Figure 1
| The secret to cutting good threads is to have a good threading tool. Threading tools must have the proper shape to cut an accurately shaped thread, but the clearance and relief angles must be correct to produce a clean thread. The shape of the thread corresponds directly with the form of the thread to be cut (Figure 1). |
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| Grinding a thread cutting tool on a pedestal grinder is not that difficult, but you need to inspect the tool carefully. Accuracy in the tool is of great importance to the outcome of the thread. The design of the high-speed, steel cutting tool for cutting 60-degree unified threads is shown in Figure 2. |
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| This tool has a 60-degree included angle and a flat on the tip which corresponds to the pitch of the thread to be cut. The larger the thread pitch, the larger the flat. The pitch is defined as the distance from a point on one thread to the same point on the next thread (Figure 3). |
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The thread pitch can be calculated by dividing the number of threads per inch into 1, or 1 over the number of threads per inch. An easy-to-understand example of this is a part with 8 threads per inch. Place the number 1 over 8 and you get 1/8. The decimal equivalent of 1/8 is .125. Therefore, the pitch of this thread is .125. The flat on the tip of the threading tool is derived from the pitch of the thread. The flat width is equal to the pitch of the thread divided by the constant of 8. For the part with 8 threads per inch and a pitch of .125, the flat width would be .125 divided by 8, or .015. The flat on the tool to cut 8 threads per inch should be 0.015. The side relief angle should have 10 to 12 degrees of clearance, much like a standard turning tool. Remember, the location of the side cutting edge angle is on the opposite side when cutting left hand threads. The front end relief will be 10-17 degrees depending on the tool holder and the hardness of the material being cut. The harder the material, the more support you want under the tool tip. If the tool tip keeps breaking off, you may have too much front clearance. There really isn’t any top rake ground into a threading tool. The top rake is usually produced by the high speed steel tool holder. If you are using a neutral holder, you may want to grind a slight amount of top rake into your tool. See your instructor concerning neutral tool holders and threading. Now, let's go over the procedure for grinding a 60-degree, right-hand, threading tool.
The first step in preparing to grind a tool is to dress the wheel. Wheel dressing is done to keep the wheel face straight, sharp, and true. A dull grinding wheel can cause heat to get built up in the tool, especially a sharp-pointed threading tool. Heat buildup in the tool is not good because it may make the cutting tool softer, and when we go to cut with it, the tool dulls prematurely. Start the wheel on the pedestal grinder. Always stand to the side when starting a grinder. Let the machine come up to speed and run for at least 30 seconds. Place the wheel dresser on the tool rest. Grip the handle with both hands. Slightly raise the handle. Bring the dresser up to the wheel. Move the dresser back and forth to get rid of the groove in the middle of the wheel face. Look down between the tool rest and the wheel to make sure that the wheel face is flat. Check the distance between the tool rest and the wheel. There should be no more than 1/8" clearance between them.
Before you begin grinding the tool, make sure that there is an ample supply of water or coolant available near the grinder. You are going to need to dip the tool into the water or coolant occasionally. Again, this is to avoid heat building up in the tool.
When you first attempt to grind a threading tool, it helps to lay out the tool. This way you can concentrate more on your technique rather than on your skill at judging angles.
Use a center gage, a scribe, and some dye to lay out the shape of the tool. You won’t need to lay out the side relief because you won't be able to see your layout line. The side relief will come as a matter of feel.
Line the side cutting edge layout line parallel with the face of the rough grinding wheel. Bring the tool into light contact with the wheel. Now take and roll the bottom of the tool into the grinding wheel to achieve the 10-degree side relief angle. Run the tool back and forth across the face of the wheel to avoid grooving the middle of the wheel. If you are doing this correctly, the bottom of the tool will be ground more than the top. As the side cutting edge angle becomes larger, you should see the side relief angle forming. We want to keep this relief at about 10 degrees. But, when you do this for the first time, you are better off having too much angle rather than not enough. Remember to cool the tool off before the layout dye burns off. The layout dye will be a good indicator of heat buildup in the tool.
As you approach completion of the side cutting edge angle, look for the sparks hitting the top of the tool. When the sparks are hitting the top of the tool, it is a good indicator that you are getting a smooth, one-facet, grind.
When we have completed the side cutting edge angle, the next portion of the tool we will need to grind is the end cutting edge angle.
The end cutting edge angle helps form the thread and prevents the end of the tool from rubbing on the work. Line the end cutting edge layout line parallel with the face of the grinding wheel. Bring the tool into light contact with the wheel. Don’t roll the bottom of the tool into the grinding wheel. Hold it flat or parallel with the side relief angle, which was ground previously. If you are doing this properly, the top of the tool will come in contact with the wheel first. Remember to cool the tool off before the layout dye burns off. As you approach completion of the end cutting edge angle, look for the sparks hitting the top of the tool. When the sparks are hitting the top of the tool and the end cutting edge angle meets the side cutting edge angle, you are finished with the rough grind.
Inspect the tool for the condition of the ground surfaces and use the center to gage the accuracy of the angles. Move to the finish grinding wheel. Lightly grind the side cutting edge angle making any minor modifications to the 60-degree angle. The need for these modifications may have become apparent when checking the tool with the center gage. Remember to roll the bottom of the tool into the grinding wheel to achieve the 10-degree side relief angle. As you approach completion of the side cutting edge angle look for the sparks hitting the top of the tool. This is a good indicator that you are getting a smooth, one facet, grind.
When we have completed the side cutting edge angle, the next portion of the tool we will need to finish grind is the end cutting edge angle.
Bring the tool into light contact with the wheel. Don’t roll the bottom of the tool into the grinding wheel. Just hold it flat or parallel with the side relief angle which was ground on previously. When you have completed the end cutting edge angle, inspect the tool for the condition of the ground surfaces, and using the center, gage the accuracy of the angles. Depending on the width of the flat, you may want to hold the tool lightly on the finishing wheel, or you can hone the flat on the cutting tool. Finally, lightly hone the cutting edges of the tool to remove any burrs that may have formed on the tool during grinding.
