IN THE WORKSHOP The point here being that unlike the amateur’s

IN THE WORKSHOP The point here being that unlike the amateur’s
IN THE WORKSHOP
Lathe Alignment
by Tubal Cain
D URING
THE FAST few weeks “the works” have
seldom been without some evidence that lathe
alignment was in progress. First, the Larch had
to be moved, and as the shop floor is far from
level any machine on its own stand must receive
attention even if the move is slight. Then, a
vintage Myford-Drummond was overhauled, to
fill the gap between the departure of the trusty
ML7 and the arrival of a Super 7. This involved
alignment of the machine “within itself” as the
headstock had been removed, as well as checking
that bed, slides and tailstock had been truly
re-scraped. Finally, the arrival of the Super 7
involved a third repeat of the exercise. In view
of the general interest in this subject, and especially of the remarks and queries in Mr. Beck’s
first article, it seemed opportune to put a few
notes on paper whilst the matter is fresh in mind.
The first matter to be dealt with, perhaps, is
“why bolt the machine down at all?” This question has been asked in the correspondence
columns of “M.E.” certainly back to 1916 - the
earliest copy I have seen. Contrary to Mr. Beck’s
impression, a lathe in conditions of zero gravity
would be unmanageable. The law that “When the
horse pulls the cart, the cart pulls the horse” is
universal, and at every start of the machine the
bed would rotate slowly round the contra-rotating
spindle. If the work were not perfectly balanced,
the headstock end would start to corkscrew ;
and, finally, the whole issue would start to precess
as a gyroscope. But the point is that the restoration of gravity would not cure these ills unless the
total mass of the machine were very large in
comparison with the forces involved. The bolting
down - or fixing - of any machine tool is necessary to contain the forces due to out-of-balance,
vibration, and starting reactions. And, of course,
to enable you to rest your elbow on it safely, turn
handles, and the like.
Now, certain machines are heavy enough in
themselves to contain these forces, and this is
especially true of some high-speed lathes, where
the work and machine must be fully balanced to
ensure that it is not distorted in the turning.
Others, like milling machines, must, as a rule, be
held down only against the vibration caused by
the cutter. At the other extreme, the large shaper,
if not securely fixed, would chase the operator
round the shop. Note, I have said “fixed”. Bolts
912
are not the only means, and I have stuck many a
large machine tool to the shop floor with glue.
The point here being that unlike the amateur’s
workshop, machines in production shops and
research laboratories must frequently be rearranged to suit the flow of work in hand. Sticking them down saves time and does not leave a
lot of holes in the floor when they are moved.
So, we “fix” the machine somehow to contain the
forces - and, incidentally, as Mr. Beck may have
found out, to stop them from falling over backwards. The C.G. of a Myford is not far from the
centreline of the rear holding-down bolt!
Second - why level? Only for convenience.
Not all machines (lathes, that is) have level beds.
Indeed, the vertical boring mill- the “roundabout”- is no more than a vertical lathe, with
the headstock in the floor and the saddle running
up and down a vertical bed. We had rows of
these in the works, machining cylinder heads for
24 in. diesel and gas engines, and some very large
ones for the big flywheels. Some production
lathes have the bed at an angle across the
machine, which facilitates chip flow away from the
work, and I have seen a lathe design in which the
axis was up at about 30 deg., with the headstock at
the high end, and the operator in effect sitting on
the tailstock.
There is no magic in “level” except that it is
by far the easiest angle to reproduce. If the lathe
bed is known to be truly level, any subsequent
setting up work- on the vertical slide, for
example-can be done using no more than a
reasonable quality spirit level. Further, it is very
useful for erection jobs to have one surface in the
shop which is known to be true and, for the
amateur, the lathe is the most convenient surface
to hand. For these reasons even in production
shops machines which were either not bolted at
all, or were to be glued down, were all equipped
with four jacking screws which bore on steel
plates on the floor.
Why four? Repeatedly in articles I have seen
the proposal that machines should have but three
supports, on the argument that there would then
be no fixing strains. Indeed, a Model Engineer
competition, held a good many years ago, to find
the “perfect” lathe showed the majority of the
drawings specifying two supports at the headstock and one at the tailstock end. This reveals
an understandable misapprehension of the purpose of the alignment process. It is assumed that
the object is to support the machine so that it is
free of all strain. In a perfect world, this might
be true, but even then the weight of the machine
parts would introduce strains not present when
the bed alone was machined. This is especially
true of the modern small lathe with a motorising
MODEL ENGINEER 21 September 1973
unit attached to the bed as a cantilever structure ;
at a guess this unit applies a torque of around
200 lb./in. to the bed of the Super-7, far more
than any cutting forces likely to be applied.
The world is not perfect, either, and the best
jigs in the world it cannot be assumed that a
lathe bed is mounted on the surface grinder
entirely free from strain. This did not matter in
the days when bed surfaces were all scraped
true, as any distortion would be corrected at the
second operation. Costs won’t permit this nowadays- and in any case, the ground surface is
superior. So that it may well be that that the odd
bed, ground perfectly true on the machine, will
spring a little when taken off.
The object of machine alignment, therefore, is
first, to re-introduce any strains that were there
when the bed was machined originally, and second
to compensate for any which have been added by
the attachment of motor, gearbox, taper-turning
attachment, and so on. It sounds a formidable
undertaking, but fortunately it can be achieved
with little difficulty. You do not, in fact, need any
instrument more advanced than a really good pair
of firm joint calipers, though dial indicators,
micrometers, and block levels will help. You can,
in fact, get tied in knots because your instruments
are too sensitive. When I first moved into the new
shop here I borrowed a precision block level in
which one division indicated a discrepancy of
one part in 24,000; moving my weight from one
foot to the other would move the bubble appreciably due to the deflection of the floor! If you
can detect 3 thou. per foot error, this is sufficient.
But, as I said before, you can do the job without.
Before dealing with the process of setting up, a
few words about bolting down. If you are using
a stand designed for the machine, it is only
necessary to ensure that this is reasonably level
and not strained when screwed to the floor. A
carpenter’s level is good enough for this job, as
the object is simply to reduce work later. If the
floor is concrete, simply fit shims under the feet
until the stand is level and does not rock. Don’t
be content with shims on one side of each bolt let the bolt be supported by shims on both sides;
the ideal is to use U-shaped ones, but one is
always in a hurry to get the machine working at
this stage! If the floor is timber, then I prefer to
use really hard plywood (wood-worm proof,
please ; you don’t want to find the shim has dissolved into dust in a year or so) and always cut a
U-shaped slot to pass each side of the bolt or
screw. Dry these by the fire, and give them a coat
of varnish. Once the stand is level, follow the
maker’s instructions with regard to bolting the
machine to the stand. There is a variety of jacking screw arrangements, and each maker has his
MODEL ENGINEER 21 September 1973
f o r adjustment $
FIG.
I
own ideas. Set the lathe on the supports ; don’t
bolt it down-leave it overnight for everything
to settle, especially if on a wooden floor.
If the machine is to go onto a wooden bench,
as mine is, then it is important to provide a
supporting and jacking arrangement which will
reduce the risk of crushing the timber. If the
machine is a modern Myford, then the purchase
of the steel drip tray and a pair of raising blocks
is a good investment. If not, or if the purse won’t
stand it, then Fig. 1 shows an arrangement that
will serve very well. A and B are large diameter
washers to spread the load over a reasonable area
of timber - about 2 in. or 3 in. diameter-with
a 1 in. dia. collar to centre them in the fixing hole
in the bench top. Note that the lower one has a
clearance hole for the screw H (which should be
at least + in. dia.), but the upper one is tapped.
The length of the screw is sufficient to allow for
the nut D (square for preference) and the jacking
nut E. F is the lathe foot, and the screw must
either be reduced in diameter at the upper end to
pass through, or (as mine always were) a smaller
stud screwed into the end of the larger screw.
G is the lathe holding-down nut. Don’t forget to
allow for washers under the nuts. If you intend
913
to fit a metal drip tray or even just a metal surface to the bench, then arrange it as at J. In use,
the large washers and the screw are loosely
assembled in the bench and the nut D finger
tight, till you have established that the centres
are correct. The large nut D is then tightened up,
really hard, and the jacking nuts E run on.
Mount the lathe, but don’t tighten it down- just
slip the nuts on sufficient to prevent it toppling
over. Leave overnight for things to settle. Then,
if need be, tighten the big nuts a little more. You
now have a four-point jacking system as good as
any. One point, however, is worth attention. Use
a coarse thread- Whitworth - for the nut D
and BSF for A and E, and make sure that A is at
the end of the thread before assembly.
Now to the setting up, the procedure being the
same, however the machine is fixed. I shall
refer to “jacking screws” throughout ; this means
a jacking nut as well, or to the insertion or
removal of shims if no jacking arrangement is
available. The word “level” refers to a block
level of sensitivity about 3 thou. per foot. If you
haven’t got one, you may be able to borrow one
from the local Technical College provided that
you sign for it - and are able to pay for it if you
drop it! Any other sort of level will be mentioned
as such. Dial indicator (or DTI) means one reading in thousandths -it just isn’t worth messing
around with the more sensitive ones ; they are
great liars, very prone to stick - and similarly a
.OOl in. (or .Ol mm.) micrometer will do. If you
have none of these, then you must get a good
pair of firm joint calipers ; without these you cannot manage; with them you can do well. If you
are going to use a dial indicator, then you will
also need a parallel test bar. You should have
one of these handy anyway ; here again the local
Technical College may oblige- it is the sort of
exercise carried out by some of the Craft and
Technician classes. Get them to make you one
about 1 in. dia. and a foot long ground between
dead centres. The diameter doesn’t matter at all
provided it is parallel and round. Such a bar is
very helpful in setting up the tailstock back to
parallel after taper turning. Failing this, use a
piece of ground stock not less than + in. dia.,
and preferably larger. Make sure it is straight ;
much silver steel is true to diameter, and round,
but not always straight.
Levelling method
Adjust all the jacking screws until the lathe
does not rock, and then set the level across the
bed at the headstock end. Adjust the screws at
the headstock only to level the bed. Set the level
along the bed, and adjust the tailstock screws
until the level bubble is central. Do this adjust914
ment with the front screw and follow with the
rear, taking out the rock. Now set the level across
the bed at the tailstock end, and adjust the two
tailstock screws. Return to the headstock again,
and repeat the whole process. Now tighten all
four lathe holding-down nuts, reasonably but not
fully tight. Start again at the headstock, but this
time you must first slacken the holding-down nut
before altering the jacking screw, and then retighten. Carry on again as before. You may have
to do this several times. If you find inconsistent
results check (a) that the level is true ; it should
read the same whichever way round it is used. If
not, use it both ways and split the difference
(b) that your floor is not so flimsy that your
weight moving around is deflecting the level,
(c) that the bench is not being rocked by your
leaning on it. In the case of (b) or (c), add more
stiffness !
If your lathe has raised Vee guides, then you
will either have to apply parallels under the level
to clear the guide rib, or set it on the cross-slide
and run the saddle back and forth along the bed.
The former is the better method if your parallels
are to be relied upon. Try them both ways round,
as suggested for the level. Once you are satisfied,
tighten all the holding-down bolts, and check the
job once more. Note that you may have to repeat
the process in a few weeks time, especially if
you have a wooden floor, as things may shift a bit
under the vibration of the machine. I usually
check my machines at pension time- once a
quarter !
Dial Indicator Method
Thoroughly clean the mandrel nose and the
three-jaw chuck, both on the screw and the jaws,
and fit it carefully. If you don’t know which is
the “preferred socket” for the chuck key, check
it now. Set your test mandrel in the chuck, gripped
full length of the jaws, and set up the DTI with
the absolute minimum of overhang of the pillar,
etc., in the toolpost. Apply the indicator to the
bar as close to the chuck jaws as you can get,
disengage the bull-wheel from the pulley, and
spin the chuck slowly, noting the variation on the
indicator. If it is more than +0.003 in., send it
back (or clear out the dirt!). Now, slacken the
chuck, and retighten at socket No. 1. Note the
reading. Slacken, tighten at No. 2 socket, and
note; repeat for No. 3 socket. The socket which
gives the smallest runout is the one to be preferred for accurate work ore thdt diameter. Unless
the chuck has a ground scroll, it may well ilo
better or worse on another size of bar. (I mark
my own chucks with yellow paint on the sock&
that does best at 1 in. dia.)
Now set the bar with about 5 in. or more stickMODEL ENGINEER 21 September 1973
i
FIG. 3
ing out, tighten with the preferred socket, and find
the high and the low point under the dial indicator. Midway between these should be “correct”.
Mark the chuck, traverse about 4 in. away and
repeat. The “correct” position should be the
same, or nearly. If it isn’t, the bar is bent. (You
must then get another ; this method won’t work
with a bent bar.) With the bar in the “correct”
position, the DTI should read the same both close
to the chuck and 4: in. away, if the bed is true.
But it may read as if the bar were .0003 in.
towards the DTI at the 4+ in. position. If so, leave
it alone. This is not only permissible, but desirable ; it means that the work is leaning towards
the tool about quarter of a thou. in 4+ in.-and
when cutting, the workpiece will bend away from
the tool, so that a parallel cut will result. If the
deflection is bigger then you must adjust the tailstock jacking screws. Front jack up if the free end
leans away from the DTI ; rear jack up if it leans
tothzrds the DTI. You should, of course, have
levelled the machine as well as you can with an
ordinary level, so that you have the convenience
in setting up later, before you carry out this
procedure. As before, it will be necessary to
repeat the check after a little while.
Turning Test Method
I always use this method after having set up
the machine by one of the other means ; it is,
after all, the final arbiter-that the machine
turns parallel. Fit the three-jaw as before, and
find a piece of stock about 6 in. long and large
enough just to fit right inside the chuck jaws ; the
larger the better. (Sorry! First set the machine
up as level as you can, and take out the rock in
the jacking screws. Tighten the holding-down nuts
MODEL ENGINEER 21 September 1973
finger tight or a little more.) Get a good hold of
the bar, but don’t strain the chuck. Take a cut
along the middle length of the bar to reduce it
about 3% in. less in diameter than the ends, S O
that it appears as in Fig. 2, where the line XX is
the closest the tool can come safely to the chuck
jaws. This need only be a rough cut. Now, with
finest power feed, take a cut along the two
bobbin ends, to take out any ovality. Say about
.005 in. or so. Remove the tool, and resharpen,
honing the cutting edge on an oilstone to a fine
finish. Set it up at centre-height and take a very
fine cut -not more than ,003 in.- across each
bobbin. Use the finest power feed you can get
with your change-wheels. (If you experienced any
chatter in the pr’eliminary cuts, you must reduce
the speed till it goes before doing this final cut.)
Measure the two bobbins with the micrometer.
If the free end is large, jack up the front jacking
screw at the tailstock end ; if small jack up the
rear. Again, if the free end is only a few tenths
large at this light cut, you may do better to leave
it alone, but it should not be more than .0002 in.
Now tighten the machine down properly, and
repeat the test, adjusting as necessary.
It is not essential to use steel for this job,
though I always try to use free-cutting steel.
Brass will be just as good, but don’t use aluminium, which may build up a “nose” on the tool,
and interfere with the cutting. I usually run the
tool dry, and it is important to keep your hands
off the machine whilst the bobbins are being cut.
Lean on the tailstock, and it will upset things!
Using Calipers
The above method can be used perfectly satis-
factorily, though not absolutely perfectly, if you
see the differenc,e, without a micrometer. It may
be difficult for the “Mike Brigade” to believe, but
if you use it properly, a caliper will detect less
than half a thou.! Note- I said “detect”, not
“measure”. I doubt if the rule and calipers can be
set to better than .Ol in. - ten thou. - but in this
case we are concerned not with the actual diameter of the bobbins but that they should be the
same diameter. Make this simple test. Chuck a
piece of 1 in. stock in the three-jaw and take a
light cut with fine feed to true it up. ‘Set the topslide over to 11 deg. ; hone up the tool edge and
set it so that it presents to the work at the correct
attitude. An advance of 1 thou. on the top slide
will now advance the tool about .0002 in. Bring
the tool close and lock the cross-slide. Take a
very fine cut with power traverse-about 10
divisions on the top slide-over a length of
about 4 in. Traverse back with the saddle and
put on a cut of one division on the top slide, and
machine about Q in. of the bobbin at this setting.
915
This will give you a workpiece with two diameters, one .0004 in. smaller than the other. Set
the calipers to the large diameter, and then try
them on the smaller. If you have set them in the
correct manner, you will easily notice the difference. The tool must, of course, be really sharp
and honed to a fine finish, or it will not cut this
small amount.
Aligning the tailstock with a mandrel between centres.
What? The correct manner? Very well, here it
is. To open the calipers, tap the hinge on the top
of the tool-post; the lighter you tap, the less it
will open. To close, tap the back of one of the
caliper blades, holding the calipers at the hinge.
It is as simple as that, and it always curls my
toes up to see chaps “setting” calipers by ramming them across the workpiece, and then applyBelow: Testing the shears of an “M” type lathe after
reconditioning.
ing them to the rule. They will be reading from
$64 in. to 1% in. out that way. Mark you, they
must be good calipers, and frankly, I find the
old “firm joint” type far better than the screw
adjusted ones. My own Sunday best pair are part
of a little set which w’ere given away by Messrs.
Chesterman at Christmas over 35 years ago-a
set of feelers, a folding rule, and a pair of
calipers, all in a little pocket case. They have been
in continuous service since then, never needed
adjustment, and have a lovely feel about them.
(My “second set” were made by my grandfather,
about 100 years ago. Made, he told me, from the
blade of a worn-out iron-ore shovel, with brass
washers on the rivets. These, too, work very
smoothly still.) You have to develop the feel, of
course, but this comes quite easily. The tool must
be held very lightly by the hinge, and when
properly set will just hesitate and then fall over
the work almost by its own weight. The workpiece
two-tenths small, and they won’t hesitate, if
properly set before. True, if unbelievable!
After which diversion, to return to the setting
up. The procedure is exactly the same as with
the micrometer, except that you use the calipers
to test the diameters of the bobbins.
It may take a little longer, as when the job is
nearly right the need to detect by feel will mean
several checks on each bobbin. Incidentally, I
always find it helps to develop increased sensitivity if I close my eyes when applying the
calipers. (Similarly when fitting a nut or some
other object which is out of sight!) If in doubt
about the sizes of the two bobbins, aim at getting that at the free end slightly the larger of the
two, for the reasons mentioned earlier.
The “No Strain” Method
This is the method advocated in a number of
books on Amateur Turning, and in some lathe
makers’ leaflets. The machine is set up with the
mandrel locked and a bar about 1 in. dia. and 8 in.
long in the three-jaw. This bar need not be true
-a piece of normal b.m.s. will do. The bed is
set as level as is possible with whatever spirit
level is available, using the jacking screws and
the procedure as for setting up with a precision
level. It is not bolted down at this stage. A dial
indicator is set up in the tool-post, about 6 in.
from the chuck, and carefully set to zero.
The cross-slide should then be locked and the
saddle too, to avoid accidental movement, but only
a very gentle application of the locking screws.
Recheck the DTI. Now tighten the holding-down
nuts. The dial indicator will almost certainly
deflect.
To be continued
916
MODEL ENGINEER 21 September 1973
LATHE ALIGNMENT
by Tubal Cain
Continued from page 916
THE
JACKING
S C R E W S at the tailstock
end until, when all holding-down nuts are tight,
the DTI still reads zero. The bed will now be
held in exactly the same geometry as applied
when it was free under its own weight.
If this method is used, it is essential that it be
followed by a recheck using the Turning Test. As
mentioned earlier, it is not safe to assume that the
free bed will not be twisted if the motor and
motorising unit is attached to the bed itself. You
can check this for yourself by setting up the test
as above, with the DTI at zero. Now remove the
motor belt, and set a block of wood under the
motor so that it is just taking the weight. Release
the motor support platform arms, so that the
motor rests on the block and is not restrained by
the tension arms. Observe the DTI. The deflection
mdicated is that caused by the motor-or part
of it, for some of the weight of the motor is still
taken by the hinge of the platform. The actual
twisting of the bed will be greater still, for there
remains the weight of the countershaft and associated components hanging on the back of the
machine.
A DJUST
Setting the Tailstock
Whilst the gear is about, it is worth spending a
little more time setting the tailstock true - though
with a new machine you will usually find it pretty
close when delivered.
First check that the headstock centre is true; if
in any doubt, set the top-slide over and machine
the centre to 60 deg. If you have a test mandrel
that is centred at both ends, and a DTI it is only
necessary to set up the latter and take readings at
both ends of the bar, adjusting the tailstock until
the readings are the same at both ends. Again, if
that at the tailstock is a few tenths towards the
tool, let it be ; the tailstock will deflect a trifle
under cuting loads. (You don’t believe it? Just
try leaning on the tailstock as if you were tired,
Test b a r
Clamp dial
indicator here
FIG
Saddle on ““worn
part of bed
x,r sp bar or ‘inilar
Block 011
cross-slide
4
MODEL ENGINEER 5 October 1973
and observe the DTI whilst you do so!) If you
have neither DTI nor mandrel, then you must
use the bobbin and turning method, and whilst
you are about it you might as well make a
proper bobbin bar for subsequent use after having set over the tailstock for taper work. Use
stock between 1 in. and 1: in. diameter, centre
both ends -make it about 12 in. to 14 in. long,
no more or you may suffer chatter -and turn
$ in. down at one end to take the carrier; file a
flat on this for the pinching screw. Face the ends,
and turn a little recess to protect the centre holes.
Relieve the bar as shown in Fig. 3. about %s in.
on the diameter, leaving the two bobbins which
should then be turned with a fine cut. It is worth
making the inner edges of the bobbins an exact
(or “aliquot”) dimension-say 10 in. to 12 in. as you can then use the bar to set up tapers with
the minimum of calculation. The procedure is
then quite simple - just take a very light cut over
each bobbin and adjust the tailstock until they
are the same diameter after cutting. When this has
been done, it is worth taking a few thou. off the
body of the bar, which will then be parallel, but
take only light cuts, as the bar itself will tend to
deflect under tool pressure.
setting UQ a worn Lathe
This is the problem which exercised Mr. Beck,
and has worried many thousands of others, too !
Fortunately, most of the wear will be on the back
and front, and on the underside of the shears,
not on the top, so that if the first few inches of
the bed nearest the headstock is avoided, the
machine may be set up using the levelling method
with reasonable confidence. The machining
method is unsafe, as both bed wear and headstock bearing wear may mislead. Fig. 4, however,
shows how the DTI method may be applied.
First, lubricate the headstock with thicker oil
than usual, to take up bearing clearance. (Don’t
forget to wash the bearings out and re-oil afterwards.) Set up a hefty bar (clamp it to a block on
the cross-slide) so that the DTI may be applied
to the mandrel in the chuck whilst the saddle is
sitting on a part of the bed that is less worn.
Most of the wear on an amateur’s lathe is on the
first few inches of the bed -indeed, I think many
model engineers have the drill chuck stuck fast
in the tailstock, and would be hard put to it to
find their tailstock centre ! Now, the main snag
will be wear in the saddle itself. This will mean
that as the saddle is traversed it will rock, bringing
the DTI nearer the mandrel as it is traversed
towards the headstock, and vice versa. To reduce
this effect, first take up some of the slack in the
gibs, but not so much as to make the saddlestiff
in the ways. Then before taking each DTI read971
ing, traverse the saddle a little towards the tailstock, using the leadscrew handwheel or rack
handwheel to choice, whichever suits you best.
This will rock the saddle over the same way each
time a reading is taken. One important point will
be observed in Fig. 4. The bar holding the DTI
is shown hanging over bath sides of the saddle.
T h i s is to prevent the weight of the bar from
rocking the saddle over in the vertical plane, the
free end acting as a counterweight.
Once you have set the machine up in this
fashion, it will pay to make a test cut on (say)
a 1 in. dia. bar in the chuck, just to see how far out
it is. You may well find that the main trouble is
slackness in the gibs causing chatter, rather than
taper turning. Of course, the proper thing to do is
to correct the wear - which, with a flat bed lathe,
is not too difficult-but that is another story!
Other Tests
You can make a complete check over the
geometry of a lathe in an evening, if you have
the mandrel, a DTI and a ground flat parallel.
These tests should be:
Spin& Arose: Internal taper true. External
register true. Face of shoulder true. End float
in spindle, nil.
H e a d s t o c k : Mandrel in taper hole, or held in
chuck. Truth in horizontal plane. Truth in
vertical plane.
Centres: Bar between centres, checked for truth
in both vertical and horizontal planes.
Saddle: Top-slide traverse parallel with bed in
vertical plane. Cross-slide at right-angles to
spindle across the bed-concavity only permitted. For this test clamp a parallel in the
faceplate - the latter should be concave.
Tailstock: Truly aligned to headstock. Spindle
when extended and clamped to be parallel to
bed in vertical plane. Parallel to lathe axis in
horizontal plane.
A 6 in. centre-height commercial quality centrelathe would be expected to hold figures about
twice those above, except those for the spindle
nose, where the normal tolerance is about .OOO5 in.
In passing, it should be noted that that maid of
all work, the 3-jaw chuck, will have a tolerance
no better than ,003 in. in manufacture, over the
range of effective diameter. The accurate w o r k
holding tool for concentricity is always the 4-jaw
chuck ; even collets are made to a tolerance!
One final word about the mounting of lathes
on wooden benches. There is no doubt at all
that these can, if precautions are not taken, warp
sufficiently to throw a machine out of alignment
in a matter of hours, but in my own shop, very
little movement takes place. First, the timbers are
very robust - the top is 3 in. thick. Second,
all joints are glued as well as bolted, and the
triangulated struts make a very rigid structure.
Most important, however, the wood used was all
stored in the shop for several weeks, under the
normal heating conditions, after being cut for
joints etc. The whole was then varnished - three
coats in all - after assembly. Geometric changes
in the bench itself are very small; the chief
trouble is floor deflection when, for example, a
heavy piece of equipment is moved in or out. If
I were doing the job again the only change I
would make would be to have the lathe on a
separate bench. This would have enabled me to
assemble the machine with access to the rear,
and then move bench and machine into place far easier than leaning over bench, machine, and
tool rack to get at an inaccessible Allen screw
in the motor platform!
At the Guildford Rally: Mr. Smith of Wickford,
Essex drives his 3 in. scale Wallis & Steevens traction
ennine.
In all cases you should expect to find a tolerance
-- it would be prohibitively expensive to manufacture machine tools so that it passed every test
“spot on”. The makers’ tolerances for the Larch
precision lathe were: Spindle nose - .OOO4 in.,
end float zero. Headstock - .0004 in. per foot,
mandrel not to lean away from the tool or towards the bed. Centres - .0004 in. towards tool
at tailstock permitted in full length of bed. Saddle
- top-slide parallel to bed to .0008 in. per foot ;
cross-slide - concavity not to exceed .0004 in.
on radius of faceplate. Tailstock - .0004 in.
rising in vertical plane or towards tool in horizontal, in its length. On the recent check test,
all measurements were within these tolerances.
972
MODEL ENGINEER 5 October 1973
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