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What Are 1-2-3 Blocks?
Few tools in the machine shop are as simple and as useful to machinists as a set of 1-2-3 blocks. Their precision-ground surfaces, standardized dimensions, and versatile hole patterns make them useful for much more than simply supporting a workpiece. Whether you're setting up a milling machine, positioning a part, creating a repeatable setup, protecting your machine table, or making an awkward measurement easier, 1-2-3 blocks can often provide a quick solution.
And if you already have a set in your toolbox, you may be using only a fraction of what they can do. In this article, we'll look at the basics of 1-2-3 blocks and several practical ways to put them to work.
Also called 1-2-3 setup blocks, they're typically made from hardened steel and precision ground on all six sides. Because their dimensions and surfaces are accurately controlled, they can serve as both work supports and reference tools.
Most 1-2-3 blocks also have a series of holes through them. Depending on the design, some of these holes are threaded, allowing the blocks to be bolted together or secured to a machine table. Their three different dimensions are one of their biggest advantages. Simply changing the orientation of a block gives you a different reference height.
Why Are They Called 1-2-3 Blocks? 
It's simply a reference to their 1" × 2" × 3" dimensions.
For example:
- Place the block on its 1" side → 1" reference height
- Turn it to its 2" side → 2" reference height
- Stand it on its 3" side → 3" reference height
That makes a 1-2-3 block an extremely convenient way to establish repeatable distances from a machine table or another reference surface.
What Are 1-2-3 Blocks Used For? 5 Practical Uses
1. Create Repeatable Machine Setups 
One of the most useful applications for 1-2-3 blocks is establishing a repeatable setup. A block can be positioned and clamped on a machine table to create a consistent reference for multiple parts. Once the setup is established, additional workpieces can be placed against the blocks in the same location.
This can significantly reduce setup time when machining multiple identical parts while helping maintain consistency from one workpiece to the next. The holes in the blocks provide even more options for securing them to the machine table, attaching components, or connecting blocks together.
2. Use Them as Parallels or Work Supports
1-2-3 blocks can be used to support a workpiece above the machine table. Place the blocks beneath the part and clamp everything securely. The blocks provide a solid reference surface while creating clearance beneath the workpiece. That clearance is especially useful when drilling or machining through a part.
Instead of allowing the tool to continue into the machine table, the space below the workpiece gives the cutting tool room to pass through. The holes in the blocks can also provide clearance or additional workholding options for certain drilling and machining setups. Always make sure the workpiece and blocks are securely supported and clamped before machining.
3. Position and Support Parts During Setup
1-2-3 blocks are excellent temporary or permanent setup supports when positioning a part:
- In a vise
- On a machine table
- Against a fixture
- Against another workholding component
Their precision-ground surfaces provide convenient reference faces for establishing height, alignment, and location. Because they're square and accurately sized, they can eliminate the need to fabricate a special support for many simple setups.
4. Check Measuring Tools
Because precision 1-2-3 blocks are manufactured to tight dimensional and squareness tolerances, they can also be useful for checking measuring tools. For example, you can use a known dimension of the block as a quick reference and compare it with a caliper or other measuring instrument.
This isn't a substitute for a properly calibrated inspection standard when traceable measurement or calibration is required, but it can be a quick way to identify a measuring tool that may be out of adjustment or producing questionable readings. Of course, the accuracy of the block itself matters. Use blocks with known specifications and keep their precision surfaces clean and undamaged.
5. Make Difficult Measurements Easier
Sometimes an oddly shaped workpiece simply doesn't provide a convenient edge for a caliper to contact. A 1-2-3 block can solve that problem. Place the block against the workpiece to create a straight, accessible reference edge.
You can then position the caliper against the block rather than trying to reach an irregular feature on the part. The different dimensions of the block can also be used to establish a convenient reference distance. It's a simple technique, but one that can make certain measurements much easier and faster.
Why Do 1-2-3 Blocks Have Holes?
The holes in a 1-2-3 block aren't just there to make it lighter. On blocks with threaded holes, they can turn a simple pair of blocks into a modular setup system.
Threaded holes can be used to:
- Secure blocks to a machine table
- Attach blocks to one another
- Build temporary fixtures
- Create custom workholding arrangements
- Establish repeatable setups
- Configure blocks as larger setup components
Some 1-2-3 block sets are specifically designed to connect together, allowing them to be configured as 2-4-6 blocks, angle plates, and other setup arrangements.
Featured at Travers: TTC G-123-MOD 1-2-3 Block Set
If you're looking for a versatile set of 1-2-3 blocks, take a look at the TTC G-123-MOD 1-2-3 Block Set, Travers Item #57-020-425.
The set includes two sets of 1-2-3 blocks, a 16-piece connector hardware pack, and a fitted case. The blocks measure 1" × 2" × 3" and feature 23 holes, including 3/8"-16 tapped holes for mounting and connecting the blocks. They're precision ground on all six sides, with a stated squareness of ±0.0001° per inch and dimensional accuracy of ±0.0002".
The modular design is what makes this set particularly useful. With the included hardware, the blocks can be configured into larger setup blocks, angle plates, and other arrangements, giving you considerably more versatility than a basic pair of solid 1-2-3 blocks.
Tips for Getting the Most from Your 1-2-3 Blocks
Like any precision tool, 1-2-3 blocks should be treated accordingly.
Keep the surfaces clean
Chips, dirt, and debris between the block, workpiece, or machine table can affect the accuracy of your setup.
Check for burrs and damage
A ding or raised burr on a reference surface can affect both setup and measurement accuracy.
Know the accuracy of your blocks
Not every 1-2-3 block is manufactured to the same tolerance. When using them for precision setup or inspection, know the specifications of the particular blocks you're using.
Secure everything
A precision block is only useful if it stays where you put it. Make sure the blocks, workpiece, and other setup components are properly secured before machining.
A Small Tool with a Lot of Uses
The 1-2-3 block is a great example of a simple machine-shop tool that can solve a surprising number of problems. Its standardized dimensions provide convenient reference heights. Its precision-ground surfaces make it useful for setup and inspection. Its holes allow it to become part of more elaborate fixtures and workholding arrangements.
Whether you're supporting a workpiece, establishing a repeatable setup, creating clearance for machining, checking a measurement, or giving a caliper a convenient reference surface, 1-2-3 blocks can make the job easier. Once you start looking for ways to incorporate them into your setups, you'll probably find that these seemingly simple blocks are one of the most versatile tools in the machine shop.
How Do You Choose the right caliper?
Like 1-2-3 blocks, calipers are a machine shop essential! Download our valuable guide on the proper selection and usage of digital, dial, and Vernier calipers. Learn the advantages and disadvantages of each, and which caliper best meets your needs and suits your application. Find information on:
- The fundamentals of calipers
- The features and appropriate use of digital, dial and Vernier calipers
- The benefits of calibrated calipers
- Ingress Protection (IP) Ratings explained
