9x20 Lathe Tailstock DRO
Last updated on
Thursday, March 26, 2020 02:26:57 PM Mountain US Time Zone


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.









Click on linked thumbnails

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

Digital Scale

Four-inch (100 mm) digital scale with data out port (China). 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.








Click on linked thumbnails

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 aluminum 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