During the Covid 19 lockdown in Singapore, all activities in shipyards came to a standstill as yard workers were not allowed to leave their dormitories. One of our clients was facing a challenge as they need to dock the vessel to render critical repair on one of the main engine whereby 2 of the M90 threaded holes on the engine block were severely damaged when dismantling the cylinder head.
The client’s default plan was to enlarge threaded holes to M130mm to receive customized inserts with M90 x 6mm through threaded holes as shown in the illustration video below.
The client attempted a self-repair to enlarge the thread hole with no avail as the tap broke inside the engine housing when it was half way through. With the next sailing legs coming near and schedule for docking not insight, the client decided to seek for insitu solutions while the vessel resides at Singapore’s eastern anchorage region.
With working range from M60 to M140 with up to 260mm in depth, the CNC (computer numeric control) threading machine that was developed in house by our engineering team was selected for this application.The machine is able to conduct boring and threading as well as milling in one single setup. With the datum point for the hole being programmable, this versatility give our client the option to reinstate the center of the hole with reference to the original PCD if it is found to be offset due to manual repair process. Being press for time due to the sailing schedule, 2 teams of technicians were mobilized to work on the preparation simultaneously:
Design, machining and testing of mounting template based on the site measurement submitted by our client through our job prerequisite form. The template functions as machine foundation, re-establish the Pitch Center Diameter and orientation of the holes as shown in photo 3 below.
Coding and testing of several G code subroutine programs to cater for different type of cutter and shaft length. As anchorage condition posed travel restriction, a wide degree of versatility is needed to provision for any possible scenario.
Upon completion of preparation and testing within 2 days, the response team was mobilized onboard without further ado and the repair scope was completed within 3 days. The hole which was damaged by the broken tap was enlarged to M140 instead.
The CNC threading machine uses high interpolation cutting speed coupled with small feed rate as shown in video 2 below to achieve high precision threaded hole as compared to conventional tapping methodology.
As the threading end mill is using replaceable insert, worn-out cutting edge can be easily replaced hence ensure the form of fine pitch thread. Manual tapping of large diameter without fixture to align the torque driver will induce perpendicular and offset error to the original hole, the common cause for tap breakage especially when the cutting edge gets blunt due to wear and tear. Taking into consideration the cost a m130 machine tap, hydraulic torque driver, number of threaded hole per tap, breakage risk and lead time, CNC threading machine can offers a cost effective and safer option for our client.