Guides / Cost
What a Large Part Machining Quote Is Actually Made Of
September 21, 2026
A quote for a 3 tonne housing is not a scaled up quote for a 300 gram bracket. The line items are different, and the ones that dominate are not the ones buyers usually argue about. If you know what the shop is actually pricing, you can change the drawing in the places where it helps and stop pushing on the places where it does not.
Machine hours are most of the number
On a small part, programming and setup are most of the cost and the cut itself is minutes. On a large part it inverts. The machine is occupied for days. A gantry mill facing a 4 metre platen, or a boring mill opening two bores on opposite faces of a gearbox housing, runs for shifts at a time, and the shop is charging for a machine that nothing else can use while your part is on it.
This is why heavy shops quote in machine hours rather than per piece, and why the hourly rate of a large boring mill is several times that of a machining centre. You are renting a scarce table.
What you can do about it: remove material that does not need to be there, and do not ask for a finish pass over surfaces that nobody touches. Surface finish called out across a whole drawing, rather than on the faces that matter, is one of the most common quiet cost adders.
Handling is a separate line, and it is real
Every time the part is turned over, someone rigs it, cranes it, sets it down, indicates it back in and proves the datums again. On a heavy part that is not a few minutes. It can be half a shift, and it needs a second person and a crane that is then unavailable to the rest of the shop.
A part that needs two setups and a part that needs five can have similar cutting time and very different prices. If your drawing forces five, the shop is charging for five.
What you can do about it: put the datums where one setup can reach as many of them as possible, and say openly which features must be in position relative to each other. A shop that knows the real requirement can often combine two setups into one. A shop reading a drawing where everything is tolerance to everything else will assume the worst.
Stock removal, not material price
Buyers tend to focus on the price per kilo of the steel. The shop is looking at how many kilos have to come off as chips.
A flame cut weldment that arrives 15 mm oversize on every face means hours of roughing before the first finish cut. A casting that arrives close to net shape, or a weldment with machining allowance only where it is needed, skips most of that.
This is also why the blank type matters so much in the RFQ. Weldment, casting, forging and solid stock lead to completely different roughing plans.
The sequence, and whether stress relief is in it
A welded or heavily machined steel structure carries internal stress. Take metal off one side and the part moves. On a 400 mm part the movement is inside your tolerance. On a 3 metre frame it is not.
The honest sequence is rough machine, stress relieve, then finish machine. It costs an extra handling cycle, an oven booking and several days of lead time. A shop that leaves it out is cheaper on paper and is transferring the risk to you.
When you compare quotes, this is the single most useful thing to check. Two quotes that look 30 percent apart are often just one quote that includes stress relief and one that does not.
Tolerance across the part, not per feature
Holding a bore to a few hundredths is routine. Holding two bores in position when they are 4 metres apart is a different problem, and it brings in temperature, foundation, machine geometry and how long the part sat before it was measured.
Large part drawings often inherit tolerances from smaller assemblies and end up asking for geometric control across a span that nothing in the assembly actually needs. Every tightened span adds machine time, adds inspection and sometimes adds a whole extra process.
What you can do about it: separate the two questions in your own head before you send the drawing. Which features need to be accurate in themselves, and which need to be accurate relative to each other. Then tolerance only the second group tightly.
Transport and packing can beat the machining
An oversized part needs a permit, a route, a lifting plan, a crate built for it and sometimes an escort. On some jobs the freight is a larger number than the machining.
This is also why the shop’s location matters more on heavy work than on small parts. A shop 500 km away that costs 10 percent more can easily land cheaper than one across an ocean.
Give the delivery city or port in the RFQ. It changes which shops are worth matching at all.
What a good heavy quote tells you
A quote worth comparing states the machining sequence, says whether stress relief is included, separates handling from cutting time, names who supplies the blank and says what is and is not included in the freight. If a quote is a single number with a lead time, you cannot compare it with anything, and the number is usually optimistic.
Frequently asked questions
Why do quotes for the same large part differ so much?
Usually because the shops read the sequence differently. One plans to rough, stress relieve and finish. Another plans to machine it in one pass and hope it stays put. Ask each quote to state the sequence and the gap explains itself.
Is it cheaper to send a weldment or a casting?
For one to five pieces a weldment is almost always cheaper, because there is no pattern to pay for. Past roughly twenty pieces a casting usually wins on machining time, since less material has to come off.
Does ordering more pieces reduce the price per part?
Less than you would expect. On a small milled part setup spreads over the batch. On a large part the machine time per piece stays high whatever the quantity, so the curve flattens quickly after the first few.
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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