Blocks, 1-2-3, Ultra Precision

1-2-3 blocks provide a precise, stable, and reusable reference for ensuring accuracy in machining, inspection, and layout tasks, making them a staple in all machine shops.

These ultra precision 1-2-3 blocks are parallel within 0.0001", flat within 0.0001" and square within 0.0001". They are up to 0.0003" oversize for lapping. 

Each set includes two matched blocks.

• The blocks have 23 holes through them. Five are threaded 3/8-16 NC, and the other through holes are 5/16" diameter. 
  
• The set includes a matched pair of 1-2-3 blocks, five 3/8-16 x 1.5" socket head cap screws and a hex wrench.
• Blocks are ground and case hardened on all sides to 55-60 Rc. 

Furnished in a fitted case.

These blocks have a myriad of uses, including, but not limited to:

• Use as parallels: Set your large workpiece on 1-2-3 blocks and clamp it to the table so you can drill through holes without damaging your mill table.
• Use as fixtures: 1-2-3 blocks can be clamped to a machine table and used to create a repeating set-up for machining identical parts.
• Check your calipers: Since 1-2-3 blocks are made to tight tolerances, they can be used to verify the accuracy of your calipers.
• Use for measuring: Hold a 1-2-3 block against your workpiece to make a protruding edge that you can measure to.  

Here is What to do with 1-2-3 Blocks by Rick Sparber that shows the many uses of 1-2-3 blocks,