Dec. 09, 2024
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Unit 6: Lathe ThreadingAfter completing this unit, you should be able to:
&#; Determine the infeed depth.
&#; Describe how to cut a correct thread.
&#; Explain how to calculate the pitch, depth, and minor diameter, width of flat.
&#; Describe how to set the correct rpm.
&#; Describe how to set the correct quick change gearbox.
&#; Describe how to set the correct compound rest.
&#; Describe how to set the correct tool bit.
&#; Describe how to set both compound and crossfeed on both dials to zero.
&#; Describe the threading operation.
&#; Describe the reaming.
&#; Describe how to grind a tool bit.
Thread cutting on the lathe is a process that produces a helical ridge of uniform section on the workpiece. This is performed by taking successive cuts with a threading toolbit the same shape as the thread form required.
Practice Exercise:
1. For this practice exercise for threading, you will need a piece of round material, turned to an outside tread Diameter.
2. Using either a parting tool or a specially ground tool, make an undercut for the tread equal to its single depth plus .005 inch.
3. The formula below will give you the single depth for undertaking unified threads:
d = P x 0.750
Where d = Single Depth
P = Pitch
n = Number of threads per inch (TPI)
Infeed Depth = .75 / n
To cut a correct thread on the lathe, it is necessary first to make calculations so that the thread will have proper dimensions. The following diagrams and formulas will be helpful when calculating thread dimensions.
Example: Calculate the pitch, depth, minor diameter, and width of flat for a ¾-10 NC thread.
P = 1 / n = 1 / 10 = 0.100 in.
Depth = . x Pitch = . x .100 = . in.
Minor Diameter = Major Diameter &#; (D + D) = .750 &#; (.075 + .075) = 0.600 in.
Width of Flat = P / 8 = (1 / 8) x (1/10) = . in.
Procedure for threading:
1. Set the speed to about one quarter of the speed used for turning.
2. Set the quick change gearbox for the required pitch in threads. (Threads per inch)
Figure 1. Thread and Feed Chart
Figure 2. Setting Gearbox
3. Set the compound rest at 29 degrees to the right for right hand threads.
Figure 3. 29 Degrees
4. Install a 60 degree threading tool bit and set the height to the lathe center point.
Figure 4. 60 Degree Threading Tool
5. Set the tool bit and a right angles to the work, using a thread gage.
Figure 5. Using the Center gage to position the tool for machining Threads
6. Using a layout solution, coat the area to be threaded.
Figure 6. Layout
7. Move the threading tool up to the part using both the compound and the cross feed. Set the micrometer to zero on both dials.
Figure 7. Compound Figure 8. Cross Feed
8. Move cross feed to the back tool off the work, move carriage to the end of the part and reset the cross feed to zero.
Figure 9. End of the part and Cross feed to Zero
9. Using only the compound micrometer, feed in .001 to .002 inch.
Figure 10: Compound feed in .002 inch
10. Turn on the lathe and engage the half nut.
Figure 11: On/Off Lever and Half Nut
11. Take a scratch cut on the part without cutting fluid. Disengage the half nut at the end of the cut, stop the lathe and back out the tool using the cross feed. Return the carriage to the starting position.
Figure 12. Starting Position
12. Using a screw pitch gage or a rule check the thread pitch. (Threads per inch)
Figure 13. Screw Pitch Gage Figure 14. Screw Pitch Gage(10)
13. Feed the compound in .005 to .020 inch for the first pass using cutting oil. As you get near the final size, reduce the depth of cut to .001 to .002 inch.
14. Continue this process until the tool is within .010 inch of the finish depth.
Figure 15. Threading operation
15. Check the size using a screw thread micrometer, thread gage, or using the three wire system.
Figure 16. Three wire measurement
16. Chamfer the end of the thread to protect it from damage.
Reamers are used to finish drilled holes or bores quickly and accurately to a specified sized hole and to produce a good surface finish. Reaming may be performed after a hole has been drilled or bored to within 0.005 to 0.015 inch of the finished size since the reamer is not designed to remove much material.
The workpiece is mounted in a chuck at the headstock spindle and the reamer is supported by the tailstock.
The lathe speed for machine reaming should be approximately 1/2 that used for drilling.
Reaming with a Hand Reamer
The hole to be reamed by hand must be within 0.005 inch of the required finished size.
The workpiece is mounted to the headstock spindle in a chuck and the headstock spindle is locked after the workpiece is accurately setup. The hand reamer is mounted in an adjustable reamer wrench and supported with the tailstock center. As the wrench is revolved by hand, the hand reamer is fed into the hole simultaneously by turning the tailstock handwheel. Use plenty cutting fluid for reaming.
Reaming with a Machine Reamer
The hole to be reamed with a machine reamer must be drilled or bored to within 0.010 inch of the finished size so that the machine reamer will only have to remove the cutter bit marks. Use plenty cutting fluid for reaming.
Procedure:
1. Grip the tool bit firmly while supporting the hand on the grinder tool set.
2. Hold the tool bit at the proper angle to grind the cutting edge angle. At the same, tilt the bottom of the tool bit in towards the wheel and grind 10 degrees side relief or clearance angle on the cutting edge. The cutting edge should be about .5 inches long and should be over about ¼ the width of the tool bit.
3. While grinding tool bit, move the tool bit back and forth across the face of the grinding wheel. This accelerates grinding and prevents grooving the wheel.
4. The tool bit must be cooled frequently during the grinding operation by dip into the water. Never overheat a tool bit.
5. Grind the end cutting angle so that it form an angle a little less than 90 degrees with the side cutting edge. Hold the tool so that the end cutting edge angle and end end relief angle of 15 degrees are ground at the same time.
6. Check the amount of end relief when the tool bit is in the tool holder.
7. Hold the top of the tool bit at about 45 degrees to the axis of the wheel and grind the side rake about 14 degrees.
8. Grind a slight radius on the point of the cutting tool, being sure to maintain the same front and side clearance angle.
Grind front Grind side Grind radius
Lathe tool bits are generally made of four materials:
1. High speed steel
2. Cast alloys
3. Cemented Carbides
4. Ceramics
The properties that each of these materials possess are different and the application of each depends on the material being machined and the condition of the machine.
Lathe tool bits should possess the following properties.
1. They should be hard.
2. They should be wear resistant.
3. They should be capable of standing up to high temperatures developed during the cutting operation.
4. They should be able to withstand shock during the cutting operation.
Cutting tools used on a lathe are generally single pointed cutting tools and although the shape of the tool is changed for various applications. The same nomenclature applies to all cutting tools.
Procedure:
1. Base: the bottom surface of the tool shank.
2. Cutting Edge: the leading edge of the tool bit that does the cutting.
3. Face: the surface against which the chip bears as it is separated from the work.
4. Flank: The surface of the tool which is adjacent to and below the cutting edge.
5. Nose: the tip of the cutting tool formed by the junction of the cutting edge and the front face.
6. Nose radius: The radius to which the nose is ground. The size of the radius will affect the finish. For rough cut, a 1/16 inch nose radius used. For finish cut, a 1/16 to &#; inch nose radius is used.
7. Point: The end of the tool that has been ground for cutting purposes.
8. Shank: the body of the tool bit or the part held in the tool holder.
9. Lathe Tool bit Angles and Clearances
Proper performance of a tool bit depends on the clearance and rake angles which must be ground on the tool bit. Although these angles vary for different materials, the nomenclature is the same for all tool bits.
&#; Side cutting edge angle: The angle which the cutting edge forms with the side of the tool shank. This angle may be from 10 to 20 degrees depending on the material being cut. If angle is over 30 degrees, the tool will tend to chatter.
&#; End cutting edge angle. The angle formed by the end cutting edge and a line at right angle to the centerline of the tool bit. This angle may be from 5 to 30 degrees depending on the type of cut and finish desired. For roughing cuts an angle of 5 to 15 degrees, angle between 15 and 30 degrees are used for general purpose turning tools. The larger angle permits the cutting tool to be swivelled to the left when taking light cuts close to the dog or chuck, or when turning to a shoulder.
&#; Side Relief (clearance) angle: The angle ground on the flank of the tool below the cutting edge. This angle may be from 6 to 10 degrees. The side clearance on a tool bit permit the cutting tool to advance lengthwise into the rotating work and prevent the flank from rubbing against the workpiece.
&#; End Relief (clearance) angle: the angle ground below the nose of the tool bit which permits the cutting tool to be fed into the work. This angle may be 10 to 15 degrees for general purpose cut. This angle must be measured when the tool bit is held in the tool holder. The end relief angle varies with the hardness and type of material and type of cut being taken. The end relief angle is smaller for harder materials, to provide support under the cutting edge.
&#; Side Rake Angle: The angle at which the face is ground away from the cutting edge. This angle may be 14 degrees for general purpose tool bits. Side rake centers a keener cutting edge and allows the chip to flow away quickly. For softer materials, the side rake angle is generally increased.
&#; Back (Top) Rake: The backward slope of the tool face away from the nose. This angle may be about 20 degrees and is provide for in the tool holder. Back rake permits the chips to flow away from the point of the cutting tool.
1. What is pitch for ¼-20 tap?
2. To what angle must the compound be turned for Unified Thread?
3. Explain why you swivel the compound in Question 2.
4. What is the depth of thread for UNF ½-20 screw?
5. How would you make a left-hand thread? This is not covered in the reading&#;think it out?
6. What Tool bit do we use for cutting thread?
7. Please describe Center Gage.
8. What do we use to check the thread pitch(Thread Per Inch)?
9. The first and final pass, how much do we feed the compound in?
10. Name four material that use to make Tool bits.
This chapter was derived from the following sources.
THREADING ON THE LATHE
Introduction
If you want to learn more, please visit our website threading in lathe machine.
This document presents some of the more common techniques for threading on the manual engine lathe.
Tap Handle
Using a tap handle is the most common method of tapping on the lathe. The workpiece is clamped in the lathe chuck, a spring loaded center (for smaller taps) or a dead center (for larger taps) is clamped in the tailstock, and the tap is held and rotated using a tap handle, as we do with the assigned parts in lab.
Figure 1a: Examples of using standard tap wrenches and spring loaded tap guide (left) or dead center (right) to tap holes on the lathe.
Figure 1b: Example of using an adjustable wrench and a live center to tap holes on the lathe.
Figure 1c: Examples of various tap handles.
Die Handle
Using a die handle is a common method of external thread cutting on the lathe. The workpiece is clamped in the lathe chuck, and the threading die is held and rotated using a die handle.
In general, round-shaped dies are for cutting threads onto a workpiece and hex-shaped dies are for chasing (cleaning up / repairing) existing threads.
Before using a threading die it&#;s important to make sure the major diameter of the shaft to be threaded matches the range listed in the Machinery Handbook. For example, a ½-20 UNF 2A thread must have a major diameter between 0. and 0.&#;. The smaller the major diameter, the easier the die will cut. In general, undersize the shaft diameter by 2% of the major thread diameter.
When using a threading die on the lathe it&#;s important to start the thread die collinear to the axis of the part, so use the body of the drill chuck for alignment and guidance. It&#;s also important to cut a generous chamfer on the end of the part to help the threading die start cutting.
Figure 2a: Examples of various threading die handles.
Figure 2b: Using the drill chuck body to align the threading die axis with the workpiece axis when starting the thread.
Rigid Tapping
Rigid tapping is the second most common method of thread cutting on the lathe. With this technique the tap or die is clamped in the tailstock using a variety of methods and threaded into or onto the workpiece under spindle power. Smaller taps up to 3/8&#; can be clamped in a keyed Jacob&#;s style chuck (NEVER a keyless chuck!). Larger taps should be clamped using a split sleeve or heavy duty tap driver, as shown in figure 3b.
Figure 3a: Rigid tapping on manual lathe. Click the image on the right for a video showing the process.
Figure 3b: Example of split sleeve tap driver for lathe tailstock (left) and heavy duty tap driver for lathe tailstock (right).
Figure 3c: Rigid die cutting on manual lathe.
Die cutting video
Single Point Threading
Single point threading involves mounting a threading tool with the proper thread profile to the toolpost and cutting the thread using multiple synchronized passes.
In general there are two types of cutting tool geometries which can be used: partial and full form profiles. Partial profile cutting geometries only cut the minor (or root) diameter of the thread, whereas full profile cutting geometries cut both the minor and major diameters of the thread profile to size. The advantage of partial profile cutting geometries is that one tool can cut a variety of thread pitches, whereas a full profile cutting geometry is only good for one particular thread pitch. The advantage of full profile cutting geometries is that the entire thread is finished in one operation, saving significant thread finishing and deburring time.
Figure 4a: Partial vs full form profile threading geometries.
The following videos explain the process in good detail. Fast forward through the parts which are not interesting to you J.
Figure 4b: Good single point threading video (left; threading starts at 18:08 time stamp) and shorter clip of thread cutting (right).
Figure 4c: Examples of properly designed thread reliefs.
The process for single point turning threads in the design lab is as follows:
1. Clamp the part in the lathe using a live center if necessary.
2. Turn the OD to the target major diameter and include a chamfer on the end at least 0.020&#; smaller than the minor diameter of the thread profile to be cut.
3. If permissible, cut a thread relief using a grooving tool (as shown in Figure 10 and the two video thumbnail images above). The thread relief should be slightly less than the minor thread diameter.
4. Adjust the gearbox levers on the front of the headstock to cut the proper thread pitch.
5. Adjust the threading tool so it is aligned parallel to the X-axis.
6. Touch off on the part and zero the X-axis.
7. Cut a light (0.001 - 0.002&#;) scratch pass across the surface of the part to be checked with a thread gage for accuracy.
8. If the pitch of the scratch pass measures correctly, begin cutting the thread to depth; start with deeper depths of cut (.010&#; in aluminum, 0.005&#; in steel) and make progressively shallower cuts as the thread gets deeper and the threading tool begins to leave a worse finish)
9. As you approach final thread size, use a fine file to carefully debur the rough edges of the major diameter (unless using a full profile insert, which deburrs the major diameters automatically, as discussed above). The major diameter should end up a few thousandths of an inch under the nominal size, according to the tolerances listed in the Machinery Handbook (e.g. 0.-0.&#; for a ½-20 UNF 2A thread). You will know when you are close to the final size by keeping track of your X-infeed value, which will end up smaller than the equivalent internal thread&#;s minimum minor diameter by the noted allowance (e.g. 0.446&#; &#; 0.&#; = 0.&#; for the same ½-20 UNF 2A thread).
10. The procedure for making an actual cut is:
a. Check the direction of the threading direction by engaging the half-nut with the tool a safe distance from the part; for this example, we will thread toward the chuck
b. Adjust the spindle speed to a low setting (100-300 rpm) depending on how brave you are J
c. Position the tool in a safe starting location to begin cutting the thread
d. Advance the tool toward the part the distance (depth of cut) you wish to cut
e. Engage the half-nut for threading (it&#;s safest to leave this engaged for the duration of the threading session)
f. Turn the spindle ON in the FWD direction and allow the tool to make a cut
g. Turn the spindle OFF before the tool reaches a shoulder (if not exists); you can use the foot brake to stop it quickly if needed; if you stop too early, simply bump the power switch to continue the cut or rotate the chuck by hand
h. Retract the tool a safe distance from the part in the X-direction
i. Turn the spindle ON in the REV direction to allow the tool to return to a safe starting location
j. Repeat steps d. thru i. until the desired minor or pitch diameter is reached.
Figure 4d: Insert comprehensive single point threading video here?
Thread Measurement
Threads can be measured at least three different ways: by checking with a mating nut or thread gage, by using a dedicated thread micrometer, or by using the three wire method.
Mating Nut or Thread Gage
Checking with a mating quality nut or thread gage is the easiest method to determine when the thread is cut deep enough. Nuts are much cheaper than calibrated thread gages, but work fine for most prototyping applications.
Figure 5a: Checking thread size using an existing, quality nut. It&#;s convenient to keep a complete set of quality nuts on rings for thread measurement (right).
Figure 5b: Checking thread size using a thread gage. Click the thumbnail for the video.
Thread Micrometer
Using a thread micrometer is the easiest way to accurately measure thread pitch diameter for comparison to the thread data listed in the Machinery Handbook or this link. However, thread mics are fairly expensive.
Figure 6a: Example of a thread micrometer (notice the v-shaped anvils) and its use measuring thread pitch diameter.
Figure 6b: (Poorly made) video on how to use a thread micrometer. Click thumbnail for video.
3 Wire Method
The three wire method uses basic geometry and three identically sized wire rods to allow the pitch diameter to be calculated using any standard micrometer measurement. The three wire method is very accurate and the cheapest method of measuring a variety of thread pitch diameters. The downside to this method is that it requires a lot of dexterity to make an accurate measurement without dropping the precision thread wires.
Figure 7a: Checking thread size using 3 wire method. Click the thumbnails for the videos.
Figure 7b: Thread wire formulas for converting between actual measurement and pitch diameter.
Figure 7c: Three wire measurement method explained. Click image for .pdf file.
Miscellaneous Points
Always use cutting oil when threading on the lathe. WD40 works well for aluminum. Oatey dark threading oil works well for steel. Chlorinated Moly-D works best for materials which are tougher to machine, like stainless and alloy steels.
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