How Do You Remove Cross-Hole Burrs in a Hydraulic Manifold Block?

Why do burrs form where drilled passages intersect?

A drill that breaks through into an open bore pushes the last of the metal outward instead of cutting it away, and that leaves a ridge at the intersection the tool cannot see or reach again.

A manifold block is mostly cross-drilled passages, and every place two passages meet is a breakthrough. One deburring supplier's 2026 article notes that burrs form on edges, holes, ports and internal features during CNC machining, drilling and casting, and that conventional methods struggle with intersecting holes and internal ports. In its words, even specialized machining services cannot guarantee effective deburring in all internal features.

The size is small but not trivial. In a Cutting Tool Engineering case study of ductile iron pump manifolds, the burrs were 0.2 to 0.3 mm high across holes of about 5 to 38 mm in diameter. Burrs of that height can still foul a spool, lodge under a seat or sit loose until flow carries them off.

The reason for caring is functional. The same supplier article says loose burrs can circulate in the oil, clog filters early, make valves stick or fail and cause internal pump wear, which in heavy equipment shows up as leakage, pressure instability and downtime. It gives no failure-rate figures, so the argument is one of mechanism, not statistics, but the mechanism is easy to follow and it is why the requirement belongs on the drawing.

What should the drawing say about internal burrs?

Say that all internal intersections must be free of burrs and loose particles, state the edge condition you want, leave the method open unless you need one, and name how the result is verified.

A note that reads only "deburr all edges" is read by most machinists as the outside edges. Make the internal requirement explicit and measurable. A workable note names four things: the surfaces it covers (all drilled passages and their intersections), the state required (no raised or loose burrs, and an edge break within a stated range), the method freedom (any method that meets the requirement, unless a method is part of your qualified process), and the proof (see below).

An edge-condition standard such as ISO 13715 gives a common way to indicate edges of undefined shape on a drawing, so the callout means the same thing to you and to the supplier. Pair it with a cleanliness requirement written to the standard your customer already uses, because a block can be burr-free by eye and still carry particles. The guide on technical cleanliness for machined parts covers how those limits are written.

Design helps more than inspection. Where the layout allows, keep drill exits into open bores on a short, accessible path, avoid exits into thin walls, and tell the supplier which intersections are critical, such as those next to a valve seat. A short drawing note on the critical few is better than demanding the same result everywhere.

How much can thermal deburring change the part and the oil path?

It can change dimensions and leaves oxide powder behind, so a block that is thermally deburred needs a washing step and a post-process dimensional check.

The Cutting Tool Engineering case study is candid about this. A pre-study showed no dimensional change, but dimensional changes appeared once the process was in production, and the user adjusted the thermal process to offset them. Burnt-off burrs also form an oxide powder that sits as a loose dry film on the surface, looks like rust and has to be removed. The usual removal is a phosphoric acid wash, which the user avoided as hazardous and incompatible with its wastewater treatment, so it used a neutral cleaning product instead.

The supplier article describes the same method as burning off low-mass burrs in a sealed chamber while the base metal is largely unaffected, and recommends pairing it with post-process cleaning, filtration and inspection. The lesson for a drawing is to treat the sequence as one process: deburr, wash, then measure the features that matter, such as port threads, sealing faces and bores that take a cartridge valve.

Thermal, electrochemical and other batch deburring methods are outside the CNC cell. Saola arranges them through partner plants it manages when a drawing calls for them, and the production location is stated on each quote.

What should an overseas machinist prove before you accept a block?

Ask for borescope or equivalent images of the critical intersections, a cleanliness report against your stated limit, and a first-article dimensional check taken after deburring.

The point of asking for proof is that nobody can see inside the block from the outside. Agree these items before the first order, not after the first complaint:

• Images of the critical intersections taken with a borescope or similar probe, on the first articles and then on a sampling basis.
• A cleanliness result against the limit and method on your drawing, with the wash and flush procedure the supplier used.
• Dimensional results taken after the deburring step, not before, for the features that deburring can touch.
• A statement of the deburring method and of any change to it, so a process change is reviewed rather than discovered.
• Packaging that keeps ports capped and the block clean after cleaning, which is as much a part of the requirement as the wash.

Block drilling and milling run as CNC milling at the lead plant in Vietnam. Send the drawing with the cleanliness limit and the intersections you consider critical, and the quote will say what can be shown and what has to be arranged outside the CNC cell. The generic methods are compared in the guide on deburring methods for machined parts.