Abnormal skills handling bore: double-sided CNC boring and crusher housings
This page is for process engineers and shop planners who bore through-holes from two sides, or who machine crusher housings where the bore must stay round and aligned. It covers the abnormal situations that show up on the floor, the measurements that tell you which one you have, and the tooling and fixturing choices that usually fix it.
What counts as an abnormal bore
A hole can measure in tolerance at one cross-section and still be wrong everywhere else.
First, decide what the bore has to be
Most arguments about a bad bore start because nobody agreed on the acceptance criteria. A Ø60 H7 bore on a crusher housing is not the same job as a Ø60 clearance hole that only has to pass a pin. Roundness, straightness, taper, surface finish and position all matter differently depending on the function. Write those limits down before the first cut.
On double-sided work the added constraint is alignment. Two bores entering from opposite faces have to meet inside the part, and the meeting point is the hardest place to inspect. A small angular error at each face becomes a step at the center. That step is what usually gets called an abnormal result, even when both ends pass their own diameter checks.
Before you touch the machine, measure the existing bore at three depths and two perpendicular directions. Six numbers tell you more than one. A round hole with a uniform taper has a different cause than a hole that is round at the top and oval at the bottom.
- 1RoundnessCheck at least three depths, not one.
- 2StraightnessA ball bar or a long mandrel shows the drift.
- 3TaperMeasure diameter at entry, middle and exit.
- 4PositionCompare to the datum called out on the drawing.
Reading the abnormal patterns
Chatter marks that repeat at a fixed spacing along the bore point to tool overhang or a weak boring bar. The spacing equals the feed per revolution, so a 0.15 mm/rev feed leaves marks every 0.15 mm. Shorten the bar, increase its diameter, or drop the speed until the marks disappear. Adding more coolant will not fix it.
A bore that comes out oval usually means the part moved. Thin-wall housings and split clamp bores deflect under chuck or vise pressure, spring back after the cut, and measure wrong. Rough the bore, release the clamp, let the part settle, then finish with light passes. On crusher housings with a split line, torque the bolts to the specified value before the finish cut and keep them torqued during inspection.
Taper that grows toward the exit is often a spindle or table alignment issue on double-sided machines. Check the machine first with a test bar before you blame the setup. A misalignment of 0.02 mm over 300 mm will show as a clear taper, and no amount of tool tuning will remove it.
Abnormal bore symptom to likely cause
Use this as a starting point, then confirm with measurement.
| Symptom | Likely cause | First check |
|---|---|---|
| Oval at one face | Clamping distortion | Release clamp and re-measure |
| Uniform taper | Spindle or table misalignment | Test bar on the machine |
| Step at the center | Angular error on double-sided setup | Face datum and re-zero |
| Repeat marks along bore | Tool overhang or weak bar | Shorten bar, lower speed |
| Oversize in one direction | Entering cut on a hard spot | Inspect raw material |
| Poor finish, size holds | Wrong insert radius or feed | Change insert geometry |
| Bore drifts off position | Fixture moved during cycle | Check clamps and stops |
Tooling and fixturing choices that hold the bore
A boring bar should be as short and as stiff as the geometry allows. The rule of thumb is an overhang-to-diameter ratio below 4:1 for steel bars. Beyond that, deflection grows quickly and the bar starts to sing. When the bore is deep, use a larger bar with a smaller insert, or switch to a damped bar if the budget allows.
For double-sided boring, a common mistake is to run both sides with the same tool and the same offsets. The two spindles are not identical. Measure each side separately and keep separate offset tables. A 0.01 mm difference in tool length between sides will show as a step at the meeting point.
Fixturing matters as much as the tool. A three-jaw chuck on a thin ring will distort it. Use a bored soft jaw set, or a fixture that clamps on a thick section away from the bore. For crusher housings, locate on the machined mounting face and two dowel holes, then clamp over the bore, not across it.
- 1Bar ratioKeep overhang under 4:1 where possible.
- 2Separate offsetsEach spindle gets its own tool table.
- 3Clamp locationClamp on thick sections, not over the bore.
Feeds, speeds and passes for a stable bore
Roughing removes most of the material, so run it hard enough to avoid work hardening, especially in stainless. Leave 0.3–0.5 mm on the diameter for finishing. That leaves enough material for the finish pass to cut cleanly but not so much that the bar deflects.
Finishing passes should be light and consistent. A depth of cut around 0.1–0.15 mm on the radius works for most steels and aluminium. Too light a cut rubs instead of cutting, which raises surface finish values. Too heavy a cut pushes the bar off line. Feed rates of 0.08–0.15 mm/rev are typical for a boring bar in steel.
Coolant should reach the cutting edge, not just the bore entry. Through-tool coolant is the most reliable option for deep bores. On aluminium, high-pressure coolant also helps clear chips that would otherwise recut and scratch the wall. For cast iron, dry cutting with air blast often gives a better finish than flood coolant.
Common questions about abnormal bores
Why does my bore measure good at the ends but bad in the middle?
This usually points to bar deflection or a chip packed in the bore during the cut. The bar bends most at mid-span, so the middle comes out undersize or oval. Check chip evacuation and reduce overhang.
On double-sided work, a step at the middle can also come from the two sides not meeting on the same axis. Verify both spindle alignments with a test bar.
Can I fix an out-of-round bore by taking another pass?
Only if the error is caused by clamping and the part springs back after release. In that case, re-clamp lightly and take a light finish pass. If the error is in the machine or the bar, another pass will copy the same error.
Measure the bore after the part is free. If it is round when free but oval when clamped, the fixture is the problem, not the cut.
What surface finish can we hold on a bored crusher housing?
A conventional bored finish lands around Ra 1.6–3.2 μm. With a sharp insert, a rigid bar and a controlled finish pass, Ra 0.8–1.6 μm is realistic on steel and aluminium.
Going below Ra 0.8 μm on a bore usually means grinding or fine boring with a dedicated head. Tell us the finish callout and we will quote the process that reaches it.
How do you inspect a bore that is hard to reach?
We use bore gauges, internal micrometers and a bore scope where needed. For tight-tolerance work, a coordinate measuring machine with a stylus long enough to reach the middle gives the full profile.
On double-sided bores, we check both ends and the meeting region. Reports can be included with the shipment on request.
Does material choice affect how the bore behaves?
Yes. Stainless grades like 316L work harden if the tool rubs, so a positive rake insert and a firm feed help. Aluminium 6061 cuts cleanly but can gall if chips are not cleared. Cast iron is stable but abrasive on inserts.
We machine aluminium, stainless, steel, titanium, copper alloys and plastics. The cutting data changes with each, and we adjust the process accordingly.
What tolerance can you hold on a bored hole?
Our general machining tolerance is ±0.005 mm on features that are set up for it. A deep bore with high overhang may need a wider band, and we will tell you when that is the case.
We inspect 100% before shipment and can provide reports. If a bore cannot be held to the drawing, we will flag it during DFM review rather than after the cut.
Send us the bore problem
Upload the drawing and tell us where the bore goes wrong. You get a quote and DFM feedback within 12 hours.
12-hour quote100% inspectionNDA on request
