The Small Holes Field Guide
The Small Holes Field Guide
Twelve checks to run before you drill under 1 mm in brass, PCB, watch plates and model parts. Fewer snapped bits, fewer ruined parts. Save this page or print it for the bench.
Start with the hole, not the bit box
A 0.5 mm hole looks simple on a drawing. At the bench it leaves almost no room for wobble, a loose part or a heavy hand. These first checks happen before the motor turns on.
Know the hole you need
Finished diameter, depth and material first. A through-hole in thin brass sheet, a 2 mm-deep hole in a watch plate and a hole in FR-4 board are three different jobs.
Buy the sizes you actually use
Small bits are consumables. If your parts use 0.8 and 1.0 mm, keep those in multiples instead of a big assortment you never touch.
Match the bit material to the job
HSS is the forgiving all-rounder for brass, plastic, wood and aluminum. Carbide holds an edge in abrasive PCB laminate but is brittle: it will not forgive wobble or a part that shifts.
Check the shank, not just the tip
A 0.3 mm bit often has a 3.175 mm (1/8 in) shank. The tip makes the hole, the shank decides whether it fits your holder. Below about 1 mm a collet grips more evenly than a three-jaw chuck.
Test the fit on scrap
A nominal 1 mm bit does not always make a 1 mm functional hole. If a pin or wire needs a close fit, drill a test hole in an offcut first.
Most "bad bits" are setup problems
A press can be sound while a loose vise or a bent bit makes bad holes. Check these before blaming the machine.
Look for runout
Put a straight shank in the spindle and turn it by hand against a fixed point. Visible wobble at the tip is worth fixing: clean the collet and nut, try another straight shank, then look at the holder.
Square the table to the spindle
Check with a small machinist square in both directions. A tilted table gives slanted holes, most visible in thicker brass and parts where a pin must stand straight.
Clamp it, never hold it
Fingers are not workholding. Use a vise or clamp the part on a flat sacrificial backing board so the force goes straight down into support.
Back up the exit side
A flat backing board stops thin sheet and PCB from flexing and reduces breakout and burrs as the bit comes through.
Let a small bit cut
Light, steady feed
You should feel the bit cutting, not forcing its way through. If it squeals, rubs without advancing or flexes, stop and check sharpness, speed and alignment.
Peck only when chips pack
For deeper holes, advance a little, retract smoothly to clear chips, continue. Never yank a spinning bit out sideways.
Ease off at breakthrough
The last fraction of a millimeter is where thin parts go wrong. Lighten the feed as you approach the back side, then remove only the burr, by hand.
What the failure is telling you
| YOU SEE | USUALLY MEANS |
|---|---|
| Bit snaps near the tip | Too much feed, packed chips or a dull edge |
| Bit snaps higher up | Side load: the part shifted or the spindle wobbles |
| Rough, oversize hole | Runout, a worn bit or no support at breakthrough |
| Melted hole in plastic | Rubbing instead of cutting: heat |
| Bit walks off the mark | Weak start or a part not held flat. Stop, don't push sideways |
Read the full guides
Each part of this field guide comes from a longer article on our blog, with more detail and examples.
| How to drill watch plates without ruining them | Read |
| How to choose micro drill bits for fine work | Read |
| Drill press alignment for accurate small holes | Read |
Tools from the guide
| Mikrabench drill presses: chuck, collet and micrometer | See |
| PCB micro drill bits, 3.175 mm shank, 10 of one size | See |
| Drill press vise and 120 x 150 mm base plate | See |
| XY Stage 60 mm, lead screw cross slide | See |