THE LATHE

2.0. ADJUSTMENTS AND MAINTENANCE

Regular adjustment and maintenance is essential for your lathe to perform at it's best. Not only will intrinsic accuracy improve, but surface finish will be better and operations such as parting off demands correctly adjusted slideways. Further, wear will be minimised with correctly adjusted and lubricated slides. You should routinely clean all slideways after each session and lubricate with a little clean machine oil. I've tried all sorts of lubricant in the past, including those based on Teflon and molybdenum disulphide (the latter called "Dri-Slide" - a fine suspension in a solvent base), and a good clean, light machine oil such as Nuto H32 is as good as any. Particular attention should be paid to the headstock and pulley bearings to ensure that they have adequate lubrication at all times.

The first items to consider are the slideways. These should be free to move but without the slightest amount of free play, but it's equally important that they should not be too tight. It is a mistake to think that simply tightening the gib strips until the slides are stiff will improve rigidity. It might, but what it will also do is prevent precise positioning, because instead of moving smoothly the slide will move in jerks as the pressure of the leadscrew overcomes the resistance. So when you try to put that last thou cut on the slide doesn't move - so you try again and take 2 thou off instead of 1! Also, tight slides will increase the rate of wear. The ideal setting will allow the slide to move fairly easily, by hand pressure alone, with no tight spots throughout it's travel.

The carriage and bedways are subject to considerable stresses in normal use, and this will in time lead to some unavoidable wear. This wear will be very much exacerbated by the presence of abrasives between mating surfaces (grit from the bench grinder if it's in the same room, dust from turning cast iron etc.,), a lack of lubrication, and poor adjustment itself which lets the loading concentrate on small contact areas of the sliding components. This being the case it is clear that such uneven wear can eventually prevent correct adjustment being achieved. An older worn lathe may require more than simple adjustment, and more serious renovation such as re-grinding of bedways and the replacement of worn parts will perhaps be necessary. So before you make a start on adjusting the slideways make an assessment of the overall condition of the lathe first, and correct or replace any worn parts. A new set of gib strips and feedscrew nuts (or even the screws themselves) should not be too expensive.

Most smaller and inexpensive model engineer lathes (this includes the Myfords) are a design compromise to produce a good product at reasonable cost. A compromise such as using the leadscrew to drive the carriage for normal turning as well as screwcutting is not conducive to longevity. More expensive lathes have separate driving mechanisms for the two functions. Nevertheless, with proper care and attention a lathe such as the Myford or Boxford can continue to give good service for 50 years and longer, so it's worthwhile doing all you can to protect your investment.

In the Myford, and other lathes where the carriage is driven by the leadscrew located on the nearside of the lathe bed, there is an unequal pressure applied to the guide surfaces - in effect the carriage is subject to twisting forces by being pulled along by one side that causes wear at the leading nearside corner and the trailing farside corner. If regular adjustments are not made this will eventually cause rounding of these edges and no amount of adjustment will then correct the free play thus produced. This effect was particularly evident with older ML7s where only the nearside shear was used to guide the carriage, and a good remedy to correct for this fault is to grind back the guide in the centre of the carriage, insert a gib strip (a hacksaw blade with the teeth ground off does well) at the rear edge between carriage and the farside shear (which was unused) and use this as the other guide surface. Another quick and revealing check is to wind the carriage from one end of the bed to the other, feeling for resistance as you go. As most turning work is done near to the headstock end this is the first part of the bed that will wear, and if adjustments are made to correct for this then the carriage will feel more resistance to movement at the tailstock end. If no maintenance has been done on the lathe for a long time this fault may not be evident until free play at the headstock end has been taken up (so beware when buying a used machine that free movement along the whole of the bed may not be an indicator of lack of wear - the unscrupulous dealer has been known to purposefully *slacken* the carriage so that this defect goes un-noticed).

2.1 The Carriage:

Starting with the carriage then, first check there is not a lot of gummy oil and grease deposits clogging up the gib strip, this is only going to attract grit and encourage wear. If there is, you'll have to strip and clean it with solvent then re-lubricate with a light machine oil. While it's out it's a good idea to check the strip is flat and parallel at this stage (do this anyway if the lathe is old or has seen hard use). Smear a little marking blue on the surface plate and rub the gib strip across it, look for wear at the ends of the strip, and stone flat if a little wear is indicated. If wear is gross replace with a new part. Note that incorrect adjustment of the carriage can affect the alignment of the cross-slide which needs to be at 90 degrees to the lathe axis to face across work correctly (or, to be more exact, set to turn *slightly* concave by a few 10ths). You may as well check for this at the same time and perhaps the easiest way is to set up in the 4-jaw chuck a true parallel of about 8" length. A parallel can be made from ms section about 3/4" x 3/8", rub on the surface plate with a little engineer's blue and carefully file of any high spots, finally finish by placing a sheet of very fine emery on the plate and rub the bar against that. With the cross-slide withdrawn set up a DTI so it touches the front surface near one end of the parallel held in the chuck, then rotate the chuck 180 degrees and check the reading on the opposite end. When the two readings agree the bar is set square to the lathe axis. If you now wind the cross-slide so the DTI tracks across the face of the bar you will immediately be able to see if the cross-slide is set correctly (actually, it's better to remove the cross-slide feed screw and push it by hand for this job, but we'll leave that until later). Slacken the lock nuts and loosen all the adjusting screws. Disconnect the clasp nut so the carriage is free to move. Assuming there are four adjusting screws back these out a couple of turns, then turn the outer two in until you feel a little resistance. Check by moving the carriage that there is no binding. Run the DTI across the bar to check the cross-slide alignment, if there is a small amount of mis-alignment this can be corrected for by tightening or loosening the two screws. When satisfied with alignment turn in the remaining central adjusting screws and check again for alignment and binding, assuming everything is OK tighten the locknuts (without moving the adjusting screws). Finally, check for free movement along the length of the bed - now that free play has been taken up this will be a good test for bed wear.

2.2 Cross-slide and Top-slide:

Checking for alignment of the cross-slide is described in the previous paragraph. The other adjustments apply to gib strips and backlash. It's best to release the feedscrew to make adjustments to the gib strips, on the Myford this is achieved by retracting the slide and undoing the two mounting screws holding the handle bracket to the front of the slide casting. Slacken all locknuts and adjusting screws and move the slide to it's middle position. Turn in the adjusting screws, starting in the middle, until you feel resistance. Only tighten those screws that adjust a supported part of the gib strip (i.e., not the end ones as the slide will overhang front and back). Push the cross-slide from one end of it's travel to the other to check there is no binding. If OK, push the slide all the way forwards and turn in the front screws, then pull back and do the same with the end screws. Check again for free movement without binding. Finally, lock the screws in position with the locknuts and re-attach the handle and feedscrew.

Free play at the handle (on the Myford anyway) is dealt with using a threaded collar which is turned to remove play and then locked in place with a setscrew. Backlash caused by wear between feedscrew and nut is more difficult to deal with. The simplest answer is to replace both components with new ones. If parts are difficult to come by a modification may be possible in which the nut is split cross-ways into two halves and arrange for one half to be fixed and the other to turn and lock in place with a screw. This works with Myfords and is effective in removing all backlash in the feedscrew. Much the same procedure applies to the top-slide though wear here is usually much less than with the cross-slide.

2.3 Tailstock: