Tap Performance Troubleshooting – Part 8: No-Go Gage Enters 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 no-go gage enters 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 pitch diameter being too large and allows the no-go gage to start in the hole. For further information on other issues such as thread quality related issues, go gage does not enter the thread, insufficient tool life, chip control, excessive wear, fractures , chip welding, and tool breakage please see other articles in this series.
When the no-go gage can be threaded into the part, it is considered out of spec and not acceptable. There can be several causes for this that we should look to for correction, such as:
- Tool holding
- Chip control
- Built up edge
- Excessive runout
Let’s take a look at each of these causes and identify a solution.
If a floating holder is being used, it is possible to have axial mis-cut due to the movement that is allowed. Axial mis-cut occurs when the cutting forces acting on the tap try to move the tap at a different rate than the machine is feeding. It works in opposite directions when comparing spiral point and spiral flute taps.
Spiral point taps have the cutting forces pushing up on the tap. As a result, the cutting forces are preventing the desired rate of movement into the part. This creates a case where the tap re-cuts the top part of the thread profile creating an unacceptable surface and an oversized thread pitch. The oversized thread pitch allows the no-go gage to be threaded into the part.
Spiral flute taps have the same condition in the opposite direction. The cutting forces are trying to pull the tap into the part faster than the desired rate of movement. This causes the axial mis-cut to occur on the bottom side of the thread profile. However, it has the same result as when it occurs in the spiral point tap: the thread pitch diameter is oversized which allows the no-go gage to thread into the part.
The first action is to make sure the tap being used is appropriate for the material. If the cutting geometry is not designed to work in the material, it could cause the axial mis-cut situation to worsen. Characteristics such as the coating or helix angle should be optimized for the material.
It is also possible to reduce the feed rate to 90%. This will ensure that the floating holder is under constant tension. Having the floating holder under constant tension will minimize the amount of movement and stabilize the tool reducing the ability for mis-cut.
Finally, if the machine spindle and controller have the capability, switch to rigid tapping. Most modern machine tool spindles have the capability to rigid tap. Floating holders or full tension/compression holders are not always necessary. If it is possible to not use these holders that allow movement, the axial mis-cut condition can be avoided completely.
Radial mis-cut can occur when having problems controlling the chips. Unequal cutting forces from chip packing or chips welding to the part can cause an oversized thread pitch diameter. Evidence of this is birdnesting, random torque spikes, fractures and tool breakage. The higher the hardness of the material and the lower breaking elongation of the workpiece material, the better the chips can be controlled. For soft structural steels, low alloyed steels of low hardness, and stainless steels, chip control can be very difficult.
The best solution for better chip control is to select a tap that is better designed to control the chips in the material being machined. For more in-depth details on controlling chips while tapping, please see a previous article in this troubleshooting series called Tap Performance Troubleshooting – Part 2: Chip Control.
Built up edge
When machining soft or gummy materials, built up edge (BUE) can occur and cause the tap to cut oversized. In these cases, action against the BUE needs to be taken because it will not be possible to cut threads within specification while the material is sticking to the tap.
The surface treatment of the tap may not be appropriate for the material being machined. If non-ferrous material is being machined, uncoated tools or even polished tools are preferred. The sharp, smooth surface makes it more difficult for the material to stick. However, if the material being machined is carbon steel, titanium, or nickel based alloys, the opposite is true. These materials require a coating that is properly specified by the tooling manufacturer. Stainless steels are usually the most common materials for this particular problem, and they of course require a unique solution. Surface treatment of the taps such as a VAP coating or steam oxide are the most preferred. The cutting edge does not get rounded over from having a coating covering it, this would hurt chip control. Yet, it is not left as unprotected compared to having it completely uncoated.
The BUE could also be a sign that the tool is simply worn. Tools that have dulled from abrasive wear generate more heat and do not machine as effectively due to higher cutting forces. In these cases, replace the tool.
Optimizing the coolant supply can have a great effect on preventing the BUE from occurring. Position the coolant lines so that they are flushing the chips as effectively as possible. Sometimes, the coolant is oriented so that it is actually washing the chips back into the hole. Ensure that this is not happening. If internal coolant is not being used, switch to a tap that accommodates internal coolant. The coolant will help with chip control. In all cases of either internal coolant or external coolant, try to maximize the coolant pressure.
Depending on a variety of issues, the tap speed may be either too high or too low. Whichever the case, make sure to optimize the cutting speed. The best resource for this is to consult with the recommendations directly from the tap manufacturer. Many manufacturers even have these resources available online.
Runout is described as a tool spinning around an axis that is not the tool’s axis. It makes the tool wobble instead of spinning symmetrically. Runout can be easily checked with a dial indicator. Zero the indicator on the tool’s diameter and rotate the tool 360°. The dial indicator should not move off of zero. If there is excessive runout, the tool will cut oversize since the tool diameter will now behave like the actual tool diameter plus the amount of runout. When the thread pitch diameter is too large, the no-go gage can be threaded into the part.
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.