Careful maintenance will increase the life and efficiency of the machine. A thorough weekly cleaning is very necessary for any machine in continuous use. It plays to check and adhere to the lubricating instructions from time to time and any defect however small it is, should be immediately rectified. Otherwise this will lead to a major breakdown of the machine, resulting in loss of production and high cost of repairs.
It is necessary to check the electrical outfit once in every six months and clean particularly the contact points. Melting or burnt strips should be removed and the links lubricated carefully. The ball bearings of the motor require cleaning and refilling with grease once in a year.
To perform this:
(a) The covers are removed.
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(b) The bearing shields are removed to leave them both accessible on the shaft. They are washed with benzoyl. Then the bearings are filled with clean bearing quality grease. The dismantled parts are to be reassembled carefully.
When replacing the damaged bearings, the new bearings should be oil heated and mounted on the shafts and greased. It is important to prevent any damage to any part either while removing or during reassembly. The oil filter and the pump are to be cleaned once in a year. The old oil is to be drained out, the sumps are to be cleaned before refilling them with fresh oil of correct specification.
A general survey of the lubricating system may be restricted to the observation of oil level marks, refilling oil periodically and to daily lubricating of all openly moved mechanisms and links and guideways.
Adjustment of Main Spindle Axis in Horizontal Plane (Fig. 13.11):
The headstock is secured on the lathe bed by five fixing studs and nuts. The studs on the right side of the head stock acts as the pivot. The other four ends have 1 mm clearance around them in their respective head stock holes. The head stock can be moved around the pivot for aligning the spindle axis in the horizontal plane.
Wherever a job turns taper, it might be due to incorrect alignment of the main spindle to the bed guideways in horizontal plane. In such cases, the spindle axis can be readjusted within permissible tolerances as per test charts.
The following equipment is required for checking and readjusting the horizontal alignment of the main spindle:
(i) A mandrel with a taper at one end to suit the taper in the main spindle bore and with a cylindrical portion measuring to a length of 300 mm.
(ii) Dial indicator.
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(iii) Magnetic stand.
(iv) Metric size spanner-set for the head stock clamping nuts.
To start with, before loosening the head stock clamping nuts, the error should be noted over the mandrel inserted in the spindle and once again after the nuts are loosened completely. If there is any change it should be noted.
Then the two hexagonal head stock adjusting screws (shown as A in Fig. 13.11) should be adjusted so that the dial indicator shows the correct reading. (To gain access to the screws A. it is necessary to remove the quadrant covers and the quadrant). Then the head stock clamping nuts are to be fully tightened and again the horizontal alignment should be checked and corrected, if necessary, by repeating this method.
Adjusting Feed Shaft Clutch (Fig. 13.12):
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To feed shaft, drive is through a cone friction clutch, which is spring loaded and is set at the works. If the longitudinal or transverse feed drive is overloaded, the clutch will slip. In case of clutch adjustment, the nut C is set so that a weight of 25 kg attached to the hand wheel of the apron, just stops the feed movement by slipping of the clutch. The spring B should have enough play for the release of the clutch.
Replacement of V Belts (Fig. 13.13):
For replacing the V-belts, the main spindle has to be removed and then the pulley sleeve is to be removed.
(a) For removing the main spindle:
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(i) The top cover of the head stock is removed.
(ii) The 8 socket head cap screws, A fixing the front cover M are removed.
(iii) From the front gear, the locking wire B and grub screw C are removed.
(iv) The rear nut D is unscrewed and removed and the collar is taken out.
(v) The two gear blocks are blocked with wooden pieces at E and F against the nearest fixed support.
(vi) A mild steel rod of ɸ 25 mm threaded at both ends along with a stirrup with a hole as shown in the diagram is used for extracting the main spindle. The rod is inserted through the spindle bore and is held at the left hand end by a collar spigotted into the spindle bore.
The right hand end of rod passes through the drilled hole in the stirrup which rests on the main casting. B tightening the nut on the right end, the spindle is extracted.
(b) Removing the pulley sleeve:
For this the shaft S is to be extracted and the gear block on this shaft is removed first, so that the pulley sleeve can be removed to the right. For this, the cover V and the adjusting piece K are removed. Using an extractor, the shaft is dismantled then. The shaft and the adjusting piece are provided with threaded holes for the extractors.
Now the pulley sleeve can be removed as follows:
(i) The locking wire H and the screw G are removed.
(ii) The ring nut and the locking washer I are then removed.
(iii) From the top, the plug J (Bearing positioning plug) is removed.
(iv) The retaining ring R and the deflector L are dismantled.
(v) With a mallet or plastic hammer, the sleeve is tapped out towards the right, till the front bearing is clear.
(vi) Place a wooden block between the bearing and casting wall and gently tap in the sleeve back. Now the front bearing is removed from the sleeve. (This may necessitate the removal of the shift fork also).
(vii) Now the sleeve is tapped out towards the right until it is free.
Caution:
Rough handling must be avoided while extracting and replacing. The spindle and pulley sleeve should release without any difficulty. The spindle can be removed and replaced without altering the bearing adjustments.
Re-Assembly of the main spindle:
When the main spindle is being reassembled the following points are to be noted:
(i) The bearings should be thoroughly cleaned.
(ii) The reduction gear is greased lightly to ease refilling on the spindle.
(iii) Care is taken to see that the direct drive dog N does not join against the sleeve. This is done by moving the lever operating the front gear continuously. This is very important.
(iv) The alignment of the outer race of the front bearing should be checked before fitting it in the housing.
(v) The positioning screw O should enter the notch in the collar freely.
Adjustment of Main Spindle Bearings:
These high precision bearings are carefully adjusted at the works. The machine is test-run and performance tests are carried out in the test floor during the final testing before the machine is dispatched. Normally, this adjustment should remain constant during several years of uninterrupted running, even at the highest permitted speeds.
In the front, the double row taper roller bearings provide the spindle axial location in both the directions. The external race is prevented from rotating since it is held in the head stock body by the front plate. In the rear end, single taper roller bearings outer race slides in head stock housing with elastic pre-load from matched set of coil springs.
Thus,
(a) The spindle and taper rollers of the front bearings are held in the taper housing to take up the axial play.
(b) Play in the rear bearing is eliminated.
(c) All expansions and contractions are allowed to take place freely towards the rear ; without disturbing the front bearing adjustment.
Temperature check:
The abnormal increase in temperature of the main bearings is a reliable guide to check the bearing adjustment as well as the lubrication.
To observe the temperature rise the cavity provided in the top of the front plate is to be filled with oil and an industrial mercury thermometer is placed in it. A non-stop run of ninety minutes at a spindle speed of 1600 r.p.m. should cause a rise in temperature of 40°C above the ambient temperature.
A higher temperature increase indicates that the spindle bearings are constrained due to over-tightening. However, a lower temperature does not mean that the bearing space requires adjustment. The run out on the spindle should be within 0.01 mm in any temperature condition of the bearings.
When adjusting the front bearing by means of adjusting nut, it is essential to proceed carefully by fractions of a turn (1 or 2 mm at the periphery of the nut). If the bearing is over-tightened, the not must be completely slackened off and the bearing freed by several light taps on the rear end of the spindle, taping against a block of wood or aluminium.
This is to be performed only by skilled personnel. After adjusting the front main bearing, the rear bearing cover is to be tightened and locked such that the run out on the spindle nose is less than 0.01 mm.
Replacing of Broken Shear Pin in Lead Screw:
The lead screw is connected to the feed box through a safety shear pin coupling. When the pin is sheared due to any overload, the lead screw will not transmit motion from the feed box. This pin can be seen by sliding out the knurled collar E found on the left end of the lead screw.
The shear pin is replaced as follows (Refer Fig. 13.14):
The split nut is disengaged. By gently tapping with a copper hammer, the knurled collar E is slid to the right. The shear pin would have broken into three pieces and the smaller piece 3 will fall off as soon as the collar is slid away to the right. Now the lead screw is rotated till the broken piece 2 comes in line with the through hole in the sleeve B.
Using a pin punch, this piece is knocked out of the sleeve. Now the sleeve is given a quarter turn so that the bushed holes in the lead screw and the sleeve come in line and the piece 1 of the broken pin is knocked out. The hole is blowed out clean. A new shear pin slightly greased to prevent rusting, is inserted, lightly and without hitting, to prevent the shear bushes being displaced.
The shear pin should be made out of bright drawn steel of tensile strength 43 kg/mm2. When cutting steep pitch threads, care must be taken that there is no sudden reversal from forward to reverse. Only shear pins manufactured of the material and dimensions specified may be used.
Adjustment of V-Belt Tension (Fig. 13.16):
Belt tension is adjusted by slackening the nut A, completely allowing the motor and speed box weight to tension the belts. Now the nut A is hand tightened and nut B is fully tightened by spanner. The speed box base plate is now held rigidly between the nuts A and B. The tension of the belt should not be increased by tightening the nut B when the nut A is loose, as otherwise the pulley bearings will be overloaded.
Adjustment of Brake (Fig. 13.15):
The bearing force could be set as follows:
Slacken the nut C. Keeping the brake pedal pressed down firmly tighten the nut D so that the brake band grips the drum. Now the pedal is released. To suit the degree of braking desired, the nut C is slightly tightened.
Trouble Shooting:
