Reaming precisely finishes the inner surface of holes. It is a precision hole-finishing process that demands dimensional accuracy of ±0.01mm relative to the hole's diameter. A reamer typically has 68 cutting edges, producing holes with high roundness. Additionally, consolidating multiple processing steps into one reaming tool can reduce both processing time and effort.
This article discusses the challenges encountered in reaming and key points to ensure the pilot hole is not wasted.
A reamer is a specialized tool used in reaming processes. It finishes the hole's internal surface by shaving with cutting edges on the tool's side, achieving precise hole diameters and surface roughness. The challenges in reaming include:
Like drills, reamers are prone to breakage if mishandled. Causes of tool breakage include poor pilot hole processing, chip clogging, small back taper, excessive wear at the biting part, and too high feed rate. Given that many reamers are relatively small in diameter (around φ20), particular attention is needed to prevent breakage due to overload.
In reaming, even a slight tool runout can cause a poor surface finish. Tool runout can result from various factors, including spindle/tool holder inaccuracies, chips, and cutting conditions.
Reaming demands higher dimensional accuracy compared to drilling. Thus, avoiding incorrect hole diameter requires more sophisticated techniques. Causes of incorrect hole size include:
Adjusting the cut-in amount and cutting conditions is essential to reduce the load on the tool while ensuring efficient chip removal.
To prevent reamer breakage, an appropriate allowance for the reamer size is necessary. A small pilot hole (i.e., a large allowance for the reamer) increases the load on the reamer, leading to breakage. Additionally, bending or misalignment of the pilot hole can also cause reamer breakage, demanding high precision in the pilot hole processing.
Adjusting "rotation speed" and "feed rate" to reduce cutting resistance helps prevent tool breakage. Generally, for harder workpieces or smaller diameter reamers, lowering the feed rate can reduce the load on the tool. Selecting cutting conditions that suit the reamer's rigidity and the workpiece material is crucial.
Adjust the "supply amount" and "supply pressure" of the coolant to ensure it reaches the machining point, flushing out chips. Especially for deep holes or blind holes, using a reamer with coolant holes to promote chip removal is necessary.
Controlling tool runout is critical to improving surface finish. The selection of peripheral equipment, such as collet chucks or tool holders, is also vital.
Generally, increasing the margin width improves the surface finish but also increases cutting resistance. Specifying an appropriate margin width for the workpiece material can achieve a high-quality hole finish. For example, a thicker margin width is better for soft materials like aluminum, copper alloys, and cast iron, while a thinner margin width suits high-hardness steel.
Reamer misalignment or runout can cause uneven contact between the reamer side and the hole's internal surface, affecting the surface finish. Especially if machining marks (cutter marks) are present inside the hole, re-tightening the collet chuck or tool holder is necessary to prevent reamer runout.
Selecting the right tool holder is essential to minimize reamer runout. For high precision surface finish requirements, considering a "shrink fit chuck" that is less prone to runout during machining is necessary.
The chamfer (biting part) is the cutting edge at the tip of the reamer that bites into the workpiece at the start of machining. If the chamfer runs out, the reamer side won't contact the hole's internal surface properly, affecting the surface finish.
To control the runout of the chamfer, ensure secure holding with a collet chuck or tool holder, and re-sharpening the tip is essential.
Tool runout and variations in pilot hole accuracy are common causes of reduced hole diameter accuracy. Controlling temperature changes during machining and ensuring an appropriate allowance are crucial to improving hole diameter accuracy.
In reaming, the reamer's cutting edges rub against the hole's internal surface at high speeds, causing significant temperature changes. Using coolant to lubricate and cool the contact surface is necessary to prevent accuracy issues and metal welding due to friction heat.
If the pilot hole is large and the reamer's allowance is small, the reamer's cutting edges may not sufficiently contact the hole's internal surface, leading to accuracy issues. It's essential to drill an appropriate-sized pilot hole according to the hole diameter and workpiece material.
Since reaming requires a certain level of skill, it's crucial to assess whether the tight tolerances are genuinely needed. If the part's performance is unaffected, consider using a carbide drill for high-precision hole drilling instead of reaming.
This article discussed challenges in finish machining with a reamer and key points to avoid wasting the pilot hole. The demand for reaming in machining high-precision parts, from automotive components to aircraft parts and molds, has been increasing, making reaming a critical process that affects the final product's quality.
