Guides / Process

The Burr Inside the Bore Nobody Can See

September 21, 2026

A brass fitting passes inspection at the shop. It assembles fine. Then it fails a leak test on the customer’s line, or worse, it passes the leak test and sheds a chip of brass into a valve seat six months later.

The usual cause is a burr in a place nobody could see.

Why brass burrs at all

Free-cutting brass machines better than almost anything. Machinability 100, short chips, bright finish. So the burr question feels like it should not apply.

It applies at one specific moment: when a tool exits material.

At the instant a drill breaks through the far wall of a bore, there is nothing behind the material to support it. Instead of shearing cleanly, a lip of metal folds over and stays attached. The better the alloy, the smaller that lip, but it does not go to zero.

On an outside edge this is trivial. A file, a tumble or a chamfering tool removes it in seconds.

Inside a bore, at the intersection with a cross hole, it is a different problem entirely.

Why the internal one is hard

Three things make the cross hole burr the expensive one.

Nobody can see it. It sits inside a bore that may be four millimetres across and thirty long. Without a borescope it is invisible, and borescoping every part at volume is not realistic.

Few tools can reach it. A file does not go round a corner. Standard deburring tools work on accessible edges. Getting a tool to the intersection of two crossing bores is a specialist job.

It is exactly where the fluid goes. The burr sits in the flow path, which is why it eventually ends up in a seat, a seal or an orifice downstream.

This is the single most common reason a machined brass fitting passes at the supplier and fails at the customer.

How shops actually remove it

Designed-in chamfers. The cheapest solution by a wide margin. If the cross hole breaks into a chamfered feature rather than a flat wall, there is far less burr to begin with. Free if it is on the drawing, impossible if it is not.

Brush deburring. Abrasive nylon or wire brushes driven through the bore. Works on accessible straight bores, less effective at a deep intersection.

Mechanical deburring tools. Spring loaded or pivoting cutters that pass through a bore and cut back on the return stroke. Effective, and they need a tool path designed for each intersection, so they add cycle time.

Abrasive flow machining. An abrasive laden putty is extruded through the part under pressure, polishing every internal passage it touches. Excellent on complex internal geometry. Specialist equipment, so a shorter supplier list.

Thermal deburring. The part sits in a sealed chamber with a fuel gas mixture which is ignited. The burrs, having a high surface area to mass ratio, burn away while the part does not. Removes every burr everywhere at once, including ones you did not know about. Batch process, so cheap at volume and expensive in small runs, and again few shops own the equipment.

High pressure water jet deburring. Fixtured nozzles direct water at specific intersections. Common in production of hydraulic and pneumatic components.

What this means for your drawing

The single most valuable line you can add to a brass fitting drawing is an explicit internal burr condition.

Missing, which is the common case:

Deburr all edges

A shop reads that as external edges, quotes a tumble, and is not wrong to do so.

Explicit:

All internal cross hole intersections to be free of
burrs and loose particles. No raised material permitted
in the flow path. Verify by borescope on first article
and on 3 parts per lot.

Now the shop quotes a deburring operation, because you asked for one. The quote goes up. The field failures go away. That is a trade most buyers would take knowingly and almost none get offered.

Design changes that remove the operation entirely

Worth checking before you pay for deburring.

Break the cross hole into a chamfer or counterbore rather than a flat bore wall. Far less burr forms.

Add a relief groove at the intersection so the exiting tool has somewhere to go.

Cross at a shallower angle where the function allows. A perpendicular breakthrough creates the worst burr.

Drill from the inside out where geometry allows, so the burr forms on an accessible face.

Reconsider the hole order. Drilling the cross hole before the main bore sometimes means the main bore cuts the burr away as it passes.

Any of those can be free. All of them are cheaper than an added operation, and much cheaper than a recall.

Where it bites hardest

Parts where this is worth real attention:

• Hose barbs and adapters with a cross drilled port
• Valve bodies with crossing passages
• Manifold blocks
• Instrumentation fittings and ferrule bodies
• Anything that will be leak tested after assembly
• Anything feeding a downstream orifice, seat or seal

If your part is on that list and your drawing does not mention internal burrs, that is the gap. See also thread standards for brass fittings, because the cross hole burr usually lives inside a threaded bore.

Have a fitting with crossing passages? Send the drawing and state the internal burr requirement in the part description. We match it to shops that have the process for it rather than shops that will discover the problem on your first article.

Frequently asked questions

Keep reading

• Brass CNC Machining: Why It Is Cheap, and Where the Cost Hides
• Swiss Turning Brass Parts and When It Is the Cheaper Route