9x20 Lathe Tailstock DRO
Last updated on Tuesday, March 28, 2023 01:16:03 PM Mountain US Time Zone

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Tailstock DRO, Digital Scale,
Scale L-bracket, Finger Bracket,
Return Spring, Scale Zero,
Display Angle Plate

Tailstock DRO

A Digital Readout (DRO) was added to the Jet
9x20 lathe tailstock. It can be set to zero at
any point with the units selected to either imperial
or metric. The scale has memory so even
when the ram's extension is changed with the
scale OFF, it still tracks distance traveled.
It has an SPC plug receptacle & could be easily
connected to the DPU-550 DRO, if desired.
However, I like the LCD (no wires) & prefer
displaying variable speed RPM on the main DRO.
Locating the scale on the backside keeps it
clear of the ram & tailstock locking levers.
Precision, repeatable depth control of
drilling operations is now simplified.

 
Metric Sizes
M4-12, M4-16, M4-20, M4-32,
M5-12, M5-16, M5-20, M5-25,
M6-12, M6-16, M12-50 (2 pack)

 
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Pre-modification baseline OEM Jet BD-920N tailstock.

Digital Scale

Four-inch (100 mm) digital scale with
data out port. Digital scale instructions.
Attributes: ideal form factor for this application,
hardened SS with four pre-tapped M3-0.5
threaded holes on transducer back plus end
brackets for precise rigid mounting, low cost.

 
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Scale L-bracket

Material for a bracket C-clamped
in place to visualize locations.

The scale is held to the L-bracket
by the two supplied M3-0.5 screws.

Backside drilled 7/32" (5/8" deep) &
tapped at 50% 1/4-20 threads for cast iron.
I chose a coarser thread because its soft
cast iron. It's sitting on a 1-2-3 block.

Left a section of corner material
on the outer edge for extra strength.

The black masking tape kept the scale from
sliding out after the right bracket was removed.

The L-bracket mount is sturdy allowing easy
manipulation of the transducer function buttons.

Finger Bracket

A finger bracket rides against the end of the tailstock's
ram. SS 6-32 screws were used as fasteners.
The ram contact area of the finger tip was precision ground.
The inside corner has a 1/8" radius for strength.
Many people use a bulky circular bracket that clamps
onto the ram. Use depends on the ram's length.
On the Jet, it would interfere with fully retracting the
ram to eject some tools which, of course, depends
on the MT2 arbor's tip length. The rigidity of a ram
clamp can also cause binding of the mechanism.
Since the finger bracket tip floats against the
ram's end, it can not possibly cause any binding.
Its employment & action are completely analogous to
that of an indicator. In general, I do not like to
lose OEM machine functionality as a result of any
modification. I retained the ram's scale & zero.

Return Spring

Underneath, a return spring maintains finger contact
with the ram. 6-32 screws were used for spring posts.
They were threaded into the brackets & locked with
nuts/washers. The screw heads retain the spring.
The spring strength must be selected to not stress
the mechanism but provide reliable, complete
 returns. This spring has a 1000 g (35 oz) pull as
measured with an RCBS Ohaus trigger scale. All
corners were rounded for safety & esthetics. There
is a micro-dab of grease on the contact area.

The finger has to be thin enough to clear all
tailstock accessories; chucks, live centers, & end mill
holders. Final dimensions: 1/8" thick, 3" long, rectangular
base is 1.5" x 1.4", & the finger is 1.55" L x 0.195" W.
Recommend using steel instead of a
luminum for the finger bracket.

Recovered 0.039" ball bearings
shown being held by a weak magnet.

Drilled a shallow 0.038" hole, super
glued & pressed-in the ball bearing.
This provides a hardened, spherical contact
point against the tailstock ram face.

Scale Zero

The three mechanical & one
digital scales zeros all coincide.

Checked it against a high quality Mitutoyo indicator
that is accurate to 0.00005" (50 millionths).
The readings matched & were repeatable even
after multiple extension/retraction cycles.
It was also compared to the main DRO (0.0001"
readout) & their agreement was remarkable.

The hardened scale was shortened by about
1-3/4" to insure fingers-on-the-knob clearance.

Display Angle Plate

The tailstock DRO works so well that I decided to slightly
angle the LCD toward me to increase its contrast.
I used the L-bracket as a fixture to hold
the 0.2665" thick aluminum material.

Plate & bracket clamped in the mill vise.
The bracket is also the fixture.

Starting to fly cut the angle.

Finished fly cut.

The wedge is 10 deg.
This is before finishing & corner
rounding using a disk sander.

Used M3-0.5 transfer screws to locate
the transducer on the angled plate.
Since the back is hardened SS, it is helpful
that it has four, pre-tapped holes for mounting.

The 3mm screw holes are perpendicular to the top
angled surface & are counterbored on the bottom.


The angle plate sandwiched in-between
the transducer & the L-bracket. The
corners have been rounded.

The two, 8-32 button-head screws slide & lock in short
slots allowing finger bracket tip-to-ram positioning.

Now the finger bracket points down a bit
but still precisely touches the ram end.
Actually, the new position moved
the finger tip to a better location.

The addition of the wedge misaligned the spring.
Note the access holes to adjust the scale bar gib.
Carefully adjust the gib screws to minimize any
sideways play to obtain highest reading accuracy &
 repeatability. A jeweler's slot screwdriver was used
to tighten the scale as it was loose out of the box.
Above the left access hole is an arrow that is on
the slide-up cover of the transducer's data port.

A longer (right) SS 6-32 screw realigned the spring.

The slight 10 deg angle gives improved ergonomics.
See the 9x20 Lathe DRO modifications.
When compared to the first photo on this page,
one can easily see the contrast increase.
Also, a low angle was selected so the DRO
would not interfere with use of the ram locking
handle & to keep the shape of the finger bracket
from having to be redesigned/fabricated.


A 3/8" 2-flute end mill in an MT2 holder being
used to make a 0.050" deep counterbore for a rivet.

Tailstock DRO, Digital Scale,
Scale L-bracket, Finger Bracket,
Return Spring, Scale Zero,
Display Angle Plate

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