Tap Performance Trouble Shooting - Part 6: Tool Breakage
A series from Walter Tools USA
The following is one part in a series of articles regarding performance problems associated with the tapping process. The solutions focus on specific actions to counter specific issues.
This article addresses remedies for the issue of Tool Breakage. We are offering suggestions and changes that can be made to counter the effect of breaking the tapping tool. For further information regarding chip control or premature tool wear, please see other articles in this series such asinsufficient tool life, chip control, excessive wear, fractures orchip welding.
When breaking taps, we need to pay attention to several different areas of the complete tapping cycle. Answering the following questions help us identify the cause of tool breakage and its solution:
1. Does the tool break when reversing?
2. Are there problems with chip control?
3. Does the tool run into the bottom of the cored hole?
4. Are the machining conditions unfavorable such as long overhangs or unstable clamping?
5. Is a suitable tool being used for the application?
6. Is the quality of the core hole good enough?
Let’s analyze each question.
Does the tool break when reversing?
Tool breakage tend to occur most often when reversing the tap at the bottom of the threaded hole. There are four main causes that we need to look at to properly identify the root cause.
Does a chip get jammed between the tool and the component?
When reversing the tap, it has to shear-off the chip root with the back part of the land. If a tool has a high chamfer relief angle and the chip thickness is small, then the chip can get jammed between the tool and the component. This can lead to tool breakage.
To remedy this, reduce the chamfer relief angle and / or increase the chip thickness so that the chip doesn’t get stringy. You can change the chip thickness by reducing the number of flutes or by using less teeth in the chamfer area. A larger chip thickness improves chip formation and makes it easier to break the chip root.
The reduction of the number of cutting edges will reduce the tool life, but if the tool life is determined by tool breakage, the tradeoff should still result in a net improvement in tool life.
Does the material spring back when stopping to reverse the tap?
When reversing, the tool is stopped for a short amount of time. If materials with spring-back effect are being machined, it can happen that the tool gets jammed from the tapped hole closing up after the cutting is complete.
Poor lubrication and tools that are not free-cutting can make this problem worse. While more negative geometry can promote better tool life and chip control, they can make the spring back condition worse in certain materials.
Free-cutting tools with high rake & clearance angles, high helix, and sharp cutting edges exert less force on the part being machined resulting in less spring back effect and produce a thread size that is truer to net size.
In extreme cases, tools with removed teeth in the guidance part can be used to reduce the ability of the material to grab the tool. But this will reduce the self-guidance capabilities of the tap.
Does the tap have difficulty shearing-off the chip root with the back part of the land?
When reversing, the tap has to shear-off the chip root with the back part of the land. If a tap with a low number of flutes and thin lands is used, the tool has a significant movement in the axial direction before shearing-off the chip root with the back part of the land. Therefore, the load that is needed to shear-off the chip root is only applied to a small section of the tooth. This can lead to breakage.
To combat this, increase the number of flutes so that more teeth can be applied to the cutting action. Also, increasing the width of the tool land will make a stronger tool that can handle the higher cutting force.
Is rigid tapping being used?
When rigid tapping, the cutting forces can be very high, particularly if tapping very deep threads or machining them at very high sfm. The torque exerted on the part when reversing, is maximized with rigid tapping.
Using a Synchronous tapping head with minimum compensation will reduce these high tapping torques which cause the tap to break. Not only does the Synchronous tapping head reduce the overall value of the torque, it also softens the transitions through the different phased of the tapping cycle. Thereby increasing the overall tool life of the tap.
Are there problems with chip control?
Evidence of problems with chip formation and chip evacuation is bird nesting, random torque peaks, fractures and tool breakage. These are all signs that the operation is suffering from poor chip control.
For detailed information on how to resolve chip control issues, see the previous article in this series on chip control.
Does the tap run into the bottom of the core hole?
When calculating the drill depth to allow a full thread to the required depth, it is important to remember that there is a lead on the front of the tap (often referred to as the chamfer area). At the bottom of the hole, the tap will not produce a full thread in this area. Therefore, the drilled hole needs to be deep enough to allow for the chamfer area below the required full thread depth.
The same problem is caused by the presence chips left in the bottom of the hole. This would have the same effect as not having a core hole deep enough by minimizing the room required for the tap and could ultimately break the tap.
The easy solution is to make the core hole deeper. However, this is not always possible due to stringent part requirements. If chips remaining in the hole are causing the problem, focus your efforts on removing these chips by using an tap drill with internal coolant to evacuate the chips during the drilling process. Finally, using a tap with a shorter lead, or fewer teeth in the chamfer area, will allow the full thread to be machined closer to the bottom of the hole and reduce the need for a deeper core hole.
Are the machining conditions unfavorable?
When having unfavorable machining conditions, such as poor synchronized spindle, worn chuck, unstable machine, unstable clamping, long protrusion, and insufficient lubrication, tool breakage can occur. Taps with high hardness, for the purpose of high wear resistance, such as solid carbide and nitride, tools require stable conditions.
If high hardness taps, such as solid carbide taps, are being used, switch to a tougher substrate such as HSS-E or HSS-E-PM. These taps are much tougher and are better suited to handle unfavorable machining conditions.
As mentioned above, using a Synchronous tapping head with minimum compensation will reduce the high tapping torques which cause the tap to break. While it is always recommended to incorporate this type of chuck, it is particularly important if breaking taps is a common occurrence.
More common-sense solutions are also applicable in cases such as this. Actions such as slowing the rpm of the application or improving the coolant situation are always prudent. Improving the coolant situation could involve increasing the coolant pressure or even increasing the concentration level.
Is the tap being used appropriately for the application?
Most tap providers offer very large selection of various types of taps. This is because many of those taps are designed for specific types of applications. They are either designed for specific materials, specific hole configurations – such as thru hole or blind hole – or even for required depth of thread. It is important to understand if the tap being used has been designed for the application it is being applied. This is one of the most common causes of taps failing.
Is the quality of the core hole good?
Problems with the core hole could be a range of issues which include diameter of the core hole, positioning of the core hole, straightness of the core hole and roundness of the core hole. Any one of these issues would have a negative effect on the tapping process.
When having problems with the drill quality, the drilling process should be examined. If the drill that is used is too long, the core hole quality regarding true position or hole straightness could suffer. A worn drill could introduce an excessive amount of heat into the drilling process which would result in a larger hardened layer (edge zone hardening). The required torque for machining the thread through a larger edge zone hardened area will increase and tool breakage can occur. If the core hole is drilled with a pecking cycle the edge zone hardness will differ depending on the depth. So it is critical to understand where the tool is breaking.
If, after all this troubleshooting, the problem cannot be solved, it may be time to consider switching the thread producing process. Thread Mills and thread formers offer many advantages over cut taps, particularly in the area of chip control.
Didn’t find a solution to your issue? For a remedy that does not involve such a direct customization, try a tap designed around a universal application range and can be appropriate for a wide range of materials. You can also find additional details in this Threading Handbook.