Tap Performance Troubleshooting – Part 7: Go Gage Does Not Enter the Thread
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 the issue of when the go gage does not enter the thread. The goal is to offer you suggestions and changes that can be made to increase the quality of the tapped hole. This particular problem typically stems from the issue of the thread being too small for the thread gage to start. For further information on no-go gage enters the thread, thread quality related issues, insufficient tool life, chip control, excessive wear, fractures, chip welding, and tool breakage please see other articles in this series.
When the go-gage cannot be threaded into the part, the part is considered out of spec and not acceptable. There can be several causes for this that we should look to for correction.
- Is the thread clean?
- Does the material tend to spring back?
- Is the tool heavily worn?
- Does the component have thin walls?
Let’s analyze each question.
Cleaning the thread
If chips or small particles remain in the thread, the gage can get stuck. This could be interpreted as the thread not being the proper size, but it’s really more of an interference issue. It’s important to clean the thread with compressed air before ever checking the thread, even for the first time. If chips or small particles are in the thread when trying to use the thread gage, it could be possible to press the chips into the thread thereby becoming “stuck” or attached to the thread. In these cases, compressed air might not be sufficient to clean the chips or interference from the thread. Cleaning should be done before gaging.
Spring back material
Some materials have a tendency to spring back after being machined. Most easily recognizable are those materials with high nickel content such as stainless steels (ISO M) and other nickel based alloys (ISO S). Because of the spring back effect, the threaded hole would have a tendency to shrink slightly after machining. This would cause the thread to be slightly too small for the gage to check properly.
To remedy this problem try to use a very free cutting tap such as Walter Tool’s Synchrospeed taps. A free cutting tap will cut the thread truer to size by efficiently cutting the chip. If the tap is not free cutting and has more of a negative cutting geometry, this will push harder on the material while cutting and make the spring back effect worse. The harder the material is pushed when cutting, the more it will want to spring back when the cutting is complete.
Freer cutting tools are essentially sharper tools with more positive cutting geometry. This means that these tools will have characteristics such as higher rake face angle, higher clearance angles, higher helix, and are uncoated. These features give the tool a sharper cutting edge and will help achieve the desired free cutting action.
It’s also possible to remedy this problem by using a tap with a slightly oversized tolerance. Taps that are considered or labeled “oversized” might be an overcorrection, the solution may only require a correction of .0005”-.001”.
With increasing wear, the tool becomes blunt and does not cut free anymore. If you had a material with a tendency to spring back, such as the ones just previously discussed, this would exacerbate the problem. But a worn tool also begins to lose size. In this case, the tap would simply be undersized to cut the desired diameter.
The immediate response should be to replace the tool. Corrective action would start with examining the tap drill operation and try to optimize it. Problems with the tap drill such as a worn tool, or incorrect cutting parameters, could cause excessive heat resulting in edge zone hardening of the drilled hole. This will have a negative effect on the tap tool life since it now has to deal with a material that has been hardened. Improving the drill performance will improve the life of the tap.
Another solution would be to look for a tap constructed of a harder substrate. Manufacturer’s do have a variance in High Speed Steel substrates and hardness based on the type of material the tap is designed to machine. It’s also possible to switch to a tap constructed from solid carbide. Solid carbide taps have a significant better wear resistance based on the substrate when compared to High Speed Steel construction, but they are also much more expensive. For more details on improving tap tool life, please see a previous article in this troubleshooting series called Tap Performance Troubleshooting – Part 1: Insufficient Tool Life.
Having components with thin walls that can be easily moved or bent, creates a problem similar to the spring back issue. Namely, the thread can shrink once the tool leaves the cut. So, the corrective actions here are similar to those listed for in the spring back materials section above.
We want to use sharp, free-cutting tools so as not to exert excessive forces on the part. Cutting the material as cleanly as possible will help reduce the effect of bending the thin wall components. Also, a slightly oversize tap may also help correct the gaging issue.
It would also be important to review the clamping. The heart of the problem with thin-walled components is keeping the parts from moving or being bent. See if it is possible to improve the clamping setup that would prevent the thin wall component from moving during the machining process.
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.