Reaming

Next to drilling and tapping, reaming is the operation most frequently performed on zinc alloy die castings, where holes are cored it is not necessary to drill before reaming.

All zinc alloys ream well over a wide range of machining conditions, yielding very clean, smooth surfaces with uniform hole diameters. Generous flooding with emulsified cutting fluids (20:1 ratio) is recommended for all reaming operations.

Reamer Design

The common six fluted reamer in high-speed steel having a 45° lead chamfer and 0° radial rake angle is generally acceptable. Both the right-handed spiral fluted geometry and straight-fluted reamers are suitable.

Some standard reamers have a land that is too wide for the best results with zinc alloys. A land about 0.4mm wide is frequently recommended, but this is often ground down to around 0.18mm or even 0.13mm to obtain freer cutting, less tendency to load and reduced heating. Wide lands tend to produce burnishing and to generate heat. If heating is sufficient to cause the hole to expand, it may be found too small after the reamer has been removed and the piece has cooled. Some consider it better practice to let the work float than to fasten it solidly while reaming.

Stepped reamers are used for simultaneously reaming coaxial stepped holes and are sometimes combined with spot-facing tools. Most of the cutting with a reamer is done by the nose, which is beveled to provide a lead, usually at an angle of about 45°. Once the initial grinding of the flutes and lands is correct, little other grinding is required subsequently except at the lead end, though in some cases, a very light cut may be taken off the radial face of the flutes. In grinding the lead bevel, a clearance angle of about 10° is advocated.

For holes of sufficient size to permit the use of inserted-blade reamers, this type is sometimes preferred, but the blades should be firmly supported.

Hole Diameter Before Reaming

The hole diameter before reaming should be approximately 97% of the reamer diameter.

Speeds and Feeds

The same speeds and feeds recommended for drilling are also generally recommended for reaming. Speeds can be reduced by as much as 50% for better diameter control.

The following table can be used as a general guide to reaming, based on a surface cutting speed of 60 to 90 m/min