1. MACH 118: Turning Between Centers

2. After completing this unit, you should be able to…
OBJECTIVES
After completing this unit, you should be able to…
Describe the correct setup procedure for turning between centers.
Select correct feeds and speeds for a turning operation.
Detail the steps necessary for turning to size predictably.
Check for taper with a test bar, and restore alignment by adjusting the tailstock.
Check for taper by taking a cut with a tool and measuring the workpiece, and restore alignment by adjusting the tailstock.

3. Setup for Turning Between Centers
To turn a workpiece between centers, support it between the dead center (tailstock center) & the live center in the spindle nose.
A lathe dog clamped to the workpiece is driven by a drive, or dog, plate mounted on the spindle nose.
Fig. I-178 Lathe dog (Micro-Mark).

4. Setup for Turning Between Centers
A workpiece cannot be cut off with a parting tool while being supported between centers
This will bind & break the parting tool and ruin the workpiece.
For drilling, boring, or machining the end of a long shaft, a steady rest normally supports the work.
These operations cannot be done when the shaft is supported only by centers.

5. Setup for Turning Between Centers
A shaft between centers can be turned end for end to continue machining without eccentricity if the live center runs true.
Shafts to be subsequently finish ground between centers must be machined between centers on a lathe.
Fig. I-180 Eccentricity in the center of the part because the live center is off center.

6. Lathe Centers TURNING BETWEEN CENTERS
The center for the headstock spindle is sometimes referred to as a live center because it rotates.
In contrast with a nonrotating dead center in the tailstock spindle.
The live center rotates, the dead center does not.
Innovations such as ball bearing tailstock centers that do rotate make this terminology confusing.
Such ball bearing centers are commonly called live centers even though they are used in a tailstock spindle and not the headstock.

7. Lathe Centers TURNING BETWEEN CENTERS
A solid “dead” center can be mounted directly in the lathe spindle.
Seat the bushing firmly in the taper & install the center.
Fig. I-181 Make sure the bushing is firmly seated in the taper.
Fig. I-182 Installing the center.

8. Lathe Centers TURNING BETWEEN CENTERS
A chuck center is often machined from a short piece of soft steel mounted in a chuck, then left in place.
The work is mounted between it & the tailstock center.
A lathe dog with the bent tail against a chuck jaw is used to drive the workpiece.
Fig. I-184 Live center being machined in a four-jaw chuck. The lathe dog on the workpiece is driven by one of the chuck jaws.

9. Lathe Centers TURNING BETWEEN CENTERS
The nonrotating tailstock (dead) center is hardened to withstand machining pressures and friction.
Fig. I-185 The dead center is hardened to resist wear. It is made of high-speed steel with a carbide insert.
Dead centers are virtually obsolete but have the advantage of greater rigidity.
Needle bearing & ball bearing centers are used extensively in machine shops & manufacturing.

10. Lathe Centers TURNING BETWEEN CENTERS
Ball bearing centers are shown here.
Fig. I-187 Antifriction ball bearing center.
Fig. I-188 Cutaway view of a ball bearing tailstock center. (DoALL Company)

11. Lathe Centers TURNING BETWEEN CENTERS
Rolling pipe centers are used for turning tubular material.
Fig. I-186 (a) The pipe center is useful for supporting a tubular workpiece. (b) Pipe center. (Courtesy of Monarch Machine Tool Company).

12. Lathe Centers TURNING BETWEEN CENTERS
Set the dog in place & tighten, adjust the tailstock so the bent tail of the dog moves freely in its slot, then tighten the tailstock binding lever.
The heat of machining will expand the work- piece & cause the dead center to heat from friction.
If overheated, the center may be ruined and may even be welded into your workpiece.
Fig. I-190 Lathe dog in position.

13. Roughing and Finishing
TURNING BETWEEN CENTERS
Roughing and Finishing
The formula to determine time for turning, boring, & facing is:
Light cuts & small feeds can waste time in roughing operations, even when higher cutting speeds are used.

14. Turning to Size TURNING BETWEEN CENTERS tab
After roughing cuts are taken, .020” to .030” should be left for finishing.
If insufficient material is left for machining, the tool will rub and will not cut.

15. Turning to Size TURNING BETWEEN CENTERS
The tool is advanced into the work, and the first of the two finish cuts is made.
Diameter is checked with a micrometer & the remaining amount to be cut is dialed on the crossfeed micrometer.
Fig. I-202 A trial cut is made to establish a setting of a micrometer dial in relation to the diameter of the workpiece.
Fig. I-203 Measuring the workpiece with a micrometer.

16. Turning to Size TURNING BETWEEN CENTERS
A short trial cut is taken (about 1/8” long) and a final check with a micrometer is made to validate the tool setting, and then the cut is completed.
Finishing of machined parts with a file and abrasive cloth should not be necessary.

17. Turning to Size TURNING BETWEEN CENTERS
Worn lathes are not dependable for close tolerances, so allowance must be made for filing.
Surface material left for filing ranges from to in., depending on the final finish and diameter.
The more material removed by filing, the more likely it is that the finish size will not be cylindrical.

18. Turning to Size TURNING BETWEEN CENTERS
When filing on a lathe, use a low speed and long strokes, and file left-handed.
For polishing with abrasive cloth, set the lathe for a high speed and move the cloth back and forth across the work.
Fig. I-204 Filing in the lathe, left-handed.

19. Turning to Size TURNING BETWEEN CENTERS
Because of the damaging effect of abrasive grains on the sliding surfaces of machinery, some shops do not allow abrasive cloth to be used at all.
Abrasive cloth leaves grit on the ways of the lathe, so thoroughly clean the ways after polishing.
Fig. I-205 Using abrasive cloth for polishing.
Fig. I-206 Using a file for backing abrasive cloth for more uniform polishing.

20. Tailstock Alignment ALIGNMENT OF THE LATHE CENTERS
The tailstock will normally stay in good alignment on a lathe not badly worn.
If used for taper turning with the tailstock offset, the tailstock may not have been realigned properly.
It is a good practice occasionally to check the center alignment of the lathe you generally use & always check the alignment before using a different lathe.
A check for taper while it is still in the roughing stage.

21. Tailstock Alignment ALIGNMENT OF THE LATHE CENTERS
Methods for aligning centers on a lathe.
In one method, the center points are brought together and visually checked for alignment.
This is not a precision method for checking alignment.
Fig. I-220 Checking alignment by matching center points.

22. Tailstock Alignment ALIGNMENT OF THE LATHE CENTERS
Another method of aligning centers is to use the tailstock witness marks.
Adjusting to the witness marks is only an approximate means of eliminating taper.
The tailstock is moved by means of a screw or screws.
Fig. I-221 Adjusting the tailstock to the witness marks for alignment.

23. Tailstock Alignment
A typical arrangement is shown below, where one setscrew is released and the opposite one tightened to move the tailstock on its slide
Fig. I-222 Hexagonal socket setscrew which, when turned, moves the tailstock provided that the opposite one is loosened.
Fig. I-223 The opposite setscrew being adjusted.

24. Use of a Test Bar and Machining
ALIGNMENT OF THE LATHE CENTERS
Use of a Test Bar and Machining
More accurate means of aligning centers are to use a test bar and to machine and measure.
A test bar is a shaft that has true centers & no taper.
No dog is necessary, as the test bar is not rotated.
A dial indicator is mounted in the tool post so it will travel with the carriage.
Contact point should be the center of the test bar.
Fig. I-224 Test bar setup between centers with a dial indicator mounted in the tool post.

25. Use of a Test Bar and Machining
ALIGNMENT OF THE LATHE CENTERS
Use of a Test Bar and Machining
Fig. I-225 Indicator is moved to measuring surface at headstock end, and the bezel is set on zero.
Begin with the indicator at the headstock end & set the indicator bezel to zero.
Fig. I-226 Carriage with dial indicator is moved to the measuring surface near the tailstock. In this case the dial indicator did not move, so the tailstock is on center.
Move the setup to the tailstock end of the test bar & check the dial indicator reading.

26. Use of a Test Bar and Machining
ALIGNMENT OF THE LATHE CENTERS
Use of a Test Bar and Machining
If no movement of the needle has occurred, the centers are in line.
If the needle has moved clockwise, the tailstock is misaligned toward the operator & will cause the workpiece to taper with the smaller end at the tailstock.
If the needle has moved counterclockwise, the work- piece will taper with the smaller end at the headstock.

27. Use of a Test Bar and Machining
ALIGNMENT OF THE LATHE CENTERS
Use of a Test Bar and Machining
As only a minor adjustment is usually needed while a job is in progress, the most common method of aligning lathe centers is by cutting and measuring.
This method usually uses the workpiece in the roughing stage.
Fig. I-227 Checking for taper by taking a cut on a workpiece. After the cut is made for the length of the workpiece, a micrometer reading is taken at each end to determine any difference in diameter.

28. Use of a Test Bar and Machining
ALIGNMENT OF THE LATHE CENTERS
Use of a Test Bar and Machining
A light cut is taken along the length of the test piece, and both ends are measured with a micrometer.
Set up a dial indicator & move the tailstock half the difference of the two readings.
Make another light cut & check for taper.
Fig. I-228 Using a dial indicator to check the amount of movement of the tailstock when it is being realigned

29. ASSIGNMENT
Read and answer self test questions for the following chapters.
Section I, Units 6,7, and 8