16.4.
Machine tools are very sensitive to impact or shock, even heavy cast inn standards are not always solid and rigid enough to withstand stresses due to falling during transportation,
and deformations may be set up. Although the machine is always carefully adjusted and aligned when on the test stand or in the assembly department of the manufacturer, it is well known from experience that erection in the workshop of the user is not always done with sufficient care and thus inaccuracies of the work may result from the faulty erection of the machine. So the machine should be carefully levelled up by means of a spirit level before starting with the actual trial tests.
Each trial measurement is based on the correct erection of the machine. No upright, base etc. can be made so rigid that it will be thoroughly free from deformation resulting from faulty erection.
Machine tools for the workshop must be able to produce workpieces of given accuracy within prescribed limits, consistently and without requiring artistic skill on the part of the operator.
For acceptance test of a machine, its alignment test is performed and to see its dynamic stability, which may be poor though alignment tests are right, certain specific jobs are prepared and their accuracy checked.
Fig. 16.15
The relative alignment of all parts of machine and the accuracy of the control devices and driving mechanisms are measured under no load condition. The result of these measurements must lie within the prescribed limits given by the manufacturer depending upon the grade of the machine tool.
A specification for the alignment tests must comply with the following general requirements :
(1) The procedure for testing standard machine tools must not require more than 6 to 8 hrs of work provided allthe tooling and measuring equipment are readily available.
(2) The permissible limits of accuracy of individual measurements must be wide enough to make economical manufacture possible while on the other hand the cumulative error of number of superimposed details should not be excessive.
The various tests performed on the milling machine are shown in (Fig. 16.16) and described below.
16.4.1.
Cutter Spindle Axial Slip or Float.
We have to distinguish between axial (or end) play and axial slip of the spindle.
End play means the indispensable freedom of a spindle moving in the axial direction to prevent it from seizing by heating. This end play is specially important on high speed machines and it should be within the prescribed limits.
Axial slip is denned as the axial spindle movement which may repeat positively with each revolution as a consequence of manufacturing errors. It is only this axial sliding movement which is to be tested, and the specified tolerance applies only to this movement.
When testing the axial slip of a spindle the feeler of the dial gauge rests on the face of the locating spindle shoulder and dial gauge holder is clamped to the table. The locating spindle shoulder is rotated and change in reading is noted. This is done at the two spots diametrically opposite to each other. The total error indicated by the movement of the pointer includes three main sources of errors.
(i) Axial slip due to error in bearing supporting the locating shoulder.
(ii) Face of the locating shoulder not in a plane perpendicular to axis of rotation. (Hi) Irregularities of front face.
Effects of this will be that in cutting spirals, the pitch will not be constant and we will get irregular pitch helix.
If the feeler touches at the same spot where the turning tool on the emery wheel has machined the spindle collar in the assembled machine, then the feeler will not show any deviation. Therefore axial slip must always be tested at two points 180° apart on the collar of the spindle.
16.4.1.1.
Eccentricity of External Diameter.
The feeler is placed on the cylindrical surface of the shoulder. The locating shoulder is rotated and any deviation in reading of dial gauge is noted.
It is due to eccentricity of the spindle in the hole in which it fits. Due to it, vibrations are produced and the cutter will float side ways and cut over, or under-size. Face mills may dig in when leading edges cease to cut.
16.4.2.
True Running of Internal Taper.
The table is set in its main position longitudinally and the mandrel 300 mm long is fixed in the spindle taper. A dial gauge is set on the machine table and feeler adjusted to touch the lower surface of the mandrel. The mandrel is then turned and the dial readings at two points are noted i.e., one at the place nearest to spindle nose and other at about 300 mm from it. For shifting the position of dial gauge from A
to B cross-slide of the machine is operated to bring the dial gauge at the bottom of the end of mandrel. There are can be two errors :
(i) Axis of the spindle and the axis of taper may not be parallel.
(ii) Eccentricity of the taper hole which, if present, should indicate same error at both the places.
The error in first case will give different readings at two places. Due to this error, cut will not be shared equally between teeth of cutters, and therefore vibrations and poor finish will result.
16.4.3.
Table surface parallel with arbor rising towards overarm.
In selecting the permissible errors of horizontal milling machines, care is taken to the fact that in the direction parallel to the cutter spindle, the work table extends towards the front face of the knee only, and never slopes down. While working, the table tends to incline downwards under the influence of the weight of work and cutting pressure, while the cutter arbor tends to deflect upwards. Great importance, therefore, has always been attached to the necessity of having the direction of table tolerance opposite to the deformation expected under cutting conditions. Parallelism between table face and the axis of the main spindle is checked as follows :
A dial gauge is set on the machine table. A mandrel 300 mm long is fitted in the spindle taper. The feeler of dial gauge is made to touch the lower surface of the mandrel. With mandrel in position (mean) the readings at the maximum travel of the table surface are observed. The stand of the dial gauge is moved and not the table itself remains stationary.
Effect of this error will be that the milled surface produced will not be square to the base and parallel cross ways.
16.4.4.
Surface Parallel with Longitudinal Movement.
For this test the dial gauge is fixed to the spindle. Feeler is directed upon the surface the machine table and latter moved longitudinally. The deviations from parallelism between the table surface and longitudinal motion are noted down. If the table is uneven, a straight edge may be placed on the surface. Due to this error the surface of the table will fluctuate up and down and cutter will not take equal cuts on the job which is clamped on the table and the milled surface will not be parallel to the base.
16.4.5.
Traverse Movement Parallel with Spindle Axis,
(a) in horizontal plane; (b) in vertical plane. The table is set in its mean position and dial gauge fixed on the table. The table is moved crosswise and any deviation on reading of dial gauge is noted with feeler on one side of mandrel in horizontal plane and under the mandrel for error in vertical plane.
Due to this error, depth of cut will vary when cross slide is moved.
16.4.6.
Central T-Slots Parallel with Longitudinal Movement.
The T-slot, particularly the central one should be well machined on the internal vertical surface throughout its length because jigs and fixtures are located by T-slots. The general parallelism of the central slot with the longitudinal movement of the table is checked by using a bracket 150 mm long with a tennon which enters the T-slot. Against the upper surface of the bracket in vertical plane the feeler of the dial gauge is located. Having fixed the dial gauge to the spindle and adjusting its feeler to the surface of the bracket the table is moved longitudinal while the tennon block is held stationary by hand and deviations from parallelism are noted from dial gauge. During the process the tennon slides along the slot, thus eliminating the effects of local error.
Due to this error, the depth of cut will not remain constant as the job will be inclined according to inclination of T-slots with longitudinal movement and the axis of job held between tail stock and index head will not be perpendicular to cutter.
ALIGNMENT TESTS ON MILLING MACHINE
| No. | Test to be applied | Test Diagram | Gauges and Method | Permissible Error | ||||
| (1) | (2) | (3) | (4) | (5) | ||||
| 1A B | Cutter spindle Axial Slip or Float Eccentricity of External Diameter |
4 B |
(A) Clock Indicator (B) Test two places at 180° |
A 0.01 mm B 0.01 mm | ||||
| 2 | Internal taper for true running | i—ii | 1 — Bi | L ^ Tnnnr-i |
Clock Indicator slowly move mandrel for maximum eccentricity (A) Nearest to spindle nose (B) At a distance of 300 mm |
A 0.01 mm B 0.025 mm | ||
| co | Work Table Table surface parallel with arbor rising towards overarm | Stationary mandrel. Clock indicators under mandrel Test table surface for max. travel |
0.025 mm per 300 mm | |||||
| A fl | r < |
k i |
■O (Tin nrrr |
|||||
| 4 | Surface parallel with its longitudinal movement. | J ii |
n ® jr 9 ‘ -i—r— *® 1 |
Clock Indicator in spindle Test table surface for max. travel |
0.04 mm upto 600 mm movement 0.05 mm over 600 mm movement |
|||
| 5 | Transverse movement parallelism with spindle axis in horizontal plane In vertical plane |
A C |
r— | Stationary mandrel Clock indicator moved transverse with table. For A test on side of mandrel and for B test under mandrel |
A 0.025 mm per 300 mm B 0.025 mm per 300 mm |
|||
| to r~- | ||||||||
| §6 4 | J | |||||||
| 6 | Centre T-slot parallel with longitudinal table movement | Clock indicator in spindle. Tenons in’T slot | 0.04 mm upto 600 mm movement 0.05 mm over 600 mm and upto 100 mm movement | |||||
| O | ||||||||
| - | ||||||||
| [ n |
||||||||
| l I | ||||||||
| 7 | Centre T-slot square with arbor. | / f- | Clock in spindle. Tenon Block Turn over method | |||||
| db==i—r*v—,—cb | ||||||||
| 8A B | Column Column ways for knee square with table. Inclination front to rear Side inclination |
Clock in spindle. Square set vertically | A 0.025 mm per 300 mm B 0.025 mm per 300 mm |
|||||
| 1 | 3=f | ® | ||||||
| $ 17 |
1—1_. : L | |||||||
| \= | ||||||||
| 9A | Overarm parallel with spindle In horizontal |
of | 1 | Clock in Spindle For A compare readings |
A 0.025 mm per 300 mm | ||||||
| 6 | on under overarm and mandrel | ||||||||||
| p | |||||||||||
| B | ^ 9 | 9 if | o | ||||||||
| o | |||||||||||
| n r- | |||||||||||
| B | In vertical plane | X | For B compare reading | B 0.025 mm per | |||||||
| 1-1_t. | on side of everarm and mandrel | 300 mm | |||||||||
| [ r |
1= | ||||||||||
| 1 | Fig. 16.16 | — | |||||||||
16.4.7.
Centre T-slot Square with the Arbor.
If the central T-slot is not perpendicular to the arbor, the key way etc. cut on the machine will not be parallel to the axis of job.
For this test table is adjusted in the middle portion of its longitudinal movement and a tennon block 160 mm long inserted in the central T-slot. A dial gauge is fixed on the mandrel, the feeler being adjusted to touch the vertical face of the bracket. Observe the reading on the dial gauge when the bracket or tennon block is near one end of table. Then swing over the dial gauge and move the tennon block so that the corresponding reading can be taken near the other end of the table. Generally two tennon blocks are used.
16.4.8.
Tests on Column,
(a) Column ways of knee square with table, inclination front to rear (b) Side inclination.
Arrange table is its central position and fix a square with an arm about 300 mm long (Angle iron type bracket placed on the surface), on the table surface and attach a dial gauge to the spindle mandrel in such a way that the feeler rests on the arm (vertical face) of the surface near the bottom edge. Observe the reading of the dial gauge. Move the table upwards abut 300 mm and again observe the dial gauge. The difference in readings is a direct indication of the error of perpendicularity of the table surface (from guiding surface) and knee support or side guiding support. The above test is conducted for two positions of the square and in first position the dial gauge touches the square in front and in second position it faces the side of the square i.e. at 90° to the first position (for testing side inclination).
If column-ways for knee are not square with the table, as the table is fed upwards in facing operation or end milling, the surface produced will not be square with the table surface.
16.4.9.
Overarm parallel with spindle
(a) in horizontal plane, (b) in vertical plane.
To check the parallelism of the overarm and the spindle, fix the dial gauge on the table and its feeler under the mandrel. Move the table crosswise and note any change in the reading. For error in the horizontal plane, repeat above readings so that feeler is under overarm and compare the two sets of readings. For vertical plane compare the readings on mandrel and side of overarm.
16.4.10.
Alignment of the main spindle with bore of the bracket of the overarm.
With the mandrel (parallel) in the bore of the overhanging bracket and gauge holder in the mandrel fitted to the spindle taper, the feeler is adjusted so that it touches the mandrel in the
bore. The main spindle is turned slowly and reading of the dial gauge is noted at four points (opposite ends in horizontal and vertical planes i.e. 90° apart).
The difference between two 180° opposite readings and other two is twice the eccentricity of the mandrel in the vertical and horizontal directions respectively.
It the axis of the bearing of the supporting bracket is not co-axial with the spindle axis, the axis of the arbor which is held in spindle and supporting bracket will not be parallel with table surface and hence the cutter mounted on the arbor will take more cut on supporting bracket side if the bearing axis is somewhat lower than spindle axis and less cut if the bearing axis is above.
