Setup guide

How to Use a 123 Pair CNC Machine

A 123 block pair is the cheapest metrology tool in the shop and the one most often used wrong. This guide walks through squaring a vise, finding an edge, setting Z on the block face, and cutting the first part without scrapping it. Written for machinists and process engineers running 3-axis and 4-axis mills.

Get Instant Quote Talk to an Engineer

±0.005 mm toleranceNo minimum order12-hour DFM reply100% inspection

how to use a 123 pair cnc machine setup on a mill table

On this page

1
Key takeaways
2
What a 123 pair cnc machine setup actually needs
3
Prepare the table and the blocks
4
Feed, speed, and step-down when the block is your datum
5
Mistakes that scrap the first part
6
When the setup should go to a machine shop
7
Step by step: using a 123 pair on the mill
8
123 block pair vs other workholding references

Quick answer

Key takeaways

The pair is a square, not a stopTwo matched blocks let you check parallelism and squareness at the same time, which a single block cannot do.

Zero from the block face, not the vise jawJaw faces wear and move. The hardened block face stays flat, so your Z datum stays repeatable across a run.

Keep them clean and deburredA 0.02 mm chip under a block tilts the whole setup. Wipe the table and the block faces before every clamp.

Do not use them as a hard stopCutting forces will slide a loose block. For production stops, bolt a dedicated stop or use a vise jaw.

Fundamentals

What a 123 pair cnc machine setup actually needs

A 123 block is a hardened, ground steel block, usually 1 × 2 × 3 inches, with all six faces square to each other within a few tenths. Sold as a matched pair, the two blocks are ground together so they share the same height. That is the part people miss: a pair is not two random blocks. The matched height is what lets you stack, span, and compare.

On a 123 pair cnc machine setup, the blocks do three jobs. They raise the workpiece off the table so a through-cut does not hit the table. They give you a flat, known reference face for a dial indicator or an edge finder. And they let you check that a vise or fixture is actually square before you trust it with a 4,000 mm part.

They are not precision vises and they are not sine plates. If your drawing calls for ±0.005 mm on a bore position, the block gets you close enough to start indicating, but the final number comes from the probe or the indicator, not from the block itself. Treat the block as a reference, not as a measurement result.

1
Matched heightBoth blocks in a pair are ground to the same thickness, so a stack is repeatable.
2
Hardened facesTypically 58–62 HRC. They resist dents from chips and tool shanks.
3
Six ground facesAny face can be the base. Rotate to pick the height you need.

Before you cut

Prepare the table and the blocks

Start by stoning the table. Run a fine stone over the T-slots and the table surface to knock down any burr or ding. A raised bump of 0.01 mm under a block will show up as a taper on a tall part. Then wipe with a lint-free cloth and a light oil. Do not use compressed air only; it pushes chips into the T-slots.

Check the blocks themselves. Run a finger along the edges and look for nicks. A rolled edge on a block is common after years of use and it will rock on the table. If a block rocks, set it aside and use the other one, or stone the edge lightly and re-check with a dial indicator on the face.

Clamp with the right force. A 123 block held by a single strap clamp can lift at the far end. Use two clamps on opposite corners of a large block, or clamp the workpiece and let the block sit under it as a spacer. Re-check with a 0.02 mm feeler gauge: if the gauge slides under the block, the clamp is not holding it flat.

Cutting data

Feed, speed, and step-down when the block is your datum

The block does not change your cutting data, but it changes how much stock you can trust. When the Z datum comes off a block face, keep the first pass shallow: 0.3–0.5 mm radial for aluminum, 0.2–0.3 mm for stainless. That gives you a chance to measure before the tool is buried in the part.

For aluminum 6061 on a 3-axis mill, a 10 mm carbide end mill runs well at 8,000–12,000 rpm and 1,500–2,500 mm/min with 0.5 mm radial step-over and 3–5 mm axial depth. For 304 stainless, drop to 2,500–3,500 rpm and 400–700 mm/min with 0.3 mm radial. These are starting points; listen to the cut and check the chip color.

If you are holding ±0.005 mm, do not take the final pass off the block datum. Indicate the finished face, reset Z from the part, and take a 0.05 mm finish pass. Thermal growth in the spindle and the part will move the number by more than the tolerance over a long run.

Use coolant or air blast to keep chips off the block. A chip that lands between the block and the table mid-run will lift the block and shift your datum. This is a common failure on long cycles where the operator walks away.

Common errors

Mistakes that scrap the first part

The most expensive mistake is trusting a block that is not seated. A single chip of 0.05 mm under one corner tilts a 75 mm tall block by roughly 0.02 mm across the face. On a part with a 0.05 mm flatness call, that is half your tolerance gone before the tool touches metal.

The second is using the block as a hard stop. Blocks are not bolted down in most setups. A 12 mm end mill at 2,000 mm/min will push a loose block if the cut direction is wrong. If you need a stop, bolt a dedicated stop or use the fixed jaw of the vise.

The third is mixing blocks from different pairs. Two 123 blocks from different manufacturers can differ by 0.02 mm in height. Stack them and your Z datum is off by that amount. Keep pairs together and mark them with a paint dot if you have several sets.

Finally, do not assume the block is still square after a crash. A dropped block can bend by a few hundredths. Re-check it on a surface plate with a dial indicator before you use it on a tight-tolerance job.

When to hand it off

When the setup should go to a machine shop

A 123 pair gets you through most 3-axis and 4-axis work. It stops being the right tool when the part needs five faces machined in one setup, or when the tolerance is tighter than the block itself can hold. At that point the datum has to come from the machine, not from a loose block on the table.

GreatLight runs 16 simultaneous 5-axis machining centers with a Ø400 mm rotary table and travels up to 4,000 × 400 × 150 mm. On those machines the setup is done with a probe and a pallet, not with hand blocks. The block pair is still useful for incoming inspection of fixtures and for squaring a vise on a 3-axis machine.

If you are not sure which route fits your part, send the drawing. We reply with a quotation and a free DFM analysis within 12 hours, and we can start production within 24 hours of a released order. Parts typically ship in 3–5 days.

We hold ±0.005 mm on turned and milled features, with surface finish down to Ra 0.2–0.8 μm when the drawing calls for it. Every part is inspected before shipment, and reports are available on request.

Procedure

Step by step: using a 123 pair on the mill

1
Step 1 — Clean and stone the tableStone the table and the T-slot edges, then wipe with oil and a lint-free cloth. Confirm flatness with a dial indicator on a magnetic base: sweep a 200 mm circle and look for under 0.01 mm variation. If it is worse, find the burr before you go further.
2
Step 2 — Seat the block and check itPlace the block on the table with the 3-inch dimension vertical if you need height. Push down and slide it lightly against a T-slot edge. Indicate the top face across 100 mm; the reading should stay within 0.01 mm. If it does not, rotate the block 90° and re-check to isolate a bad face.
3
Step 3 — Square the vise against the blockClamp the vise loosely, then butt a 123 block against the fixed jaw and the table edge. Tighten the vise bolts in a cross pattern, re-check with a dial indicator along the jaw, and adjust until the jaw runs within 0.01 mm over 150 mm. This is the step that saves the most scrap.
4
Step 4 — Set the Z datum on the block faceBring the tool or probe down to the top face of the block. Touch off and set Z zero. For a probe, use a 2 mm stylus and a feed of 100–200 mm/min. For a edge finder, run at 1,000 rpm and touch until the tip kicks, then offset by half the tip diameter.
5
Step 5 — Find X and Y from the block edgesUse the 2-inch and 3-inch faces as your X and Y references. Touch off on each face with the same edge finder and same rpm so the offsets cancel. Record the numbers in the setup sheet before you move the table, not after.
6
Step 6 — Verify with a test cutCut a 0.5 mm deep, 20 mm long pass on a scrap area or a non-critical face. Measure the result with calipers and a micrometer. If the part is off by more than 0.05 mm, re-check the block seating and the vise squareness before running the program.
7
Step 7 — Remove the block and re-verifyOnce the part is clamped in the vise, take the block out. Re-indicate the part's top face to confirm the Z zero did not shift when the block was pulled. On tall parts, a shift of 0.02 mm is common if the clamp was over-tightened.

Reference

123 block pair vs other workholding references

Pick the reference that matches the job, not the one that is closest to hand.

Method	Best for	Typical setup time	Main limitation
123 matched pair	Flat parts, vise squaring, Z datum	5–15 min	Not a hard stop under load
Angle plate	Tall parts, 90° faces	15–30 min	Needs indicating every setup
Precision vise	Repeat runs of small parts	10–20 min	Jaw wear shifts the datum
Tooling plate	Multi-part pallets	30+ min first time	High initial cost
Probe + macro	Complex 5-axis parts	2–5 min per part	Needs a probe and post support

The block gets you set up. The probe gets you to tolerance.

Use a matched 123 pair to square the vise and set a repeatable Z datum, then indicate the part before the finish pass. If the part needs five faces in one setup or tighter than ±0.01 mm, send it to a shop with 5-axis capacity.

Get Instant Quote Talk to an Engineer

FAQs

Frequently asked questions

Can I use a 123 block pair on a 5-axis machine?

You can, but it is usually the wrong tool. On a 5-axis machine the part is normally held in a vise or on a pallet that has been probed and set as a work offset. A loose block on the table will not survive the rotary motion.

Use the block for incoming checks and for squaring a fixture before it goes on the machine. The datum for the cut should come from the probe.

How often should I re-check a 123 block pair?

Check the seating every setup, and check the block itself whenever it has been dropped, used as a stop, or run without coolant for a long cycle. A quick indicator sweep across the top face takes under a minute.

For tight work under 0.02 mm, verify on a surface plate before the job starts.

What tolerance can I realistically hold off a block datum?

On a clean, seated block with a good vise, ±0.02 mm is a realistic number for a first setup. Getting to ±0.005 mm means indicating the part after clamping and taking a finish pass off the part itself.

The block gets you close. The indicator gets you to tolerance.

Do I need a matched pair, or will one block do?

One block works for a simple Z raise or a single edge reference. A matched pair is better because the two blocks share a height, so you can stack them, span a gap, or compare two faces at once.

If you are buying for a shop, buy the pair. The cost difference is small.

How do I keep chips from getting under the block?

Use an air blast aimed at the block base, not just at the cut. Run coolant if the material allows it. On long cycles, stop once or twice and wipe the area.

A 0.02 mm chip under a block is enough to scrap a tight-tolerance feature.

Does GreatLight use 123 blocks in production?

We use them for fixture setup and incoming checks, and for squaring vises on 3-axis and 4-axis machines. High-volume and tight-tolerance work runs on probes and pallets.

If your part needs a specific setup method, tell us on the quote form and our engineers will confirm it.