Revised: 9/02/2012 Subject to Revisions
The first part selected for machining is the Sail Panel Pivot Bearing. One bearing is located at top and bottom of the sail panel to enable the sail panel to rotate freely. The bearing accepts a rounded pivot shaft end located at either end of the sail panel assembly. The round end of the pivot fits loosely inside the bearing with clearance to keep the sides of the shaft from contacting the bearing. The ball end of the sail panel pivot rests on a cone shape inside the bearing.
Finished Part Design Isometric
The pivot bearing has a large diameter shoulder portion that lies on top of the wood radial arm. A short shaft portion is press fit into a hole near the end of the radial arm. The hole in the top of the bearing holds and supports a pivot pin protruding from the bottom or top of the sail panel. The pivot has a rounded end that contacts the bottom of the pivot bearing. The pivot fits loosely in the hole allowing the sail panel to freely rotate with limited friction.
Piece of raw 3/8" diameter 6061-T6 aluminum rod stock
Sixteen pivot bearings are needed, one each near the end of each radial arm located at the bottom and top of each of the eight sail panels. The bearings are fabricated from a 3/8" diameter rod of 6061-T6 aluminum. Later, they may be made of other metals if necessary such as brass or steel depending on how well the bearings and sail panel pivots work with each other.
Before installing the raw stock, a dead center is installed in the headstock taper collet and the normal cutting tool tip aligned to the center. The cross slide axis (x axis) of the CNC controller is then set to zero.
Aligning Cutter Point to Dead Center
CNC Controller Screen - X Axis Set to Zero
A length of 3/8" aluminum rod at least 1" long is used to make the pivot bearing. The rod will fit through the headstock center hole so the rod can be much longer to facilitate making a number of parts in series.
The first step is to face off the end to make it square with the lathe. Only enough material is removed to make the end flat and even.
Facing Cut CAD Isometric
A normal cutting tool is then placed in contact with the cut-off face and the feed (y axis) of the CNC controller set to zero.
The diameter is then reduced to 0.360" for a distance of 0.300" from the cut face.
Top Diameter Cut CAD Isometric
The diameter of the end is reduced to 0.200" for a feed depth of 0.150" completing the mounting shank cut.
Shank Cut CAD Isometric
After cutting a chamfer of ~ 0.010" is trimmed from the face end to facilitate installation of the pivot bearing into the wood radial arm.
Part Being Run on Lathe Using CNC Program
The next step is to use the cut-off tool and separate the pivot bearing part from the stock at a distance slightly greater than 0.100" from the end of the 0.200" cut. At this point the cut-off part would be set aside and another part made.
A total of 16 pivot bearings are needed. The author will add a rear cut-off tool to the Sherline lathe and program the part fabrication using the CNC controller. One program will be used to cut the part as described above including the cut-off operation. The author will then move a new portion of rod out from the center of the 3-jaw chuck into contact with the cutting tool. The cutting tool will then be used to remove a new face and the entire process repeated except that only the feed (y-axis) will be set to zero for the next run of the CNC program.
First Three Pivot Bearings Run With CNC Program
A CNC program was written in G-Code language commonly used for machine tools. The program performs all cutting steps except the first facing cut and at the moment the cutoff step.
Portion of CNC Program to Make Sail Panel Pivot Bearing
Once the 16 parts are made (and probably a spare or two) the stock rod will be removed from the lathe. Then each of the pivot bearings will now be inserted into the 3-jaw chuck by the small diameter section.
Cut-Off Part CAD Isometric
The tail stock of the lathe will have a Jacobs chuck installed and a center drill installed. Each of the pivot bearings will be center drilled a short distance to prepare to drill each pivot bearing.
After center drilling, a pilot drill such as 3/32" will be used to initially drill a 0.180" deep hole in each of the pivot bearings. The reason for doing all parts is that the set-up and alignment will require significant time while mounting each pivot bearing in the chuck will take less time and is gauged by the tips of the chuck fingers.
#29 (0.136") Drill Hole CAD Isometric
Finally a #29 (0.136") drill will be installed in the tailstock chuck, aligned for the cut depth and each part drilled the same 0.180" depth.
After drilling each part a manual chamfer of ~ 0.01" will be made to facilitate insertion of the sail panel pivot pins later.
0.01" Top Chamfer CAD Isometric
Completed Part CAD Isometric
The completed part isometric from the top view is shown above. The bottom of the hole is a 135 degree cone set by the end of the drill bit.
Building Pivot Bearings in Quantity
After creating the G-Code program the author decided to incorporate a rear mount cut-off tool and holder on the cross slide along with the normal cutting tool. The two tools are on opposite sides of the lathe cross slide approximately equal distant from the center and nearly opposite one another. This allows the pivot bearing part to be built from first cut through cut-off automatically using the CNC program.
Once the new tool arrived and was mounted, the G-Code program was modified to include the cut-off procedure at the end and automatic alignment of the cutting tool to support placement of the next portion of stock rod material. The program modification included the use of parameters that support quick modifications based on the position of the cutting tools as well as a control parameter to support diameter calibration based on the first part made.
With those modifications to the lathe and G-Code program twenty-one new parts were made with much improved accuracy. The critical diameter of the portion that inserts into the wood radial arm was now controlled to 0.2000 to 0.2005 inches, a very tight tolerance. These tight tolerance parts should now be able to be press fit into suitable diameter holes in the arms.
Sherline Lathe With Two Cutting Tools On Cross-Slide
In the above photo the normal cutting tool is nearer the bottom while the cut-off tool is above. Both tools move in the X axis (vertical in photo) together. They are separated sufficiently to provide room for the work being turned. The photo was taken as the G-Code program was operating and the first cuts being made with the normal cutting tool.
View of Cutting Operation on Lathe While Smaller Diameter Cuts Were Being Made
The smaller diameter cut to 0.200 inches required a total of nine passes removing 0.020 in. per pass for 1-7 and 0.010 for 8 and 9. The outer diameter of the bearing is 0.360 in and slightly smaller than the stock bar. This shot was also taken during a CNC run.
Small Diameter Cut On Final Pass
Part of the Sail Panel Pivot Bearings Being Turned
A total of twenty-one bearings were ultimately made. Eight are shown above part way through the part fabrication series.
Five Set-Up Parts and Calibration Part
Five unusable parts were built in the process of proofing the program and lathe set-up procedures. The lack of a rear cut-off tool on the cross-slide required that for each part the normal tool holder had to be removed and the regular cut-off tool holder installed in it's place. This necessitated a re-alignment of the normal tool holder when it was returned to the cross slide for the next part.
The multiple calibration procedures resulted in poor tolerances for the parts, running up to several thousandths of an inch despite great care. The main difficulty is that no good reference point exists that supports better than several mils definition. It is very, very difficult to align to the dead center used to locate the center of rotation of the lathe.
The addition of the rear cut-off tool means that both tool holders remain fixed in location and accurate compensation for any slight position errors can be incorporated in the G-Code program. This is done by cutting a test part, measuring it accurately, determining the error and plugging the error value into the program. The error is canceled out numerically by the program placing cutting tool very accurately for subsequent parts.
Example Accuracy Exhibited After Numerical Compensation In G-Code Program
Most of the parts made using numerical compensation in G-Code were within 0.0005 in of target value. The photo above shows an example part made to target value. The calipers used are accurate to 0.0005 in and read to the nearest 0.0005 in. The combined error is about 0.001 in. The parts made are expected to be within a one mil tolerance.
Example Measurement Showing 03605 Readout
The parts all measured to within 0.001 in worst case and mostly better than 0.0005 in. Many measured exact target value. The numerical compensation method in G-Code works very well. Since most movements of the lathe are fully automatic the fabrication process went rapidly. The only manual operations were to adjust the turning rpm and position the stock bar for the next part. The G-Code program stops and displays a message for the operator indicating what step to take before resuming the program.
Twenty-One Accurate and Six Set-Up Parts Along Side the Lathe
After the part run was completed the above photo shows the collection of good parts in groups of three on the right and set-up parts on the left.
Bottle of Partially Completed Pivot Bearings on Print
The above photo shows a bottle of the the partially finished parts laying on the drawing. The drawing was an output of the 3D design program - Alibre Pro. Each of the parts will need to have the large diameter portion trimmed to correct thickness and the hole drilled. These operations will also be done on the lathe. They are relatively simple operations and will likely be done manually rather than using G-Code.
Close-Up View of Partially Finished Bearings
The large diameter portion will be trimmed to a controlled thickness and have a hole drilled part way through the part to a controlled depth. The drill will form a cone at the bottom of the hole which will become the bearing surface. Each of these steps will be performed on the lathe. A simple facing operation will trim the thickness while a tailstock mounted set of drill bits of increasing diameter will be used to drill the hole in steps.
Each part will be chucked in contact with the surface of the chuck fingers being held by the small diameter. A facing cut relative to the chuck finger surfaces will be made to trim the part. Each part will be mounted and face cut with a common cross-slide position so that all parts match.
A flat file was used to make a small radius on the top edge to eliminate any sharpness.
Each part will be chucked in contact with the surface of the chuck fingers being held by the small diameter. A facing cut relative to the chuck finger surfaces will be made to trim the part. Each part will be mounted and face cut with a common cross-slide position so that all parts match.
A flat file was used to make a small radius on the top edge to eliminate any sharpness.
A flat file was used to radius top edge
Close Up of Part With Top Radius
Part Before Applying Flat File to Make Radius
Drilling begins with a center drill to located the middle of the diameter. This is done using the tailstock. The center drill leaves a starting dimple in the center of the face cut.
Center Drill Used To Make Accurate Center Pilot Hole
Center Drill Held in Jacobs Chuck Attached to Tailstock
Center Drilling Underway
After the above step the smallest of a series of drills will be installed in the Sherline Mill Jacobs chuck. The depth will be calibrated for the drill tip position. This is done using a calibrated reference part that has a measured 0.1000" top portion thickness.
Each part will be installed and drilled to the calibrated depth. Following that a slightly larger size drill bit will be installed and the procedure repeated on each part. A series of four drill bits of increasing size (#45 (0.082"), #38 (0.101"), #30 (0.128") and #29 (0.136") were used to reach the final hole diameter. The depth of each bit was driven down 0.180"from the calibrated top surface.
Selected Part With Blue Marking On Bottom Has Exact 0.1000" Top Edge Thickness
Each part will be installed and drilled to the calibrated depth. Following that a slightly larger size drill bit will be installed and the procedure repeated on each part. A series of four drill bits of increasing size (#45 (0.082"), #38 (0.101"), #30 (0.128") and #29 (0.136") were used to reach the final hole diameter. The depth of each bit was driven down 0.180"from the calibrated top surface.
Sherline Mill Driven Via CNC To 0.180" Depth
Each Part Placed in Mill Vice For Drilling
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Eighteen Good Parts and Standard Thickness Part With Blue Dot On Bottom
Working with CNC G-Code for this project has expanded the author's understanding of automatic machining. The machining of this part has greatly increased understanding of setting feed rate and spindle speeds to optimize chip formation for good cutting operation. The use of parameters and decision steps in G-Code has improved ability to achieve tighter tolerance using measurements after machining and adjustment parameters in the program to trim final cuts.
The next part of the project will be to tackle the wood radial arms. These will be made of red oak obtained at the local home improvement store. The mill will probably be the major tool employed to cut part outline and bore critical holes. A special purpose boring tool may be needed for the 0.200" holes for the sail pivot bearings just completed. They will be mounted in a snug fit hole and glued in place.
Another key metal part to tackle will be the limit stops. The stops consist of two parts, one fixed and the other pivots to allow the sail panels to move freely in one direction and restricts them in the other. The moving part is pushed down by the sail panel sliding up the ramp surface in the free direction while the panel hits the end of the ramp in the other direction and cannot move past it. Those parts will be fabricated using the mill mainly. The pivot for the ramp is a small machine screw.
Another metal part that may be made before the limit stops is the end braces that go between the tips of the radial arms. These are simple square stock with a cut down interface with the next brace so they overlap. Two overlaps meet at each radial arm tip and are held in place with a machine screw that passes through the two and the radial arm. A nut and washer on the bottom secures the screw.
Several complicated metal parts are used on the sail panels, the top and bottom pivots and the bottom and top sail frames. These will require a combination of lathe and mill work.
All of these parts will keep the author quite busy for the next many weeks. Progress on this project is interrupted by efforts to build two other models mentioned in the companion blogs.
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