Spare Part Without a Drawing: What Your Supplier Needs to Reproduce It
When a part with no drawing fails, what decides whether the copy works is not the shop's machine: it is what you hand over. This guide covers what to send and in what order, which dimensions really matter, and when the part should not be reproduced as-is.
A part fails, the line stops and nobody can find the drawing. The equipment maker no longer exists or takes weeks to answer, the part number is in no catalog, and all there is is the broken part in your hand. At that moment the first reaction is to send a photo to a shop and ask how much it costs to make another one.
That is where the problem starts. The new part can come out well machined and still not fit, or fit and last half as long. It is almost never the shop's machine: the shop did not get what it needed to know what it was copying. This guide is for the maintenance lead who has to order the spare part: what to hand the supplier, in what order, which dimensions really matter, and when the part should not be reproduced as-is. How reverse engineering is done on the shop side is covered in another article; this is the side of the person ordering the part.
In Summary
- What decides whether the copy works is what you hand over, not the supplier's machine.
- Hand it over in this order: the part or its pieces, scaled photos, the material, the function of each surface, and the machine context.
- Reproducing without a drawing is not copying: it is deciding which dimensions are functional and measuring them well; the rest are manufacturing dimensions.
- If the part failed by design, do not copy it as-is: fix material, treatment or geometry first.
- At Radii, spare parts without drawings are part of custom tooling and MRO for plants: send the part and what you know about it, and get a quote with inspection included.
1. Why a photo is not enough
A photo helps the supplier understand what kind of part you mean and give you a rough cost. It does not help manufacture it. What a photo does not give:
- Fit diameters. A shaft pressed into a bearing and one that rotates freely look identical in a photo.
- Threads. Pitch, diameter and type (metric, UNC, UNF, BSP) cannot be guessed by eye.
- Flatness, perpendicularity and concentricity. They are what make the part seat and rotate properly.
- Size. Without a scale reference in the frame, not even the overall dimension is reliable.
- Wear. The broken part does not have its original dimensions: it lost material exactly where it worked.
With only a photo, the supplier has two options: quote on assumptions that are expensive to correct later, or ask you for everything that follows. Better to send it from the start.
2. What to hand over, and in what order
The order matters because each item tells the supplier how to read the next.
1. The physical part, or all of its pieces
It is the source of truth. If it broke into pieces, send all of them, including the small ones: the fracture surface tells how it failed, and a fragment that looks useless may be the only place where a dimension survived without wear.
- Do not sand it or "fix" it before sending it. Light cleaning to remove grease is fine; removing material erases information.
- If there is a twin part without wear on another machine, another station or in the storeroom, it is a better reference than the broken one. Send it or measure it.
- If the part cannot leave the plant, item 2 becomes mandatory and the supplier will have to measure on site or from what you send.
2. Scaled photos
Of the part and, above all, of where it mounts:
- The part alone, from several angles, with a ruler or caliper visible in the frame.
- The part in place (or the space where it went), showing which other parts it contacts.
- Close-ups of threads, keyways, holes and any engraved marks.
- The failure area, up close.
3. The material, or what is known about it
- Part number and equipment manual. The material is often in the spare parts list or an assembly drawing.
- Marks on the part: engraved numbers, colors, casting stamps.
- Behavior: whether it is magnetic, whether it is very hard under a file, whether it has a visible coating (chrome, black oxide, anodizing).
- If none of that exists, the material is identified in a lab. The ASTM E1476 guide describes the methods to identify and verify alloys, from chemical analysis to nondestructive methods. Hardness is measured separately: the same steel behaves very differently depending on its heat treatment.
Tell the supplier how much of this is certain and how much is a guess. "We think it is 4140" and "the manual says 4140" lead to different decisions.
4. The function of each surface
It is what helps the most and what almost nobody sends. Mark on the photos (or a hand sketch) what each area does:
| Surface | Example of what to say |
|---|---|
| Fits with another part | "This diameter goes into a bearing", "this hole takes a dowel pin" |
| Rotates or slides | "It rotates inside a bronze bushing", "it slides on a guide" |
| Seals | "An o-ring goes here", "this face seals against a gasket" |
| Seats or locates | "This face sits against the base", "these two holes line up with the plate below" |
| Takes load or impact | "The cylinder strikes here every cycle" |
| No function | "This face touches nothing" |
With this table the supplier knows where to spend precision and where not to. Without it, they have to measure everything with the same care or guess.
5. The machine context
- Make, model and year of the equipment, even if the maker no longer exists.
- What part it mates with, and whether that mating part is also worn.
- Operating conditions: temperature, coolant, dust, cycles per hour, loads.
- Why it failed and how long it lasted. This is the information that decides section 5.
- Whether someone already modified it in the plant. A part that maintenance reworked years ago is no longer the original.
3. Functional dimensions versus manufacturing dimensions
This is the idea that separates a copy that works from one that does not: reproducing without a drawing is not copying every measurement, it is deciding which measurements matter.
- Functional dimensions: the ones that define how the part mounts and works. Fit diameters, hole spacings that line up with another part, threads, seating and sealing surfaces, perpendicularity of a seating face. If one of these is wrong, the part does not fit or it fails.
- Manufacturing dimensions: everything else. Corner radii that touch nothing, thicknesses in unloaded areas, finishing chamfers. The shop can adjust them to its process without affecting function.
Why the distinction matters:
- The broken part lies about the functional ones. The areas that work are exactly the ones that wore. Measuring a worn shaft diameter and copying it gives a shaft with play. The functional dimension is rebuilt from the mating part (the bearing, the bushing, the bore it goes into), not from the damaged part.
- Fits are defined, not measured. If a diameter goes into a bearing, what has to be specified is the fit class per ISO 286 (and the tolerance it implies), not the micrometer reading on a worn part. The site's ISO 286 fits calculator helps translate the class into values.
- Precision costs money. Calling every dimension at the tolerance of the functional ones makes the part more expensive without making it work better.
The result of this step should be a drawing: functional dimensions with their tolerance and datums, manufacturing dimensions with a general tolerance, material and treatment. For geometric tolerances (flatness, position, concentricity), the common language is ASME Y14.5. That drawing is the real deliverable: the next time the part fails, nothing has to be repeated. How measurement and modeling are done on the shop side is in the guide to custom parts for discontinued machinery without the original drawing.
4. What you ask the supplier for
Handing over the information well is half of it. The other half is asking for the right things back:
| Ask for | Why |
|---|---|
| The drawing they created, with the functional dimensions marked | It lets you order the next part from any supplier, not just this one |
| What they assumed and why | If they rebuilt a dimension from the mating part or assumed a material, you need to know |
| Dimensional report of the delivered part | Confirms the functional dimensions came out within tolerance |
| Material certificate, and heat-treatment certificate if applicable | Confirms that what was ordered is what arrived |
| First article before the batch if you will order several | A trial in the machine is worth more than any measurement |
A supplier who cannot give you the drawing or explain their assumptions leaves you where you started: depending on a physical part for next time. It is one of the filters for qualifying a tooling and MRO supplier, and on a part without a drawing it weighs more than on any other order.
5. When NOT to reproduce the part as-is
The question almost nobody asks: why did it fail? If the answer is "by design," copying the exact part is manufacturing the next failure.
| Signal | What it means | What to do |
|---|---|---|
| Failed from wear after a long time | The part reached the end of its life | Reproduce as-is, restoring the functional dimensions |
| Always breaks at the same spot | Stress concentration or missing material there | Fix the geometry (radius, thickness) before manufacturing |
| Now lasts much less than before | The material of replacements changed, or the machine's conditions changed | Review material and treatment; ask what changed in the process |
| Maintenance already modified it in the plant | The "original" is no longer the original | Find out what was changed and why before copying it |
| The mating part is also damaged | The failure may come from the other part | Inspect and, if needed, replace both |
| Fails from corrosion or temperature | The material does not match the conditions | Change material or coating |
In any of these cases, tell the supplier from the start. A good supplier does not just copy: they tell you when an exact copy is not a good idea and what they would change. And if you change something, put it in the new drawing and in the machine history, so the next person does not "correct" it back to the version that failed.
Frequently Asked Questions
What do I need to send a supplier to reproduce a spare part without a drawing?
The physical part or all of its pieces, scaled photos of the part and of where it mounts, whatever you know about the material, the function of each surface (what it seats on, what rotates, what seals) and the machine context: make, model, what it mates with and why it failed. With that, the supplier can separate the dimensions that matter from the ones that do not and quote without guessing.
Are photos of the broken part enough?
For a first cost estimate, sometimes. For manufacturing, almost never. A photo does not give fit diameters, threads or flatness, and without a scale it does not even give size. If the part cannot leave the plant, the minimum is to measure the interface dimensions with instruments and send photos with a ruler or caliper in the frame.
What are functional dimensions and why do they matter more than the rest?
They are the ones that define how the part mounts and works: fit diameters with the mating part, hole spacings that line up with another part, threads, seating and sealing surfaces. The rest are manufacturing dimensions: the shop can adjust them without affecting function. Reproducing well means measuring the functional ones precisely and not spending on the rest.
How do I find out what material the original part is?
First, with whatever exists: part number, equipment manual, engraved marks, the original supplier. If there is nothing, a lab can identify the alloy with chemical analysis or with nondestructive methods like those described in the ASTM E1476 guide. Hardness is measured separately, because the same steel behaves differently depending on its heat treatment.
When should the part not be reproduced as-is?
When it failed by design rather than wear: if it always breaks at the same spot, if it lasts much less than it used to, if someone already modified it in the plant, or if the machine's operating conditions changed. Copying it as-is repeats the failure. In those cases, fix the material, treatment or geometry before manufacturing.
Conclusion: the spare part is won or lost before it reaches the shop
When there is no drawing, the quality of the new part depends on what you hand over: the complete part, scaled photos, the material, the function of each surface and why it failed. With that, the supplier can do what really matters: decide which dimensions are functional, rebuild them from the mating part and leave you a drawing for next time. And if the part failed by design, the best spare is not the exact copy.
Radii reproduces spare parts without drawings through its network of audited shops in Mexico, with dimensional inspection of the delivered part. If you have the broken part and what you know about it, upload it to Radii and get a quote.