TAPER TURNING GAUGE
Device for setting
precise taper angle on the top slide of a lathe.
Copyright – P. J.
Smith
But
permission is given to distribute this material in unaltered form as long as it
is not sold for profit.
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GENERAL
There are various ways to turn tapers on a lathe.
It is not the aim of this short paper to expound on all the possible
methods with their virtues and failings.
If the machine is large enough, the taper may be cut using the top slide
swivelled around to the appropriate angle.
Precise setting of the angle to turn accurate tapers is very much harder
than it appears.
If there is an accurate pattern to copy, such as a Morse taper setting
bar, it may be set up between centres and a dial gauge traversed its edge.
By trial and error, the angle
may be adjusted so no variation in the dial gauge reading occurs during a
traverse of the taper edge.
Adjusting and checking the angle is not as quick as it sounds because
angular adjustments upset
previous readings, and the traverse must be started again.
The device to be described was evolved so quick and repeatable settings
can be made. It works very well,
even in the absence of an accurate pattern. I would not be without it.
BACKGROUND
Recently, when cleaning out
a drawer, I came upon 8 drawings I made about 15 years ago. Some were incomplete. They were the result of an article I had
begun to write for a magazine on machining and model making about a device I
made in 1979. It is different from
every other device I have ever seen described, and, in my opinion, quite
superior in operation.
After some correspondence
with the editor, I became dissatisfied with their policy. I saw no reason why my work should end up
with them owning the copyright to the material, so simply switched off. Maybe it was arrogance on my part, but I
simply did not like their attitude.
Here is a short version of what I would have originally written, but
with the benefit of many more years of testing and hindsight.
This material is copyright. But I do give permission for it to be
copied, distributed,
and made freelyavailable, as long as it is in
unaltered form and the author is acknowledged.
Copies may not be sold for profit without the
permission of the author.
Sometimes more than a year
passes without my using the device.
But, whenever it is used, I never cease to be amazed at just how well it
works, and how such a simple device tames the setting of taper angles in a
lathe. Many other contrivances I
designed were consigned to the scrap heap years ago. Some are better left unmentioned.
This one, however, seems more valuable every time it is used, and, with
hindsight and experience, cannot be improved in any significant way.
The turning of tapers using
the top slide may not be so useful with very small lathes because of limited
travel. This device was originally made
for a large lathe with sufficient top slide travel to turn a No 4 Morse
taper. Taper turning from the top slide
of very small hobbyist lathes will be more limited, but I still think owners of
such a lathe may find the device invaluable.
This could be exactly what many machinists are looking for.
If you are interested, build the device and pass on this web information
to other interested parties.
PRINCIPLE
Graduated, angular scales on machine
tools usually are an order of magnitude less accurate than the sine bar
principle. Anyone who has done much precision
taper turning will be painfully aware of
this.
The
sine bar has for years become one of the preferred ways to set accurate
angles. It is normally used by
machinists in conjunction with the table of a milling or some similar machine,
but there is no reason it has to be with respect to a plane, horizontal, table.
The device to be described simply uses a modification of the sine bar in a
vertical plane, using the tailstock barrel as the reference direction [1].
Examine
the photographs below. This is a very
old lathe having an exposed dovetail topslide.
Although considerable wear is evident on this machine, the topslide
dovetails have been reground and are very accurate. The rest of the machine, despite wear, can be coaxed to produce
quite accurate work, when one understands the errors present. I now have a more modern lathe, but, because
this machine can swing 24 inches, and I have made many attachments for it, this
machine tool has been retained, and is still used.
The
photographs show a device clamped to the dovetail topslide, with two identical
vertical setting posts. These are
placed in contact with the tailstock barrel.
By means of a precision spacer, the topslide may be set accurately to
any angle.
The Gauge, which is held on, or
clamps to, the top slide, is shown in more detail below.
If there is no dovetail to clamp
around, the bottom bar may be omitted.
Then the two identical
vertical bars simply project below
its base and contact on the side of the body of the topslide.
This particular device was made
from a piece of plate steel accurately machined with two flat parallel
surfaces.
In this implementation, the rods
were chromed steel from some old shock absorbers. Chromed rod is ideal as it
is both rust and wear
resistant. The brown gunk is an added
rust preventative measure to beat tropical corrosion.
For each common taper, a setting
ring is made. These shown above are for
Morse 3 and Morse 4, and another quick release taper I use
to hold grinding wheels in a tool
and cutter grinder. I do not know
exactly what it is, but having the ring gauge, it is instantly repeatable.
By the way, different Morse tapers
do not use the same angle. I suspect
this is a direct result of
errors when originally
derived. For exact taper angles, you
should look up machinist’s handbooks.
The
diagrams below may be more informative.
Recently one of these devices was
made for another lathe in which the rods were set in place with locktite. The plate was bored in a mill. There is no need to have a specific rod spacing
as long as it is accurately measured after assembly. The perpendicularity and parallelism (most important) of the rods
should be checked.
After final measurement, stamp the
center to center rod spacing on the top of the base. The best way to measure center to center distance is to average
the ID and OD measurements. For the one
above, this is 3.4735 inch, which has been stamped on the base. This is just visible in the
photographs. Obviously, the longer the
baseline, the more accurate will be the setting, but you will be limited by
length of tailstock barrel and topslide body.
It would be convenient to use 5 or 10 inch or 100 mm but, with the
convenience of hand calculators, it does not matter very much.
More details are shown below. If you have a topslide with square corners,
you may leave off the bottom rod.
The example photographed above had
the bottom rod held in a vise while very small dobs of weld were applied to
hold it in place.
Since welding ALWAYS distorts the object, these MUST be extremely
small or else some other method used.
Although this shows the
perpendicular rods held in place with clamp screws, I have simply pressed or
locktited them in place.
METHOD OF USE
1 Calculate the distance D.
You will need to look up the angle of the taper. Now perform the required calculation using
D = SIN ( half angle ) x R
Where D is the setting to be applied to the
sine bar and
R is the radius of sine bar – in this case the
distance between the centres of two vertical bars.
Be very careful that you have the HALF ANGLE,
and beware that the angle MUST be halved before finding it’s sine.
You
must never find sine of the entire angle then halve this value.
I
recently did a quick calculation for a taper and double checked the result. It was obvious there was an error. Investigation revealed the calculator had
been inadvertently set to Grads instead of Degrees. Don’t make a similar error.
In my opinion, such options as Grads and Radians have no place on a hand
held calculator unless there are interlocks to prevent errors like this. Anyway, I am perfectly capable of dividing
by 180/pi or 10/9 if I want these other angular measures. The screen display is usually so small that
any mode setting like this is almost invisible to see in practice. Double check things like this, so there is
no waste of materials and effort. All
this is more reason to make specific setting rings for each taper – the risk of
errors diminishes enormously.
2
Make a setting ring with this wall thickness
which easily drops over the posts. The
only critical thing is the wall thickness and it is best made a loose fit.
There are of course other setting methods possible, but the ring is most
convenient for long term use, quick repetition, and less prone to error. See later for alternatives.
3 Adjust the topslide angle. Mount the device on the topslide,
loosen the angle adjusting bolts, and adjust the angle, until the post and ring
are all in firm contact with the tailstock spindle. You might like to use paper between the contact points as an
indicator.
If the topslide is free to rotate, as you
gently use the cross slide to advance the posts into contact, the angle will
set itself automatically.
Alternatively, when the ring falls is a good indication of loss of
contact. Finally, clamp up the
topslide. It is firmly held to the
correct angle while the bolts are tightened by the post contact. Now proceed to cut a perfect taper !. If the topslide has an angle scale, you will
usually find it is quite inaccurate – certainly it was never intended for
precision of this order.
Alternate setting methods
My own experience shows that the setting
rings are much easier to use than any other system, but for infrequently met
tapers,
it may not be worth making settings rings.
The following are other methods that can be used.
Parallelism of enclosed top slides
One should
not simply assume the edge of a topslide is exactly parallel to its direction of
motion. Since the method of setting has
the potential for extreme precision, this possible source of error should be
tested. The critical factor is easily
checked, by mounting a dial indicator so it runs against the edge of the slide,
as the slide is traversed. The
indicator should remain unchanged.
If the
topslide edge is misaligned, or if the edge has been significantly damaged, it
may be worth fixing two buttons on its edge, at the appropriate spacing, to
contact the posts, as shown below.
These could easily be machined, or ground in situ, as the slide is
traversed.
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