9x20 Quick-Change Tool Post (QCTP) & Tool Bits
Last updated on Sunday, March 29, 2020 07:12:36 AM Mountain US Time Zone


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.









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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
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
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 t he 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
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 in all dimensions.

An auxiliary handle can be screwed into the QCTP body.

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Tool Bit Types & Shapes
lathe_tools_std_shapes.jpg (28873 bytes)
Lathe tool standard shapes. Single point lathe tool angles


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 (China).

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).


A SWGCR Right-hand 90 deg WCMX-50308
B SDJCR Right-hand 93 deg DCMT-0070202
C SCLCR Right-hand 95 deg CCMT-060204
D SDNCN Right-hand 62.5 deg DCMT-0070202
E SCLCL Left-hand 95 deg CCMT-060204
F LW Threading JCL-15-120
G QA Cutoff JCQ-3

Carbide insert tool bits in 1/4" & 1/2" shanks.

Carbide insert tool-bit holders (negative 5 deg rake), from Dorian Tools (USA) left to right:
MTGNR, MTJNR, MTCNN (negative 10 deg rake), MTJNL, MTGNL.

This 5/8" (15.875mm) wide, negative 5 deg rake tool bit used to put high stress on this machine but now that it has
been upgraded with a variable speed DC motor & timing belt/pulley system, it easily makes no-chatter cuts.

Negative-rake tool-bit holders in QCTP height-adjustable dovetail mounts.
These use TNMG-style, 1/8" thick, zero relief angle inserts which have six usable tips, each.
The middle holder is an MTENN type having an insert with a 1/32" radius tip.

Different radius insert tips can be used e.g., 1/64" or 1/32". The holders have 1/2" hardened shanks.
TNMG-style inserts are the most economical to use due to six tips, apiece.
Their popularity & high industry demand drives their cost down.

5 deg negative-rake insert tool bits.
See Insert Nomenclature Chart & Indexable Tool Holder Nomenclature Chart

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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 close, precision fit to the ground shank.

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. 

A Delrin turning & 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 hypodermic needle.

Deep parting a Delrin spacer.

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

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













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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.





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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 the 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 has eliminated 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 &
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.

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






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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