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V-Block Sizes: Pairs, Clamping and Which Block to Buy
A V-block is a hardened, ground block with a 90 degree channel cut through it, and it exists for one reason: to hold round or awkward work so it can be marked, drilled or milled without moving. On its own a single block steadies a shaft. Used as a matched pair, it locates a cylinder on two points along its length so the axis stays true, which is what turns a shaky setup into an accurate one. The range here runs from a small bench block up to a heavy 100 mm block, and the decision comes down to the diameter you need to hold, the clamping method and how much accuracy the job demands.
What decides which V-block you need
- Workpiece diameter. The channel has to be wide enough that the work sits in the V rather than bridging the shoulders. A block that is too small cannot hold a large shaft, and one that is far too big puts small work deep in the valley where the clamp cannot reach.
- Block size and mass. A heavier block resists vibration and stays put on the table under an interrupted cut. Light blocks suit marking out and inspection where nothing is pushing against them.
- Matched pairs. Two blocks ground together share the same channel height, so a shaft supported on both sits parallel to the table. Unmatched blocks introduce a taper that shows up as a drilled hole that is not square to the axis.
- Clamping method. Blocks are either clamped with a strap over the top or gripped between the V and a screw clamp through the body. Strap clamping suits larger diameters, screw clamping suits smaller work and repetitive jobs.
- Hardness and finish. Hardened and ground faces hold their accuracy and resist the burrs that a rough casting picks up. The ground finish also lets the block sit dead flat on a surface plate.
- How it is used. Marking out, inspection and light assembly ask for accuracy and flatness. Drilling and milling ask for mass, solid clamping and a block that will not creep.
Which size suits which diameter
| Block | Practical work range | Best at | Limit |
|---|---|---|---|
| 60 x 58 x 40 mm | Small shafts and pins | Marking out, small parts, tight benches | Light for milling; limited clamping depth |
| 75 x 74 x 55 mm | Medium shafts and tube | General workshop drilling and layout | Needs a solid table under heavy cuts |
| 100 x 96 x 75 mm | Large shafts and heavy bar | Milling, drilling and heavy assembly | Too bulky for small, delicate work |
| 60 x 70 x 50 mm | Small to medium round work | Bench work and light drilling | Modest mass for interrupted cuts |
| 2-3/4 x 2-3/8 x 2 in | Imperial sizes and shafts | Inch-based work and mixed benches | Small channel limits large diameters |
Notice that the footprint grows faster than the channel. Going from a 60 mm block to a 100 mm block roughly triples the mass, and that extra mass is what you are buying when the cut gets heavy. If the block is only ever used for marking a shaft before it goes in a lathe, the small blocks do the job and take up a fraction of the bench.
The V-blocks in this guide
Every option in this range
Five blocks, all with the same 90 degree channel and the same job of holding round work. Prices are per block, and the spread reflects the size of the casting and the amount of grinding in it.
| Option | Nominal size | Type | Best for | Price |
|---|---|---|---|---|
| Compact block | 60 x 58 x 40 mm | Single block | Small parts, marking out | $135.96 |
| Mid-size block | 75 x 74 x 55 mm | Single block | General drilling and layout | $195.95 |
| Large block | 100 x 96 x 75 mm | Single block | Heavy work and milling setups | $227.94 |
| Bench block | 60 x 70 x 50 mm | Single block | Bench drilling and light milling | $75.98 |
| Imperial block | 2-3/4 x 2-3/8 x 2 in | Single block | Inch-based shafts and pins | $75.98 |
Using a V-block properly
- Clean both faces first. A single chip under the block tilts it, and a tilted block drills a crooked hole. Wipe the block, the table and the work before anything is clamped.
- Seat the work in the V, not across it. The work should touch both flanks of the channel. If it bridges the shoulders, the block is too small for that diameter.
- Clamp over the centre of the V. A strap clamp offset to one side lifts the work out of the channel on one flank and the axis moves.
- Use a pair for anything long. One block supports a shaft at a point; two blocks define a line. For drilling along an axis, always use a pair.
- Check with a dial indicator before cutting. Rotating the work in the V and reading the top face takes seconds and catches a misaligned block before a drill does any damage.
- Do not use a V-block as an anvil. The channel edges are ground and hardened, and a hammer blow on the shoulder is what puts a burr on the face that then has to be stoned off.
Marking out, drilling and inspection
In marking out, the block does the work of holding a shaft steady while a height gauge or a scriber runs along the top. Because the channel centres the shaft, the top of the work sits at a repeatable height, so a series of shafts can be marked without re-setting the gauge for each one.
For drilling, the block is the difference between a hole that is on centre and one that wanders. Clamped in a machine vice with the work sitting in the channel, the drill meets a surface that is square to the axis, so the point does not skate. Where the work is long, two blocks set parallel to each other hold the axis true along its whole length.
In inspection, blocks are used to support a shaft for a runout check. The work is rotated in the V and a dial indicator reads the top face, which shows bend, ovality or an off-centre feature. The accuracy of that reading depends on the channel being straight and the block sitting flat, which is why blocks intended for inspection are ground all over rather than only in the channel.
Keeping a V-block accurate
- Stone off burrs, do not file them. A fine oilstone laid flat on the face removes a raised burr without cutting a hollow.
- Keep it lightly oiled. A film of oil stops rust in the channel, and rust is what destroys the fit of the work in the V.
- Store blocks on a wooden or rubber pad. Steel on steel in a drawer is how ground corners get knocked.
- Check the pair together. If two blocks are used as a pair, stand them side by side and sight along the channels. A block that has been dropped shows up immediately.
Frequently Asked Questions
Do I need one V-block or a pair?
For marking a single point or holding a short part, one is enough. For anything where the axis has to stay true along a length, use a pair.
Are V-blocks magnetic?
Standard blocks are not. They are held in a vice or on the table with clamps, or used with a separate magnetic base for indicator work.
Can I use a V-block on a milling machine?
Yes, provided the block is large enough and clamped properly. Use the heavier blocks for milling, where the cut pushes against the work.
What angle is the channel?
Ninety degrees, which is the standard because it centres round work of any diameter on the same axis, so the top of the shaft stays at a repeatable height.
How do I check a V-block for accuracy?
Set it on a surface plate, place a known straight bar in the channel and read along the top with a dial indicator while rotating the bar. Movement in the reading is bend in the bar or wear in the channel.
Buying the right one
Start with the diameter you actually hold most often and the mass your machine can carry, then decide whether the work needs one block or a matched pair. The V-blocks in this range cover five sizes from a compact bench block to a heavy 100 mm pattern.
