Can't I just machine a standard finished bush to a custom size?

No. Cutting the bore of a finished sintered bush smears metal over the pores and seals in the oil, destroying its self-lubricating action. Start from a blank, which is designed to be machined and then re-impregnated.

Which oil should I use to re-impregnate a machined blank?

Match the matrix: TURBO T100 for BP25 bronze and FP20 iron, and METADOP for SO16 alloy. Use PE1152 for food-contact parts, or the PE1116 high-temperature grade (about −40 °C to +150 °C) for hot service.

Do I need special coolant when machining blanks?

Prefer to machine dry or with minimal fluid. Flooding with water-based coolant fills the pores, displaces the impregnated oil and can rust FP20 iron. The oil already in the blank lubricates the cut.

Hollow or solid blank for a bush?

Use a hollow blank when the finished bore is large — it saves boring time and material. Use a solid blank for thrust washers, discs and small bushes, or wherever you machine from the centre outward.

Machining Self-Lubricating Bearing Blanks (Hollow & Solid)

The standard Bushingo catalogue already lists several hundred sintered bush sizes, but real machines occasionally need a bore, length or outside diameter that no standard part matches — a long liner, an odd wall thickness, a thrust washer, or a self-lubricating disc. That is what bearing blanks are for. A blank is an oversized, fully sintered and oil-impregnated slug of BP25 bronze, FP20 iron or SO16 alloy that you machine to your own geometry while keeping the self-lubricating porosity right through the section.

Hollow versus solid blanks

Hollow blanks are thick-walled tubes intended for bushes, sleeves and liners. Bushingo lists five hollow sizes in each material, all 120 mm long: Ø Int. 38 – Ø Ext. 70, Ø Int. 45 – Ø Ext. 105, Ø Int. 80 – Ø Ext. 145, Ø Int. 80 – Ø Ext. 175 and Ø Int. 85 – Ø Ext. 105. Starting from a pre-bored blank saves material and boring time when the finished bore is large.

Solid blanks are plain cylinders for parts that start from the centre or have no through-bore — thrust washers, wear pads, cam followers and short bushes. Seven solid sizes are offered per material: Ø 20 – Long. 40, Ø 30 – Long. 50, Ø 45 – Long. 90, Ø 54 – Long. 110, Ø 70 – Long. 120, Ø 105 – Long. 120 and Ø 145 – Long. 120. Choose the nearest size above your finished dimensions so you can clean up every face.

Why blanks are machinable but finished bushes are not

This is the point that catches newcomers. A finished sintered bush should never have its bore cut, because a cutting tool smears metal across the pore openings and seals in the oil, destroying the very porosity that makes the bush self-lubricating. Blanks are different: you machine them to size and then restore the surface and the oil charge afterwards. The porosity runs through the bulk of the material, so a freshly cut face is still porous — provided you cut it correctly.

Cutting rules for a porous matrix

Keep tools sharp. Use positive-rake, keenly honed carbide or HSS. A dull edge burnishes rather than cuts and closes the surface pores — exactly what you are trying to avoid.
Take light feeds and depths of cut. Fine finishing passes shear cleanly; heavy cuts tear the sintered grains and smear the surface.
Machine dry, or with minimal fluid. The impregnated oil already lubricates the cut. Avoid flooding with water-based coolant — it is drawn into the pores, displaces the lubricating oil and, on FP20 iron, invites corrosion.
Favour speed over pressure. Let surface speed do the work so the tool does not push and burnish the bore.
Deburr gently. Avoid heavy abrasives or wire wheels that rub the pores shut.

Re-impregnate after machining

Machining removes the oil-rich surface layer and leaves swarf and heat behind, so a machined blank must be re-charged with oil before service. Degrease the part, then re-impregnate — the reliable method is a hot-oil soak under vacuum, so trapped air leaves the pores and oil is drawn in as the part cools. Match the oil to the matrix: TURBO T100 for BP25 bronze and FP20 iron, METADOP for SO16 alloy. Where the part contacts food or potable water, use the food-grade PE1152 charge; for temperature extremes, the PE1116 high-temperature grade covers roughly −40 °C to +150 °C.

Final sizing

For a bushing bore, finish to size with a sizing (burnishing) pin rather than a final cutting pass. A hardened ball or plug pushed through the bore cold-works it to a precise, smooth and still-porous surface, leaving the ideal running finish. Remember too that a machined bush bore will close slightly when the part is later pressed into its housing, so size it after fitting or allow for the close-in.