Quick-Change Tool Post (QCTP) & Tool Bits
Old modification design (top), M14-1.5 x 120mm bolt (middle) & new design (bottom).
My first QCTP modification (some years ago, before I had a
larger mill
with a boring head), was accomplished
by turning down the Phase2+ OEM 14mm post to fit into the original
cross slide hole, but it was a weak design.
Adapted a
Phase2+ wedge-type QCTP,
Model 250-111.
Wedge-type tool posts are purportedly more accurate & repeatable
than the piston type.
The Quick-Change Tool Post allows fast tool changes,
exact tool bit height
& position/angle adjustments.
A long, 22mm Craftsman box wrench is used to
loosen & tighten the
hardened tool post
nut for rotational positioning.
Full-sized lathes have a large T-slot to mount the tool post. The Jet BD 920N is too small for
a T-slot but this mod parallels that design.
The new design uses a machined M14-1.5 x
120mm long bolt (same size as the Phase2+ OEM post) with a counter-bored cross slide.
To keep the bolt head as large as possible, it was turned down just enough to
remove the hex flats.
Also replaced
all the gib
M4-0.7 x 10mm set screws with higher
quality parts. The lock
nuts are stainless steel.
Replaced the three compound bearing plate
Philips-head mounting screws with stronger,
hex cap-head bolts.
These bolt heads had to be ground down enough to allow
sufficient clearance when the plate is clamped down to the cross slide.
Used
a (centered) 1-in. end mill to cut a relief in the dovetail to keep the
bolt head as large as possible.
This clearance cut to the dove tail does not seem to have affected the
compound's mechanical
performance.
The counter bore is about 0.83" & the clearance hole is a very close fit to the
14mm bolt.
The counter-bore depth is the same as that used by the original bolt post.
To allow complete seating into the hole, the bolt-head & counter-bore edges were
properly chamfered.
The machined bolt head is an interference fit so it had to be hammered in using a
brass rod.
This was the first test part fabricated & it was flush to the
surface. I made a second part with a longer bolt.
Also, the head was machined a little thicker (not flush) for more strength & it used the available
2mm slide clearance area between the bottom of the slide & the top of the
dove-tail base.
Note the small amount of brass that was left on t he bolt head surface.
Used a
rotary
table to mill the four corners of the compound's raised, square tool post platform.
Rounding the corners allows the QCTP dovetail
tool holder to be adjusted lower than the surface of the platform,
thereby enabling adjustment of larger tools (e.g.,
scissors knurling, cut-off tool,
½" bits) at or below centerline of the work
piece.
The compound slide shown reassembled with the 14mm bolt & rounded
base corners.
Tool post bottom is shown
with a tool holder in place.
The holder dovetail corners are what have to clear the base.
Used the hardened
Phase2+ OEM flanged nut with a hardened 14mm washer which reduces tool post turning during
tightening.
Maximum tool bit size is ½-in. The
increased rigidity of the QCTP mount is outstanding.
When combined with a negative-rake carbide insert
tool bit holder, steel-cutting performance has dramatically improved.
I have now increased the size of the compound pivoting base to make it about 50% larger than the OEM in all dimensions.
QCTP tools left to right, top row: an MT-2 arbor holder with a drill chuck, ¾" & ⅝" (with a split insert) boring bar holder, a conventional knurling tool (also holds a bit), two scissors-type knurlers. Left to right, bottom row: small boring bar, a cut-off blade holder & tool bit holders. Some of the tool-bit holders have a V-groove to hold a round shank like those found on a ⅜" boring bar. The cut-off tool holder was ground along the inner top corner to relieve the wider top of some cutting blades. The blade's side sits flush against the holder & is now perfectly vertical relative to the part.
Carbide insert tool bits in ¼" &
½" shanks. More about tool bits
Carbide insert tool-bit
holders (negative 5° rake), from
Dorian Tools (USA) left to right:
MTGNR,
MTJNR,
MTCNN (negative 10° rake),
MTJNL,
MTGNL.
These use TNMG-style, ⅛"
thick, zero relief angle inserts which have six usable tips, each.
Different radius insert tips can be used e.g., 1/64th in. or 1/32nd
in. These holders have ½" square hardened shanks.
TNMG-style inserts are the most economical to use due to six tips apiece.
Their popularity & high industry demand drives production up & cost down.
For quantities ≥10, import inserts are $1.29 each or 21½ ¢ per tip (Jan 07).
Negative-rake tool-bit holders in QCTP adjustable-height dovetail mounts.
The middle holder is an MTENN
type having an insert with a 1/32nd
in. radius tip.
Negative-rake
insert tool bits.
See the Insert Nomenclature Chart &
Indexable Tool Holder
Nomenclature Chart
Tool Bit Height Gage
Aluminum height gage used to quickly set tool bit height using a QCTP.
Right side for standard settings & left side for upside-down, backside bits.
Facing cuts were made on a collet-held piece until no center nub remained.
That height then operationally defined the tool bit height for the gage.
A Dorian Tool, negative rake TNMG carbide insert tool bit holder MTGNR is shown.Tool bit should be at or slightly below center for turning &
at or slightly above center for boring. The gage bottom rests on the cross slide.Tool Bits
Lathe tool standard shapes
Single point lathe tool angles
Left-hand (HSS & brazed carbide), straight (carbide), threading (carbide), right-hand (carbide & HSS), blank (HSS).
The straight tool bit works well with the fly cutter on the mill as do the round nose & left-hand bits.
Small cut-off, 45º chamfer, round-nose & two views of an small boring bar.
The chamfer tool can chatter. Better to use the compound.
Left-hand & right-hand ¼" shank tool bits with carbide inserts.
Inserts (top & bottom), 2.2mm retention screw & metric star wrench
Approximate cutting speeds for different materials.
Converting part size & RPM to surface feet per minute (SFM)
