10 Questions You Should to Know about threading in lathe machine
Unit 6: Lathe Threading – Manufacturing Processes 4-5
11 Unit 6: Lathe Threading
OBJECTIVE
After completing this unit, you should be able to:
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' 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.
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
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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.
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A Few Quick Threading Questions... | Home Model Engine Machinist ...
1) I've heard that the cutting tool should have a flat tip 1/8 the size of the thread pitch. Does anyone do this, and is it necessary?
2) Where could I find some charts that show all the info I need for the thread (pitch diameter, major and minor diameters for OD and ID threads), basically everything I need. The thread I need to cut is 1/2-28, which isn't a standard UNC or UNF thread, so I'm having trouble finding the information I need.
Thanks for the help guys!
formula156 said: 1) I've heard that the cutting tool should have a flat tip 1/8 the size of the thread pitch. Does anyone do this, and is it necessary?
I personally don't, because IMO It's to difficult a task to perform on a manual grinder accurately.
formula156 said: 2) Where could I find some charts that show all the info I need for the thread (pitch diameter, major and minor diameters for OD and ID threads), basically everything I need. The thread I need to cut is 1/2-28, which isn't a standard UNC or UNF thread, so I'm having trouble finding the information I need.
A copy of the Machinery's Handbook will have most normal & fine threads. However what your after is an ultra fine thread, and can be found here:
http://www.efunda.com/DesignStandards/screws/unified.cfm?start=64&finish=147 When I cut a thread (internal or external) I grind a V tool with a sharp point. I cut the thread to depth, then open out the flat on the bottom using side cut removing from the left flank of the tool (the tool is at full thread depth for each pass).
This is easier if you use the direct approach method with the compound parallel to the lathe bed but also do-able using the compound at 29 1/2 deg using jiggery-pokery with the cross slide to move the tool sideways.
I always make the external thread first, then fit the internal thread to it.
Hope the above is clear.
Al
I'll run this by you guys, and if you could correct me (or hopefully agree) I'd appreciate it.
For a 1/2-28 Class 2 fit, I could do a .499 Exterior thread major with a .456 minor. For the interior thread, I could do a .500 major and .461 minor. This is with everything rounded to the nearest thou. The would mean the majors are only .001 apart and the minors are .005 apart. Should the major and minor have the same difference between them?
Should I perhaps cut the external thread as I specified, start the internal with the major I specified, and just keep advancing the internal thread till I get the fit I am happy with?
Dick,
I understand how you would flat the bottom. Do you flatten in 1/8 of the pitch? I assume you thread and flatten the enternal first. Then, you thread the internal, test the fit, and then flatten the minor of the internal thread?
Thank guys!
Oh, when grinding a threading tool (external for example), you grind the side to a 60° point, and put a little rake on the point. There is no grinding on the top, correct?
Oh again! When advancing the compound at an angle, I can do the trigonometry to figure how far to advance it, but is there a simple formula or a chart with this info.
Do you guys do a couple final passes advancing the cross slide to clean up the threads? When opening out the thread, I remove material until the flat on the crest of the bolt (external) is 1/8 of the pitch gauged by comparison to the correct feeler guage.
The flat at the root of the thread on the bolt should be (perfect world) 1/4 pitch. It is very difficult to measure internally which is why I make the internal to fit the one I can see rather than vice versa.
If you need to make threads to be as backlash free as possible, the clearances you have stated will work but may be very tight in getting to rotate. The slightest burr may cause it to stick. Also which materials you are cutting bears on the finish which can make them tight.
If you need interchangeability between several parts a little extra clearance may help as there will always be some minor differences which may add to cause sticking.
Grinding: top rake depends a lot on the material, brass doesn't need any. You will need side clearance to take into account the pitch of the thread on the left side (the bottom of the thread you have just cut angles to the right and on larger threads can impinge on the tool).
I generally don't do trig to work out when at full depth, if possible I leave a narrow diameter strip at the start of the thread which is at the minor (external) or major (internal) size. When the tool just marks this I know I am on size. If need be this can be machined off afterwards or sometimes just leave it on if it can't be seen or doesn't affect utility
Again Hope this helps
formula156 said: 1) I've heard that the cutting tool should have a flat tip 1/8 the size of the thread pitch. Does anyone do this, and is it necessary?
IMHO grind the tool to the correct included angle, with no top rake. Stone the point to remove the sharp tip. Start cutting.
2) Where could I find some charts that show all the info I need for the thread (pitch diameter, major and minor diameters for OD and ID threads), basically everything I need. The thread I need to cut is 1/2-28, which isn't a standard UNC or UNF thread, so I'm having trouble finding the information I need.
A 28 tpi thread will have the same depth of thread regardless of the diameter on which it is being cut and in this case, external thread = 0.022" deep and internal thread = 0.019 deep.
Hope this helps ??? ???
Best Regards
Bob
Maryak said: A 28 tpi thread will have the same depth of thread regardless of the diameter on which it is being cut and in this case, external thread = 0.022" deep and internal thread = 0.019 deep.
Hope this helps ??? ???
Best Regards
Bob
Where did you get those figures from? The charts I have seen contradict that. And, what about the major diameters? If I buy something externally threaded 1/2-28 is it going to have a .500" major with .022" deep threads?
I am making a flash suppressor for a rifle, and I am planning to thread the barrel and the part, but I want the part to fit on anything that already has 1/2-28 threads.
Thanks!
EDIT: Nevermind! I wasn't thinking correctly. Given a major and minor, I need to divide by two to find the cutting depth. I've been looking at this too hard I think, and I confused myself.
But...one question still remains...if the thread I need is 1/2-28, will the external thread have a major diameter of .500? Damn, I guess I am still slightly confused. I guess what I am asking is, how do I figure my outer diameters for the internal and external thread. I understand the rest from there.
formula156 said: If I buy something externally threaded 1/2-28 is it going to have a .500" major with .022" deep threads?
Yes
But...one question still remains...if the thread I need is 1/2-28, will the external thread have a major diameter of .500? I guess what I am asking is, how do I figure my outer diameters for the internal and external thread.
Yes and the internal thread will have a bore equal to 0.500" - 0.038" which is 0.462"
Is the thread you are after 1/2" UNEF?
Hope this helps ???
Best Regards
Bob
Maryak said: Yes
Yes and the internal thread will have a bore equal to 0.500" - 0.038" which is 0.462"
Is the thread you are after 1/2" UNEF?
Hope this helps ???
Best Regards
Bob
I think 1/2-28 is considered a Unified Ultra Fine Thread.
From what you said, that would make the external and internal majors the same at .500, which would be an extremely close fit. But the external minor would be .456 and internal minor .462. Sorry to sound redundant but that doesn't seem right, unless cutting the external thread brings the external major down a few thousands (if the left and right sides of the points of the thread overlap.
Again, sorry to keep asking the same question.
Thanks!
DickDastardly40 said: If I have your question correctly about the overlap, the bolt (external) will have a minor diameter of 0.456 and the nut (internal) 0.462. Therefore the bolt is smaller and you have 0.006 clearance.
Right...but what about the major diameters of both the internal and external. If they are both .500, there would be no clearance. That's the part I am unsure about. Unless cutting to the correct minor depth brings the major depth down a little from where it started.
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