Precision Ground Aluminium Rod & Bar

What makes ground bar different from standard bar

Every round bar starts as an extrusion. Extruded bar to EN 755-3 or ASTM B221 is economical but carries diameter tolerances of a tenth of a millimetre or more. Drawing the bar through a die (cold finishing, EN 754-3 or ASTM B211) tightens this considerably. Centreless grinding is the final step: the bar passes between a grinding wheel and a regulating wheel, and a thin skin is removed until the diameter, roundness and finish meet the target.

The result is stock that a sliding-headstock (Swiss-type) lathe can grip in its guide bushing along its whole length. Swiss machines support the bar a few millimetres from the cutting tool, so any variation in diameter either jams the bushing or lets the bar chatter. Bar feeders for fixed-head automatics have the same problem at high spindle speeds: a bent or oval bar whips in the feed tube, limits speed and damages the machine.

Ground bar also removes the surface layer that carries die lines, small laps and the coarse grain sometimes found at the periphery of extruded bar. That gives a cleaner surface for parts that are used straight off the bar with only facing and cross-drilling, such as pins, dowels, shafts, pistons for pneumatic cylinders and roller cores.

Diameter tolerances: ISO 286 shaft grades explained

Ground bar is normally specified to an ISO 286 shaft tolerance grade. The letter h means the upper limit is the nominal diameter and all variation is undersize, which is what a guide bushing needs. The number is the tolerance grade: the smaller the number, the tighter the band. The table shows the standard ISO 286-2 limits for the diameter ranges most often ground in aluminium.

Limits from ISO 286-2. 1 µm = 0.001 mm. Use our fit and tolerance calculator for other sizes and grades.

For comparison, cold-drawn bar to EN 754-3 or ASTM B211 is typically held to a band of several hundredths of a millimetre, and extruded bar is looser still. A common commercial specification for ground aluminium bar is h8, with a flat −0.02 mm band also widely requested; across 10 – 30 mm that flat band falls between h7 and h8. Grades of h7 and h6 can be produced by additional grinding passes but cost more and need careful handling, because aluminium is soft enough to be marked in transit.

Specify what your process actually needs. A Swiss lathe with a rotary guide bushing usually runs well on h8 or h9 bar; parts that are used on the as-supplied diameter as a fit in a reamed hole may need h7 or h6.

Straightness, roundness and surface finish

Straightness is measured by rolling the bar on a surface table or between vee blocks and reading the gap or dial indicator deflection over a stated length. Write both the figure and the measuring method on the order: "0.2 mm over any 1,000 mm" and "0.2 mm total over the bar length" are not the same requirement.

Roundness and diameter are measured with micrometers or laser gauges at several points along the bar. For critical work, ask for a statistical summary of the diameter readings for the lot rather than a single nominal figure.

Which aluminium alloys are supplied as ground bar?

Ground bar is made from alloys that machine well and hold their size after grinding. Typical properties below are Aluminum Association / ASM values for the temper; the governing minimums depend on the order standard and are reported on the mill certificate.

Two notes on free-machining grades. Classic 2011 and 6262 contain lead and bismuth to break chips; lead content is restricted under the EU RoHS and ELV directives, so lead-free variants (for example 2011B or bismuth-tin versions of 6xxx) are increasingly specified. If your parts go into European electrical or automotive products, state the compliance requirement on the order. See our free-cutting aluminium bar guide for details.

Standards for ground aluminium rod

There is no separate standard for "ground" aluminium bar. Material, temper and mechanical properties come from the bar standard; the tighter dimensional limits are added on the order or drawing.

Ground bar sizes and weights

Weight per metre for 6xxx alloys at 2.70 g/cm³ is shown below. 2011 (2.83 g/cm³), 2024 (2.78 g/cm³) and 7075 (2.81 g/cm³) bars are 3 – 5 % heavier for the same diameter.

How precision ground bar is produced

1. Billet and extrusionHomogenised billet is extruded to a round slightly over the drawing size, then solution heat-treated where the temper requires it.
2. DrawingThe bar is cold drawn through one or more dies, which sizes it, improves the surface and work-hardens it; heat-treatable grades are aged after drawing to reach T6, T8 or T9.
3. StraighteningRoll straightening brings the bar close to final straightness and, together with stretching, reduces residual stress so it does not bend when ground or machined.
4. Centreless grindingOne or more passes between grinding and regulating wheels remove a thin layer and bring the diameter, roundness and finish into the specified grade.
5. Chamfering and cleaningEnds are chamfered and the bar is cleaned of grinding fluid and swarf.
6. Inspection and packingDiameter, roundness, straightness and surface are checked, results recorded, and bars are packed so they cannot touch each other in transit.

The order of drawing and ageing matters: grinding a bar that still carries large residual stresses can make it bow later when one side is machined away. Stress-relieved tempers such as T351, T451 and T651 are therefore preferred where they exist for the alloy.

Ground bar compared with cold-finished and extruded bar

If every surface of the part will be turned anyway and the machine is not a sliding-headstock type, cold-finished bar is usually the economical choice. If the OD is a functional surface or the machine needs a consistent bar for the bushing, ground bar reduces scrap and set-up time enough to justify the extra cost.

Where precision ground aluminium rod is used

• Medical and dental devices: instrument handles, housings and small shafts turned on Swiss lathes, often anodised afterwards
• Pneumatics and hydraulics: piston rods for light-duty cylinders, valve spools and guide pins, where the OD runs in a seal
• Electronics and optics: connector bodies, camera and lens mounts, sensor housings and standoffs
• Automotive: brake and transmission valve components, ABS parts and fittings made in very high volumes
• Aerospace and defence: pins, bushings and fasteners to AMS-QQ-A-225 in 2017, 2024 and 7075
• Industrial equipment: textile and printing rollers, dowels, locating pins, linear guide shafts for light loads

What these parts share is that the bar diameter is either left as supplied or turned with a very light pass, and they are made in long runs where scrap and machine stoppages dominate the cost. For one-off or low-volume work machined all over, ground bar rarely makes economic sense.

How to specify ground bar, and common mistakes

• State the tolerance grade, not just "ground". Write "Ø12 h8 (0/−0.027)" so supplier and inspector use the same limits.
• Define straightness and how it is measured, per metre or over the full length.
• Match the temper to the process. A soft temper (T4, O) may burr in the bushing; T6, T8 or T9 grinds and machines more cleanly.
• Check the free-machining additions. If parts must be lead-free, ask for a lead-free grade and the analysis on the certificate.
• Plan for anodising. Ground surfaces anodise well, but anodic film grows on the diameter; allow for coating thickness on fits.
• Order the right length. Bars that do not suit the feeder magazine leave long remnants and waste material.
• Protect the surface. Ask for individual sleeves or interleaving; a scratched bar can fail a tight-tolerance bushing.

Related ground bar, alloys and machining guides

Ordering ground aluminium bar from Metline

1. SpecifyAlloy and temper, diameter with tolerance grade, straightness, surface finish, length, quantity and the governing standard (ASTM B211, AMS-QQ-A-225, EN 754).
2. QuoteWe confirm availability, inspection plan and delivery terms (EXW, FOB, CIF or DAP).
3. InspectDiameter, roundness and straightness are checked against your limits before packing.
4. DocumentEN 10204 3.1 mill test certificate with chemistry and mechanical properties, plus dimensional report on request.
5. ShipSleeved bars in rigid export boxes or wooden cases, by sea or air worldwide.

Precision ground aluminium (aluminum) rod: frequently asked questions