When a Large Welded Face Should Be Ground Instead of Milled
Most machined weldments get their flat faces from a mill. On large plate faces there is a second route that many buyers never consider, and on the right part it is both flatter and cheaper.
What Blanchard grinding is
A Blanchard grinder is a vertical spindle rotary surface grinder. The part is held on a large rotating magnetic chuck and a segmented grinding wheel, often several hundred millimetres across, comes down onto it. The whole face passes under a wide abrasive swath rather than being covered by a cutter walking back and forth.
The result is a flat face with a distinctive crosshatch pattern, produced in one operation, with the part held flat on a chuck rather than clamped in a fixture.
Where it beats a mill
Large flat areas. A 2 m by 1 m mounting face is a lot of passes for a face mill. A rotary grinder covers it in a fraction of the time because the swath is wide and the whole surface is being worked at once.
Flatness on a thin or flexible part. Grinding removes material with far lower cutting forces than milling. A plate that would deflect under a face mill and spring back can be ground without being pushed.
Getting through mill scale and weld spatter. Hot rolled plate arrives with scale that is abrasive and murder on carbide. Abrasive does not mind. On a weldment made from as-rolled plate, grinding the face is often cheaper than milling it for exactly this reason.
Hard material. Abrasion resistant plate and hardfaced surfaces can be ground. Milling them is expensive and sometimes not practical.
Surface finish on a mating face. A ground face with a fine finish beds against another face better than a milled one, which matters on machine bases, die shoes and mounting plates.
Where a mill wins
Anything that is not a flat face. Bores, pockets, slots, tapped holes, steps. A grinder makes one flat plane. Everything else needs a spindle.
Parts with interrupted or raised features. A face broken up by bosses and ribs is awkward on a rotary chuck and straightforward on a mill.
Small faces. Setting a small part on a large chuck to grind a 200 mm pad is not efficient. Mill it.
Where the face and the bores share a datum. If the flat face and the bore positions must be tightly related, doing both in one machine setup removes an error source. Splitting the work between a grinder and a mill adds a setup and a relocation.
The combination that is used most often
On real machined weldments the two are not rivals. The common sequence is: weld, stress relieve, grind the large datum face flat, then take the part to a mill or a horizontal boring mill and cut the bores and the remaining features from that ground face as the datum.
That gives the machinist a genuinely flat, scale free reference to sit the part on. A frame set on a ground face is located properly. A frame set on an as-welded face is sitting on whatever the highest points happen to be, and every bore cut from it inherits that error.
If your part has one big datum face and a lot of features referenced to it, this sequence is worth asking about by name.
Part size and practical limits
Rotary grinders come in a wide range of chuck sizes, and the largest are genuinely large. What limits a given shop is chuck diameter, the height under the wheel and the weight the chuck and the crane will take.
Two more practical points. A magnetic chuck needs ferrous material, so stainless and aluminium weldments need mechanical clamping or a fixture, which changes the job. And the part has to sit stably on the chuck, so a frame with a warped underside may need that side skimmed first.
What to put in the RFQ
Ask for the outcome, not the machine. “Face A flat within 0.05 mm, surface finish Ra 1.6 or better, after stress relief” lets each shop choose the cheapest route it owns. Specifying “Blanchard ground” when a mill would do the same job rules out shops that would have quoted well.
The exception is when you specifically want the ground finish for bedding or appearance. Then say so, and say why, so the shop does not substitute.
Have a large welded plate face with a flatness callout? Send the drawing and we will match it with shops that own both the grinder and the mill.
Frequently asked questions
What flatness can Blanchard grinding hold on a large weldment?
Shops commonly quote flatness in the range of a few hundredths of a millimetre over a large plate, and the achievable number depends on the part size, the section stiffness and whether the part was stress relieved. Ask the specific shop for its capability on a part your size rather than relying on a general figure.
Is grinding more expensive than milling?
Per square metre of finished face, grinding is often cheaper on large flat areas because the wheel covers a wide swath in one pass. Milling wins where the face is small, interrupted or accompanied by bores and pockets that need a spindle anyway.
Does the part still need stress relief before grinding?
Yes, for the same reason as milling. Grinding removes material and releases locked-in stress. A frame that was not stress relieved can come off the grinder flat and be bowed an hour later.
Need a quote for this part?
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