Tap Performance Trouble Shooting - Part 5: Chip Welding

August 5, 2019 10:30 am

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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 Chip Welding.  We are offering suggestions and changes that can be made to counter the effects of chip welding.  For further information regarding chip control or premature tool wear, please see other articles in this series such as insufficient tool life, chip control, excessive wear, fractures or tool breakage.

Chip welding occurs when the workpiece material begins to stick to the tapping tool.  This reduces the effectiveness of the tapping operation and can ultimately lead to the end of tool life.  Depending on where we see the chip welding, we can take specific action to address the issue.

Typical Chip Welding issues are typically caused by one of the following problems:

1. Jammed Chips

2. Chips welding in the flank area

 

Let’s take a look at them and see how to solve them.

 

Jammed Chips

Chips jamming

Chips Jamming when the tool is reversed can cause chip welding and ultimately break the taps.

 

Cause:

The picture above shows a chip that has been jammed while reversing. This type of welding can occur when machining Al alloys and using taps with high relief angles.

This failure mode can occur when the chip thickness is low, and they are not being controlled properly due to their stringy nature.

When machining work piece with short chipping materials (e.g. grey cast iron) it can occur that very small chips are getting jammed and lead to the chip welding.

This type of chip welding does often lead to tool breakage.

 

Solution:

Find a method to increase the chip thickness and reduce the rake angle.  The reduced rake angle will prevent the chips from getting jammed.  And the higher chip thickness will make the chips easier to control and keep in the flute area.

Here’s what we recommend:

  • Reduce relief angles designed into the tap
  • Reduce number of flutes on the tap (chip thickness is increased)
  • Reduce chamfer length or number of teeth in the chamfer area (chip thickness is increased)

 

Chip Welding in the Flank Area

Chip Welding in the Flank Area can cause premature end of tool life

 

Cause:

Friction between the work piece and the tool can cause chips to weld onto the tool in the flank area.  This results in premature tool wear and can end the life of the tool.  Additionally, this type of wear could also be caused by a chemical affinity between the tool and the workpiece material: ex. TiN coating in Titanium materials.

 

Solution:

There are several different approaches to remedy this problem.  But the main idea is to improve the overall process by controlling the chips, reducing the heat being produced during the tapping process, and ensure that the tool material is appropriate for the work piece material.  Try to incorporate as many of the following actions as possible:

 

  • Choose a tool with higher flank relief angles to reduce friction
  • Choose a suitable coating / surface treatment based on the workpiece material
  • Improve the coolant supply by adding internal coolant if not using already. If internal coolant supply is already being applied, increasing the coolant supply pressure or even the concentration level of water soluble coolant to increase lubricity.
  • Reduce the cutting speed to lower temperature and therefore reduce chip welding
  • Make sure that the tap cuts free by using tools with a high helix angle and a high rake angle. This results in a tool with sharp cutting edges.

 

Steam Oxide Tools in Aluminum Alloys to control chip welding

 

Taps that have a steam oxide surface treatment, may look like the material deposits on the rake face are worse than an uncoated tool, but they are in fact the same

 

When machining threads in Aluminum alloys, a sharp cutting edge is desired.  Using a tool that is uncoated or a tool with a steam oxide treatment helps to maintain a sharp cutting edge because there is no coating to round over the cutting edge.

After a few threads, both tools show material deposits in the chip flutes. These deposits are generally more visible on the steam oxide tools due the dark color of the material.  An analysis of the tools may lead to a determination that there is more build up on the steam oxide treatment than the uncoated tools.  This, however, is not the case.  The material built up on both tools is very much the same.

In both cases, function and thread quality is not affected by these deposits. Accuracy to gauge is also comparable and there are neither built up edges nor chip welding in the flanks. The deposits eventually level off and do not tend to increase with further usage of the tools.

However, chip formation is better when using steam oxide treated tools.  Because of the friction created by the steam oxide, it does a better job of curling the chip.  See the chip comparison analysis in the photos below.  You can see after the same number of threads, the steam oxide tools are doing a better job of curling the chips.

Using the steam oxide surface treatment on taps in Aluminum alloys does a better job of controlling the chips over an uncoated tool.

 

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.

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