Each year I strip down the cross saddle and carriage
from the lathe bed for maintenance check. My
ML7 is now well
over 30 years old and has been used for model making as a hobby and invariably
most of the work is done near the headstock end causing the usual wear. You can
easily appreciate this wear by traversing the carriage along the bed with the
saddle gib strips a good fit at the headstock end and feel it tighten up as you
move towards the tailstock end. The bed on my ML7 is induction hardened which
does minimize wear but cannot eliminate it.
The first task is to check the top surface of the slide ways for any raised areas caused by dents and scratches. Unfortunately each year presents a few more, usually the result of carelessness such as dropping the chuck key or tool holder. Whilst this can look bad it isn't a major cause of worry so long as the raised areas are eliminated. I do this using a honing stone and afterwards ensure it is well cleaned. The dents act as oil reservoirs and so long as they are few and small in area no problem, only personal pride is hurt!
First a word of caution. The following truing-up process will only work if the lathe bed has been regularly checked and maintained. If wear builds up without attention then the only remedy is a trip to Myfords for a bed re-grind and that can prove expensive and in my case incredibly inconvenient. The following process is really a good clean-up of the bed and very small surface-wear adjustments using a hone. If the bed ways are soft then scraping could achieve the same results but only very, very small amounts need to be removed each time, just enough to take-out any binding. Incidentally, I believe to continue using a machine that binds is probably the worst thing you could do as this binding will continue to get appreciably worse after each adjustment to compensate for slackness near the headstock end. The checking and remedial process is necessary to ensure the bed slide ways are truly parallel. Take note of the bearing surfaces (X). With my lathe the bearing surfaces are the front and rear but on some it follows the true narrow guide principle.
___________________ X1________________________
______________________________________________
____________________ R ________________________
____________________ X2_______________________
In any event you need to note the surface which is unused (R) and thus in perfect condition as originally machined, in my case the rear of the front slide. Using this as a reference first check along the whole length the front slide (R to X2) for any variation in width. I use a digital calliper for this which enables the first reading to be set to zero and all subsequent readings show a plus or minus. The calliper is accurate to 0.0005 but even this accuracy may be too course to measure the wear. Take great care to get true readings. The likely amount of wear is probably just a few tenths of a thou and so the caliper is not always precise enough to measure exactly. If the caliper is locked after measuring mid-way along the bed then you can reasonably judge the variance in sizes down to very small amounts. This is not a process to hurry and re-checking of results needs to be done frequently. Using a honing stone any variances where the calliper starts to bind can be eliminated. The stone will only remove the smallest amount and should be done extremely sparingly and such that the squareness of the slide is maintained. In practice these 'high points' will almost always be at the tailstock end reducing gradually to about mid position. I am always surprised how quickly this task is accomplished and again you must ensure very thorough cleaning afterwards. If the wear is substantial then this method will not work and could make things a lot worse. I only do this check once a year and then only if the table movement towards the tailstock starts to show binding. If regular attention is paid to checking for any wear or binding then the method outlined will work well, it has for me. You now have the front bed slide perfectly parallel and can now tackle the rear slide in the same way to address the bearing surface by measuring over the front and back bearing surfaces (X1 to X2). Once this is completed the slides are perfectly parallel to the original register. The acid test after assembly of the now thoroughly cleaned slideways is confirmation that the carriage will now move evenly from end to end.
The steel plates beneath the saddle which prevent lift need to be inspected for wear. Since the bearing surface is only about 1/4" wide some wear is often seen. I am fortunate to have a surface grinder and these plates are given a quick re-surface and honed with a stone to ensure a smooth surface. Reassemble adjusting the gibs as Myford recommend. My method is to take out all play and back off the adjustment screw a minute fraction of a turn. It is important that all play is eliminated with no undue pressure on the bearing surfaces otherwise wear will soon occur. A point worth a mention is the adjustment of the front saddle gib screw nearest the tailstock end. This can easily be stressed so I back this off by about 1/8 to 1/4 turn. Lubricate all saddle points with the less than perfect oil gun and test the results! The objective is to have a slide movement that is silky-smooth all along its travel but show no hint of sideways movement or binding. All this takes time and usually I find that after adjustment and say a few days use the slide needs additional adjustment. I think this is down to the slides, gibs and screws all bedding in. Once again a word of caution, using an incorrectly adjusted slide will cause wear quickly and produce poor surface finishes plus a loss of accuracy due to 'sticky' slides.
Traversing the saddle the full length of the bed will now feel smooth with no tightening. A job well done. I have fitted a piece of thin brass sheet 2 wide to the front of the saddle which protects the slides. The underneath of the sheet has a piece of felt glued to wipe swarf away.
Finally, the need to ensure the lathe is turning parallel. To do this you need a test piece (I keep these on hand for this job) about 7-8" long and about 1" diameter. The middle 5 inches are turned down to by about 0.050" on the diameter. Mount the test piece in the three jaw chuck. I have a centre-pop on the bar to align to the No. 1 chuck jaw. Take a 0.002" cut across both collars without the tailstock and measure. If the diameter of the collar at the free end is larger than the front collar, increase the height of the front foot at the tailstock end, or under the back if smaller. This is where the Jacking feet come into use. No need to buy the Myford version as a pair can easily be made from 1.25" square mild steel. Now that the lathe turns parallel I then set another test-piece between centres. This is about 12" long and 1" diameter relieved in the middle 9". Again take a 0.002" cut over both collars and measure. Adjust the tailstock set-over. I only make a change to the set-over for this test as it is a long job to get it exactly right. This was one of the main reasons I designed and built the Taper Turning Attachment.
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