Drill, Ream, Bore or Helical-Mill a Tight Hole: What the Reported Tolerances, Finishes and Costs Actually Support

Rule out drill-only before you compare any quote

Start with the fit class on the drawing, not with the price. A drilled hole is reported at roughly ±0.1 mm, and one source puts the range at ±0.1–0.2 mm^16,19,23,24.

An H7 callout is far narrower than that: Ø20 H7 spans 20.000–20.021 mm, a 21 µm band, against a drilled hole at 100–200 µm¹⁶. That is roughly five to ten times looser than the band the drawing allows, computed from the cited figures.

So a hole that must finish at H7 cannot be finished by drilling alone, however rigid the machine. The reported values come from unknown-tier pages and are typical figures, not limits, but the gap is wide enough that the conclusion holds for the H7 case^16,19,23,24.

Treat drill-only as a roughing or pilot operation and budget the second operation into the quote from the start.

Reaming or boring: match the operation to the fit class

Reaming and boring both clear the H7 bar, so the choice between them turns on how tight the bore must be and how well it must sit on its datums. Reaming is reported at ±0.01–0.05 mm, which covers H7/H8, and it improves roundness and finish as it sizes the hole^16,24.

Boring is reported tighter at ±0.005–0.02 mm with higher concentricity to datums, and it costs more cycle time^16,24. One source states the rule plainly: reaming takes a hole to H7, boring reaches H6 or tighter¹⁹.

That is the line to apply to your drawing. If the callout is H7 and the hole is a dowel or bearing bore, reaming is the reported cost-effective route.

Reserve boring for the tighter class or where concentricity to a datum is the controlling requirement^16,24. Two practical points sit behind the table.

Reaming needs stock left for the reamer — one source puts the allowance at 2–4% of the hole diameter, so the pilot drill size has to be defined on the drawing or process sheet¹⁶.

And the reported ranges are typical values from unknown-tier pages, so ask the shop to confirm the specific band it will hold on your material and depth before you release the RFQ^16,24.

When the finish callout alone forces a second operation

Size tolerance is not the only thing that rules out drill-only. Reported Ra improves in a clear order: drilling at 3.2–6.3 µm, reaming at ≤1.6–0.8 µm, boring at ≤0.8 µm²⁴.

Reaming is also reported to improve finish to ≤1.6 µm¹⁶. That matters when the drawing carries a sealing or bearing surface.

A functional finish callout — one source cites Ra ≤0.8 µm for seals and bearings — can force a second operation even where the diameter tolerance is loose enough for a drill^{16 24}. Check the finish requirement against the drilling band before you accept a drill-only quote.

And note that no Ra value is reported for helical milling, so that route cannot be judged on finish from this evidence²⁴.

What the tolerance and finish are buying in cycle time and price

The cost ranking follows the tolerance ranking. Drilling is reported as very low cost and time, reaming low-to-medium, boring medium-to-high²⁴.

Reaming is described as more cost-effective than boring for most H7 bores, with boring carrying the higher cycle time¹⁶. The mechanism is not shop padding.

Tighter tolerances require slower speeds and more passes, and finer finishes require additional finishing operations, so the extra time is a direct consequence of the callout²². That is why the same hole can be quoted at very different prices depending on the fit class and Ra you specify.

Use that when you read a quote. If a shop prices an H7 bore at drill-only rates, the operation is missing.

If it prices a boring operation where reaming would hold the callout, you are paying for capability the drawing does not need^16,24. No numeric cycle times or tooling costs are published for these operations, so the ranking is the only comparison the evidence supports.

Checks to make before the RFQ is priced

The process name does not guarantee the result. Rigidity, runout control and matched cutting data are reported as prerequisites for holding hole tolerance and finish, so confirm them rather than assuming them¹².

• ✓Confirm the machine setup is rigid, with the workpiece in a stable fixture and rigid tool holders.
• ✓Ask how tool runout is controlled — precision collets and inspected holders are the reported method.
• ✓Confirm spindle speed and feed are matched to the material and the operation, not left at a default.
• ✓Confirm coolant or lubrication is applied, particularly for deep-hole drilling and reaming.
• ✓Ask the shop to state the diameter band it will hold on your material and depth, since the published ranges are typical values.
• ✓Confirm the reaming stock allowance and the pilot size are defined on the process sheet.

Material and geometry that add a hidden operation

The workpiece can change the plan before the quote is fixed. Stainless steel and titanium are reported to increase drill wander and heat, which pushes the shop toward carbide tooling, through-coolant and careful pilot sizes²⁴.

Aluminum and brass ream well, but too high a feed can bell-mouth the hole²⁴. Geometry matters too.

Hard alloys, difficult chip control, long overhangs and deep drilling are reported to raise machining difficulty and cost²⁷. If your hole is deep relative to its diameter or sits far from the fixture, expect the shop to add operations or time, and ask it to say which.

Agree the inspection method before the order

A hole is only proven to the callout if it is measured to the callout. Inspection for tight holes may require bore gauges, surface finish measurement, a CMM, first article inspection and final inspection records²⁹.

Hand tools that suit a general tolerance are not sufficient evidence for an H7 bore. Decide with the shop which of those apply to your feature and get it into the order.

For critical parts, buyers should confirm inspection requirements before placing an order, which may include dimensional reports, surface finish checks or first article inspection documentation²⁹. That is the record you will rely on if the bore is challenged later.

Where the sources disagree, and what to verify

The disagreement is in the width of the reported bands, not in the ranking. Drilling is consistently the loosest, reaming next, boring tightest^16,19,23,24.

The narrower figures come from pages that publish a single typical value, while the wider ones come from pages that publish a range. So neither is wrong — they are different ways of stating the same capability.

Treat every figure in this article as a reported typical value from unknown-tier pages, not as an industry limit or a mandatory grade choice^16,19,23,24. The practical consequence is that you should not accept a published range as the shop's commitment.

Ask for the band it will hold on your specific hole, in your material, at your depth, and put that number in the order.

Before you request a quote

• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Request the complete specification sheet with units, test conditions and configuration assumptions.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.