Why One Setup Is Cheaper Than Three on a Two Tonne Frame
On a small machined part, the cost is mostly cutting time. On a two tonne weldment it is mostly handling. The spindle is turning for a small fraction of the hours you are paying for.
That is why the most useful question about a large weldment is not how many features it has. It is how many times the part has to be moved.
What a setup actually costs
Picture the sequence for one re-fixture on a heavy frame:
- Stop the machine, remove the clamping.
- Get a crane and a slinger. Lift a 2 tonne part off the table.
- Re-orient it, set it down on the new face, pack and clamp it.
- Indicate it in. Find the datums again, sweep a bore, check square, adjust, re-check.
- Reset the work offsets and prove out.
None of that is cutting and all of it is charged. On a large part it can run to hours, and it repeats for every setup.
There is a second cost that does not appear as hours. Every relocation adds a tolerance stack. Features cut in setup one and features cut in setup three are related through two re-indications. If the drawing requires them to be tightly related, the shop has to work to a tighter number in each setup to protect the relationship, or use a more accurate and slower alignment method.
The machines that do this work
Horizontal boring mill. The classic weldment machine. The spindle is horizontal and the part usually sits on a rotary table, so several vertical faces can be reached by indexing the table rather than re-slinging the part. For a frame with bores on multiple faces this is often the difference between one setup and four.
Bed or bridge mill. Vertical spindle, large table, good for large horizontal faces and features on the top of a part. Straightforward and common.
Gantry mill. The part stays still and the bridge moves over it. Very large envelopes, good for long parts such as beams and rails.
Rotary grinder. Not a mill, but it belongs in the plan. On a large flat datum face, grinding rather than milling can be flatter and cheaper, and it gives the machinist a proper reference to work from.
The right machine is the shop’s decision. What you need to give them is the information to make it.
What to put in the RFQ
Three numbers rule shops in or out before anything else:
Overall envelope. Length, width and height of the finished part.
Finished weight. What the table and the crane have to take.
Which faces carry machined features. This decides whether a horizontal machine with a rotary table can do the job in one hit or whether the part must be turned over.
Add one more if you have it: which features must be tightly related to which. That is what tells the shop where it can afford a re-setup and where it cannot.
Designing for fewer setups
This is where a buyer or a designer can move the number substantially.
Put machined features on as few faces as possible. A frame with bores on two opposite faces and nothing elsewhere is a fundamentally cheaper part than one with features on five faces.
Keep tightly related features on the same face. If two bores must be coaxial within 0.05 mm, having them on faces that can be reached without re-slinging the part is worth redesigning for.
Allow through features rather than blind ones from both sides. A through bore machined from one side, or line bored, beats two blind bores cut in separate setups and expected to line up.
Give the part feet or pads that a fixture can sit on. A frame that will not sit stably has to be packed and shimmed, which is slow and introduces error.
Machine the primary datum first, then work from it. Covered in the GD&T guide. A part located from a real machined face is a different proposition from one located from as-welded surfaces.
Line boring and in situ options
Where a line of bores through a long frame has to be coaxial, line boring puts a single bar through all of them and cuts them in one pass. It removes the alignment problem rather than managing it.
Some shops also have portable machining capability, so a very large fabrication can be machined in place rather than moved to a machine. It is specialised and it is worth knowing exists when a part is genuinely too large for any table.
What this means for the quote
When quotes come back far apart on a large weldment, the machining plan is usually why. One shop is doing it in two setups on a horizontal machine with a rotary table. Another is turning the part over four times on a bed mill. Both are honest quotes for the same drawing.
That is a good reason to ask, on a significant part, how many setups the shop is planning. The answer explains the price and it tells you which quote carries the tolerance risk.
Send the envelope, the weight and the drawing and we will match it with shops whose machines take the part.
Frequently asked questions
Why does an extra setup cost so much on a heavy part?
It is not the cutting. It is crane time, re-fixturing, re-indicating the part to find the datums again, and the tolerance stack that comes from relating features cut in two different positions. On a two tonne frame a re-setup can be hours before the spindle turns.
What machine types handle large weldments?
Bed and bridge mills, horizontal boring mills and gantry machines, depending on the shape. A horizontal boring mill with a rotary table is the classic weldment machine because it can reach several faces by indexing the table rather than re-fixturing the part.
How do I know if my part fits a shop's machine?
Give the overall envelope and the finished weight in the RFQ, and ask the shop for machine travels and table load capacity rather than machine names. Those two numbers rule shops in or out faster than anything else.
Need a quote for this part?
Send the drawing. We match you with up to 3 shops that weld and machine it in house, and we reply within 24 business hours.
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