Metal buying guides / Fit and processing
Drilled, Tapped, or Countersunk? How to Specify Holes
A bolt size doesn't fully describe a hole. The hole may let the fastener pass through, provide threads for it, or make room for its head. Tell the shop what the connection needs and specify the hole feature accordingly.
Updated
Quick answer
State what each hole must do: let a bolt pass through, hold threads, or seat a fastener head. Include its size, location, depth where needed, and the face being worked. A bolt name alone does not define the hole.
For a plate with several holes, the request also needs their count and locations. A photo showing approximately where they go is useful context, but it doesn't establish the dimensions.
A through hole goes all the way through the part. A blind hole stops inside it. A tapped hole has internal threads for a screw or bolt. Usable thread depth is the length of complete thread the fastener can engage, which can be less than the total drilled depth.
Identify the hole type
On a small screen, scroll the table sideways to see every column.
| Feature | What it does | Information to include |
|---|---|---|
| Plain drilled hole | Creates an unthreaded opening | Diameter, location, through or depth |
| Tapped hole | Provides internal threads | Full thread callout, location, through or usable depth |
| Countersink | Forms a conical recess | Angle, diameter or defined seating requirement, side |
| Counterbore | Forms a cylindrical recess | Diameter, depth, side, and underlying hole |
A countersink and counterbore are different geometries. Use the correct term or send the fastener specification and a section sketch so the intended seat can be resolved.
Don't select a drill diameter from the bolt name alone
A clearance hole needs room for a fastener to pass. A tapped hole is prepared for a particular thread. Neither is fully described by “hole for a quarter-inch bolt.”
For a thread, include pitch or threads per inch as well as diameter. For example, 1/4-20 and 1/4-28 are different thread descriptions. If a fit class is specified, keep it in the callout. Don't add one arbitrarily because it makes the drawing look more complete.
For a blind threaded hole, distinguish usable thread depth from total hole depth. If you're unsure how to define the connection, identify the fastener and the required engagement for review by the designer or shop.
Locate the pattern from clear references
Choose labeled reference edges or a defined centerline. Give each hole center's location from those references. Mark the face being viewed, especially if the plate has a countersink on only one side.
An example request might say: “Four through holes, diameter and coordinates per drawing B; countersinks on face A only, fastener drawing attached.” The actual drawing still needs the missing dimensions. This wording is a structure for the request, not a hole design.
Include surrounding features
Show edges, bends, neighboring holes, and any hardware that could interfere. Features close to a bend may affect the forming plan; Protolabs' sheet-metal guidelines discuss nearby-hole deformation. Specific limits depend on the material and process.
Say whether the part is supplied bare or finished. If the hole or thread must meet a requirement after coating, make that condition explicit.
Request the complete operation
Rapid Metals lists drilling, tapping, and countersinking among its reviewed processing options. Other features also need scope confirmation.
Send the dimensioned drawing with the material, thickness, count, and required date. Compare the accepted quote against the whole feature description, not just the word “drill.”