Machine for accurately cutting screw threads

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A screw-cutting lathe is a machine (specifically, a lathe) capable of cutting very accurate screw threads via single-point screw-cutting, which is the process of guiding the linear motion of the tool bit in a precisely known ratio to the rotating motion of the workpiece. This is accomplished by gearing the leadscrew (which drives the tool bit's movement) to the spindle with a certain gear ratio for each thread pitch. Every degree of spindle rotation is matched by a certain distance of linear tool travel, depending on the desired thread pitch (English or metric, fine or coarse, etc.).

The name "screw-cutting lathe" carries a taxonomic qualification on its use&#;it is a term of historical classification rather than one of current commercial machine tool terminology. Early lathes, many centuries ago, were not adapted to screw-cutting. Later, from the Late Middle Ages until the early nineteenth century, some lathes were distinguishable as "screw-cutting lathes" because of the screw-cutting ability specially built into them. Since then, most metalworking lathes have this ability built in, but they are not called "screw-cutting lathes" in modern taxonomy.

History

The screw has been known for thousands of years. Archimedes described the water screw, a system for raising water. Screws as mechanical fasteners date to the first century BC. Although screws were tremendously useful, the difficulty in making them prevented any widespread adoption.The designers of screw-cutting lathes aimed to solve this problem with their machines in such a manner that would enable the production of screws cheaply and efficiently. It would be these qualities of screw production that enabled the utilization of screws in an industrializing world.

Early wooden screws

The earliest screws tended to be made of wood, and they were whittled by hand, with or without the help of turning on a lathe with hand-controlled turning tools (chisels, knives, gouges), as accurately as the whittler could manage. It is likely that sometimes the wood blanks that they started from were tree branches (or juvenile trunks) that had been shaped by a vine wrapped helically around them while they grew. (In fact, various Romance words for "screw" come from the word root referring to vines.[1]) Walking sticks twisted by vines show how suggestive such sticks are of a screw.

Early metal screws

Early machine screws of metal, and early wood screws [screws made of metal for use in wood], were made by hand, with files used to cut the threads. One method for making fairly accurate threads was to score a rod using an inclined knife with a wrap halfway around the rod, the knife being precisely angled for the proper pitch. This was one of the methods Maudslay used to make his early leadscrews.[2] This made the screw slow and expensive to make, and its quality highly dependent on the skill of the maker. A process for automating the manufacture of screws and improving the accuracy and consistency of the thread was needed.

Earliest lathes with machine-guided toolpath for screw-cutting

Lathes have been around since ancient times. Adapting them to screw-cutting is an obvious choice, but the problem of how to guide the cutting tool through the correct path was an obstacle for many centuries. Not until the late Middle Ages and early modern period did breakthroughs occur in this area; the earliest of which evidence exists today happened in the 15th century and is documented in the Mittelalterliche Hausbuch.[3] It incorporates slide rests and a leadscrew. Roughly contemporarily, Leonardo da Vinci drew sketches showing various screw-cutting lathes and machines, one with two leadscrews.[3] Leonardo also shows change-gears in some of these sketches.[3]

In the succeeding three centuries, many other designs followed, especially among ornamental turners and clockmakers. These included various important concepts and impressive cleverness, but few were significantly accurate and practical to use. For example, Woodbury discusses Jacques Besson and others. They made impressive contributions to turning, but the context in which they tended to work (turning as a fine art for rich people) did not channel their contributions toward industrial uses.[3]

Henry Hindley designed and constructed a screw-cutting lathe circa . It featured a plate guiding the tool and power supplied by a hand-cranked series of gears. By changing the gears, he could cut screws with different pitch. Removing a gear permitted him to make left-handed threads.[4]

Modern screw-cutting lathes (late 18th to early 19th centuries)

The first truly modern screw-cutting lathe was likely constructed by Jesse Ramsden in . His device included a leadscrew, slide rest, and change gear mechanism. These form the elements of a modern (non-CNC) lathe and are in use to this day. Ramsden was able to use his first screw-cutting lathe to make even more accurate lathes. With these, he was able to make an exceptionally accurate dividing engine and in turn, some of the finest astronomical, surveying, and navigational instruments of the 18th century. [5]

Others followed. Examples were a French mechanic surnamed Senot, who in created a screw-cutting lathe capable of industrial-level production, and David Wilkinson of Rhode Island, who employed a slide rest in . However, these inventors were soon overshadowed by Henry Maudslay, who in created what is frequently cited as the first industrially practical screw-cutting lathe. According to Encyclopaedia Britannica, &#;The outstanding feature of Maudslay&#;s lathe was a lead screw for driving the carriage. Geared to the spindle of the lathe, the lead screw advanced the tool at a constant rate of speed and guaranteed accurate screw threads&#;. Bryan Donkin in took Maudsleys design and refined it further with his screw cutting and dividing engine lathe, which utilised a mechanism for compensating for inaccuracies in the leadscrew. Joseph Whitworth, a disciple of Maudslay, created a design that, through its adoption by many British railway companies, became a standard for the United Kingdom and the British Empire. Called British Standard Whitworth (BSW), it is the world's first national screw thread standard.[7] These tools were also exported to continental Europe and the United States. They permitted the large-scale, industrial production of screws that were interchangeable. Standardization of threadforms (including thread angle, pitches, major diameters, pitch diameters, etc.) began immediately on the intra-company level, and by the end of the 19th century, it had been carried to the international level (although pluralities of standards still exist).[citation needed]

In the late 19th century Henry Augustus Rowland found a need for very high precision screws in cutting diffraction gratings, so he developed a technique for making them.[8]

Present day

Until the early 19th century, the notion of a screw-cutting lathe stood in contrast to the notion of a plain lathe, which lacked the parts needed to guide the cutting tool in the precise path needed to produce an accurate thread. Since the early 19th century, it has been common practice to build these parts into any general-purpose metalworking lathe; thus, the distinction between "plain lathe" and "screw-cutting lathe" does not apply to the classification of modern lathes. Instead, there are other categories, some of which bundle single-point screw-cutting capability among other capabilities (for example, regular lathes, toolroom lathes, and CNC lathes), and some of which omit single-point screw-cutting capability as irrelevant to the machines' intended purposes (for example, speed lathes and turret lathes).

Today the threads of threaded fasteners (such as machine screws, wood screws, wallboard screws, and sheetmetal screws) are usually not cut via single-point screw-cutting; instead most are generated by other, faster processes, such as thread forming and rolling and cutting with die heads. The latter processes are the ones employed in modern screw machines. These machines, although they are lathes specialized for making screws, are not screw-cutting lathes in the sense of employing single-point screw-cutting.

See also

References

Bibliography

11

Unit 6: Lathe Threading

OBJECTIVE

After 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.

Lathe Threading

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

Thread Calculations

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.

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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.

Reaming

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.

Grind a Lathe Tool bit

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

Cutting tool Materials

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 tool Nomenclature

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.

UNIT TEST

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.

Chapter Attribution Information

This chapter was derived from the following sources. 

Want more information on china cnc turning center manufacturers? Feel free to contact us.

• Lathe derived from Lathe by the Massachusetts Institute of Technology, CC:BY-NC-SA 4.0.
• Cutting Tool Terminology derived from Lathe Cutting Tools &#; Cutting Tool Shapes by the Wisconsin Technical College, CC:BY-NC 4.0.
• Cutting Tool Terminology derived from Cutter Types (Lathe) by the University of Idaho, CC:BY-SA 3.0.
• Centering derived from [Manual Lathes Document]