Lathe Modification: Tumbler Reverse & Hi-Low Gears
This is a very ambitious
modification that is still in the planning stages. The Variable Speed
DC Motor adds tremendous flexibility to the lathe, but most agree that
the user will still be required to change speeds with the belts and pulleys.
In addition, the Lathemaster 9x30, like most 9x20 lathes, lacks the ability
to reverse the leadscrew, so it can't cut lefthand threads nor can we
use the leadscrew to feed left to right. The goal of this pair of modifications
is to eliminate gear and pulley changes except for thread cutting once
and for all.
To solve these problems
I'm envisioning two levers that operate two different gearing mechanisms.
The first lever operates a Tumbler Reverse of the kind often fabricated
for these lathes. The second lever provides Hi-Low Gears, which basically
offer a high-low speed range. In the lower range, we'd find the gearing
reduces speeds by a factor of 4.7:1 or so. My plan is to employ the ingenious
geometry of the Tumbler Reverse to also implement a Tumbler Back Gear.
For a good pictorial description
of how a tumbler mechanism words, see Steve
by reviewing the lathe's normal gear layout:
Pulley Stack 1: This stack drives the spindle
Pulley Stack 2: This stack is on the motor
Pulley Stack 3: These are the "Low Gears"
Change Gears: For cutting threads
The range of
speeds available by default is:
Stack 2 (Direct from Motor): 620, 1000, 2000 rpm
Stack 3 (From "Low Gears"): 125, 210, 420 rpm
The low gears
are provided by the toothed timing belt that seems to have a range of
We can see
the different pulleys in the stacks in this photograph:
Speeds on pulleys are low, medium, and high, left
range of these pulleys is 0.62:1, 1:1, and 2:1. Based on all of these
ratios, when we take the middle pulley and the 2 (motor) stack, we can
see the motor nominally runs at 1000 rpm.
Okay, now let's
look at how we reverse the leadscrew in the normal factory-approved manner.
Consider the following diagram:
Factory-approved Reversing Mechanism
would have use remove the Idler gear to a new location below and slightly
to the left (direction of red arrow), and install a reversing gear next
to it whenever we want to reverse. It makes it possible to reverse, but
it sure ain't easy! Note that the gear on the spindle, the Reversing Gear,
and the Idler Gear are all the same size, about 42mm in diameter. This
is to ensure that the gear train runs at the same speed regardless of
which direction we're running it in.
take a look at possible ways of adding a tumbler reverse to this geartrain.
two tumbler mechanisms. The first would be a fairly standard tumbler reverse,
done just as one commonly sees it on any 9x20 lathe. The second would
tumble between taking power from Stack 2 (the direct drive motor stack)
and Stack 3 (the low gear stack). Our new Variable
Speed DC Motor has a maximum speed of 4500 rpm. Using the aforementioned
tumbler mechanism with the same ratios already on the lathe, and the assumption
we will turn our motor no more slowly than 120 rpm, we should be able
to deliver the following speed ranges:
High Gear (Direct Motor Drive, Stack 2): 120 - 4000 rpm
Low Gear (Off Stack 3): 25 - 840 rpm
the extra tumbler gears and pulleys will be a straight 1:1 ratio to the
spindle. The modification would involve removing the pulley stacks from
the motor and "low gear" area, leaving the timing belt intact,
and replacing said stacks with gears that interact with 2 gears on the
tumbler plate. Because we use 2 gears, we keep the same direction of rotation.
One gear tumbles between the 2 stack spindles, and the other sits at the
pivot with a single pulley on it taking belt motion back to the lathe
spindle. I'm guessing I can cannibalize the middle (direct drive) pulley
from one of the two stacks and drive the lathe spindle belt with that.
quite an improvement over the stock lathe speed ranges. We have extended
the high end by 2000 rpm and lowered the bottom end from 125 to just 25
rpm. Any slower and we would do just as well to hand crank the spindle
(the subject for another modification I'm sure!). Here is what the geometry
for the Hi-Low Tumbler might look like:
Hi-Low Tumbler Geometry
Tumbler Geometry is the classic triangular planform. A gear takes power
off either Stack 2 (Direct Motor Drive) or Stack 3 (Low Gears). It is
pivoted in the vicinity of the current idler mechanism. I am envisioning
this gear turning a second gear at the pivot that has a pulley on it that
will mesh perfectly with the middle (direct drive) pulley on the spindle.
Since it is located at the pivot point, belt tension remains a constant.
We'll need to create a mechanism for releasing the tension so the belt
can be changed. This could be achieved with an arrangement similar to
the current idler, which would likely have to be relocated. A Hi-Low shift
lever would extend straight upward from the pivot. Push the lever forward
to get "Hi Gear" and pull back for "Low Gear". Sounds
intuitive enough, eh?
Now let's look
at the potential for a Tumbler Reverse mechanism. Having seen from the
diagram above how things work, Lathemaster provides with a harder challenge
than the normal 9x20 lathes in fabricating such a mechanism. Typically,
we can pivot around an idler gear and bring our tumbler into play. In
this case, the threading gear train has no such idler gear, as all the
gears are in a vertical row. Instead, we may be able to pivot around the
spindle, as follows:
Pivoting a Tumbler Reverse Around the Spindle
We could probably
also have pivoted around the Change Gear, but is is moveable, and so that
would not be a satisfactory solution. In fact, we can pivot at any point
along a line drawn so that it intersects Spindle and Change Gear centers
as long as we're far enough from the gap between the two gears to provide
a gentle enough engaging arc. We could also design this system as a sliding
"linear" engagement where the two gear sets move on a line instead
of a pivoting arc, so we should think about what makes the most sense.
It might also work to pivot on that line slightly above and to left of
the spindle too, and use a fork-shaped plate that goes around the spindle
and holds our gears.
As for the
gears, what's needed is to introduce 2 new gears. In the "Forward"
mode, we essentially swap our original Idler from below the line between
Spindle and Change Gear to above, in mirror fashion. In "Reverse",
we swing the "Forward" Idler up out of the way and bring the
normal Factory Reversed Idler geometry up from below. Tricky, eh? In order
to accomplish this magic, we shall need to order a spare "Reverse"
Idler from Lathemaster, fabricate a plate to mount all these gears to,
and make sure we can attach that plate so that it pivots around the spindle.
No mean feat!
While I'm at
this, I plan to install a sensor assembly to provide spindle tachometer
readings to the Shumatech DRO. It will
be essential to be able to determine the exact spindle speed (no longer
a function of gears and pulleys!) in order to ensure accurate thread cutting.
Judging from the picture above, there should be plenty of room to affix
some sort of optical or magnetic sensor.
Gears: 1 for each pulley stack, 2 for tumbler plate
and Mounting Hardware
1 Spare Reversing
Gear from Lathemaster
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