Custom machine parts — how to order a replacement when there is no documentation

The machine is standing idle, the manufacturer no longer exists or quotes a three-month lead time at a price close to half a new line. Having custom machine parts made is everyday reality in maintenance — the machining itself is rarely the problem; almost always it is the input data: where do you get dimensions, tolerances and material when there is no documentation.

In this post we break the topic down into three starting scenarios: you have a drawing, you have a worn part, or you have nothing but the machine. For each one we show what to prepare, where the traps are, and what the road from enquiry to a finished replacement looks like.

Custom machine parts: three starting scenarios

Everything depends on what is physically lying on the table when you write the enquiry. In practice there are three situations:

• there is a technical drawing (paper, a scan, sometimes the machine's technical manual),
• there is a worn or damaged part that can be measured,
• there is nothing left except the machine the part fell out of or seized in.

Each scenario means a different amount of preparatory work and a different risk. The less data, the larger the share of the cost that goes into measurement and documentation rather than the turning or milling itself.

Scenario 1: you have a drawing

The best case. Even an old paper drawing from the nineteen-eighties contains what no measurement will ever fully reconstruct: nominal dimensions, tolerances, fits and the material specified by the original designer.

What to do before sending the enquiry:

1. Scan the drawing legibly, including the title block and the notes in the margins.
2. Check that the drawing matches the version of the part actually installed in the machine — manufacturers introduced changes without updating the manual.
3. Add the material grade, or note that the supplier should select an equivalent.
4. State the quantity and whether the part is critical for keeping the plant running.

Watch out for units and standards: older drawings are often specified with steel grades no longer produced (e.g. steel 45 to the old Polish PN standard instead of C45). A good supplier will select a modern equivalent. We described the complete data set for an enquiry in the post on what files to send for a CNC quote.

Scenario 2: you have a worn part — measurement and reverse engineering

The most common case in maintenance. A worn shaft, bushing, chain sprocket or lever ends up on the measuring table and becomes the reference sample. The supplier measures the geometry, reconstructs the 3D model and the drawing, and only then does machining start. We described the whole process — from calipers to 3D scanning of complex shapes — in more detail in the post on reverse engineering and part reconstruction.

What to watch out for when reconstructing dimensions from a worn sample

This is where most mistakes are made. A worn part by definition no longer has its nominal dimensions — it has the dimensions left after a few thousand hours of operation.

• Wear allowance. A shaft journal that measures 39.85 mm today was most likely designed as 40h7. Copying the measured value perpetuates the wear in the new part.
• Fits. A diameter on its own says nothing — what matters is the pair: journal and bearing, bushing and pin. Always measure or provide the mating component, so that the fit is reconstructed rather than a single dimension.
• Unworn zones. The reference dimension is best taken where the part did not work: under a retaining ring, in a groove, on an unloaded end.
• Deformation. A part that failed may be bent or battered. Measuring ovality and runout before reconstructing the geometry is a duty, not an option.
• Material and heat treatment. A file and a spark test on a grinder give a rough idea of hardness, but for critical parts it is worth testing hardness and composition — or deliberately choosing a material better than the original.

A practical tip: do not clean or polish the worn part before sending it. Contact marks, discolouration and the wear pattern tell a machinist more than many a written description.

Scenario 3: only the machine is left

The part seized, shattered into small fragments, or the previous service crew took it with them. What remains is the machine and the seat the part worked in. This is the most labour-intensive scenario, but a feasible one.

The dimensions are then taken from the surroundings: bearing seat diameters, hole spacings, mating components that survived. The manufacturer's parts catalogues help (even without dimensions they show the geometry), as do photos of a twin machine in the plant or fragments of the destroyed part. In such cases a measurement visit or disassembly of the assembly is usually needed — work on the boundary between machining and maintenance service.

It is also worth asking whether to reconstruct the part one-to-one. Since new documentation is being created anyway, weak points can be improved: add lubrication, change the material, strengthen the cross-section that kept cracking.

Decision table: which scenario and what to prepare

Whatever the scenario, one rule applies: the more context (the part's function, operating conditions, what was wearing), the fewer assumptions on the supplier's side and the lower the risk that the replacement comes back for rework.

How the job proceeds step by step

Whatever the scenario, the process on the supplier's side looks similar. It is worth knowing, so you know what you are waiting for and what you can speed up on your side.

1. Enquiry with a complete data set: drawing, sample or measurements, material, quantity, the part's operating context.
2. Technical analysis: the process engineer checks feasibility, selects the material and a possible equivalent, and points out gaps in the data.
3. Quote with a lead time — with a complete enquiry, without extra rounds of questions.
4. Documentation rebuild: 3D model and manufacturing drawing; with a sample, corrected for wear and fits.
5. Machining: turning, milling, and where needed heat treatment and finish grinding.
6. Dimensional inspection of the key diameters and fits, then dispatch or collection.

From the customer's perspective the biggest time savings sit in steps 1 and 2: a complete enquiry can shorten the whole job by several days, because every follow-up question about material or a fit is another exchange of messages. It also pays to decide on the quantity straight away — making a second piece alongside the first costs a fraction of a separate order six months later, because the documentation and the machine setup already exist.

Make new or recondition?

Not every worn part needs to be made from scratch. A shaft with one worn journal is often cheaper to weld overlay and regrind, and a housing with an oval seat can be bored out and bushed. The break-even point depends on material cost, the number of operations and how long the machine can stand still. We take that calculation apart separately in the post on reconditioning or making a new part.

Rule of thumb: simple parts in structural steel are usually quicker and safer to make new; large parts, parts in expensive material or with long machining times more often pay off to recondition.

Summary

Having custom machine parts made is first and foremost a data problem, not a machining problem. With a drawing all you need is a good quote. With a worn sample the key is measurement with a wear correction and reconstructing the fits, not copying dimensions one-to-one. Without a sample the dimensions are taken from the machine — slower, but it works.

Have a part that needs to be made? Send a drawing, photos, or simply describe the situation via the contact form — we will prepare a quote within 48 hours and advise which route to take from sample to finished replacement.