Deep-hole tapping on titanium alloy parts is very challenging. If a part is close to completion, the scrap will be scrapped due to the damage of the tap, and the loss is very large. Therefore, to avoid scraping, it is required to use the correct tool and tapping technology. Machining a deep-hole thread means that a long time is required for contact between the tool and the workpiece. At the same time, more cutting heat and greater cutting force will be generated during the machining. Therefore, tapping in small deep holes of special materials (such as titanium metal parts) is prone to tool breakage and thread inconsistency.
To solve this problem, two solutions can be used:
- (1) Increase the diameter of the hole before tapping;
- (2) Use a tap designed for deep hole tapping.
Increase the diameter of the hole before tapping
Proper thread bottom hole is very important for thread machining. A slightly larger threaded bottom hole can effectively reduce the cutting heat and cutting force generated during the tapping process. But it will also reduce the thread contact rate. The National Standards and Technical Commission stipulates that in deep holes, only 50% of the full height of the thread is allowed to be tapped on the wall of the hole. This is especially important when tapping small holes in special materials and difficult-to-machine materials. Because although the thread contact rate decreases due to the decrease in the height of the thread on the hole wall, due to the increase in the length of the thread, the thread can still be reliably connected. The diameter increment of the threaded bottom hole depends mainly on the required thread contact rate and the number of thread heads per inch. Based on the above two values, the correct thread bottom hole diameter can be calculated using an empirical formula.
Cutting parameters
Because titanium metal parts are difficult to machine, cutting parameters and tool geometry need to be fully considered.
1.Cutting speed
Because titanium alloys have large elasticity and deformation rates, relatively small cutting speeds are required. When machining small holes in titanium alloy parts, the recommended circumferential cutting speed is 10 to 14 inches per minute. We do not recommend the use of smaller speeds, as that will result in cold work hardening of the workpiece. In addition, it is also necessary to pay attention to the cutting tool damage resulting in cutting heat.
2.Chip pocket
When tapping in deep holes, the number of tap grooves needs to be reduced to increase the chip-holding space of each groove. In this way, when the tap is retracted, more iron filings can be taken away, reducing the chance of tool damage due to iron clogging. But on the other hand, the increase of the chip flute of the tap reduces the diameter of the core, so the tap strength is affected. So this will also affect the cutting speed. In addition, spiral fluted taps are easier to evacuate chips than straight fluted taps.
3.Front corner and rear corner
A small rake angle can increase the cutting edge strength, thereby increasing the tool life; and a large rake angle is beneficial for cutting metals with long chips. Therefore, when machining the titanium alloy, it is necessary to comprehensively consider these two factors and choose the appropriate rake angle. The large clearance angle can reduce the friction between the tool and the chip. Therefore, the taper angle is sometimes required to be 40 °. When machining titanium, a large clearance angle is ground on the tap, which is beneficial to chip removal. In addition, fully ground taps and tapped back taps also facilitate tapping.
4.Coolant
When machining special materials, it must be ensured that the cutting fluid reaches the cutting edge. In order to improve the coolant flow, it is recommended to open a cooling groove on the back of the tap. If the diameter is large enough, consider using internal cooling taps.
