Processing technology of titanium alloy deep hole thread

It is difficult to machine deep hole thread for special material parts.For example,deep hole tapping on a titanium alloy part is very challenging.It is very uneconomical to scrap a part that is close to completion due to the scraping effect caused by the broken tap.Therefore,in order to avoid scraping,it is required to use correct cutting tools and tapping technology.
First of all,we need to define what is deep hole and why it needs special consideration.In drilling,those holes with a depth greater than 3 times the aperture are called deep holes.Deep hole tapping means that the tapping depth is more than 1.5 times of the tap diameter.For example,when a tap with a diameter of 1/4"is used to process a thread with a depth of 3/8",this situation is usually called deep hole tapping.
Machining a deep hole thread means a long time of contact between the tool and the workpiece.At the same time,more cutting heat and greater cutting force will be generated in the machining process.Therefore,tapping in small deep holes of special materials(such as titanium parts)is prone to tool damage and thread inconsistency.
To solve this problem,two schemes 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
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 in the tapping process.But it will also reduce the thread contact rate.
The national standards and Technical Committee stipulates that in deep holes,only 50%of the full height of the thread is allowed to be punched on the hole wall.This is particularly important when tapping holes in special materials and difficult to machine materials.Because although the thread contact rate decreases due to the reduction of the thread height on the hole wall,the reliable connection of the thread can still be maintained due to the increase of the thread length.
The diameter increment of threaded bottom hole mainly depends on the required thread contact rate and the number of thread heads per inch.According to the above two values,the correct diameter of threaded bottom hole can be calculated by using empirical formula.
2.Cutting parameters
Because titanium parts are difficult to machine,cutting parameters and tool geometry need to be fully considered.
cutting speed
Because titanium alloy has large elasticity and deformation rate,it needs to adopt relatively small cutting speed.When machining small holes in titanium alloy parts,the recommended circumferential cutting speed is 10~14 inches/minute.We do not recommend using a lower speed,because that will lead to cold work hardening of the workpiece.In addition,it is also necessary to pay attention to cutting heat caused by tool damage.
Chip holding tank
In deep hole tapping,it is necessary to reduce the number of tap slots to increase the chip holding space of each slot.In this way,when the tap retreats,it can take away more iron filings and reduce the chance of tool damage caused by iron filings blockage.On the other hand,the increase of the chip holding groove of the tap reduces the core diameter,so the strength of the tap is affected.So this will also affect the cutting speed.In addition,spiral groove taps are easier to remove chips than straight groove taps.
Front and rear corners
Small rake angle can improve the strength of cutting edge,thus increasing the tool life;Large rake angle is conducive to cutting long chip metal.Therefore,when machining titanium alloy,it is necessary to comprehensively consider these two factors and select the appropriate front angle.
Large back angle can reduce the friction between tool and chip.Therefore,sometimes the back angle of the tap is required to be 40°.When processing titanium metal,a large back angle is ground on the tap,which is conducive to chip removal.In addition,fully ground taps and taps with blade back shovel grinding are also conducive to tapping.
Coolant
When machining special materials,it must be ensured 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,internal cooling tap can be considered.
3.Application examples
An aircraft parts manufacturer needs to conduct deep hole tapping on a part.The material of this part is grade 7 titanium alloy.During machining,the circumferential cutting speed is 13 inches/minute,and coolant is used at the same time.
In order to ensure the accuracy of parts,the operator should replace the tap in time before it is blunt.When the tap is worn,the sound produced in the cutting process will change.By listening to these sounds,the operator can determine the number of threaded holes that can be machined before the tap is worn before machining.
The factory has two tapping stations on each tapping equipment,equipped with the same tap.When one of the taps is worn,it can be easily and timely replaced.