Cross-Hole Deburring: Choosing a Brush That Removes the Burr Without Scratching the Bore
Industry guide · 7 min read · updated 2026-10-30
Where a cross-drilled hole breaks into a main bore — in hydraulic manifolds, valve bodies, fuel and oil galleries, crankshafts — drilling leaves a burr on the inside edge of the intersection. It is hidden, often several diameters deep, and it is exactly the kind of loose metal that later breaks off into a hydraulic circuit.
Machinists argue about the best fix because there isn't one answer for every part. This guide explains where brushes fit, how to pick between an abrasive nylon brush, a flexible hone and an end brush, and how to avoid the most common complaint: a clean intersection but a scratched bore.
Why cross-hole burrs are hard
- The burr sits inside the part, at the point where the second drill breaks through — you can't see it or reach it with a hand scraper.
- The intersection edge is a three-dimensional saddle, not a flat circle, so a countersink or chamfer tool only cuts part of it.
- Any tool that reaches the burr also touches the finished bore wall on the way in and out, which is where the scratching comes from.
Where brushes fit — and where they don't
Brushes are a finishing method for small, thin burrs and edge blending. The filaments flex into the intersection and wipe across the edge from many angles, so they deal with the saddle shape that rigid tools miss. They are not a stock-removal method: a heavy rolled burr, or a burr thicker than the filament can bend over, needs a cutting tool first (a back-deburring or cross-hole deburring cutter), then a brush to blend the edge.
For very high volumes of blind or deeply buried intersections, production lines also use non-mechanical processes such as thermal or electrochemical deburring. Brushing remains the most common choice where parts are run in moderate batches on the same machine that drilled them.
| Situation | Typical approach |
|---|---|
| Small, thin burr at an accessible intersection | Abrasive nylon bore brush or flexible hone, run through the main bore |
| Burr at the entry edge of the cross hole | Abrasive end brush or wheel brush on the outside edge |
| Heavy or rolled burr | Cut with a deburring tool first, then brush to blend |
| Bore finish must also be improved | Flexible hone, which also leaves a plateau finish on the wall |
| Deep, blind internal intersections in high volume | Consider thermal or electrochemical deburring |
Brush types for intersecting holes
- Abrasive nylon tube / bore brushes — twisted-in-wire brushes filled with grit-loaded nylon. Run through the main bore, the filaments flex into the intersection and cut the burr with the grit rather than with wire tips. Example: our spin-grit abrasive nylon tube brush (silicon carbide 320#, for tube IDs from 1/2 in to 1-1/4 in).
- Flexible hones — abrasive globules on flexible filaments. Self-centring in the bore; they break the intersection edge and finish the wall at the same time. Example: our aluminium oxide flexible hone brush (25 mm, 120 grit). See also flexible hone brushes.
- Abrasive end brushes — for burrs at the mouth of a cross hole on an outside face, where a straight-on brush can reach. Example: our silicon carbide abrasive end brush. More in end brushes.
- Wire brushes — cut faster but are the most likely to leave wire marks or a secondary burr on soft alloys; use them on steel parts where the bore finish is not critical.
Why abrasive nylon doesn't scratch the way wire does
A wire brush cuts with the hard tip of each wire. On a soft or finished bore those tips can leave visible marks, and on aluminium or brass they can roll a new, smaller burr. Abrasive nylon cuts with grit bonded through the filament; the nylon flexes away from the wall under load, so the cutting action concentrates on edges that stick out — the burr — rather than on the flat bore surface. That is the reason machinists choose it for deburring finished parts.
It is not zero-contact. Too coarse a grit, too much interference or too long a dwell will still dull or mark a fine bore. The process tips below are about controlling that.
Choosing the filament: silicon carbide, aluminium oxide or ceramic
"Ceramic vs abrasive nylon" is a common point of confusion: ceramic-grain filament is itself an abrasive nylon — the difference is the grain carried in the nylon, not the base material. Details in our guides to silicon carbide vs aluminium oxide filament and ceramic abrasive filament.
Grit follows the burr: a coarser grit removes a larger burr faster but leaves a rougher edge, a finer grit blends and finishes. For cross holes where the bore finish matters, start with the finest grit that still removes the burr in a reasonable cycle.
| Grain | Best for | Notes |
|---|---|---|
| Silicon carbide | Aluminium, brass, cast iron and general deburring | Harder, sharper grain; the most common abrasive filament |
| Aluminium oxide | Steel and stainless; finer finishing | Tougher grain that fractures less; gentler finish on steel |
| Ceramic grain | Hardened steels and tough alloys, heavier burrs | Fastest-cutting option; used where SiC or AO cut too slowly |
Sizing the brush to the bore
Bore brushes and hones are made slightly oversize to the bore so the filaments stay in contact as they pass the intersection. Too little oversize and the brush skips the burr; too much and it presses hard on the wall, which is when scratching and fast filament wear start. Each brush style has its own recommended oversize, so give us the bore diameter, the cross-hole diameter, the depth to the intersection and the material, and we will size the brush to it.
Reach matters as much as diameter: the brush section has to travel past the intersection, so the overall length and stem must suit the depth of the cross hole from the part face.
Process tips that prevent scratching
- Enter and leave the bore with the brush rotating; stopping the spindle inside the bore is a common cause of spiral marks.
- Keep the speed within the brush's rated limit and avoid excessive speed — more speed is not a faster cut on a small filament brush. See our RPM and surface speed guide.
- Use a short, consistent dwell at the intersection and a fixed number of passes, so every part gets the same treatment.
- Use coolant or honing oil where the maker recommends it — especially with flexible hones — to carry swarf away from the wall.
- Flush the part afterwards: loose burr fragments and abrasive dust must not stay in a hydraulic gallery.
- Check the first parts with a borescope or by sectioning a scrap part before settling the cycle.
What to send for a quotation
- Part material and hardness.
- Main bore diameter and depth; cross-hole diameter and its distance from the part face.
- Size of the burr (a photo of a sectioned part helps) and the required bore finish.
- Machine and holding: CNC spindle, drill or hand tool; shank or thread type.
- Expected batch size, so we can suggest a construction that suits the volume. Send it through Request a Quote.
Frequently asked questions
- What is the best brush for deburring cross holes?
- For small burrs at an intersection you can reach through the main bore, an abrasive nylon bore brush or a flexible hone is the usual choice: both flex into the saddle-shaped edge and cut with grit rather than wire tips. Heavy burrs need a cutting tool first, with a brush to blend afterwards.
- How do I deburr a cross hole without scratching the bore?
- Use abrasive nylon or a flexible hone instead of wire, choose the finest grit that still removes the burr, keep the brush only slightly oversize, enter and exit while it is rotating, and keep dwell time short and consistent.
- Is a ceramic brush different from an abrasive nylon brush?
- Ceramic-grain brushes are abrasive nylon brushes that carry ceramic grain instead of silicon carbide or aluminium oxide. Ceramic cuts fastest on hard alloys; silicon carbide is the general-purpose option; aluminium oxide gives a gentler finish on steel.
- Can a brush remove a large burr?
- Not efficiently. Brushes are for small burrs and edge blending. A large or rolled burr should be cut first with a deburring tool, then brushed.
- Can you make a brush for my specific bore and cross-hole size?
- Yes. Send the bore and cross-hole diameters, depth to the intersection, material and machine type, and we will quote a brush built to those dimensions.
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Need help choosing or a custom spec?
Send us your machine, drawing or requirement — we'll match the right brush and quote, usually within one business day.