Surface Plates, Gauge Blocks, and Height Gauges
Set up a flat reference, build an exact size from gauge blocks, and take height readings on a surface plate without damaging your standards.
Table of contents
Most measuring tools need something to measure from. In an inspection room, that something is a surface plate for flatness and a gauge block for size. Together they let you check other tools, set up comparisons, and measure heights that no hand tool can reach. Treat them as the standards they are, because every reading you take from them inherits any damage they carry.
What You Will Be Able to Do
- Explain why a surface plate is a reference, and care for it correctly.
- Choose and build a gauge block stack for a given size.
- Wring gauge blocks together and take them apart safely.
- Take a height measurement with a height gauge.
- Set up a part on parallels, angle plates, and 1-2-3 blocks.
The Surface Plate
A surface plate is a thick slab of granite, lapped very flat, on a stable stand. It gives you a flat plane to measure from. Granite is used because it holds its shape, does not rust, and tends to chip rather than raise a burr if something hits it.
Plates come in grades of flatness. The grade and size together set the flatness tolerance, and a bigger plate is allowed more total error.
| Grade | Common use | Overall flatness on an 18 x 24 inch plate |
|---|---|---|
| AA | Laboratory | About 0.000075 inch |
| A | Inspection room | About 0.00015 inch |
| B | Toolroom | About 0.0003 inch |
Plate Care
- Clean the plate before and after use. Use the plate cleaner your lab approves, or denatured alcohol, and a lint-free cloth. Grit grinds the surface as parts slide over it.
- Cover the plate when it is not in use.
- Set parts down gently, and lift them instead of sliding when you can.
- Spread your work across the plate. Using one spot all the time wears a low area.
- Never set hot parts on it. Heat on one face makes the plate bow, and that uneven heating is called a thermal gradient. Keep it away from sun and heaters too.
- Never drill, hammer, tap, or file on it. It is not a workbench.
- Keep it on its proper stand, with the supports where they belong, because a plate can sag if it is propped the wrong way.
Plates wear and need scheduled recertification, often once a year. Report any new chip.
Gauge Blocks
Gauge blocks, also called Jo blocks, are rectangular blocks of steel, ceramic, or carbide made to an exact length. They are the working link between your tools and the national length standard. A good block is certified to within a few millionths of an inch, on the order of 0.000004 inch for a fine grade.
You use them to check micrometers and calipers, set comparators and height gauges, and act as a master for a known size.
Grades run from calibration quality down to workshop quality, and the names depend on the standard. ASME B89.1.9 lists K, 00, 0, AS-1, and AS-2, and ISO 3650 lists K, 0, 1, and 2. Keep the finest set for checking other blocks and use workshop blocks for daily setting.
Building a Stack
A set has blocks in steps, so a few of them can make almost any size. Use as few blocks as you can, because each joint and each block adds a little error. Work from the smallest digit first.
Worked example: build 1.4837 inches from a common 81-piece inch set.
| Step | Pick a block | Left to build |
|---|---|---|
| Start | 1.4837 | |
| Clear the 0.0007 | 0.1007 | 1.3830 |
| Clear the 0.003 | 0.1330 | 1.2500 |
| Clear the 0.250 | 0.2500 | 1.0000 |
| Finish | 1.0000 | 0 |
Check by adding: 0.1007 + 0.1330 + 0.2500 + 1.0000 = 1.4837.
Wringing
Wringing joins two clean blocks so tightly they cling together. A very thin film of moisture or oil and the molecular attraction between very flat surfaces hold them. The joint adds so little that it is usually ignored.
- Wipe both faces with a clean cloth, and check for nicks and burrs.
- Set the two blocks together at a cross angle, or overlap the ends.
- Press lightly and slide or twist them into line. You will feel them grab.
- To take them apart, slide them. Never pry.
Do not leave a stack wrung for long. Trapped moisture can corrode the faces and the blocks can stick. When you finish, clean each block, give steel blocks a light coat of rust preventive, and return it to its slot. Bare fingers leave acid and oil, so handle blocks as little as you can. Keep them at room temperature, because your hands warm them and change their length.
Height Gauges
A height gauge is a sliding measuring head on a heavy base. It measures up from the plate, so the plate is its zero. Vernier models read to about 0.001 inch. Digital models commonly read to 0.0005 inch or 0.0001 inch.
To measure a step height:
- Clean the plate, the gauge base, and the part.
- Bring the scriber or probe down onto the plate and zero the gauge.
- Set the part on the plate, with the reference surface down.
- Raise the head above the feature, then lower it gently to touch the feature.
- Read the height. For a close check, use a test indicator on the head and set it with a gauge block stack, then compare the part to the stack.
A height gauge can also scribe layout lines, as long as you use a scriber meant for it. Slide it by its base and keep the base flat on the plate.
Comparing to a Stack
Comparison is the most accurate way to measure on a plate. Instead of reading the gauge's own scale, you make the indicator read zero on a gauge block stack of the target size. Then you move to the part. Any change in the reading is the difference from the stack. Your error is then mostly the block's error, which is tiny.
Worked example: a stack of 2.3000 inches zeros the indicator. On the part, the indicator reads +0.0004. The part height is 2.3004 inches.
Setup Hardware
| Item | What it does |
|---|---|
| Parallels | Matched ground bars that raise a part evenly |
| 1-2-3 blocks | Ground blocks 1 x 2 x 3 inches for propping and stepping |
| Angle plate | Holds a part at a true 90 degrees to the plate |
| V-block | Cradles a round part |
| Sine bar | Sets an exact angle using gauge blocks |
Wipe each one before use. A chip under a parallel tips everything above it.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Using the plate as a bench | Wear and nicks that ruin every later reading |
| Setting a hot part on granite | Bowing, and a false reading until it settles |
| Leaving blocks wrung overnight | Corrosion, stuck blocks, a costly recertification |
| Prying a stack apart | Scratched or damaged measuring faces |
| Forgetting to zero the height gauge on the plate | All readings off by the same amount |
| Measuring with dirt under the part | The whole setup tilts |
Check Your Understanding
1. You need a 0.8125-inch reference. Which rule guides your stack?
Use the fewest blocks you can, clearing the smallest digit first. Each block and joint adds some error.
2. A freshly machined part is warm and you are in a hurry. Can you set it on the granite to check it?
No. Let it cool first. It would warm one face of the plate and also read large.
3. After an hour at the bench, a gauge block stack is stuck together. What do you do?
Slide the blocks apart gently, never pry. Then clean them, protect them, and put them away.
4. An indicator zeroes on a 1.5000 stack and reads -0.0006 on the part. What is the part height?
1.4994 inches.
Key Takeaways
- A surface plate is a reference for flatness, not a bench. Keep it clean, covered, and away from heat.
- Grades AA, A, and B set how flat a plate must be for its size.
- Gauge blocks are certified to millionths. Use the fewest you can.
- Wring blocks by sliding them, never leave them wrung, and never pry them.
- Zero a height gauge on the plate. For the best accuracy, compare to a gauge block stack.