Machining technology of titanium alloy deep hole thread
It is difficult to process deep hole thread of special material parts.For example,deep hole tapping on a titanium alloy part is very challenging.It is very uneconomical to scrap a part near completion due to the scraping effect caused by the damage of the tap.Therefore,in order to avoid scraping,it is required to use correct cutting tools and tapping technology.
First,we need to define what deep hole is 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 easy to produce tool damage and thread inconsistency.
In order to solve this problem,two schemes can be adopted:(1)increasing the diameter of the hole before tapping;(2)Use a tap 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 also reduces 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 tapped 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.
Because titanium parts are difficult to machine,it is necessary to fully consider the cutting parameters and tool geometry.
Because titanium alloy has large elasticity and deformation rate,it is necessary to adopt relatively small cutting speed.When machining small holes in titanium alloy parts,the recommended circumferential cutting speed is 10~14 inches/min.We do not recommend using a lower speed because it will lead to cold work hardening of the workpiece.In addition,pay attention to the 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 diameter of the tap groove is increased,but the strength of the tap groove is reduced.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,so as to increase 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 rake angle.
Large back angle can reduce the friction between tool and chip.Therefore,the back angle of tap is sometimes 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 back grinding are also conducive to tapping.
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,internal cooling tap can be considered.
An aircraft parts manufacturer needs to conduct deep hole tapping on one part.The material of the part is grade 7 titanium alloy.During machining,the circumferential cutting speed is 13 inches/min,and coolant is used at the same time.
In order to ensure the accuracy of parts,the operator shall replace the tap in time before it is blunt.When the tap is worn,the sound generated in the cutting process will change.By listening to these sounds,before machining,the operator can determine the number of threaded holes that can be machined before the tap is worn.
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 replaced conveniently and timely.