Dowel Pin Hole Tolerance: Reamed, Bored or Interpolated Holes
Two dowels 100 mm apart locate a fixture plate to a base. The holes are reamed to H7, the pins are standard hardened m6, and the assembly binds halfway down before a single bolt is tightened. Nothing is out of tolerance. The parts are simply toleranced in a way that guarantees interference at the extremes, and nobody multiplied the errors by the distance between the pins.
Dowel pin hole tolerance is a system problem: hole size, hole position, pin size and the geometry of the joint all act together. Getting one of them right does not help if the other three are left at general tolerance.
Three ways to make the hole
Reaming is the default. Drill 0.2 to 0.3 mm undersize, then ream at 0.3 to 1.5 m/min with 0.05 mm/rev feed. A good reamer holds H7 and 0.005 mm roundness in aluminum or steel, but it follows the drilled hole: if the drill wandered 0.05 mm, the reamer fixes the diameter and leaves the position error.
Boring is single-point work with an adjustable head. It corrects position because the tool is set from the machine's coordinate system rather than following a pilot hole, and holds true position of 0.005 to 0.01 mm. It is slower — two to four minutes per hole — and needs a rigid setup and a hole of at least Ø10 mm.
Interpolation, or helical milling with an end mill smaller than the hole, sits between the two. A Ø4 carbide end mill can produce a Ø6 hole to 0.01 to 0.015 mm on a 3-axis milling center, and because the toolpath is a program, the position can be corrected after measuring without changing tools. For holes over Ø12 mm it is usually the most accurate method available on a VMC.
A dowel that carries shear load needs a tight fit and true position. A dowel that only aligns a cover needs neither.
Position tolerance, and the bonus you are already getting
A position callout of Ø0.05 mm at maximum material condition on a Ø6 +0.012/0 hole is not a fixed zone. At MMC the hole is 6.000 mm and the zone is Ø0.05. If the hole comes out at 6.012, the 0.012 departure from MMC is added to the zone, giving Ø0.062. That bonus is real and it is why a slightly large hole with a slightly off position can still assemble.
The trap is the reverse case. A hole at exactly 6.000 with 0.05 mm of position error is at its limit, and its mating hole has its own 0.05 mm limit in the opposite direction. Two parts at their extremes need 0.1 mm of clearance, and an m6 pin in that hole provides 0.004 to 0.012 mm of interference.
For a two-pin joint, do not tolerance both pins as round holes at tight limits. Use one round hole and one diamond (relieved) hole, or one round hole and one slot. The diamond pin removes the over-constraint in the direction of the hole spacing, so the second pin only has to control rotation. This single change eliminates most binding complaints on pinned joints.
See also: Bearing Bore Tolerance: Fit Classes, Roundness and Thermal Growth and Drone Frame Machining: Billet Center Plates, Ribs and Motor Patterns.
Press dowels, slip dowels and pull dowels
Pin size and hole size must be selected as a pair. The table gives the bands we use for a Ø6 dowel; scale the reasoning, not the numbers, for other diameters.
| Dowel type | Pin tolerance on Ø6 | Hole size | Interference | Typical use |
|---|---|---|---|---|
| Standard m6, press | +0.004/+0.012 mm | 6.000 to 6.008 mm | 0.004 to 0.012 mm | Permanent location, non-serviceable joint |
| Slip fit h6 | 0/−0.008 mm | 6.012 to 6.020 mm | 0.004 to 0.012 mm clearance | Serviceable joint, hand assembly |
| Pull dowel, tapped | +0.004/+0.012 mm | 6.000 to 6.008 mm | 0.004 to 0.012 mm | Location plus extraction thread |
| Shoulder dowel | m6 on the shank | Reamed to the shoulder | Press on the shoulder | Wear surface or pivot |
| Coiled spring pin | Compressed in the hole | 6.000 to 6.100 mm | Radial compression | Light location, vibration damping |
Two practical notes. A press fit in aluminum loses roughly 0.008 mm of interference at 60 °C because the hole grows faster than the pin, so a 0.006 mm press in a 6061 housing is a slip fit in service. And a slip-fit dowel needs retention — either a shoulder, a snap ring, or a threaded pull hole — because a 0.01 mm clearance pin will walk out under vibration.
Depth, venting and blind holes
Engage 1.5 to 2 times the pin diameter in steel and 2 to 3 times in aluminum; more adds drilling time and nothing else. Make the hole 1 to 2 mm deeper than the pin so it cannot bottom and hold the joint open.
Blind holes trap air. A close-fit dowel in a blind hole either stops short or pushes back out overnight. Drill a Ø1 mm vent through the bottom, or leave 0.5 mm of relief by drilling deeper than the flat-bottom depth with a 135° point. Coolant and assembly grease cause the same hydraulic lock.
Deburr both ends. A burr at the mouth holds the pin 0.05 mm off the surface, enough to tilt a plate and spoil the parallelism of the joint. Chamfer 0.2 × 45° and check with a 10× loupe, not by feel.
Why pinned assemblies fail
Failures cluster into five patterns:
- Hole spacing error. Two holes each ±0.01 over 100 mm produce up to 0.02 mm of differential, which exceeds the clearance in a press fit. The second pin never goes in without a hammer.
- Position error from drilling. The reamer followed a wandered drill: round, on size, wrong place.
- Hole too shallow. The pin bottoms, the joint sits open, and the bolts bend the plate when tightened.
- No vent. The pin hydraulically locks and is found 2 mm proud after the press.
- Slip fit with no retention. The pin walks out and the joint loses location after a week of vibration.
Each maps to a drawing callout: true position, a note to bore or interpolate rather than ream for critical joints, a depth with a minimum clearance below the pin, a vent hole, and a retention feature.
Getting it made right
Specify the dowel standard and the fit, not just "Ø6 hole". State true position with the material condition, depth and clearance below the pin, chamfer, and whether the hole is reamed, bored or interpolated. If the joint is serviceable, say so: that changes the fit and the price.
Reaming adds seconds per hole, boring adds minutes, and a wrong callout costs a fixture plate. Send the dowel part number with the drawing and we will check the hole size against the actual pin tolerance before quoting — send the file here, or see the tolerance range we hold across materials.
Frequently asked questions
What hole size should I use for a 6 mm dowel?
6.000 to 6.008 mm for a standard m6 press dowel, or 6.012 to 6.020 mm for an h6 slip dowel. Do not use a generic H7 band for a press fit: the top of H7 at 6.012 turns the press into a slip.
Should dowel holes be reamed or bored?
Ream for production holes up to Ø10 mm where position is not critical. Bore or interpolate when true position must be better than 0.02 mm, when the hole is over Ø12 mm, or when you need to correct position after measuring.
How deep should a dowel hole be?
Engage 1.5 to 2 times the diameter in steel and 2 to 3 times in aluminum, then add 1 to 2 mm of clearance below the pin. Deeper engagement adds no strength and makes the hole harder to clean.
Why will my second dowel not go in?
Hole spacing error, almost always. Two tight-fitting pins over-constrain the joint. Use a diamond pin or a slot for the second hole, or open up the position tolerance and rely on the first pin plus the bolts for location.
Related reading
Send a drawing for a quote and DFM review.
STEP, IGES, X_T, PDF or DWG up to 50 MB. Tell us quantity and finish, and the quote comes back with tolerance and process notes, not just a price.