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Possibilities of tool clamping
Lathe tools must be firm and safetly clamped. Insufficient clamping may lead to rejects or accidents. By the clamping claw or the lathe tool holder, the lathe tool is fixed well even with difficult cuts.
Figure 19. Clamping possibilities of
the lathe tool
clamping jaw;
lathe tool holder;
rotatable fourfold lathe tool holder
With the help of the quadruple lathe tool holder, up to four lathe tools can be set up for the respective work.
Thus, the clamping and unclamping of the lathe tools and the work-pieces for the individual operations is no more necessary. As soon as a new workpiece shall be worked, the hand lever is unlocked, the tool holder is turned and the lever is tightened again with the exact position of the individual lathe tools being secured by an index.
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Clamping must be firm and safe. Insufficient clamping may cause rejects, breaking of the lathe tools and accidents. |
Pay attention to the following points:
- The lathe tool must always be in dead central position (centre of the tailstock/setting gauge).- Height adjustment by sheet metal backings (must be accurately plane and clean; use a few thick backings instead of many thin ones; the length should be at least three quarters of that of the tool shank).
- Do not adjust the height by placing sheet metal under the rear end of the lathe tool.
- Use of the quadruple lathe tool holder.
The multiple-cutting edge boring tools are put by their taper shank into the machine taper of the tailstock spindle.
Then, they are advanced towards the workpiece which is rotating with the work-driving spindle with the help of the hand wheel.
If the external taper of the drill and the internal taper of the quill do not match, the difference - if the internal taper is larger - can be balanced by reducing sleeves.
External and internal tapers must be absolutely clean, otherwise the drill rotates in the quill and destroys the internal taper.
In addition, it would, in such case, not be at centre with the rotation axis.
Larger drills are protected against the above mentioned kind of rotation by putting on a work driver.
Small drills are clamped in the drill chuck, which, then, is put into the quill by its taper shank.
In some cases, the automatic feed is used for moving the boring tool forward. For this purpose, the drill is clamped on the carriage with the help of a holding device.
Drill axis and rotation axis must be accurately congruent.
Figure 20. Boring block
1 boring block, 2 lathe tool holder, 3 twist drill
Possibilities of clamping the workpieces
The three-jaw chuck serves for clamping short workpieces quickly, safely and centrically.
Figure 21. Three-jaw chuck
1 three-jaw chuck with turning jaws, 2 three-jaw chuck with boring jaws
With the help of the four-jaw chuck, quadrangular and octagonal parts are clamped.
Figure 22. Four-jaw chuck
In order to achieve an exact true running of the parts, soft gripping jaws are used which are bored true to size of the respective workpieces.
Small, irregularly shaped workpieces are often clamped in the four-jaw chuck with individually adjustable chuck jaws.
They can be quickly aligned. The chuck jaws can be moved in common as well as individually,
For clamping smaller workpieces and for working from the bar, especially on turret lathes and automatics, the collet chuck is used.
Figure 23. Collet
1 clamping body, 2 workpiece, 3 lathe spindle, 4 clamping tube, 5 handwheel
Figure 24. Step chuck
1 basic chuck, 2 workpiece, 3 lathe spindle, 4 clamping tube
Large, flat bodies of revolution such as disks and lids but also square and rectangular as well as irregularly shaped workpieces are - nearly without exception - are clamped flying on faceplates.
Figure 25. Faceplate
A very economical method of clamping is that on the mandrel, which is also called expansion arbor. It is put into the cleaned internal taper of the lathe spindle.
Figure 26. Arbor
1 clamping body, 2 taper plug, 3 workpiece, 4 lathe spindle
Premanufactured arbors can be adapted to various diameters by the lathe operator himself. They are especially suited for clamping levers and other irregularly shaped parts with a bore hole and save the use of complicated fixtures.
Other methods of clamping may be applied according to each respective kind of workpiece.
Clamping mistakes and their causes
Frequent clamping mistakes are:
- Too long projecting of the lathe tool - lathe tool is springy.
- Wrong setting-up at centre by too many backings and/or non-aligned backings - this causes chattering, canting, tool breaking,
Figure 27. Lathe tool too much
projecting
1 wrong clamping
- Inclination of the lathe tool due to overraised shank the consequences are: displacement of the cutting edge angles, the spindle of the clamp nut is tightened in an oblique way.
Figure 28. Wrong setting at centre
1 too many backings, 2 inclined position of the lathe tool due to overraising of the shank, 3 distortion of the cutting edge angles, 4 spindle of the clamping nut not straight
- Wrong position of the clamping jaw and/or locking screws: the lathe tool is fixed only by one edge - the service life of the tool is reduced.
Figure 29. Wrong position of the
clamping claw/clamping bolts
- Wrong position of the lathe tool: The tool is forced into the workpiece - risk of rejects.
Figure 30. Wrong position of the
roughing lathe tool
1 arrow points to the direction in which the tool is forced into the workpiece, workpiece becomes a reject
- Unclean quill: Foreign matters are between the tapers, which leads to rejects and tool breaking.
Figure 31. Foreign matters between
the tapers
1 twist drill, 2 quill, 3 foreign matter
- Too much projecting of the workpieces and/or unsufficient fitting to the clamping jaws - tool breaking due to chattering.
Rejects are also caused by canting of the workpiece.
Risk of accident when loosening the workpiece.
Figure 32. Workpiece projecting too
much/ill-fitting clamping jaws
1 supporting ring
How can workpieces be
clamped?
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What is the purpose of clamping in soft clamping
jaws?
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Determination of the settings (cutting values)
The settings (cutting values) resulting in the best possible economy can be determined only for one definite turning operation on one certain lathe. Cutting speed “v”:
The speed v is the distance s covered in one unit of time, i.e.
The distance s can be expressed by any unit length, e.g. in mm or in m or in km. The unit of time forming the basis of the measurement of the speed can also be chosen in any measure of time, for instance in h (hours), in min (minutes) or in s (seconds).
For the purpose of a uniform determination of the cutting speeds in the turning process, the distance s is expressed in m and the time t in min.
|
v |
s |
t |
|
m/min |
m |
min |
The same applies to boring, counterboring, reaming, milling, broaching, planing, slotting and shaping, for which the cutting speeds are always given in m/min.
Pay attention to the fact, that sometimes cutting speeds are given in m/s, though.
If a workpiece completes one full rotation about its axis and if the diameter of this workpiece is d, each individual point on its surface covers a distance of
s = d x p
If the same workpiece does not rotate only one time but n times about its axis, the distance each individual point of its surface covers is n times as long, i.e.
s = n x d x p
Since the diameter d enters this equation in mm, but the distance s shall be determined in m (1 m = 1000 mm), the equation reads as follows:
|
s |
n |
p |
d |
|
m |
- |
- |
mm |
If the same workpiece again rotates n times about its axis, and if these n rotations are completed in exactly 1 min, it has the rotational speed of n per minute or n/min, which can also be written as follows: n 1/min or n r.p.m.
The latter way of writing is used to indicate rotational speeds.
Since with the number of rotations completed in one minute the unit time of min has entered the equation, we now have no more just the distance s, but the distance s covered in one minute, that is to say, we are now dealing with a speed v.
Thus, the equation reads like this:
|
v |
n |
d |
p |
|
m/min |
r.p.m. | |
- |
For example:
A workpiece of a diameter of d = 200 mm rotates along the lathe tool at n = 30 r.p.m. What is the peripheral speed v?
The peripheral speed of the workpiece, at the same time, is the speed at which the turning is removed, i.e. the cutting speed.
Rotational speed “n”: As is to be seen, there care certain relations between the cutting speed v, the diameter d and the rotational speed n.
A certain diameter and a certain cutting speed result in:
|
n |
v |
d |
p |
|
r.p.m. |
m/min |
min |
- |
For example:
A disk of Ø 200 mm shall be rough-machined at v = 100 m/min. The rotational speed n is to be determined.
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Given: d = 200 mm, v = 100 m/min |
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Required: n = r.p.m. |
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|
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159 r.p.m. |
In planning pay attention, that the cutting speed, when reaching the centre, moves towards 0, i.e. the mean diameter is used for calculation.
With individual parts, a greater diameter may be chosen. With series, a smaller one may be taken in order to avoid frequent regrinding and setting-up.
Time is saved this way.
Cutting depth “a”:
The cutting depth a in mm is the amount by which the lathe tool is fed before beginning a cut.
If possible, the entire machining allowance shall be removed in only one roughing and one finishing operation. With plain turning, it is calculated from the raw diameter dr and the final diameter d of the workpiece according to the following equation:
Feed “s”:
The feed, or better feeding distance s in mm is the progress of the lathe tool is the direction of the cutting movement after one rotation of the workpiece. It is selected from the in-feed scale of the lathe, which is determined by the feed gear, according to the proportion a : s.
By the cutting depth a and the feed s, the sectional area of chip F is determined:
F = a x s
Therefore, for an economical turning, a certain chip proportion a : s must be observed.
With a great feed, a large quantity of chips is achieved - at the same time, the performance of the machine is increased.
Further aspects in connection with the selection of the feed are
the surface quality and the material of the workpiece. What diameter is chosen
for the calculation of the cutting speed with
finishing?
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What is the equation for calculating the cutting
speed?
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