9x20 Lathe Compound Bracket & Angle Plate Jig
Last updated on Tuesday, March 19, 2019 05:42:24 AM Mountain US Time Zone


Solid Compound Bracket, Compound Angle-Plate Jig, Jig Alignment Animation,
Taper Angle Measurement, Compound Bracket Archive, Compound Pivot, Annular QCTP Spacer 

Solid Compound Bracket

Fabricated a compound bracket that is now just one piece of steel.

Test fitting parts.

Low tool post to cross slide deflection while cutting-off a 1" mild steel rod.
The added support increased tool post rigidity while lowering chatter.

Compound Angle-Plate Jig
 A jig holds 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 of the different precision angle setup plates against the compound's front edge.
Three, counter-bored clearance holes were made for the 6-32 SS attachment bolts.

Hardened, 1/4" thick, precision-ground angle setup plates; 1, 2, 3, 4, 5, 10, 15, 20, 25, & 30 degrees.

Click on linked thumbnails

Use 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.

It is good practice to activate the E-stop button while setting up the lathe.

First, snug only a left (or right) bolt before making the fine adjustment/alignment then tighten the other two
bolts & recheck alignment. The 6-32 bolt holes were sized for a free fit (#25) to allow for this adjustment.

Jig Alignment Animation

Jig alignment animation.

Detail of corner relief & toe clamp.

A 1 deg 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 pedestal bracket. This extra clearance was
needed for only the small angle plates (1 to 5 degrees). If the top plate had been made wider, the relief cut would
 not have been needed. 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.

Detail of the 1/4" 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 clamping area is exactly 1/4" deep, same as all of the plates.

A 1/4-20 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 deg angle setup plate placed into the jig bracket.

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

Click on linked thumbnail

The compound was set at 3 deg to make a 6 deg included angle taper cut on a 1/2" steel rod.
The taper shown is after smoothing & polishing with progressively finer grit sandpapers
each alternately applied at right angles to effectively remove any machining marks.

Taper Angle measurement


To verify the accuracy of a cut after using the angle plate jig to set the compound, the taper's nominal 3 deg angle
was measured. The part is resting on a precision, 1/4"-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.
Move the Y-axis back & forth until a peak on the round part is indicated. Then the indicator was zeroed &
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 deg = ATAN (HEIGHT" / LENGTH")     or     ANGLE deg = ATAN (Z" / X")

The calculated angle was 3 deg 0' 53.25". A good value considering the tolerance of the angle
plate is +/-10' & the effects of the other tolerances (parallels, vise, mill alignment, part shank), too.
This calculation is analogous to that for sine bars.
Note the calculator must be set to degrees.

Click on linked thumbnails

Here a 45 deg 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.

Compound Bracket Archive
The photos below archives earlier bracket designs & modifications.

centering_piece.jpg (50186 bytes)
Early website scanned photos showing the plate's center being drilled. Initial 1" drilled hole.

 one_inch_drill.jpg (56225 bytes)
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
center the piece before tightening the bolts. Also made a bracket using a 6-inch, 4-jaw chuck.

boring.jpg (48265 bytes) 
Counter bore started.

boring_2.jpg (51375 bytes)
A boring operation shown using a right-hand 1/4" 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 a compound angle-plate jig, instead.

rotary_table_plate.jpg (54172 bytes)
Part mounted onto a rotary table using a miniature hold-down clamp set. Machined the round corners & bolt
 recess using a 5/16" carbide end mill. I have since eliminated the bolts recess cuts in an updated plate.

6-in, 4-jaw chuck holding a plate to make a thicker bracket.

Compound bracket bolted into place using M6-1, 10mm length, SS hex bolts to keep all the wrenches metric.
The mounting bolts screw into long, steel nuts that are about 0.2" x 0.4" x 1".
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.

Compound Pivot

Have now increased the size of the compound pivoting mechanism of several 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.

Annular QCTP Spacer

Decided to add a Delrin collar in-between the base of the compound & the bracket for a test evaluation.
Delrin is very slippery & it was used to allow easier compound rotation.

The disk, already 3.1" in diameter, was centered in the 4-jaw chuck & then cut down to 0.305" thick.

Drilled out a 1.5" hole before boring to design size.

Bored open to 1.9" ID.

Chamfered the inner edges with a razorblade while turning slowly.

The close fitting collar is shown installed in-between the parts. A tight setup with increased rigidity.

Shown parting at the lowest spindle speed (146 RPM) a 1" soft steel rod using a carbide insert tool bit.
Always lockdown the carriage when performing a cutoff operation.

The prototype collar worked so well I decided to make an aluminum piece. Increased the OD to 3.2".

Fly cutting the cylinder on the mill.

1.5" drill bit quickly removed a lot of material prior to boring.

Bored a 2.1" ID hole.

Very tight fitting metal collar installed under the compound. There is adequate clearance to
loosen & tighten the four hold-down plate bolts using a 10mm open-end wrench. Chamfered the
four bottom edges & corners of the compound base to assure smooth rotation when unclamped.
An added benefit is that the collar effectively keeps all of the swarf out of the pivoting mechanism.

I then added six, 4-40 screws (every 60 deg) to securely fasten the top spacer ring to the base plate. 

Solid Compound Bracket, Compound Angle-Plate Jig, Jig Alignment Animation,
Taper Angle Measurement, Compound Bracket Archive, Compound Pivot, Annular QCTP Spacer