Rear Hubs (Nov 2016)
The initial steps for making the rear hubs was to turn the six castings (left and right hand, inner, centre and outer castings) down to finished thickness and to mate with each other about the recesses in the centre part and the spigot on the outer and inner parts. The rough castings were first held in the four jaw chuck and aligned by eye to get the alignment correct to extract the full size part from the cast material. The casting were originals from Reeves and to be fair if I had just bought them I would have sent them back! The central grooves were already wider that the drawings specified and the outer profiles were also less than the specified dimensions. So from the outset these hubs would have to be made to suit the materials.
The initial steps for making the rear hubs was to turn the six castings (left and right hand, inner, centre and outer castings) down to finished thickness and to mate with each other about the recesses in the centre part and the spigot on the outer and inner parts. The rough castings were first held in the four jaw chuck and aligned by eye to get the alignment correct to extract the full size part from the cast material. The casting were originals from Reeves and to be fair if I had just bought them I would have sent them back! The central grooves were already wider that the drawings specified and the outer profiles were also less than the specified dimensions. So from the outset these hubs would have to be made to suit the materials.
All casting were faced off and reamed in the four jaw and then fitted to a 5/8” stub mandrel to bring them to finished widths. The drawings were updated slightly to increase the length of the spigot and matching recess in order to ensure a secure engagement between the parts.
A jig was then made to hold the hubs sections concentric on a rotary table fitted to the mill. The jig consisted of a pin that aligned the 5/8” central hole to the centre of the table and allowed a M6 cap head to pass through a top hat section which encapsulated a M6 cap head and clamped the hub parts down. The jig was sized so that all orientations of the hub could be clamped down and subsequent machining completed.
A jig was then made to hold the hubs sections concentric on a rotary table fitted to the mill. The jig consisted of a pin that aligned the 5/8” central hole to the centre of the table and allowed a M6 cap head to pass through a top hat section which encapsulated a M6 cap head and clamped the hub parts down. The jig was sized so that all orientations of the hub could be clamped down and subsequent machining completed.
The next stage was cutting the spoke a grooves. To get the right width 50% of pressed spokes were draw filed on all edges and taken to a consistent size. This was a mega time consuming process that took about 20mins a spoken – hence only 50% got done and the rest will get done when the enthusiasm returns! A consistent width was achieved and then this was transferred to the hubs.
The front spokes had been cut using a CNC program to cut all slots in one session, however the end result was that some grooves were larger than others due to backlash in the mill, thus the rear grooves were cut in four separate settings on each face, and only cut parallel to the X/Y axis. To do this the centre hub was fitted to the rotary table and the centre indexed to the mill spindle. The table was then rotated to what could be measured/assumed/guessed would be the best angle to machine the outer profiles and the part locked down. A test groove was machined along the X axis following a few roughing cuts and a few finishing cuts taking the control point + and – of the Y0 to achieve the width of the groove. Once the correct Y +- values were identified, the numbers were committed to a short G code program and the rest of the grooves machined at 45deg positions.
The front spokes had been cut using a CNC program to cut all slots in one session, however the end result was that some grooves were larger than others due to backlash in the mill, thus the rear grooves were cut in four separate settings on each face, and only cut parallel to the X/Y axis. To do this the centre hub was fitted to the rotary table and the centre indexed to the mill spindle. The table was then rotated to what could be measured/assumed/guessed would be the best angle to machine the outer profiles and the part locked down. A test groove was machined along the X axis following a few roughing cuts and a few finishing cuts taking the control point + and – of the Y0 to achieve the width of the groove. Once the correct Y +- values were identified, the numbers were committed to a short G code program and the rest of the grooves machined at 45deg positions.
The outer/inner parts were then clamped to the freshly machined centre section and as the mill was still indexed the holes were drilled and tapped for the countersink screws connecting the parts together and the various features unique to each faces. At this stage the outer profiles should have been machined prior to removing the job from its clamping arrangements – but that is hind sight for you!
It was initially planned to machine the outer profiles using the 4th axis and running the cutting tool normal to the outer face, however, in development of the G Code this turned out be too complex, thus the hubs were returned to the mill and indexed to the rotary table (using the spoke slots as a reference face) and then the outer profiles cut using the side of an end mill following a CNC program. This process took a while as initially the G code was post processed from a 3D model (AutoDesk Fusion360) based on the correct sizes from the drawings, however, when cut it was obvious that the finished size needed to be smaller else all the faces would not be machined as the casting were too small! This was an iterative process as the model had to be reduced is size, the code post processed and the cuts made to see if all the material had be cut. The finished size of the other side of the hub then had to be changed to make the error consistent – very frustrating!
It was initially planned to machine the outer profiles using the 4th axis and running the cutting tool normal to the outer face, however, in development of the G Code this turned out be too complex, thus the hubs were returned to the mill and indexed to the rotary table (using the spoke slots as a reference face) and then the outer profiles cut using the side of an end mill following a CNC program. This process took a while as initially the G code was post processed from a 3D model (AutoDesk Fusion360) based on the correct sizes from the drawings, however, when cut it was obvious that the finished size needed to be smaller else all the faces would not be machined as the casting were too small! This was an iterative process as the model had to be reduced is size, the code post processed and the cuts made to see if all the material had be cut. The finished size of the other side of the hub then had to be changed to make the error consistent – very frustrating!
The central grooves were the most complex to machine and these were conducted on the 4th axis in a hand written program that moved a milling tool down through the groove whilst rotating the hub to keep the tool point 90degs to the working surface! Thinking about how to write the G Code for these grooves took far longer than the actual machining time but allowed for the size of the grooves to be adjusted to (again suit the material) and was an interesting part of the project!
The basics for the hand written code was to support the hubs in a mandrel between the 4th axis chuck and the tailstock and keep the tool (an end mill) normal to the groove, then move the part and tool simultaneously so that the control point follows the contour round the groove.
The basics for the hand written code was to support the hubs in a mandrel between the 4th axis chuck and the tailstock and keep the tool (an end mill) normal to the groove, then move the part and tool simultaneously so that the control point follows the contour round the groove.
rear_hub_grooves.xlsx | |
File Size: | 31 kb |
File Type: | xlsx |
(Please note, I have included the excel file as an example of the steps I took to make this feature. This file is not described well enough to be used by anyone else in its current state - sorry. One thing that can be taken from it though, is how to use Excel to build up lines of code based on a formula linking one control point to the next...)
The code was developed in MS Excel by identify the key input parameters (diameters that could be measured), then splitting the required path into sections where formulas could be used to track the control point along the required path from the start of the section to the end of the section. The formulas were then changed through the next section and so on. The code was then built up as an X Y Z function of a step change in A (0.1deg). Mach3 would then interpolate a straight path between the steps in the four coordinates and manage the tool velocity.
The key difficulties encountered were:
The most complex feature of the code was modelling the path as the tool rounded the outer radius of the diamond shaped path as the path relevant to a change in A needed to take into account the function of two radius at fixed point in space whilst keeping the tool normal to the surface. As the tool move round this section the Y axis actually changes direction, which is v difficult to comprehend!
The end result was very good, however mega time consuming. The key parameters needed to be change several times as not only was the material too small to deliver the specified size, the features of the casting were also miss aligned compounding the necessary error. The surface finish was good and in practice I ran the final program twice on the same settings to relieve the spring cut and ran it very low feed rates to improve the finish.
Finally all the corners were rounded off with small files and the main features of the hubs were complete.
The code was developed in MS Excel by identify the key input parameters (diameters that could be measured), then splitting the required path into sections where formulas could be used to track the control point along the required path from the start of the section to the end of the section. The formulas were then changed through the next section and so on. The code was then built up as an X Y Z function of a step change in A (0.1deg). Mach3 would then interpolate a straight path between the steps in the four coordinates and manage the tool velocity.
The key difficulties encountered were:
- Thinking in four dimensions
- Trying the visualise the tool path
- Keeping the tool cutting in a conventional milling strategy
- Trying the generate lead ins and lead out that didn’t destroy the part
The most complex feature of the code was modelling the path as the tool rounded the outer radius of the diamond shaped path as the path relevant to a change in A needed to take into account the function of two radius at fixed point in space whilst keeping the tool normal to the surface. As the tool move round this section the Y axis actually changes direction, which is v difficult to comprehend!
The end result was very good, however mega time consuming. The key parameters needed to be change several times as not only was the material too small to deliver the specified size, the features of the casting were also miss aligned compounding the necessary error. The surface finish was good and in practice I ran the final program twice on the same settings to relieve the spring cut and ran it very low feed rates to improve the finish.
Finally all the corners were rounded off with small files and the main features of the hubs were complete.
Drive pins, cotter pins and collars
The drive pins were a simple turning exercise. I decided to make them from stainless steel rather than mild steel, mainly to prevent corrosion in the future and I had a few pieces of stainless that were about the right diameter. The ends of the pins were turned on a taper to suit the drawings and then finished with file to produce the rounded profiles.
The drive pins were a simple turning exercise. I decided to make them from stainless steel rather than mild steel, mainly to prevent corrosion in the future and I had a few pieces of stainless that were about the right diameter. The ends of the pins were turned on a taper to suit the drawings and then finished with file to produce the rounded profiles.
The collars were again machined using the mill. A simple 3D model was created in Fusion360 and a tool path exported. A stainless plate was then super glued to a sacrificial piece of aluminium and held in the vice on the mill. The through hole was drilled first and a 8BA bolt fitted to provide some extra clamping force then the collar were machined. The corners were broken with a smooth file and the faces finished on fine (2500 ish) paper.
pliers.
pliers.
The collet pins were simply cut from a sheet of steel shim using kitchen scissors. Then formed by bending the strips round a drill bit and tidied with square ended
Hub caps
The rear hub caps were turned up from the bronze casting and for the main part were a simple turning exercise, using a stub mandrel. The complex part was machining the logo on the outer faces. The tiny scale of these features is currently beyond the accuracy of my CNC mill, thus this parts will have to wait….
The rear hub caps were turned up from the bronze casting and for the main part were a simple turning exercise, using a stub mandrel. The complex part was machining the logo on the outer faces. The tiny scale of these features is currently beyond the accuracy of my CNC mill, thus this parts will have to wait….