Deep hole drilling methodology for high yield strength Inconel 718 >1000 MPa

Abstract

Drilling of deep holes on heat-resistant superalloys like Inconel 718 can be found in specialized engineering applications involving extreme temperatures. Such exotic materials, especially at high yield strengths pose indomitable challenges in drilling deep holes. Poorly designed drills for pilot hole drilling and deep hole drilling can lead to catastrophic failures at different stages. Inappropriate selection of drilling parameters and/or drilling sequence will either drives the drills to degrade and break rapidly on one hand or adversely affect the overall productivity on another. Eventually, these incapability will result in irreparable hole straightness and costly material scraps. In this paper, a comprehensive drilling solution on general machining centers for deep holes with depth- To-diameter ratios ranging 10 to 100 is proposed. To study compatibility of drills for pilot hole drilling and deep hole drilling, three twist drill designs with distinct geometries such as point angle and chisel edge were used. Other than initial tool-work engagement efficacy, effects of bottom hole geometries on process stability using four types of single-lip deep hole drills are investigated. An intermediary reaming is performed to achieve tight IT7 tolerance. As a result, a novel parameter - 'drilling engagement ratio' is derived as a quantitative tool design guideline for deep hole drilling. Following that, practical implementation of single-lip drills are studied in detail, in conjunction with controlled edge geometries. It is found that low drilling feeds deteriorate drilling stability and lead to high wear rate as a contrary to common industry practice. Chips generated at low feed rates are more prone to break, but irregular in shape. Such irregularity improves with increasing feed rates, alongside tool wear but fails to break at high feed above 0.04 mm/ rev - resulted in ultimate tool failure. Based on extensive experimental observations, an optimal set of drilling parameters and drill designs is recommended.,.