A machinist needed a 5/8 in hole in a part that would later hold a boring bar, so it had to sit on center. Gun drilling would have meant remaking the parts, which tapered too thin at one end. The advice he got on a CAM forum: spot drill, drill about 0.015 in under size, bore the first quarter inch to 0.001 in under size, then ream.
Why bore before reaming? Because a reamer follows whatever hole it’s given. Ream when the hole is already in the right place and straight, and what’s left is size and finish. Bore when position or straightness still needs fixing. Do both when you need both.
Reaming and boring compared
| Reaming | Boring (single-point) | |
|---|---|---|
| Fixes position and straightness | No, it follows the existing hole | Yes, within the rigidity of the setup |
| Size and finish | H7 is a common catalog grade | Set by adjusting the head |
| Stock it needs | About 2–5% of diameter | Can take uneven stock |
| Speed once set up | Fast: many edges | Slower: one edge |
| What it asks of the machine | Low runout, no spindle play, rigid clamping | Rigid spindle, short overhang, accurate positioning |
Bore, then ream combines the two: the boring pass sets the axis and leaves a small, even stock, and the reamer finishes to size, at the cost of one more operation.
1. Position and straightness: a reamer can’t fix them
A reamer cuts mainly with the short chamfer at its tip. The body behind it doesn’t cut; it guides, using the hole it has just made as a bushing. Writing in Cutting Tool Engineering, Erez Speiser puts it bluntly: reamers follow the existing hole path, and if the pre-drilled hole is misaligned they copy the mistake.
A boring bar has one edge and nothing riding on the hole wall, so it cuts where the spindle takes it. Bored to clean-up, a drifted hole comes out on the axis the program asks for. That’s why the forum advice bored the entry first, or on a mill, interpolated it with an end mill: the reamer then starts in a hole that’s already on location.
The catch is that your bore is only as straight as the machine and the setup. Its accuracy comes from the axis positioning and from how little the bar deflects, not from the hole.
2. Size, roundness and finish: where reaming earns its place
Give a reamer a straight, well-placed hole and it sizes it fast and consistently. Sandvik Coromant lists H7 as the hole tolerance for its CoroReamer 435. On a 10 mm hole, H7 is a 0.015 mm band under ISO 286.
The stock you leave decides whether you get there. Speiser’s rule of thumb is 2–5% of the diameter, around 3%: 0.2 to 0.5 mm on a 10 mm hole. The forum’s 0.015 in on a 5/8 in hole works out to 2.4%. Sandvik warns that too little stock is the common mistake: the reamer rubs instead of cutting, wears quickly and loses diameter. Too much overloads the cutting chamfer.
3. Speed and setup time
Once it’s dialled in, a reamer is hard to beat. BIG DAISHOWA says certain reamers offer productivity a single-point boring tool simply can’t match. It also says the dial-in is what turns most reaming jobs back into boring jobs: time saved in the cut goes on setting the reamer in a floating holder or adjusting guide pads, and the semi-finishing tool before the reamer often ends up being a boring tool anyway.
Cost works the same way. A reamer is a fixed-size, perishable tool; a boring head takes indexable inserts and adjusts across a range of diameters. For many sizes in small batches, boring usually wins. For one size in volume on a stable pre-hole, reaming usually does.
4. What each process asks of the machine
Reaming is sensitive to anything that moves the tool off the hole axis. Sandvik’s checklist includes a spindle with no play, a short tool overhang and a rigid workpiece. Its reaming troubleshooting guide traces tapered bores to spindle or tool runout and a pre-drilled hole out of line with the reamer, and oval bores to a workpiece that isn’t stable in the fixture.
Boring asks for stiffness and positioning. A boring bar bends like a cantilever, so its deflection climbs roughly with the cube of its overhang. A short, stiff assembly and a rigid spindle matter most, and the hole’s location is only as good as the axis that put it there. For long bars in steel, the taper matters too; our comparison of BT40 and BT50 goes into that.
Which one does your hole need?
Answer these three questions about the hole on your drawing:
- Is the pre-hole already in position and straight? If yes and only size and finish remain, ream.
- Does the drawing control position, straightness or alignment with another bore tighter than your drilling holds? Then bore: the entry if the hole just needs to start on location, the full depth if straightness matters.
- Do you need H7-class size on a hole that also has to be on location? Bore to leave a small, even stock, then ream.
How we’d set up a machine for precision holes
For aluminium valve bodies, housings and plates with reamed holes, a V-series machining center with three linear guideways is the usual choice. For boring in steel and cast iron, the L series, with linear guides on X and Y and a box way on Z, gives the stiffness a boring bar needs, and the larger L-series models take BT50 for long bars.
Before any machine ships, its X, Y and Z axes are checked and compensated over full travel with a Renishaw laser interferometer. That gives the V and L series a positioning accuracy of ±0.005 mm per 300 mm and repeatability of ±0.003 mm per 300 mm, which is what the location of your bored holes depends on.
The spindle is checked too. A 300 mm test bar goes in the spindle, and runout at its far end must stay within 0.008 mm. That’s the same 300 mm limit set by a draft Chinese specification for high-precision vertical machining centers.
- Through-spindle coolant, to flush chips out of blind holes ahead of the reamer, available as an option
- A workpiece probe, to measure the bore in the machine before the part comes off, available as an option
- BT40 or BT50 spindle, matched to the boring bars and reamer holders you plan to run
Reamers and boring heads are your choice; we’ll match the spindle and taper to them. See the full range on our machining center page.
Send us the part drawing with the hole tolerances, material and batch size through our inquiry form. We’ll recommend the machine and configuration, estimate the cycle time for the hole sequence and quote it.
FAQ
What tolerance can a reamer hold? H7 is a common grade in reamer catalogs, for example Sandvik’s CoroReamer 435. That’s a 0.015 mm band on a 10 mm hole.
What is lobing in a reamed hole? It’s a hole that’s out of round in a regular pattern of lobes. A reamer with evenly spaced teeth can whirl in the hole and leave N+1 or N−1 lobes, where N is the number of teeth, as one reamer patent describes. Reamers with unequally spaced edges are designed to break up that whirl.
Is boring more accurate than reaming? It depends on which error you mean. Boring controls position and straightness; reaming gives tighter, more repeatable size and finish on a hole that’s already straight. That’s why precision bores are often bored, then reamed.
Sources
- Erez Speiser, Reaming Basics: Drilling Performance, Cutting Tool Engineering, February 2026
- Sandvik Coromant, CoroReamer 435, How to use CoroReamer 435 and Troubleshooting for reaming
- BIG DAISHOWA, FAQs: Setting Up Boring Tools
- US Patent 6,379,090 B1, Force balanced irregular pitch reamer (background on lobing)
- ISO 286-2, tolerance grades for holes (H7 values)
- China Machinery Industry Standardization Technology Association, T/CAMS Technical specification for inspection and acceptance of high-precision vertical machining centers (draft for comment), test G11
- CHANSIN CNC V- and L-series specifications (guideways, spindle tapers, accuracy) and factory inspection
