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Machining a Large Part In-House or Sending It Out

September 26, 2026

The question comes up whenever a part outgrows what a company already owns: buy the machine, or send the part out. It is a capital decision dressed up as a machining decision, and it is worth treating as one.

What owning the machine actually costs

A horizontal boring mill or gantry mill is not a small purchase, and the price tag is the smallest part of the real cost.

Floor space and foundation. A floor type borer or a large gantry needs a foundation rated for the load, not just a slab. That is a facilities project before it is a machining project.

Crane capacity. The machine is only useful if something can lift the part onto it. A shop buying a machine rated for 40 tonnes still needs a crane that can move 40 tonnes, at the right hook height, in the right bay. See table load and crane capacity.

Programming and setup skill. Running a large machine well is a different skill from running a standard machining centre. Workholding, datum strategy and indicating a four metre part all take time to learn, and mistakes are expensive on parts this size.

Utilization. This is the number that decides everything else. A machine that runs one part a month still needs someone to maintain it, insure it, and keep the skill current on it between jobs.

What the machine has to earn back

Work through the arithmetic before the decision, not after.

  1. Forecast the real volume. Not the optimistic number. How many parts, of roughly what size, over the next two to three years.
  2. Price the machine, the foundation, and the crane upgrade if one is needed. Add commissioning and the training curve.
  3. Estimate machine hours per part using the part envelope, not just its weight. See when a part outgrows a machining centre.
  4. Compare the payback period against outside quotes for the same volume. If outside quotes for the forecast volume cost less than the machine over its useful life, the machine is not paying for itself, it is a fixed cost with a comforting shape.
  5. Check what else the machine could run. A machine bought for one product line that can also take on other internal work, or outside jobs to fill idle time, changes the arithmetic. A machine that can only ever cut one part does not.

Signs that point toward buying

  • The part is central to the product and volume is steady, not a one-off program
  • The company already has the crane capacity and floor loading, so the marginal investment is the machine itself
  • The design changes often enough that round trips to an outside shop cost real schedule time
  • There is other large-part work in the building that can share the machine

Signs that point toward outsourcing

  • Volume is low, seasonal, or tied to a program that might not repeat
  • The part is a one-off, a prototype, or a repair
  • The company does not already have the crane and floor rating, so the real cost is a facility upgrade, not a machine
  • The part needs a machine class the company would only ever use for this one job, such as a vertical turning lathe for a single ring or casing

The middle case: both, at different volumes

The two options are not mutually exclusive over time. A company proving out a new product often outsources the first parts while volume is uncertain, then brings the work in-house once the forecast firms up and the machine pays back inside a reasonable window. The reverse also happens: a machine bought for a program that ended sits idle until the company decides outside work no longer makes sense to chase, and the part moves to an RFQ instead.

Either direction, the decision holds up better when it is revisited against the actual volume that showed up, not the volume that was forecast at the start.

What this means for an RFQ

If the outsourcing side of the arithmetic wins, the next question is not “which shop” but “how many shops should see this drawing.” That is a separate decision, covered in how many shops should quote a large part RFQ.

Frequently asked questions

What utilization justifies buying a boring mill or gantry mill?

There is no fixed number, but shops that carry these machines profitably usually keep them running on a mix of internal and outside work, because very few single companies generate enough large-part volume on their own to fill one. If the honest forecast is a handful of parts a year, the machine will sit idle most of the time.

Does outsourcing mean losing control of the design?

No. The drawing, the tolerances and the acceptance criteria stay yours either way. What changes is who owns the machine risk: tool wear, setup time, and the capital sitting on a balance sheet.

Can a large part start in-house and move outside later?

Yes, and it is common in the other direction too. A part built in-house while volume is uncertain can move to an outside shop once demand is proven, or the reverse if volume grows enough to justify buying the machine.

Need a quote for a part like this?

Send the drawing with the envelope and finished weight. We match you with up to 3 shops that take parts this size and reply within 24 business hours.

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