What is a machining pre-form?
A pre-form is the shape a machined part starts from. Every machined component begins as a stamped blank, a length of round bar or tube, or a bought-in near-net blank such as a forging, a cold-extruded or cold-headed blank, a casting, or a sintered powder-metallurgy blank. That choice is usually made before anyone picks up a tool. It decides how much material has to be removed, how much of the original grain flow survives into the finished part, how many operations the machine shop needs, and where the porosity or the hard spots will be. A near-net pre-form costs more per kilogram than bar but removes most of the chips, so the total part cost frequently falls even though the input material went up. For a part machined from bar, removing 70 percent of the input as swarf is normal; from a forging, the same part may leave 15 percent.
Why the pre-form decides the part cost
Machining cost is dominated by the volume of material removed and the number of setups, not by the price of the raw stock. A near-net pre-form attacks both: less material to cut means shorter cycle times, fewer tools consumed and less waste to dispose of, and a pre-form that already carries the boss, the web or the flange can remove an entire operation from the route. The trade-off is tooling and minimum order quantity, because a forging die, an extrusion punch or a sintering tool is only economic above a certain annual volume. Below that volume bar stock is cheaper, even with the extra chips. The point of reviewing the pre-form at the DFM stage is to find out which side of that line the part sits on while the drawing can still be changed.
The pre-forms, and when each one wins
- Stamped blank — made in-house. Blanking, piercing and forming run on Balford's own presses from 25 t to 350 t, and the blank is then machined in the same plant. One review covers both steps and there is no hand-over between suppliers, which makes this the shortest route for a flat or lightly formed profile that carries a machined feature.
- Round bar. The default for rotational parts at low and medium volume: no tooling, no minimum order, any diameter the bar feeders take. Its cost sits in the chips, so a part machined from bar that removes most of the input is the first candidate for a near-net blank.
- Steel tube. Chosen where the bore is already a hole — sleeves, spacers, bushes and housings keep the inside diameter as bought and finish only the functional surfaces. Tube wall thickness and concentricity become part of the drawing review.
- Hot forging (红冲) — a billet is heated until it glows and forged in a die. Grain flow follows the part contour, so it is strong in the directions that carry load, and it suits parts with a forged boss, lever or eye that would be wasteful to cut from solid. As-forged tolerances are loose compared with machining, so forged parts are normally finish-machined on the functional features.
- Cold extrusion (冷挤压) — the billet is forced into shape at room temperature. Dimensional accuracy and surface finish come out far closer to finished than with hot forming, material utilisation is high, and the cold working raises the yield strength of the surface. It suits symmetrical parts: sleeves, cups, bushes and housings.
- Cold heading (冷镦) — wire is upset into a head or a shoulder at room temperature. It is fast, produces almost no waste, and is the standard route for fasteners, pins, rivets and shouldered terminals. The limit is shape: cold heading makes good use of material where the form is essentially a body with an enlarged end.
- Casting (铸件) — sand, investment or die casting. Casting is the only route that produces complex internal cavities economically, and sand casting carries almost no tooling cost, which makes it the right answer for one-off and low-volume parts. In exchange the tolerances are coarser than the other routes, and porosity has to be controlled and inspected rather than assumed away.
- Powder metallurgy (粉末冶金) — metal powder is compacted in a die and sintered. It is near-net for small, complex, high-volume parts and it can produce controlled porosity on purpose, which is why it dominates self-lubricating bearings and some magnetic components. The limits are part size, thickness and impact strength, and the tooling cost is only justified at volume.
Key point
The decision is usually made by four numbers: annual volume, finished part size, how symmetrical the shape is, and how much strength the part needs in a specific direction. High volume and a symmetrical shape point at cold extrusion, cold heading or powder metallurgy. A shaped boss or a load direction points at hot forging. Low volume and a complex internal cavity point at casting. No tooling budget points at bar.
Which of these Balford carries out, and which are subcontracted
Balford machines in-house, and it also stamps and draws in-house. Stamped blanks are produced on our own presses, so for that route both the blank and the machining sit with one supplier. Round bar and steel tube are bought as material. The near-net blanks that we do not make are bought from qualified subcontractors: hot forging (红冲), cold extrusion (冷挤压), cold heading (冷镦), casting (铸件) and powder metallurgy (粉末冶金). That split is stated deliberately, because the pre-form supply chain is part of the process route a customer signs off, and a supplier that describes only the operations it performs itself has described half a part. Balford is not a forge, a foundry or a powder-metallurgy plant, and the page does not pretend otherwise. The supplier list for these pre-forms is being extended; the route quoted for a part names the pre-form it uses.
Each subcontractor is bound in writing to confidentiality terms at least as strict as the customer's own NDA gives Balford, and receives only the information needed for its process step. The process route quoted for a part names the pre-form and the subcontracted steps it uses, and the identity of the subcontractor is disclosed on request.
What to send for a pre-form review
- The finished part drawing — the pre-form decision follows from the finished geometry, not from what the current supplier happens to buy.
- Annual volume and the expected programme life — this is what decides whether tooling is economic at all.
- The material — some grades are only available in certain forms, and a grade that cannot be forged may cast well.
- Where the strength has to be — the load direction and any fatigue or pressure requirement, since that is what grain flow and porosity affect.
- Which surfaces are finished and which are as-formed — a pre-form is only worth buying near-net if the as-formed surfaces are acceptable somewhere on the part.
- The current route — if the part is machined from bar today, the comparison is straightforward and usually pays for itself in the first year at volume.
What we do not claim
Balford is not a forge, a foundry or a powder-metallurgy plant; those steps are bought in from qualified subcontractors and are named as such. The press range of 25 t to 350 t and the 250 mm maximum deep-draw diameter describe the stamping and drawing side of the plant, not the subcontracted pre-forms. Tolerance of 0.001 mm is only quoted for selected critical machined features where the process, geometry, material and inspection conditions allow, and against the drawing. Quality management runs to ISO 9001:2015, with IATF 16949-aligned controls for the automotive programmes we serve; IATF 16949 certification is not claimed.
Balford has built more than 4,000 single-hit drawing dies since 2000, operates a 12,000 m2 plant in Zhuji, Zhejiang with 500+ machines and 120+ engineering and production staff, and produces roughly 50 million parts a year. Machining drawings are reviewed and quoted, usually within 24 hours, with DFM observations returned alongside the price — including a recommendation on the pre-form when the volume and the geometry justify one.
Frequently asked questions
Should my machined part be forged or machined from bar?
It depends on how much material you are currently throwing away and what the part has to do. If the finished part is a shaped body with a boss or a lever, a forging arrives much closer to the finished geometry and the machining time falls sharply; above roughly a few thousand pieces a year the forging tool usually pays for itself. If the part is small, simple and the annual volume is low, bar stock wins because it carries no tooling cost. Send the drawing and the annual volume and the comparison can be made on real cycle times rather than on the price per kilogram.
What is cold extrusion used for?
Cold extrusion forms the billet at room temperature, so the finished blank comes off the tool with a good surface finish and tolerances much closer to finished than hot forming gives. It suits symmetrical parts — sleeves, cups, bushes and drawn-type housings — and it raises the yield strength of the surface through cold working, which is useful where the part sees pressure or wear. Where a part is currently machined from solid and is essentially a body of revolution, an extruded pre-form is usually worth comparing.
Do you cast or forge the pre-forms yourselves?
No. Balford machines in-house and also stamps and draws in-house, so stamped blanks come off our own presses — but hot forging, cold extrusion, cold heading, casting and powder metallurgy are bought from qualified subcontractors, and round bar and tube are bought as material. That is stated because the pre-form is part of the process route a customer approves. Each subcontractor is bound to confidentiality terms at least as strict as the customer's NDA gives Balford, receives only the information needed for its process step, and is named on request.
When does powder metallurgy make sense?
When the part is small, complex, needed in high volume, and either benefits from controlled porosity or cannot be produced economically by other means. Sintering is near-net, so machining is often limited to a bore, a face or a thread, and the tooling cost is only justified once the annual volume is high. It is not a route for large parts, for thin sections, or for components that see impact loading, because porosity that is useful in a self-lubricating bearing is a liability in a part that has to absorb a shock load.