- Tips on machining 2018/05/23 UP
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Tips on Machining Vol. 18
Tapping trouble and solutions
Part 1 (Preventing breakage)
- Tag
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- Preventive maintenance
- Milling
Tapping is often the final machining process performed, making it very risky if failure occurs. This is especially true for M4 or smaller taps, which are thin and have low rigidity, and can often cause issues with breakage and machining precision faults.
We sometimes receive inquiries about tapping on topics such as sudden breakage, thread tearing, and thread gauge go/no-go abnormalities. In this series of articles, we will be discussing tapping breakage, thread defects, and thread accuracy faults. We will begin with tap breakage.
● Dealing with tap breakage
If a tap breaks, it must be pulled out. Some methods for removing a broken tap include striking it with a sharp pin so that it turns in the opposite direction, or drilling a hole in it to remove it (Sandvik hard cut drill). For large workpieces, electric discharge machining can be used to remove a broken tap.
Removing a broken tap is a lot of work, and the material may even need to be discarded. This is why it is so important to know what causes tap breakage and how to prevent it from happening.
Figure 1-2: Breakage
Figure 1-1: Chips entangled in a tap
Figures: Hard cut drill work procedure
● What causes tap breakage and how to prevent it
One cause of tap breakage is using the improper pilot hole diameter, tap type, or coating in situations where machining conditions have yet to be set, such as when fabricating a prototype. One way of preventing trouble at this point is to review the tap and cutting conditions.
In contrast, unanticipated and sudden breakage can sometimes happen on mass production lines. This occurs when the tap bites into chips during machining, which causes an instantaneous and sudden rise in cutting torque that exceeds the rigidity of the tap. The causes of a tap biting into chips can be broadly classified into the following three categories. We will provide solutions for each.
A: Chips enter the pilot hole (chips enter in between the tap and pilot hole)
B: Chips are pulled up when tapping through and become entangled in the tool
C: Chips become tangled when raising the tap after machining to the specified depth
Case A: Chips enter and remain in the pilot hole
A tap can break due to chips entering the pilot hole. This occurs when chips on the workpiece fall into the pilot hole and are bitten into by the tap as it approaches the bottom of the pilot hole (it becomes clogged with chips). There are two solutions.
(1) Review coolant usage method
When machining a pilot hole, applying coolant around the spindle may cause chips on the workpiece to enter the pilot hole. This is especially true for standing machining centers. The solution is to use through spindle coolant and to machine while blowing chips away. This will almost completely prevent chips from entering and remaining in the pilot hole.
(2) Review pilot hole cleaning method
Dry machining is a type of machining where coolant is not used, for applications as such as casting. During dry machining, the operator uses air blowing to clean the pilot hole prior to tapping. Using an oil hole holder, which supplies air instead of coolant, is effective in such cases. It cleans the inside of the hole automatically without any operator input required, which can help ensure that cleaning is performed.
Case B: Chips are pulled up when tapping through and become entangled in the tool
When using a point tap for through hole threading, if there is insufficient space between the workpiece and jig, chips may become entangled with the tap because the chips will not be ejected when they make contact with the chips that have accumulated on the jig. The solution is to ensure sufficient space to allow for a steady flow of chips.
Case C: Chips become tangled when raising the tap after machining to the specified depth
When using a spiral tap for blind hole machining, raising the tap can sometimes cause chips to become entangled. This is caused by chips becoming entangled with the tool due to a short tool protrusion or narrow space between the holder and jig, or due to a thick holder preventing chips from escaping. This can be resolved by switching to a tool with a longer protrusion and using a thinner holder, so that there is sufficient space between the tool and jig for chips to escape.
As discussed above, applying a bit of ingenuity prior to machining is the key to preventing tap breakage caused by the tap biting into chips. It is important to ensure that chips are not brought down into the pilot hole.
If you have any concerns or questions about tapping, please feel free to contact us. One of our experienced engineers will be happy to work with you on a solution.