Sinteris’ customer, a major remanufacturer, was looking to reduce the cost of producing an automotive chain sprocket without compromising its quality. Sinteris presented its case for green machining – performing secondary finishing before sintering rather than after.
The use of sinter-hardening powders has been widely used in PM parts to eliminate heat treatment operations. Very high hardness can be obtained through sinter hardening, however this higher hardness made subsequent machining very difficult. This issue has prevented sinter-hardening materials from being used on parts where a secondary machining operation is required.
A common example is the timing sprocket, which has a groove at the middle of the teeth, which typically has to be formed by a secondary machining. Traditionally, this type of part was processed with the following procedures: compacting → sintering → groove turning (plunge cut) → heat treatment → honing. For good machinability, powders of medium carbon content are usually selected, and free machining aids (e.g. MnS) are usually added in the mix. Typical problems with this process have been well recognized, including quick cutting tool wear, frequent cutting tool change and adjustment, heat treatment distortion, and discoloring.
Green machining enables production of this type of part from sinter-hardenable materials. Because the particles are not metallurgically bonded, the unsintered parts can be machined with little force. However, green strength has to be high enough (>3000 psi / 21 MPa) to survive the stress of clamping and machining. This high green strength cannot be easily obtained through conventional PM techniques. (Most green machining tests have been conducted on warm compacted parts.)
To overcome the problem of achieving sufficiently high green strength, Sinteris adopted a new Canadian-developed polymeric lubricant. Unlike traditional waxes and metallic stearates, this lubricant significantly enhanced green strength when added into the powder mix. For example, green strength was two times higher compared to EBS wax when compacted at a temperature of 55°C.
Semi-production trials demonstrated that this temperature could be easily reached throughout a green part by cold compaction. Even higher green strength could be achieved through a subsequent curing process, i.e., up to 7000 psi (48 MPa) at a density of 6.8 g/cm. The high green strength provided by this lubricant makes green machining possible for the conventional PM process.
Green machining tests were performed on several timing sprockets. The material used was a sinter-hardenable powder mix of QMP ATOMET 4601 and the new high green strength (HGS) lubricant. The sprockets were pressed to a density of ~6.8 g/cm3 by cold compaction. Green machining (turning a groove along the middle of the teeth) was performed on both as-compacted and cured parts. Results showed that the HGS lubricant provides sufficient green strength for clamping and machining operations, especially after curing.
The machined surfaces appeared to be smooth and the edges remained integrated. The green-machined sprockets made of sinter-hardenable powder eliminated heat treatment, providing much longer tool life and improved size tolerances.
- Longer cutting tool life: With this technique, one cutting edge has finished more than 18,000 grooves without showing any hint of damage as compared to only ~120 grooves with the traditional process. Considering that one cutting edge can be reshaped twice, a total of only 360 sprockets can be finished with one cutting edge through the traditional process. The down time of the green machining operation was then reduced to nearly zero compared to frequent cutting tool change and adjustment of the traditional process. The green machining process also eliminated cutting fluids.
- Elimination of heat treatment: Green machined sinter-hardenable parts eliminated heat treatment operations except tempering. The finished parts have better size tolerance and favorable appearance.
- Burr-free edges: With the traditional process, burrs formed along the machined edges during machining operation. These burrs are difficult to remove because of their locations. A sizing operation had been used following machining to remove the burrs. This operation is no longer necessary for the green-machined sprockets since no burrs formed along the machined edges.
- Reduced cost: Although material cost for sinter hardening powder is higher, and a curing operation was added to the process, the overall cost of manufacturing the client’s sprocket was still reduced by a minimum of 15% per piece.