A drawing checklist for welded machine bases

When five shops quote the same frame and the numbers land within 8% of each other, the drawing is good. When the spread is 60%, the drawing is ambiguous and each shop has resolved the ambiguity in its own favour — the low bidder simply resolved it most aggressively. You will discover which assumptions they made during the first article inspection, which is the worst possible time.

This is the checklist we run drawings against before they go out to a shortlist. The drawing is the first document in our guide to machine frame fabrication sourcing.

1. Separate the functional geometry from everything else

Mark, explicitly, which surfaces carry function. On a typical machine base that is a short list: linear rail seats, spindle or column mounting faces, bearing bores, and the reference edges used during assembly. Everything else is structure.

Tolerance the first group properly and release the second group to a general fabrication tolerance — ISO 13920 class B or C covers most structural work. Applying a precision general tolerance to the entire drawing is the single most expensive habit in machine base design, because it silently pulls sixty non-functional features into the machined-surface budget.

2. Establish machined datums

Three machined datum features, mutually perpendicular, referenced by every functional dimension. Not an as-welded edge. Not a cast face. Not “centreline of part,” which does not exist on a fabrication until someone decides where it is.

If the natural datum surfaces are also functional surfaces, say so — that is normal and it lets the shop plan a single setup. What you cannot do is leave the reference implicit and expect every shop to pick the same one.

3. Write the weld callouts properly

Every weld gets a symbol: type, size, length, pitch for intermittent welds, and a field-weld or all-round flag where relevant. “Weld all round” scrawled across an assembly view is not a specification, and the shop that prices it cheapest is the one that read it most loosely.

State the governing standard once, in the notes: AWS D1.1, ISO 3834-2 or EN 15085, according to what your customer or market requires. Add the qualification requirement — welders qualified to the standard, WPS and PQR available on request — because that is the clause that separates export-capable shops from the rest.

Then name the critical welds. A table of three to six joints, each with the required NDT method and acceptance class, is worth more than a blanket note and costs a fraction of blanket NDT. Structural joints carrying spindle or axis loads belong on the list. Cable tray brackets do not.

4. Specify the stress relief route, in sequence

Not “stress relieve.” The cycle, the standard, the position in the routing relative to rough and finish machining, and the chart as a deliverable. A frame relieved after finish machining has been ruined politely. This is set out at length in stress relieving welded machine bases, including a process note you can lift.

Include the machining allowance to remain at the time of treatment — 3 mm on all surfaces is a common and workable figure for medium frames.

5. State the machining allowance on the weldment drawing

If you are issuing separate weldment and finished-part drawings, the weldment drawing must carry the allowance explicitly. Shops will otherwise fabricate to nominal and discover during machining that there is nothing left to cut, at which point the part is scrap and the argument about whose drawing was wrong takes three weeks.

6. Say what the part is for

A sentence in the notes — “base for a 3-axis gantry mill, X travel 2,400 mm, spindle 15 kW” — changes how a competent shop reads the entire drawing. It tells the welding engineer where the load paths run and therefore which distortion to control, and it lets them flag a design problem you would rather hear about at quote stage.

Shops that never receive this context stop thinking about the part and start executing the drawing literally. That is a worse outcome for you than for them.

7. Lock down the surface treatment

Blast grade, primer, topcoat, dry film thickness, and which surfaces are masked. Machined surfaces almost always need masking and a rust preventative instead of paint, and if the drawing does not say so, some fraction of frames will arrive with paint on the rail seats.

Sa 2.5 blast, zinc-rich epoxy primer at 60 μm, 2K polyurethane topcoat at 60 μm, machined surfaces masked and protected with a removable preservative — that is a specification. “Paint blue” is not.

8. Define inspection and its evidence

Name the hold points: fit-up before welding, post-weld dimensional check, post-stress-relief check, pre-shipment final. For each, say what gets measured and what document arrives with the part.

Require the dimensional report to reference your datums. A report full of measurements taken from features the shop chose is unreadable against your drawing, and reconciling it eats more time than the inspection saved.

Photographs at fit-up cost nothing and settle most later disputes in a single email.

9. Handling, lifting and shipping

Mark the lifting points on the drawing, with the rated load. Frames get dropped, twisted or lifted from the wrong face far more often than anyone admits, and a 4-tonne weldment slung from an unintended point can lose its geometry on the way to the crate.

Specify the crating and whether machined surfaces get a desiccant wrap. Sea freight is thirty days of salt air.

10. Say what you will not accept

One short note listing the disqualifying conditions — no weld repair without written approval, no substitution of plate grade without approval, no shipment before the inspection report is accepted — removes an entire category of argument. A shop that dislikes those clauses has told you something useful for free.

Before you send it

Run the drawing against the list, then send it to two or three shops rather than one, and compare not only the numbers but the questions. The shop that comes back with three sharp questions about your datum scheme is reading the drawing. The one that returns a price by lunchtime and no questions has not opened it.

That reading is the first thing we do here: drawings that come in through the RFQ form get a manufacturability review — datums, allowance, weld callouts, plate availability — before any shop sees them, and the reply comes back within 24 business hours. The cost-driver breakdown explains which of these callouts move the price and by roughly how much, and the capability envelope sets out what the network can actually build.