If you want your lathe machine to work perfectly, you need to check whether your lathe is perfectly aligned or not !!
These tests which are carried out on a metal lathe to check the accuracy is known as lathe alignment test (or lathe accuracy test or lathe alignment procedure).
Lathe alignment test (or lathe alignment procedure) can be classified into two types.
- Geometric tests
- Practical tests
Geometric test on metal lathe includes checking the relationship between different components of lathe when it is idle. These tests are performed using simple measuring instruments along with a few special accessories.
Geometric lathe alignment tests generally include tests like checking the parallelism of the spindle axis and lathe bed, intersection of headstock and tailstock centers, etc.
The main aim of these alignment tests is to check the accuracy of the work done on the machine.
The accuracy of the final workpiece depends on the accuracy of the machine, but it also depends on the other factors like correct mounting or workpiece, type of cutting tool, quality of material, work holding devices, as well as skills of lathe operator.
Also the practical tests on the lathe are costly. Hence many machine owners prefer geometric tests.
Lathe Alignment Tests
You will see the metal lathe alignment procedure mentioned below. I have explained each lathe alignment test with a line diagram and brief explanation about each.
#1) Is Spindle Axis Parallel to Bed?
For performing this test, three equipment are needed, and that are dial gauge, stand and mandrel.
The dial gauge should be graduated in 0.0005 in. divisions and it should have a light measuring pressure.
The mandrel used in this test generally has a diameter of 1 in. and its length is 12 in.
Its taper end is fixed in the headstock spindle and it is accurately grounded on centers so that the parallel and taper parts are concentric.
The dial gauge is attached with the carriage of the lathe such that its plunger touches the surface of the mandrel at one end (see above image).
The spindle is rotated slowly and its mean reading is found on the dial gauge.
After noting the dial gauge reading, the carriage is moved towards the head stock and readings are taken at the other end of the mandrel.
In this way, the observations are made in both horizontal and vertical plane.
While performing this test in a vertical plane, there will be small error which occurs due to sagging of the mandrel. This deflection of the mandrel can be calculated by using the following formula.
Deflection = ωL4/8EI
Where, L is the length of mandrel in inches.
#2) Is Cross-slide Perpendicular to Spindle axis?
There are two methods to check whether the cross slide is perpendicular to the spindle axis or not.
One method is the “geometric” method and the other method is “practical” test.
In the geometric method, the straightedge is clamped on a cross slide as shown in the above image.
This straight edge is perpendicular to the spindle axis and it is in the horizontal plane.
The faceplate is mounting the lathe chuck and the plunger is attached with this faceplate.
The carriage is adjusted in such a way that the plunger touches the straightedge.
The straightedge is adjusted such that the readings of plunger at side A and side B becomes equal.
The cross slide is then moved in the direction perpendicular to the spindle axis.
During this process, the plunger reasons are noted down.
If there are any changes in the plunger readings, then the cross slide is not perpendicular to the spindle axis.
The second method is a practical method in which a small finished cut is given on a cylindrical workpiece. After this, the straightness of the diameter is checked across the finished surface.
This is done by placing two equal slip gauges at opposite ends of the diameter and trying the third equal slip gauge under the straightedge near the center of the job.
This slip gauge should enter because the only permissible departure from the straightedge is a slight concavity.
The amount of concavity can be measured by finding the slip gauge which can fit in the center of the workpiece.
#3) Is the Pitch of Lead Screw Accurate?
The setup for testing the accuracy of the lead screw is given in the above line diagram.
The pin with slightly increased diameter as shown in image is attached with the faceplate as far as possible from the center.
Block B is clamped on the lathe bed and block A stands on the block B which forms the stop for the pin to locate the spindle in a same angular position each time the readings are taken.
It is placed in the position where the enlarged diameter of the pin will press on its upper end.
On the right side of block B, you can see the small ball shouldered on it which forms a contact point for the length bar (L).
The saddle is attached with the dial gauge such that the dial gauge is in contact with the other end of the length bar (L).
To perform the test, a suitable gear ratio between the lead screw and spindle is selected and the saddle traverse lever is engaged.
The spindle is rotated in the reverse direction for about 180° and then it is rotated forward again until the pin presses firmly on the stop-piece.
At this position, either the readings are noted or the dial gauge is set to zero.
The length bar is now removed and a shorter bar is substituted. The stop-piece is then moved away and the spindle rotated through the calculated number of turns which will move the saddle a distance equal to the difference between the lengths of the bars, the stop-piece being put into position during the last revolution. With the pin pressing on the stop-piece, a second reading is taken on the dial gauge.
Any variation between this reading and the first one represents an error.
Proceeding in this way, with bars of gradually decreasing length, a record of the errors over the portion of the lead screw selected is obtained.
If sufficiently small intervals are chosen, a graph
of errors can be drawn.
It is suggested that intervals of one inch be used, and that one of these be subdivided (using slip gauges on the end of the length bar) in order to obtain a more detailed record over a few revolutions of the screw.
In this test, any errors in the gearing between the lead screw and the spindle are included in the results obtained. The overall error should not exceed 0.001 in. per foot.
#4) Is Line of Centers Parallel to the Bed?
In order to test the parallelism of lathe center lines, this test is performed.
The mandrel is held between the lathe centers (head stock center and tail stock center).
The dial gauge is attached with the carriage as shown in the above image.
The dial gauge moves along the lathe axis and the readings are noted on the dial gauge.
If the tail stock has a set-over arrangement, then the test is performed only in the vertical plane.
The error should be less than 0.0005 in. per foot, at the tailstock end.
#5) Is Lathe Tailstock Parallel to the Bed?
The tailstock quill is attached to the lathe tailstock and it is clamped tightly.
Rigid clamping is essential as the clamping force affects the position of quill.
The plunger is attached with the carriage as shown in the line diagram above.
The carriage is fed over the horizontal axis of the lathe bed and the plunger readings are noted down.
The plunger is first attached such that it is above the quill and then in the other test the plunger is arranged sidewise.
The end of quill should rise by an amount not exceeding 0.001 inch.