9x20 Quick-Change Tool Post (QCTP) & Tool Bits
Last updated on Tuesday, September 19, 2023 04:27:40 PM Mountain US Time Zone

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Quick-Change Tool Post (QCTP), Tool Bit Types & Shapes,
HSS, Brazed Carbide, Carbide Insert Tool Bits,
Tool Bit Holders, Carbide Insert Cutoff Tool, T-bar Cutoff Tool,
Tool Bit Height Gauges, Storage, Surface Feet per Minute

The lathe's cross-slide bed
must be level in both directions.

Quick-Change Tool Post (QCTP)
Adapted a Phase2+ wedge-type QCTP, Model 250-111.
Wedge-type tool posts are purportedly
more accurate & repeatable than the piston type.
Quick-Change Tool Posts allow 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 counterbored 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 US 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.
Phase2+ wedge-type Quick-Change Tool Post Set manual.

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 & 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.


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 counterbore is about 0.83" & the clearance
hole is a very close fit to the 14mm bolt.
The counterbore depth is the same
as that used by the original bolt post.
To allow complete seating into the hole, the bolt-head &
counterbore 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
the bolt head surface from the installation.

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,
cutoff tool, 1/2" bits) at the 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 have to clear the base.

Used the Phase2+ OEM hardened, flanged nut.
Maximum tool bit size is 1/2". 
The increased rigidity 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
making it about 50% larger than the OEM.

An auxiliary handle can be
screwed into the QCTP body.

AXA 9-12" swing
BXA 10-15" swing
CXA 13-18" swing

Tool Bit Types & Shapes

Lathe tool standard shapes.
Single point lathe tool angles.

 HSS

High Speed Steel (HSS) tool bit blanks.
Top to bottom: square 1/2", 3/8", 5/16",
1/4", 3/16", 1/8" plus round 3/16" & 1/4".
 
CAUTION: the metal dust from
grinding tool bits is very hazardous.
Wear proper protective eye & breathing gear.

HSS; small cutoff, 45 deg chamfer, round-nose &
two views of a small boring bar.
The chamfer tool can chatter.
Better to use the lathe's compound.

O-ring cutters.

Brazed Carbide

1/4" shank brazed tool bits: right-hand,
square-tipped, threading, left-hand.


Assorted brazed carbide tool bits: left-hand,
threading, square-tipped, cutoff & right-hand.
A left-hand bit cuts left to right & a
right-hand bit cuts right to left.

C2 Grade (brazed) carbide is for cast iron & non-ferrous
materials; C5/C6 Grade is for steel & steel alloys.

Carbide Insert Tool Bits

C-6 carbide insert, 1/4" shank, tool bit holders.
Key, left to right: A, B, C, D, E, F (threading), & G (cutoff).

Tool Bit Holders

QCTP tools left to right, top row: an MT2 arbor holder with
a drill chuck, 3/4" & 5/8" (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 cutoff 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 3/8" boring bar.
The cutoff 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.

Note this Phase2+ tool holder has
a V-groove to hold round shanks.

This 3/8" round to 1/2" square, slit, steel adapter
eliminates set screw marring on the shaft. The hole
was reamed for a very close, precision fit to the
ground shank, then the slot was cut.

Right-hand, left-hand tool, & straight tool holders.
Shank size is 5/16" & Length is 3".

Carbide Insert Cutoff Tool

GTN-3 carbide insert cutoff tool bit holder SLTHR12.7-3.
Insert held in V-grooves, only. Tip is 0.122" wide.
After cutting, the tool pries out a fully seated insert.
The hardened shank is 0.498" high x 3.25" long x 0.39" wide.
Capacity 0.7" cutting depth or 1.4" diameter stock.

Caution: Brittle carbide can shatter during (high-stress)
cut-off operations. Always wear eye protection.

 

Good performance in soft steel
using heavy oil as a lubricant.

Making two simple bronze bearings. The 39/64"
drill bit was followed by a 5/8" chucking reamer. 

Delrin part being cut-off.

A live-center chuck holds a Chapman hex bit to
support removing material under a button-head bolt.

Cutting fluid being applied
using a needle oiler.

Deep parting a Delrin spacer.

Deep parting at 180RPM demonstrates the variable DC motor
modification's low-speed high-torque capability.

Left piece parted with a little cutting oil whereas
the right part was cut-off using lots of oil.
Of course, cutting oil greatly extends toolbit life.

T-bar Cutoff Tool

Adapted the Taig T-bar cutoff tool to work on the 9x20.
The blade is positioned at the back & upside down.
With a front cutoff tool, forces push the blade downward
below centerline & into the part possibly causing
chatter. With a back cutoff tool, forces pull the blade
upward above centerline & away from the part possibly
reducing chatter. For max rigidity during cutoffs, keep
the blade as short & close to the chuck as possible.
Use plenty of the proper cutting fluid.

Exploded view - The tool post & base are mild steel.
T-shaped (cross-section profile) cutoff blade shown.
The blade is clamped using a small plate & 2, 10-32
bolts/washers. There are relief cuts for T-shaped blades.

Mounted rear view - An aluminum guide indexes the tool post.
The OEM T-bar design had only two, plate-clamping
10-32 bolts, so I added two more for additional strength.
Since the blade clamp plate is symmetrical, I just rotated
it & used a transfer punch to locate the extra bolts.

The front base M6-1 bolt (right) is forward of the post
bolt for maximum leverage against the upward pull.
The rear base bolt (left) uses a hardened washer.
Two, M6-1 steel T-nuts are used in the cross-slide slot.

A center significantly increases setup rigidity.

The side plate on the left is held on by four,
counterbored 8-32 hex-head screws.

The side plate functions as a bracket-in-tension
against upward pulling forces during cutting.
A good example of a bracket-in-tension is that of a
playing card; easy to crush but hard to pull apart.
The top edge of the side plate contacts the bottom
edge of the blade clamp to lend additional support.
Opened-up the two clearance holes for the main 6mm
bolts to 6.7mm to allow small, rotational adjustments.
The rigidity of this setup is very good;
no chatter, 50-400 RPM in aluminum.

Cutoff performance is enhanced especially now that
the lathe can turn low RPM using a powerful
variable-speed DC motor & timing belt pulley setup.
This cutoff operation is being performed
at 60 RPM but easily works up to 400 RPM.
Absolutely no chatter & the cuts are clean.
The variable speed DC motor driven spindle has
plenty of torque throughout its 50-1000 RPM range.
It is my belief that, in part, cutoff-induced chatter could
have been due to macroscopic fluctuations of the
spring-loaded idler pulley when under heavy spindle
loads. The removal of the OEM idler belt/pulley
system & subsequent replacement with a strong
timing belt helped eliminate cutoff-induced chatter.

Tool Bit Height Gauges

Aluminum height gauge 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 operationally defines the tool bit
height for the gauge. A Dorian Tool, negative rake
 TNMG carbide insert tool bit holder MTGNR is shown.
The height-gauge bottom rests on the cross slide.

Another quick tool bit height setting method is
to gently pinch a small rule between the tip & the work
piece, then adjust the height until it is vertical. If the
rule's top tilts toward you, then it is low & vice versa.

Using a center gauge to set up
the 60 deg tool bit cutting angle.

Storage

A multi-compartmented storage bin holds &
organizes the many different lathe tool bits.

Surface Feet per Minute

Converting part size & RPM to
surface feet per minute (lathe SFM)

Approximate cutting speeds for different materials.
Surface Feet/Minute (SFM) Chart 1     SFM 2     SFM 3

Quick-Change Tool Post (QCTP), Tool Bit Types & Shapes,
HSS, Brazed Carbide, Carbide Insert Tool Bits,
Tool Bit Holders, Carbide Insert Cutoff Tool, T-bar Cutoff Tool,
Tool Bit Height Gauges, Storage, Surface Feet per Minute

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