Machining a deep hole thread means long periods of contact between the tool and the workpiece. At the same time, more cutting heat and greater cutting force will be generated during the machining process. 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 adopted:
(1) Increase the diameter of the hole before tapping; (2) Use a tap specially designed for deep hole tapping.
1. Increase the diameter of the hole before tapping
Suitable thread bottom holes are very important for thread processing. A slightly larger thread bottom hole can effectively reduce the cutting heat and cutting force generated during tapping. But it also reduces the thread contact rate. Because although the thread contact rate decreases due to the reduction in thread height on the hole wall, a reliable thread connection can still be maintained due to the increase in thread length. The diameter increment of the threaded bore depends primarily on the required thread contact rate and the number of thread starts per inch. Based on the above two values, the correct thread bottom hole diameter can be calculated using empirical formulas.
2.Cutting parameters
Because titanium parts are difficult to machine, cutting parameters and tool geometry need to be fully considered.
a. 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 peripheral cutting speed is 10 to 14 inches/minute. We do not recommend using slower speeds as this can lead to work hardening of the workpiece. In addition, attention should also be paid to cutting heat caused by tool damage.
b. Chip flute
When tapping deep holes, it is necessary to reduce the number of tap grooves to increase the chip space of each groove. In this way, when the tap is retracted, more iron chips can be taken away, reducing the chance of tool breakage due to clogging of iron chips. But on the other hand, the enlargement of the tap flute reduces the core diameter, so the tap strength is affected. So this also affects the cutting speed. In addition, spiral flute taps are easier to chip than straight flute taps.
c. Front and rear angles
A small rake angle can improve the strength of the cutting edge, thereby increasing tool life; while a large rake angle is beneficial for cutting metals with long chips. Therefore, when processing titanium alloys, these two factors need to be considered comprehensively to select an appropriate rake angle. A large relief angle reduces friction between the tool and the chip. Therefore, the tap relief angle is sometimes required to be 40°. When processing titanium metal, grind a large relief angle on the tap to facilitate chip removal. In addition, fully ground taps and taps with relief ground are also beneficial to tapping.
d. Coolant
When processing special materials, it is necessary to ensure that the cutting fluid reaches the cutting edge. In order to improve the flow of coolant, 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.
