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9x20 Compound Bracket & Angle Plate Jig

A piece of annealed, free-machining, 303 stainless steel was selected for plate material. The surface was covered with red layout dye to make the marks & cuts easier to see. The locations of the four, 6mm mounting bolt holes & the center were laid out using a surface plate & height gage. Mount piece onto the faceplate that has been accurately faced using carbide tool bit. Drill out a large hole, 1" in this example. Lock the carriage when facing. There is a scrap plate under the part to avoid cutting the faceplate. Use the tailstock to quickly find the pre-drilled center the piece before tightening the bolts. Also made a bracket using a 6 inch, 4-jaw chuck.

 
A boring operation shown using a right-hand, ¼" carbide tool bit.

Bottom of the bracket shown. Before removing the part from the faceplate, chamfer the inner edge so the protractor corner fits well. The corners are rounded to allow the compound clearance when pivoting. The protractor is too inaccurate to use so I chose to not expose it & thereby not weaken the plate. I use my compound angle plate jig, instead.

Part mounted onto a rotary table using a miniature hold-down clamp set.
Machined the round corners & bolt/nut recess using a 5/16" carbide end mill.
I have since eliminated the bolts recess cuts in a newer plate.

Compound bracket bolted into place. I used 10mm length SS hex bolts (M6-1 thread) to keep all the wrenches metric.
The mounting bolts screw into long, steel nuts that I made; they are about 0.2" x 0.4" x 1" with a 5mm hole tapped to 6mm threads.

New, thicker plate with no bolt recesses; minimal flex.
A lathe must have a functional, pivoting compound for such
operations as threading, chamfering & short tapers.

Have now increased the size of the compound pivoting mechanism in all dimensions by about 50%..

Larger bearing surfaces, less flexing & the incorporation of full-sized, M6-1 cap-head screws.
The center hole was precision reamed to 15/32" & the lapped onto the compound's bottom indexing post for a super-tight fit.
The inner pivot edge has to have a chamfer to allow a good fit into the plate counter-bore.
As was the original, three roll pins have been added. OEM is shown on the right.
The plate flexes much less now that the pivoting foot is so much larger, probably because it's nearer to the clamp-down bolts.

Compound Angle Plate Jig
 A jig is described that can hold different precision ground angle plates against the lathe's compound for accurate taper-cutting setups.

A base clamps to the cross-slide table to hold & position the upper bracket.
The centrally relieved area allows the compound's mounting plate to clear the base.
The upper bracket holds any one of the different precision angle setup plates against the compound's front edge.
Three, counter-bored holes were made for the 6-32 SS attachment bolts.

I used a dial test indicator mounted in the spindle to make the final alignment.
The lathe spindle is engaged at its lowest speed to hold it steady.
CAUTION: DO NOT TURN THE LATHE ON.
First, snug only a left (or right) bolt before making the fine
adjustment/alignment then tighten the other two bolts & recheck tram.
The 6-32 bolt holes were sized for a free fit (#25) to allow for this adjustment.

Detail of corner relief & toe clamp.

A 1° angle is held by the jig & toe clamp.
The jig was designed to hold this smallest angle plate thus all others work, too.

Another view showing the relief required to clear the compound bracket.
This extra clearance was needed for only the very small angle plates (1° to 5°).
The left side is longer than the right side which had to clear the cross slide's gib adjustment screws.
The cross slide's top edges are heavily chamfered so no relief cuts were needed on the bracket.

Detail of the ¼" thick toe clamp.
The thumb screw is a plastic cap pressed onto a 6-32 cap-head bolt.
A brass washer is under the clamp knob.
The channel is exactly ¼" deep, same as all of the plates.

A ¼-20 set screw clamps the base to the cross-slide table.
The end plates were bolted on using 10-32 flat-head screws & then the surfaces were fly cut.

Note how the square plate corner fits into the relieved corner.

A 3° angle setup plate placed into the jig bracket.

Compound is rotated as the jig is (iteratively) brought into tight contact.
The compound is then locked down. The toe clamp clears the (rotating) tool post.

The compound was set at 3° to make a 6° included angle taper cut on a ½" steel rod.
The taper shown is after smoothing & polishing with progressively finer grit sandpapers
each alternately applied at right angles to effectively remove the machining marks.

Here a 45° angle plate is held against the compound's top, back edge.
The compound gib adjustment screws are on the front side for this cut.
The angle plate bottom edge must clear the gib adjustment screw nuts, too.

The corner has since been relieved to assure that the
angle plate seats completely against the vertical edge.

CAUTION: DO NOT TURN THE MILL ON.

To verify the accuracy of a cut after using the angle plate jig
to set the compound, I measured the taper's nominal 3° angle.
The part is resting on a precision, ¼"-thick parallel & is clamped by the vise.
A dial test indicator is mounted in the mill quill using a 3/16" R-8 collet.
The quill was moved down so the indicator touched the part.
I moved the Y axis back & forth until a peak on the round part was indicated.
I then zeroed the indicator & the DRO's X & Z axes.
The quill was retracted & the X axis moved (1.5000") on the DRO.
The quill was then lowered until the dial indicator zeroed &
the DRO's Z-axis value (0.0790") was noted.

ANGLE° = ATAN (HEIGHT" / LENGTH")     or     ANGLE° = ATAN (Z" / X")

The calculated angle was 3° 1'.  A good value considering the tolerance of the
angle plate is ±10' & the effects of the other tolerances (parallel, vise, mill tram, part shank), too.
This calculation is analogous to that for sine bars. Note the calculator must be set to degrees.