Guide
Reamed dowel holes and tight holes: what the shop needs to know before it quotes
A plate with a few reamed dowel holes is the most common precision feature on a machine-builder's drawing and the most common source of pre-quote questions. The holes locate a mating part, so the diameter and the pattern matter more than anything else on the print. A shop quotes them differently from clearance holes, and it will ask about inspection, so it pays to know what the callout means and what it costs.
Send a drawingUpdated October 8, 2026
Why dowel holes are reamed
A drilled hole is not round enough or straight enough to locate a dowel pin to a few ten-thousandths. The shop drills undersize, often bores or interpolates to get the hole straight and on position, then reams to the finished diameter. The reamer sets the size and the finish; the operation before it sets the position. On a press-fit side the hole is reamed to a slight interference; on the slip side it is reamed a few tenths over the pin.
That is why the drawing calls a dowel hole as a diameter with a unilateral tolerance, for instance 0.2500 reamed, plus 0.0006 minus 0.0000, rather than a plus or minus band. The pin is a standard size and the hole is sized against it.
What the shop is really pricing
- The tolerance on the diameter: a reamed hole with a few tenths of allowance is a different operation from a drilled clearance hole; it adds a tool, a pass and an inspection.
- The position tolerance of the pattern: a hole pattern held to 0.001 across a 30 or 40 inch plate needs a machine with a travel and an accuracy to match, and a plate that stays flat on the table.
- Perpendicularity: a dowel hole that is not square to the face binds the pin. It is a function of the setup and the plate's flatness.
- Inspection: at these tolerances the shop checks with pin gauges and, on larger patterns, a CMM. If you need the numbers on paper, say so; it is a line on the quote.
What to put on the drawing
| Callout | What it tells the shop | What is missing without it |
|---|---|---|
| Diameter with its own tolerance | Ream to this size, this allowance | The title block default is applied, which is not a dowel fit |
| Press or slip | Which side of the pin the hole is sized to | The shop asks, or picks slip |
| Position tolerance with datums | How exact the pattern is and from where | Holes are placed to the default and the mating part may not fit |
| Depth and whether it is through | Blind reaming needs a different tool and chip clearance | Through is assumed |
| Which holes match which mating part | The shop knows which holes must be made together in one setup | A pattern split across two setups may not align |
| Finish before or after | A plated or coated plate changes the hole size | Plating closes a reamed hole by a few tenths |
Questions a shop will ask on a large plate
On plates around 30 to 40 inches, expect to be asked whether the pattern will be verified on a CMM, whether the plate is to be stress relieved before final machining (a plate that moves after machining loses its hole pattern), what the surface finish on the mounting face is, and whether the holes can be finished after the plate is anodized or plated. These are the questions the Amerigo drawing read surfaces on a request so you can answer them once instead of shop by shop.
Questions buyers ask
- Can a sheet metal shop make reamed dowel holes?
- Usually not to a few tenths. A reamed dowel hole is a machining operation. On a formed sheet metal part that needs one, the shop either machines the hole after forming or sends the part out, and the quote says which.
- Should I ream the holes or press in bushings?
- If the plate will be re-pinned many times or is a soft material like aluminum, a hardened bushing in a reamed hole wears better. For a steel plate pinned once at assembly, a reamed hole is the normal answer. Either way, call it out.
- What does Amerigo do with the tolerance on my drawing?
- It reads it as drawn, lists it in the request's open questions when the drawing does not say press or slip, and never changes it. The tolerance the shop holds is the one on your drawing, confirmed on its quote.
About Amerigo. Amerigo is the sourcing network for American manufacturing. More about Amerigo