What is deep hole tapping?
First of all, it is necessary to clarify the definition of "deep hole". In drilling, holes with a hole depth greater than 3 times the hole size are usually called deep holes. For the tapping process, deep hole tapping refers to the tapping depth exceeding 1.5 times the diameter of the tap. For example, when using a tap with a diameter of 1/4 inch to machine a thread with a depth of 3/8 inch, it is typical for deep hole tapping.
The difficulty of deep hole tapping
Deep hole threading means that there is a long period of contact between the tool and the workpiece, creating more cutting heat and more cutting force in the process. When the processing object is titanium alloy and other special materials, and it is a small diameter deep hole, it is easy to cause tool damage and thread size inconsistency, which seriously affects product quality.
Two core solutions
To solve the above problems, the following two effective measures can be taken:
Increase the diameter of the pre-tapping hole
The right thread bottom hole diameter is a prerequisite for ensuring tapping quality. Increasing the diameter of the bottom hole can effectively reduce the cutting heat and cutting force during tapping, although it will correspondingly reduce the contact rate of the thread.
According to the National Committee on Standards and Technology, the allowable thread height is only 50% of the full height in deep hole machining. This standard is particularly important for small hole tapping of special materials such as titanium alloys and difficult-to-machine materials. Although the thread height has been reduced, reliable connection performance is still guaranteed due to the increased thread length.
The diameter increment of the threaded bottom hole depends primarily on the desired thread contact rate and the number of threads per inch. Based on these two parameters, a reasonable bottom hole diameter can be accurately calculated using empirical formulas.
Adopt taps designed for deep hole tapping and optimize cutting parameters
Titanium alloy materials have the characteristics of high elasticity and high deformation rate, so they must be specially designed for cutting parameters and tool geometry.
Two core solutions
Cutting speed
A lower cutting speed is recommended, with a circumferential cutting speed of 10~14 inches/min. Too high a speed will aggravate tool wear, while a low speed can easily lead to cold work hardening of the material. At the same time, it is necessary to be wary of the accumulation of cutting heat caused by tool breakage.
Chip tank design
When tapping deep holes, the number of grooves of the tap should be appropriately reduced to increase the chip holding space of each groove, which is convenient to take away more chips when retracting the tool and reduce the risk of tool damage caused by chip blockage. However, it should be noted that the increase in chip chamber will weaken the core diameter of the tap and affect its strength, so it is necessary to balance the cutting speed. In addition, spiral groove taps have better chip evacuation performance than straight groove taps.
Anterior and posterior angles
Rake angle: small rake angle can improve the cutting edge strength and extend the tool life; The large rake angle is conducive to cutting long chips. When machining titanium alloys, it is necessary to consider comprehensively and choose the appropriate rake angle.
Trail Angle: Increasing the back angle reduces friction between the tool and the chip. In titanium alloy processing, it is recommended to reach a rurality angle of 40°, and with a fully ground tap and blade back shovel design, it helps to improve the chip evacuation effect.
Coolant
When machining special materials, it is important to ensure that the cutting fluid reaches the cutting edge sufficiently. To improve coolant flow, it is recommended to install a cooling groove on the back of the tap blade. If the tap diameter is sufficient, an internally cooled tap can also be used for more efficient cooling and lubrication.
By reasonably increasing the diameter of the thread bottom hole, selecting special taps, and optimizing the cutting parameters, the reliability and economy of deep hole threading of special materials can be significantly improved, and the risk of scrapping parts can be effectively reduced.
