The economics, plainly
A part machined from solid pays for programming, fixturing, the billet, every cutting operation and inspection, and on a small batch the fixed costs dominate the price, as our cost guide sets out. A catalogue part paid for all of that once, at volume, in a factory built for exactly that part. When it is nearly right, the gap is typically one or two operations: two tapped holes, a length cut, a keyway, a bore opened to a bearing fit.
So the comparison is not "machined part versus catalogue part". It is "full machining job versus stock part plus one operation". The second wins by a wide margin on most brackets, shafts, plates, spacers and fittings, and it also inherits the vendor's material documentation and repeatability: batch two starts from the same stock item, not from a re-read drawing.
What can be modified, and what should not be
| Modification | Typical parts | Watch for |
|---|---|---|
| Add or thread holes | Brackets, plates, angle, enclosures | Edge distance and wall thickness at the new hole |
| Cut to length | Shafts, linear rails, extrusions, tube, leadscrews | Hardened rails and screws need the right cutting method and end finishing |
| Flats, hexes, slots | Shafts, fittings, adjustment brackets | Plated or anodised surfaces are cut through and may need re-finishing |
| Counterbore, countersink, keyway | Plates, hubs, couplings, pulleys | Keyway depth against a hardened hub or thin wall |
| Bore, ream or turn to size | Housings, bushes, spacers, sleeves | Concentricity to the existing features; datum from the original part, not a drawing that never existed |
| Cross-drill | Pins, shafts, clevis parts | Breakout burrs on a hardened part |
| Finish after modification | Stainless (re-passivation), aluminium (anodising), plated steel | Modifying through a coating exposes bare metal; plan the finish, not just the cut |
Two categories deserve a straight no, or at least an engineer before a machinist. Safety-rated items, lifting components, pressure fittings, structural fasteners, carry a vendor rating that a modification can void; changing them needs engineering sign-off, not just capability. And where the vendor already sells a configurable version with your holes or your length, Misumi being the obvious example, buy that: it will be cheaper than modifying the plain one. A good supplier tells you both things at the estimate stage.
Vendor-sourced or free-issue: two ways to start
Vendor-sourced. You give the part number and the supplier buys the stock part. The advantage is one contract covering the part and the modification, with one party accountable for the whole thing. TrueNorth buys catalogue parts in its own name from RS Components, Farnell, Misumi, Accu, Bolt and Nut, Fabory, Cromwell, Swagelok, Parker and others, so the vendor's lead time becomes our problem to chase.
Free-issue. You already hold the parts, so you send them. This is normal UK practice and it works well with three habits: a packing note with quantities, the marked-up drawing in the box, and an agreed scrap allowance before any tricky operation on parts you cannot easily replace. Expect the supplier to check quantity and condition on receipt and to return unused spares. The clock on the modification starts when the parts arrive.
What to send with the request
- The vendor part number or listing URL, so the stock part is unambiguous.
- A marked-up drawing. The vendor's PDF with dimensions and positions marked on it is enough; a STEP file with the modified geometry is better. Our file guide covers formats.
- Tolerances where they matter. A bearing bore needs its fit stated; a clearance hole does not. The tolerance guide explains why the difference is money.
- Quantity and honest usage. The setup for the operation is paid once, so five parts cost little more than one, the point of our MOQ guide.
- What the part does. One sentence tells an engineer whether the modification is safe and which features are the real ones.
Who does this in the UK
Most machine shops will modify a part you bring them if you can find one with a slot for a small job, and you then manage the vendor order, the delivery, the inspection and the responsibility yourself. Instant-quote platforms generally cannot take it at all, because their pricing starts from a CAD file of a part they make from solid, not from a stock item they must buy first.
TrueNorth Engineering runs it as a managed supply service, the model of a manufacturing coordinator applied at its smallest scale: send the part number and a marked-up drawing to the modified standard parts page, get an estimate within 48 hours, and we buy the stock part or receive your free-issue, place the work in our vetted UK network, inspect it, and ship in 5 to 10 working days from the part arriving, with the vendor's certification and our inspection report. One contract, non-conforming parts replaced at our cost, and a straight answer when the right move is to buy the configurable version instead.