade name is AS2. This roughly means that the tolerance is plus or minus 50 millionths. Most mold making, tool and die or precision machining shops have a master set, which is AS1, on hand to use for calibration.

Gage block materials

There are 3 common materials used to make gage blocks: steel, carbide and ceramic. The steel is by far the cheapest, but lacks durability. The carbide is vastly more durable, but also more expensive. The ceramic is extremely durable, but also very brittle and expensive. These factors must be considered when choosing a gage block set for the shop.

Most toolmaking shops, CNC machine shops, precision medical machine shops, aerospace machine shops, and micro precision machining operations use the steel blocks. It is just too difficult to justify the expense of the carbide or ceramic blocks.

Gage block manufacturers

There are several companies that make quality blocks, and many making very inexpensive but low quality sets. The reputable companies include: Starrett, Webber, Johannson, Do-All, Mitutoyo and Brown and Sharpe.

A good company offers gage block calibration as a service, and the blocks are traceable to the NIST. The cheap ones are probably made by children in the developing world, don't expect any customer service there!