Oster Power Threading Handbook
General Thread Cutting Procedures
- Comply with all of the safety precautions outlined in your machine manual and in the safety notes section of this manual.
- Insure that a good grade of cutting fluid is in the machine and that it is clean. Clean the chips out of the machine and clean the oil filter.
- Make certain that the machine and die head are clean. Clean oil on the surfaces and inside most of the components is not a problem, but if chips, pipe lacquer, or scale are allowed to accumulate in the machine components, problems will result. It is wise to occasionally disassemble the die head and remove any accumulation of foreign material. This practice will increase the life of the die head and promote better threads in that the die head will open easier, and operate more consistently. Excess accumulation of sludge in the machine components can be prevented by draining the oil out of the reservoir and cleaning the sludge out of the bottom of the reservoir. The oil which has been drained off can then be put back in the machine.
- Install proper dies in the die head insuring that no chips or other foreign materials are in the die slots. Proper dies are ground with the right rake and throat angle for the material being cut and are built for the diameter of the material. The dies should move freely in and out of the die head but shouldn't be sloppy from side to side. Make certain that the dies are in proper order in the die head. Set the die head to the proper diameter following the procedure outlined in the operator's manual and lock the die head adjustment in place. With the die head locked on diameter, the dies should float radially in the head over a small distance. They should not have excessive travel from side to side. The exact amount of these movements is variable depending on the manufacturer.
- Clean the chuck jaws with a brush and insert the material to be threaded in the chuck. Most dies are ground to cut mild or free machining steels. Other materials may require dies which are ground specifically for that material. Stainless steels, non-ferrous metals, cast iron, and plastics are examples of materials which will probably require that the dies be ground especially for that purpose. Make sure that the dies in the machine match the material being cut.
- Larger machines generally have a transmission which is used to change the spindle speed. Following the manufacturer's instructions for the diameter and material to be threaded, make sure that the transmission is set at the right speed. Generally, the larger the diameter, the slower the machine should operate. The more the material being threaded tends to work harden, the slower the threading speed should be. Tougher materials generally require slower speeds as well. If you are in doubt about which speed to use, choose a lower one. More accurate threads and extended die life are the result of slower threading speeds. When you consider that the actual difference in time for threading from one speed to another is very little compared to the time required to replace a thread which has been torn by excessive speed, the lower speed may be more profitable in the long run.
In most smaller machines, there is no operator speed control. These machines typically use a universal type motor which slows down as torque increases and thereby limits the machine speed to reasonably safe levels. In this type of machine the larger the dies used, the slower the machine will turn.
- When the machine is turned on, and the material is fed to the die head, the material should center itself in the die head easily. If it does not, chances are that the machine is misaligned and should be repaired. In machines which do not have a lead screw, use adequate starting pressure as described in the Starting Pressure section of this manual. Continue to exert enough force on the carriage so that the dies do not have to pull the carriage and tools along with them. The helix angle of the dies (q.v.) is one of the most important characteristics of the die segment in that this is what moves the dies forward over the material. Any other forces which retard or advance this motion against the helix angles will result in deformed threads. This is true for all threads in general, but is particularly important in soft materials and small pitch threads.
- The flow of good cutting fluid to the die head should be such that the cutting surfaces of the die segments are flooded. As a general rule, there is no such thing as too much oil at the die head. Oil can be conserved by setting the machine so that the pipe being threaded is sloped slightly downhill. This prevents oil from running down the pipe and being lost. Use only cutting fluids designed for chaser type threading. Other cutting fluids may work well in other applications, but for the service expected in chaser type threading, only the best will do. See the Thread Cutting Fluids section of this manual for a description of whats expected from good cutting fluids.
- When cutting threads, stop occasionally to check the condition of the dies. Make certain that they are not getting dull, or getting material fused or welded to the cutting edges. If either of these conditions exist, stop immediately. Continuing to operate with dull or fused dies will result in ruined threads and very likely, ruined dies. Correct the problem by sharpening the dies or by stoning the top surface of the cutting edge to remove welded material. If these conditions persist, or if the dies seem to wear quickly, it would be wise to change to cutting fluid more suited to your needs. If your inspection shows chipped or broken teeth, repair them by following the instructions in the Chaser Grinding section of this manual.
- Check the completed threads frequently. Ideally, gauges should be used to check the profile and size of the thread. If you do not have gauges for the thread being made, a matching nut or coupling may be used with caution. Make sure that the coupling or nut is the right size. Many commercially available fittings are outside the acceptable industry standards, and often variations in size occur within the same lot of parts. By using micrometers, many bolt and other straight thread profiles can be checked against the thread standards in our Reference Charts section of this manual. Occasionally inconsistent size is caused by the operator not opening the manually operated die head at the right time. In automatic die heads, the operating linkage must be kept clean and in good operating condition. The operating linkage senses the end of the pipe, so it is important to keep the cutoff ends chamfered and free of burrs.
- If problems persist with the threads generated, refer to the Troubleshooting Guide. In order to analyze the problem, it will be necessary to look carefully at the thread or series of threads. If the leading flank of a thread is deformed, it is probably caused by something different than if the receding flank is deformed. If the deformation is only on the first few threads, the problem is different than if the deformation exists over the full length. As with other troubleshooting, careful observation is important.
- As has been mentioned in other parts of this manual, successful chaser type threading is only the result of paying attention to the various details that go into it. The machine, material, cutting fluid, dies, and operator must all function well to get good results. By knowing what to look for, and understanding the principles of the threading operation this complex operation can become trouble-free and easily applied.
- It is wise to understand the characteristics of the material you are attempting to thread. Some materials have an extreme tendency to work harden. Some high carbon, alloy and stainless materials fall into this category. Do not attempt to thread work hardening materials if they have been cut off with a roller type cutoff device. When in doubt, use a blade type cutoff with either roller backups or brass shoes to hold the material. Work hardening can be a serious problem with some materials and if proper precaution is not taken, bad threads, broken or chipped dies, and ruined material can result.
- Never attempt to thread material that has been flame cut or welded in the area of the thread. These processes may result in hard spots in the material which can quickly ruin dies. If the material is fully annealed after flame cutting or welding, the hard spots will be softened and no problem should result. Hard spots can also be generated by abrasive cut-off devices. This is particularly true of carbon and alloy steels. Water cooling of the cutoff process helps, but still may not totally prevent localized hardening. When in doubt, anneal the material.
If the machine is outfitted with a blade type cutoff device, make sure that it is well supplied with cutting oil. Feed the cutter blade into the material slowly and continuously. Plunging the cutter into the material can result in broken blades. Sharpen the blades often and maintain the cutter relief angle supplied by the manufacturer. When cutting stainless steel or other material which tends to work harden, the guide shoes of the cutoff device should be made of soft materials such as brass.
When using a roller cutoff device, feed the wheel into the material slowly but firmly. There are great frictional loads and compressive loads on the wheel which can lead to damage or premature failure of the wheel if the cutting pressures are too great. Make certain that the cutter assembly is at right angles to the axis of the pipe being cut. If it is not the roller will tend to cut in a spiral and the pipe can be ruined. Lubricate the roller and the backup rolls of the cutter with grease or heavy lubricating oil frequently. This helps to reduce heat and the forces required to complete the cut. It also improves the life expectancy of theses components.