The challenge began when a local turret clock expert came to me and asked if I could machine a new Hour and Minute Hand for a clock he was working on. The Hour Hand was around 14” long and the Minute Hand some 18” long. Here is the Fusion 360 view of the Minute Hand drawn on a stock backing plate.
Here is the Fusion 360 view of the minute hand drawn on a stock backing plate
Clearly an 18” length was way outside the 440 table X movement (10”) so a plan was needed.
My conclusion was that I needed to be able to accurately step the stock across the tooling table and then take two or three bites at the profile machining. What follows would almost certainly benefit from a video but sadly I am not set up for this.
Fusion 360 Inputs
I decided to make the minute hand first as this was the longest and therefore the largest challenge. More experienced machinists and Fusion users may have a better way to do this but here are my steps:
- I chose an arbitrary size for a stock rectangle that would encompass the final hand profile in X, Y, and Z. Let’s go metric at this point…. 80mm x 480mm x 7mm.
- I sketched a rectangle to the X and Y dimensions in Fusion and extruded this up for 4mm.
- I drew a sketch of the Hand on the ‘top’ surface of the extruded rectangle.
- I extruded the hand sketch up for 3mm above the base rectangle to now give a hand that was 7mm deep overall when cut down to the stock lower surface.
- I extrude cut the hand mounting hole at 8mm to match my tooling table hole size.
- Down the side of the stock and outside of the hand outline I placed a number of 4mm holes spaced at 25mm. Not all these would be totally necessary but I wanted some positional flexibility as I moved the stock across the table and I was not sure at this stage what my offsets for each cut would need to be.
It is important at this stage to explain that the following process is not easy to replicate unless you have a tooling plate with a healthy matrix of tooling holes (rather than just the table T slots). A great use for SMW Fixture Plates! My tooling plate has M8 holes on a 25mm matrix across the whole area of the plate with 4mm clearance holes set within each 25mm square. These small tooling holes are only within the X and Y limits of the Tormach machining envelope and help remind me when setting up a job.
Here is a view of the plate showing the M8 holes, the 4mm tooling pin holes and the counter bored mounting holes to match the T slots on the 440 table. While I refer to the tooling pins as 4mm, the holes in the plate are 3.7mm so I could use 4mm silver steel rod for the pins with each having a 3.7mm turned shoulder.
My Tooling Plate showing the 25mm matrix and the 4mm tooling pin matrix
The Magic (Stock Contours!)
I now had the hand sketched out which could be cut with a 2D profile (if only I could cut 18” in X across the milling axis). What comes to the rescue is breaking this down into 3 sections using Stock Contours. To do this, you sketch boundary shapes representing your machine travels. These are ‘drawn’ on the top surface of the stock. These will not be readily apparent when viewing the model but are there to be picked up in CAM. The CAM accepts the full profile of the 2D shape of the Hand (using Contour Selection) but will only undertake to cut within the boundary boxes defined by the Stock Contours.
These two types of selection are slightly confusing. I regard Contour Selection as being a real tangible aspect of the design geometry whereas Stock Contour is an imaginary area relating to the basic stock. This can be the whole stock boundary or a section of the stock as defined by a sketch. I sketched three Contour Selection boxes to have arbitrary but overlapping dimensions sufficient to give me a shared number of the model tooling holes in each. Note that Fusion automatically codes the lead in and lead out of the tool in each section of machining activity.
There is a good two part description of Contour Selection here
Each of these areas can be regarded as separate jobs, and the mill has no knowledge of how they interrelate to each other. It is important therefore that they each have their own setup and XYZ reference point coinciding with an identifiable feature. This feature must be common to the particular Contour Selection area being machined and also the previous section. The following images show the three sections and the Setups.
First Setup showing the Stock Contours tab and the outlined machining area
Setup 2 showing the Stock Contour boundary boxes
Setup 3 showing the Stock Contour boundary boxes
Fusion 360 CAM showing each section operations
Mounting and Locating the Stock for the first Ops
- I mounted and clamped a length of brass bar at the rear of the tooling table and checked that its front face was parallel to the X movements. This would provide a solid Y reference to slide the stock against while sliding it across the tooling table.
- The first section to be machined would be the hand boss end so it was necessary to decide which of the tooling plate M8 holes would be the centre hole of the hand boss. To do this I loosely placed my stock onto the table up against the brass bar and jiggled it around to see which hole on the left hand side of the machining footprint of the tooling table best matched. By having the stock width oversize I was not too limited in my choice.
- Having chosen a particular tooling plate M8 hole as the hand boss centre I now needed to make the spindle sit over this hole as X- and Y-zero. I removed the stock and put my laser centring tool into the Tormach spindle. This has a laser diode offset from the spindle axis and when rotated by the spindle motor, creates a red ring of laser light on the object below. By moving the spindle up and down the size of the red light ring can be varied. This is a wonderfully simple but accurate way to set an object centre by eyeball. You can find details of this elsewhere on my blog. I moved the X and Y to centre the red circle on the chosen M8 hole.
- When content that I had concentricity between the spindle and my chosen M8 hole I zeroed X and Y. I did a double check by manually moving the spindle to X32.5 Y32.5 which brought the laser over the first tooling hole I had sketched on the model. I rechecked concentricity once again and this looked very good. (An alternative method for this setup would be to use a Haimer or similar touch off probe) See our article oh purchasing a Haimer!
- I placed the stock with a sacrificial sheet under it on the table and clamped it in place and zeroed Z to the stock top surface.
- I ran the first stages of CAM on Section #1 which was the 8mm clearance hole on the Hand central boss and the first six 4mm tooling pin holes.
- I removed the stock and using the witness marks in the sacrificial under board I put sloppy clearance holes in the sacrificial board to match the 8mm and the first 4mm tooling hole.
- I dropped the sacrificial board back onto the tooling table over the tooling pin and then mounted the stock in place using both the first tooling pin and the M8 hole to locate it accurately in position. I fastened the stock in place with a M8 screw and various clamps. This combination of M8 and tooling pin locked the stock in place against the brass bar. The M8 hole was used as the first setup reference location in the Fusion CAM. The tooling pin I had chosen would stay in place on the tooling table and be used to reference the subsequent two sections of machining.
- I ran the profile for this first section of the Hand with the CAM Heights tab set as cutting from Stock Top to Stock Bottom -1mm. This would ensure a full cut through the stock into the sacrificial board.
Laser Centering Device
Laser Centering Device in Action
Stock and Sacrificial Board
- I de-mounted the stock and moved it to the left across the table so that the tooling pin in the table now located in the sixth tooling hole drilled along in the stock (there is nothing magic about the sixth hole, it just turned out to be the best location to maximize the machining envelope).
- Knowing the geometry of the tooling plate I manually moved the spindle once again using direct MDI GCode inputs to X32.5, Y32.5 and then zeroed the X and Y at this position. This was dead centre of the tooling pin I had chosen to use as the stage 2 reference.
- Problem – I wanted to use this pin as X0Y0Z0 which was fine for X and X but Z was wrong on the Fusion model as this was sat at 4mm, not 7mm. I edited the model in Fusion by creating a boss around the particular hole and extruded it up 3mm to give me a correct model Z zero. This explains the boss around this hole (and also on the eleventh hole which I chose as the reference for the third section).
- I ran the first operation in the second setup. This drilled more tooling holes in the stock ready for me to choose for the third section and then ran the 2D profile of this middle section of the hand.
- No more tooling holes were needed to be drilled in this section as the previous operations had created enough to choose from. The eleventh hole looked the best placement fit for maximum X machining length and the stock was placed over the tooling plate pin. This pin location was still X0Y0 from the previous operation. This tooling hole on the model also needed me to sketch and extrude a boss around it to correct the Z height.
- Once Z was zeroed, the only machining operation in this set up was the final hand profile to complete the job.
Almost There! Running Section 3
Note that care is needed to ensure that both the hand AND the cut away stock are firmly clamped to the table or the stock will be thrown by the spindle as breakthrough depth is reached.
The machining of the minute hand has worked to my plan with a lot learned in the process. The continuity of the entry and exit tool witness marks from each individual machining section were minimal. On an object this size and mounted high up and so far away from people such minor witness marks would be of no consequence.
I am quite pleased with how it has turned out. I used the same process to machine the hour hand with the same results.