In one of the older annual Dropbox archives, there was an improvised lathe
turning setup by Ted Edwards (very bright guy, a former RCM regular), where
he added a plate to the cross slide to extend the toolpost forward (toward
operator), and to the left of the cross slide, to turn the circumference of
a large diameter part.
BTW, I forgot to mention earlier, wrt machining chuck adapter backplates for
spindles with flange mounting, the flanges generally have a protruding
shoulder (raised center section) which registers the chuck body to be
The shoulder also adds to the security of the mounting, so that the chuck
isn't just held in place by studs or bolts passing through the spindle
Machining a chuck backplate (or other accessory) would include cutting a
matching undercut recess to closely fit the flange feature.
plate, on a 9" lathe]
Geometrically, a lot of other shapes would work, such as some
triangle and wedge cross sections. Of course, the fixed-radius
curved-cross-section trepanning tool that you mention probably
would be strongest and best for thin cuts. However, at radius
3.25" cutting .25" deep with a .5"-high cutoff blade, it would
work to grind a 5-degree [*] side relief (and 1 degree of back
relief, per toward
the end). Also, you might want to trepan to a slightly smaller
radius than the finish ID and then finish by boring, depending
on what gives a better or quicker result.
[*] For a trepanning blade of height h at radius r, the
interference at the outer bottom edge is about (h^2)/(2*r).
With a .5"-high blade, this comes to 0.038" at 3.25" radius
and to 0.042" at 3".
I've snipped DoN's other comments; take them as predecessors
to following suggestion: If the ring is for a sun-dial, holes
on the back won't show, so you could drill holes .15" deep to
seat upon pins in a mandrel plate attached to the faceplate.
The pins would withstand shear forces, allowing much heavier
cuts than you could take when depending on glue or tape to
withstand those forces. One could tap the holes, use loctite
and studs, etc, but presumably not worthwhile to do so.
Yes -- you could make one by grinding a HSS tool bit to the
proper radii -- including clearance.
[ ... ]
Yes. Keep the lathe speed slow, and the cut depth small to keep
the forces down.
Hmm ... for the drilling, you could make a special holder which
held a thrust bearing assembly with a bore larger than the drill which
you would use to start the hole. You could crank the carriage into
position (with the bearing well centered) to maintain extra pressure
while drilling, since you don't need the carriage for the actual
Hmm ... another way to hold it fairly concentric (assuming that
your stock is truly round and near the desired size) would be a ring
which slips over the OD of the workpiece and (with less than 1/2" radial
size around the workpiece) and drill and tap four holes at 12:00
o'clock, 3:00 o'clock, 6:00 o'clock, and 9:00 o'clock and clamp it to
the faceplate with two steel bars going from 12:00 o'clock to 3:00
o'clock and from 6:00 o'clock to 9:00 o'clock so they don't interfere
with the spindle -- and the ends have to be cut to clear the bed. This,
perhaps augmented by the double-sided tape, would hold things firmly in
Hmm ... two rings -- one which is a fit on the faceplate, and
the other on the (larger) workpiece, held together with bolts might make
getting things centered a bit easier.
Anyway -- with these approaches, you could use higher spindle
speeds than you would find safe with just the tape.
In any case -- use alcohol or acetone to scrub down the surface
of both the workpiece and the faceplate to make sure that they are both
free of oil and grease for maximum adhesion of the tape.
One of many ways. I've suggested some others above which take
more time making fixtures, but which make sense if you are making
multiples of these (e.g. the scales for your sundials).
It depends. super glue forms a rather brittle joint, so if you
have an interrupted cut or otherwise bump things you could break the
bond. The double-sided tape -- especially sufficient area of
double-sided foam tape -- will give a very good grip, and be more immune
[ ... ]
Yes -- the torque needed goes up fast with increasing diameter.
The rotary table and a "slot drill" (two flute endmill) of
perhaps 1/4" diameter will probably work fairly well.
Actually -- you *could* -- barely -- with modified jaws. It is
just more difficult to make jaws for a 4-jaw than soft jaws for a 3-jaw
which is already fitted with two-piece jaws.
Think of the special soft jaws available for the Taig/Peatol
lathe. Those will handle a workpiece significantly larger than the
chuck body. For a 5" chuck -- just make large pie jaws to replace the
hardened top jaws -- *if* the machine comes with a chuck with two-part
Hmm ... also, this sounds like an argument for a gap-bed lathe.
The first few inches of the bed can be unbolted and lifted out to allow
working with a larger diameter workpiece. There is some debate whether
the removed section can ever be put back as precisely as it was
positioned when the bed was first ground, but there are things where the
gap bed is a winner.
Lots of ways to do it -- depending on the lathe, and how many
of the same size workpiece you are making.
What you are making is relatively thin so you don't have to
worry about it clearing the carriage.
The larger the diameter, the more grip surface you have between
the double-sided tape and the workpiece and faceplate.
But in any case, I would not consider 550 RPM to be a good idea.
First -- if the tape does let go, it could throw things rather
Second, 6.5" diameter means 935 SFM, which strikes me as a bit
fast unless you are using something like carbide tooling and a free
cutting mild steel like 12L14.
Based on the cutting speeds chart for HSS tooling in the
Jorgensen steel book:
4140 120 (annealed)
Stainless steels below
That is getting tricky...
I like that.
Acetone for the faceplate. Alcohol for me...
[ ... ]
I have not considered that aspect...
I do. All the time :-)
Those will handle a workpiece significantly larger than the
The problem arises with the Taig also. None of the chucks will hold a 4"
piece. I do the insides with a boring head on the mill and finish on the
[ ... ]
No, no! I did say I shall try with *4"*. Yes, 550 rpm (the lowest the Taig
will go) is way too fast for that too, but I have managed so far. The only
way it works for facing is a sharp HSS tool with a small point radius. And a
very slow and steady hand as one gets to the outside. To do the inside I
need to bore only about 2" diameter. OTOH there is probably no point doing
that as the mill method works just fine and it will provide me with only
limited information about how the big piece would behave.
Granted. But if I followed that I would be cutting nothing bigger than 1"
diameters. The frightening thing about it is that to face the piece (1018)
in question (8" diameter) one should have a lathe with low speed of 60.
Back to cranking the handle...
I will save this for future reference, thanks. Right now my plates are
pre-fabricated but who knows, in future this may be the way to go.
I routinely attach the armature to the back of the face just that way. It
took me a few attempts to reliably drill a 0.15" hole without going right
through (the current faces are 0.189" thick) and to drill deep enough to get
at least two threads of 10-32. In fact this was another way I contemplated
but I did not think of the shear pins which would certainly simplify
Much obliged, this is helpful.
I've been reluctant to suggest the way I machine large arcs, but you
seem to have some experience now.
I attach a pivot post to the table of the mill, drill the blank to fit
on it, attach a long handle and manually rotate the blank into the end
mill. Yes, the end mill is likely to grab and try to spin the work,
more so if there is any play at the pivot. That isn't too serious
while the blank is still a polygon because the end mill quickly
reaches a gap. Once the arc is nearly continuous the depth of cut has
to be very small. I use a small endmill at low speed, extend the quill
all the way down and don't lock it so I can knock the handle upward,
and set the drive belts quite loose, but it's still as dangerous as
working freehand on a drill press. The worst one I've done this way
was a circular tee slot, after the milling force damaged my undersized
I mostly do this to round the ends of linkage bars and stop before the
arc is tangent to the straight edge.
wider hinge part was first clamped upright in the milling vise,
with a shaft through the hole to position it atop the jaws, and milled
almost to size across the top. A larger diameter piece could be
clamped to an angle plate. The disk will vibrate if not well supported
close to the cut.
If the disk can be clamped firmly to the mill table, raised on
spacers, it can be cut nearly to size much more safely by rotating it
slightly between cuts. The frequent reclamping is incredibly tedious
but it would get your 8" disk close enough to finish with a light cut
on the 9" lathe.
For even larger circles I set them up on a shaft and round them with
an angle grinder, held so it crosses at an angle and spins the disk as
it cuts. That's how I shaped the front tire for the sawmill. I haven't
tried yet, but I think a disk could be turned fairly accurately on a
wooden frame with a lathe bit on an X-Y table which moves crosswise,
parallel to the axle. Think of an old foot-pedal grindstone frame. You
could spin the disk with a sanding drum in a drill.
The right way is probably to buy the largest rotary table that fits on
your mill. The disadvantage compared to a gap-bed lathe is that you
can't detail the edges as much, you'll be limited to end mill profiles
like vee grooves and corner rounders.
Those would be the GT18 steering sectors?
I have to digest this...
I think you mentioned this to me before. In fact I believe it was the
inspiration for this marvel of technology:
I did not have as much luck facing with the grinder. I made some plastic
jaws for a chuck on my woodlathe and used an angle grinder with a sanding
attachment but for some reason it did not work too well either. I have a
feeling that the thing has to spin at a lower rate than 300 rpm to give the
grinder chance to work (I am talking face work here).
Still, the on-going problem is the *inside* of the doughnut.
I am certainly considering it as a viable option. Realistically I cannot get
anything bigger than 6" rotary table on my mill and even that is probably
pushing it. OTOH if one could do things that way it is a $1000 difference in
capital expenditure :-)
BTW can you put a motor drive on a rotary table so it turns at a constant
[ ... ]
[ ... ]
O.K. But the latter *after* you are done with the machining for
the day. :-)
[ ... ]
Hmm ... I would have to go down and set up the chuck with the
extruded jaws to be sure -- but remember that you can bore a recess in
the jaws up to within perhaps 1/4" from the ends of the fingers.
But you could *make* pie jaws from solid aluminum plate at need
to have the extra reach needed.
And if the swing of the lathe is the problem, add one or two
riser blocks between the headstock and the bed. Those riser blocks will
stack if you have (or make) two of them.
[ ... ]
Hmm ... replace the motor with a DC motor and a proper speed
controller, or a small three-phase motor and a VFD to run the motor at
perhaps 1/4 speed. (about 140 RPM).
Or -- you would swap in motors which could be run slow enough to
get proper speeds.
Hmm .... the current motor has what RPM? 1800 RPM, 3600 RPM?
There are 900 RPM (four pole) motors which will get your first reduction
Or -- to a proper variable speed motor -- either three phase, or
Or -- use uncoated carbide inserts to survive the higher speeds.
The Taig swing is 4-1/8" ...
I considered that. Those who know better than I told me that the rigidity
declines quite significantly. Machining 4" with the existing setup is a
constant struggle with rigidity as it is.
Or for the same money just get a bigger lathe. I went through this some time
ago. The torque at the low speed is a big problem with the Taig. Even as it
is I have to lean on the motor some times to stop it stalling. It is what it
is: A little lathe for little things.
I tried those. I thought they were horrible. Got much better results with
properly ground HSS.
This is all done on the un-modified Taig:
(well, just the step - much milling there also)
There has been a learning curve :-)
In any case, I really have no idea what is a reasonable expectation of a
good finish achievable by a lathe of whatever size. This 1' beast was turned
by a professional. The concentric ridges are apparent. Can one ever get rid
of them? Can one make them look uniform without a power-feed? Should one go
back to facing with sandpaper and just do the edges on a lathe/mill? What is
the meaning of life...
Some rotary tables can be motorized to run continuously while milling, but I
expect that some of the light duty RTs should probably just be used for
I was using a motorized RT for a while, and it seemed to work very well
(hobby, not production speeds & feeds). I used a small Oriental Motor
gearmotor, chain and sprockets and a Phase II 6" horizontal RT.
The RT was turning very slowly, less than 2 RPM, maybe considerably slower.
It's a method I wanted to try, for cutting disks from sheet/plate stock
without center holes.
That piece fit on the lathe after sawing it roughly to size. I clamped
the square blank flat on the bandsaw table with tee bolts to cut off
the corners. Then I set the saw upright and followed a scribed circle
as far as the wide blade allows, then opened up the kerf with a chisel
to make clearance the next cut.
The upright pivot post is held in the vise, the disk rests on 1-2-3
blocks which raise it above the jaw tops. Tee slot clamp bolts clamp
the disk onto the 1-2-3 blocks, leaving a small space in between to
mill tangents to the OD. It's slower than sawing because of all the
reclamping but doesn't risk cutting in too far by accident, and leaves
only a little metal for the lathe to remove.
The ladder-shaped frame of my sawmill is the model. Both wheels needed
truing and the front one considerable flattening. I used the
crosspieces at the inner ends of the wheels for tool rests, ie to hold
the grinder steady.
You could make a similar frame out of wood by laminating 2x4's with
the ends alternating to form glued finger joints. I'd make two side-by-
side rectangular bays, the second providing a tool rest for faceplate
The faceplate could be a large pulley faced with plywood. The bearings
on the motorcycle wheels were tight enough to use one as a lathe
faceplate, though the protruding axle interferes. If you did that you
could drive the tire by friction or remove it and run a vee belt on
the rim. I used the sprocket only because I needed to transmit several
horsepower, it required a separate countershaft.
I think an X-Y table would be stiff enough for a tool rest. I acquired
one plus the headstock from a scrapped SB lathe and a brake lathe
tailstock to build a large wheel lathe, but haven't needed to set it
I should be able to the very same tomorrow (once I found where the knocking
noise is coming out of the Craftex). BTW, when you got your band saw, did
you change the oil in the gearbox right away or did you trust the Chinese
and left it for a year or so?
Another one to digest :-)
I was thinking that if I return to red neck technology I would probably make
the ring spin horizontally this time. But anything will be considered. The
important thing is to spin the work piece quite slowly otherwise the grinder
works quite inefficiently, at least on the face. The RedNeck lathe v5.01
went at less than 100 rpm and could have gone slower.
I found that an X-Y *vise* was capable of holding lathe tools quite firmly
and take large cuts. Here it is with a boring bar holder I made:
I have to add that the wobble was all in the chuck: The spindle had a runout
of only about a 0.001" :-)
That 4-jaw setup was actually quite impressive. Extra points for feeding the
boring bar and cutting metal while holding the video camera to make the
That 4-jaw chuck looks like the one that's supplied with the 9x20 lathes.
The wobble doesn't exist on the lathe spindle because the spindle includes a
raised shoulder feature to register the chuck at a nearly perfectly square
I read quite a few remarks from disappointed new 9x20 lathe owners, wrt the
4-jaw chuck. I found the chuck to be suitable for a lathe of that size and
It's not like a 9x20 is intended to turn a 50 pound workpiece at 1000 RPM.
There aren't any multi-part bearing assemblies (roller, ball) in the 6"
Phase II RT that I used, just plain polished (finely ground) steel sufaces
against machined cast iron, with the parts well-oiled with 80 or 90W gear
Since old lathes could spin hundreds of RPM (with lighter oil), I suppose a
RT could be run fairly fast if it's designed and built well enough.
The table's support surfaces are significantly different than a lathe
spindle as the two machines are designed for different
Some RTs are 40:1, others are higher turns per rotation. Worm reductions
typically have slow output speeds although heavy duty models can handle some
If one were inclined, the worm of a RT can be disengaged in many models (or
removed), so the table stem could be driven from the back side or bottom,
depending upon the orienation (of the RT, not the operator).
As long as the RT isn't routinely subjected to the side loads of turning
down diameters, push knurling and other typical lathe operations, a RT may
be suitable (just speculation) for occasional cutting operations similar to
facing cuts on lathes.
If the table's stem support/bearing surfaces wear, the accuracy of the RT is
diminished, so an overhaul may be necessary, possibly requiring boring and a
bushing or sleeve installation.
I traded away my X-Y vise and couldn't remember how far the ways
protruded beyond the jaws, so I didn't mention it. The one you have
looks to be of better quality, I bought mine around 1980 and it may
have been cast from leftover Great Leap Forward backyard iron.
I bought a second-hand Delta 4x6, so I'm not sure what the gearbox had
in it originally. It was full though black when I changed it, after
reading warnings here. The new oil has stayed fairly clean.
It has adjustable blade guides and the Delta logo on some small parts
so they did add some value over the generic saws. I strengthened the
base and put on larger wheels but otherwise it was reasonably well
made and cuts straight as long as it's on the flat basement floor.
When I use it outdoors on the uneven driveway I have to move it around
until both sides of the handle end of the base touch down at the same
time, and then check that the blade rises perpendicular to the table
with a square. That's OK since the outdoor cuts are structural steel
for welding and don't need to be as accurate. Indoors it cuts square
vertically to within ~0.005" per 1".