Reamer Allowance & Hole-Prep Chart

Reamers finish a drilled hole to a precise size and smooth surface. The pre-drill must be undersize by the reamer allowance so the reamer peels a small, consistent chip on each tooth. Too little stock → the reamer burnishes and chatters; too much → it deflects and cuts oversize. Use hand-reamer allowances for hand-tapping through brass and light-gauge stock; machine-reamer allowances for lathe / mill / CNC work where the reamer is rigidly held and spinning.

Hand reamer — imperial

Reamer Ø	Pre-drill (undersize by)	Pre-drill size	Notes
Up to 1/8"	0.003 - 0.005"	Just under nominal	Very little stock — brass preferred
1/8 to 1/4"	0.004 - 0.006"	E.g. #4 drill before 1/4 reamer	Use cutting oil; clear chips every 2-3 turns
1/4 to 1/2"	0.005 - 0.010"	E.g. 15/32 before 1/2"	Check alignment — hand reamers wander on hard spots
1/2 to 3/4"	0.008 - 0.012"	E.g. 23/32 before 3/4"	Torque can be high — clamp work securely
3/4 to 1"	0.010 - 0.015"	E.g. 63/64 before 1"	Very slow rotation; use both hands on the T-handle
1 to 2"	0.012 - 0.020"	E.g. 1-61/64 before 2"	Reamers this size are hard to hand-ream — prefer machine

Machine reamer — imperial

Reamer Ø	Pre-drill (undersize by)	Typical pre-drill	RPM factor
Up to 1/8"	0.005 - 0.008"	e.g. 3/32 before 7/64	~2/3 of drill RPM
1/8 to 1/4"	0.008 - 0.012"	e.g. 15/64 before 1/4	~2/3 of drill RPM
1/4 to 1/2"	0.012 - 0.016"	e.g. 15/32 before 1/2	~2/3 of drill RPM
1/2 to 3/4"	0.016 - 0.020"	e.g. 45/64 before 3/4	~1/2 of drill RPM
3/4 to 1"	0.020 - 0.025"	e.g. 31/32 before 1"	~1/2 of drill RPM
1 to 1-1/2"	0.025 - 0.031"	e.g. 1-29/32 before 2"	~1/3 of drill RPM
Over 1-1/2"	0.031 - 0.040"	—	~1/3 of drill RPM

Metric reaming allowance

Reamer Ø (mm)	Hand allowance	Machine allowance
Up to 3 mm	0.08 - 0.15 mm	0.10 - 0.20 mm
3 to 6 mm	0.10 - 0.20 mm	0.15 - 0.30 mm
6 to 13 mm	0.15 - 0.30 mm	0.20 - 0.40 mm
13 to 25 mm	0.25 - 0.50 mm	0.40 - 0.60 mm
25 to 50 mm	0.40 - 0.80 mm	0.60 - 1.00 mm
Over 50 mm	0.80 - 1.20 mm	1.00 - 1.50 mm

Reaming speeds (starting SFM, HSS reamer)

Material	SFM	% of drill SFM	Feed per rev
Aluminum	150 - 300	60-80%	0.004 - 0.012"
Brass / bronze	100 - 200	60-80%	0.004 - 0.012"
Cast iron	40 - 60	60-70%	0.004 - 0.012"
Mild steel	40 - 80	60-70%	0.004 - 0.012"
Alloy steel	30 - 60	50-70%	0.003 - 0.010"
Tool steel	15 - 40	50-60%	0.003 - 0.008"
Stainless	15 - 30	40-60%	0.002 - 0.008"
Titanium	10 - 25	40-60%	0.002 - 0.006"
Plastic	100 - 250	50-70%	0.005 - 0.015"

Finish allowance — grinding, honing, lapping

Process	Typical stock / side	Resulting tolerance	Resulting Ra
Rough grinding	0.005 - 0.020"	±0.0005"	32-125 μin
Finish grinding	0.002 - 0.005"	±0.0001"	8-32 μin
Precision grinding	0.0005 - 0.002"	±0.00005"	4-16 μin
Honing	0.001 - 0.010"	±0.0001"	4-32 μin
Lapping	0.0001 - 0.001"	±0.00002"	0.5-8 μin
Superfinishing	0.0001 - 0.0005"	±0.00001"	0.5-4 μin
Electropolishing	0.0002 - 0.001"	loss of size	4-32 μin

Operating tips

Rigidity is everything: a reamer that can wobble will cut oversize. Use the shortest possible reamer for the depth; hold in a solid-mount chuck, not a floating holder unless necessary.
Run reamers at half to two-thirds the SFM of an equivalent drill; feed at 2-3× the drill feed rate. Light pressure + aggressive feed = clean shearing chip.
Never back a reamer out while rotating forward under power — the flutes pick up material and score the hole. Stop the spindle, then reverse or retract.
Use flood coolant or cutting oil — reamers generate heat quickly and work-hardening materials (stainless, Inconel) will "glaze" the flutes in seconds if dry.
For close-tolerance holes (< 0.0005" tolerance), rough with a drill two sizes undersize, bore to within 0.005" of final, then ream.
Through-holes: retract reamer at full feed to avoid the heel rubbing. Blind holes: use a bottoming / end-cutting reamer.
Chatter marks are almost always a tool-holder issue, not the reamer. Check spindle runout and collet chuck grip before re-grinding.

Reamer types

Chucking (machine) reamer	Straight shank with 45° bevel lead, flutes helical or straight. Most common in the shop.
Hand reamer	Long taper lead (1-2") at the front so it self-aligns when turned by hand. Requires more allowance than machine reamer.
Taper-pin reamer	1/4" per foot taper. For taper pin holes (drive pins, alignment dowels).
Shell reamer	Hollow reamer that mounts on an arbor — cheap way to stock large sizes.
Adjustable reamer	Blades are moved radially to hit any size in a small range. Handy for repair; slow to set.
Expansion reamer	Single-size reamer with a hollow body + plug to expand diameter 0.005-0.015" for hand-fitting.
Carbide-tipped	Brazed carbide tips on each cutter. 3-5× life of HSS on abrasive materials; more expensive.
Solid carbide	Whole body is carbide; 10× life of HSS on hardened materials. Brittle — no chatter tolerated.
PCD (polycrystalline diamond)	Diamond-tipped for aluminium and composites. Up to 20× life of carbide on Al-Si alloys.

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