Tap Performance Troubleshooting – Part 9: Thread Quality Related Issues

October 29, 2020 11:15 am

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A series from Walter Tools USA

The following article is one part in a series of articles regarding quality problems associated with the tapping process.  The solutions focus on specific actions to counter specific issues.

In this post we’ll provide remedies for several issues which relate to the overall thread quality.  The goal is to offer you suggestions and changes that can be made to increase the quality of the tapped hole.  For further information on tapping related issues such as go gage does not enter the thread or no-go gage enters the thread,  insufficient tool life, chip controlexcessive wearfractures , chip welding, and tool breakage please see other articles in this series.

Poor surface finish

The quality of the surface finish produced on internal threads could be poor enough to make the thread out of spec. The part could have a specific surface finish requirement that must be met. Or the surface finish could be so poor that it actually affects the way the  go gage threads into the part. Either way, there are corrective actions that can be taken to improve the surface finish.

Poor surface finish could be so bad that it actually causes the go gage to not properly thread into the part.

Built Up Edge (BUE) is typically the number one problem causing poor surface finish.  It can also cause the tap to cut the thread pitch diameter oversized. This problem was addressed in the previous article. Please reference that article for troubleshooting suggestions.

The wrong cutting geometry, for the particular material being machined, could also cause poor surface finish, either too sharp, or too negative. In a previous article – Tap Performance Troubleshooting – Part 2: Chip Control – multiple changes were highlighted that would help control the chip. However, most of these changes can have a negative effect on surface finish. A careful balance of feeds and speeds needs to be achieved so that consistent process reliability can be maintained. Using an online resource to optimize these cutting parameters is typically a good place to start.

Burr at entry of thread

Rolling a burr on the top side of the threaded hole could cause the  go gage to not enter the thread, even though a good thread was produced. If the top of the hole is not chamfered prior to tapping, adding one will help correct this problem. Chamfering the top of the threaded hole is generally a good idea anyways. Not only does it help correct this problem, it also improves tool life and the overall quality of the threaded hole. There are even tools available to add chamfering capability to standard solid carbide drills so that cycle time and tool changes are not affected.

Burrs at the entry of the threaded hole can typically be resolved by adding a simple chamfer to the top of the hole before tapping

If the top of the hole already has a chamfer and the process is still generating a burr at the top of the hole, the quality of the chamfer may not be good enough. The chamfer could be uneven or not centered on the hole. Ensure that the chamfer is centered evenly on the predrilled hole. If this still doesn’t correct the issue, try varying the included angle of the chamfer. It may be possible that the chamfer angle is conflicting with the lead in angle of the tap chamfer area and is causing a burr to roll.

Burr at exit of thread

If a through hole is being tapped, it may also be possible to get a burr at the exit of the hole. The first thing to check here is when the burr shows up. After using the tap drill, check to see if the burr is there. If so, then it is obviously caused by the tap drill and it should be addressed there. If not, then it is most likely created by the tap. In this case, the exit hole should be chamfered after the tap drill. The same quality check should be considered regarding the size and shape of the chamfer as was discussed above when talking about the burr at the entry of the thread. Finally, it could be simply caused by a worn tool. Corrective action would be to replace the tool with a new one which has a clean, sharp cutting edge. Check out the previous article titled Tap Performance Troubleshooting – Part 1: Insufficient Tool Life, to learn how to prolong tool life.

Chip root does not get cutoff

In blind hole applications, the tap has to deal with the issue of shearing off the chip root at the bottom of the hole. The chip root is created at the moment the tap is stopped for the intent of reversing. The material that was being machined at that exact moment is still connected to the parent material.

Chip root

When reversing the tap, the chip root, left attached to the parent material, needs to be dealt with by the backsife of the tap land.

Not cutting the remaining chip root is normally more common in materials with difficult-to-machine properties, but is also a function of the tap being used. Using a tap with a small number of cutting flutes, such as two flutes for example, will create a thicker and more difficult chip to remove.  If the tap has a high clearance angle, this would help the tap run faster, but would make it more difficult to cut the chip root. It could allow the chip to get pinched between the tap and the part resulting in the tap breaking. Cutting the chip root in harder materials also becomes much more difficult due to the hardness of this chip. In these cases, a special tap, with special geometry to deal with this hard chip, may be required.

Conclusion

If the above mentioned troubleshooting techniques did not solve the problem, it’s possible that a different threading technique may need to be used such as thread forming or thread milling.

Didn’t find a solution to your issue? For a remedy that does not involve such direct customization, try a tap designed around a universal application range that can be appropriate for a wide range of materials like Walter Tools’ TC117 / TC217 Advance. These universal taps provide excellent process reliability, outstanding chip control, and high productivity in a wide range of materials. Check out this video that demonstrates the performance of these unique tools.

 

 

Check out this Threading Handbook to learn more thread production technology, trends, and innovation, or to find the right tool for your needs.

Author:  Luke Pollock, Product Manager at Walter USA, LLC.

 

1 Comment

  • Rubi says:

    In the early 1950s I was an apprentice fitter and turner in the mechanical workshop of the city of Cape Town South Africa. Another apprentice had made up 40 pieces of screwed bars, 1.125″ diameter screwed length 3″, which he had made up on our threading machine. The free end was then bent 90 degrees. Only then did they find that he had been careless and that the nuts would not go on. As he was now ill, I was told to free the threads. Because of the bent end I could not put back into the threading machine and now had to do each one by hand. And all we had was a dienut! I started working with a lot of oil and a long spanner and was making no progress except for sweating. But I was lucky. One of the fitters came to me, winked and said: do it another way. Make the nuts larger. Take the number one tap and wrap brass shim around but leave one flight free and use a lot of oil. The shim will now push the tap off center and as you screw it in to the nut, the free flight will cut into the thread and increase the nut thread diameter. It worked perfectly and within a few hours I had 40 nuts which screwed onto the bars very well.
    This is of course a true story for maintenance fitters and may help somone in the future.

    After completing my apprenticeship I went on to study mechanical engineering.

    Reuben Sluszny BSME (Cape Town) MSM (Boston) born 1934.

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