Here is a cutaway drawing of the trunnion assembly, showing the major components. I’ve removed some of the steering head linkage for clarity. The unit is an air/oil spring, with air pressure acting as the spring, and hydraulic fluid being forced through orifices to act as the damper. In addition, the steering bellcrank on top turns a steering “blade” that passes through the stanchion (moving cylindrical part), so that steering input is connected to the nose wheel. The amount of air pressure needed depends on the weight on the nose wheel, and for this airplane it’s no more than about 45 psi in the extended position.
Now we’ve mounted the fixture used in operation two onto the 4th axis (rotary). The part is again bolted and pinned to the fixture, and this arrangement allows us to machine the main bore and also rotate the part to 90, 180, and 270 degrees to add mounting holes, threads, etc. (Please forgive the rust visible on the 4th axis unit. It’s gone now, thanks to an Ospho(tm) phosphoric acid treatment that completely fixes the problem.)
I roughed the main bore using a 3/4″ diameter drill from both sides, followed by a 1-1/4″ stub drill from both sides, then an endmill using circular interpolation to get within .010″ of the final size. Finish boring was with a fancy Wohlhaupter boring head, which allowed us to bore 6″ deep and hold roundness and size with .0002″. A real pleasure to use a beautifully balanced tool like that.
The next picture shows the boring tool in operation, with the usual coolant squirting everywhere. Must have cooling and lubrication. The threads were cut earlier with a thread mill using helical milling. (I made a thread plug gauge previously to check the thread size as we finished it). Boring was the last step, so we could make necessary fine size adjustments and repeat the operation as necessary.
We used a boring cycle on the Fadal Mill that bores to depth, stops the spindle, shifts slightly away from the work, then retracts out of the hole. This is more accurate and leaves a better finish than the usual “G85 bore-in, bore-out” cycle. Most CNC machines have something similar.
This shows the finished bore, after the part has been anodized (Type I), and we have pressed two IGUS(tm) plastic bearings into the bore. The grooves visible in the bearings are to prevent hydraulic fluid from locking and possibly shifting the bearings.
This shows the preparation for the second operation. We have bolted our fixture plate down to the mill bed and aligned the dowel pins parallel to the X-axis. The use of precision dowel holes put in during the first operation allows us to maintain control of where we are when we turn the part over. The part is held down by 4 socket head capscrews and aligned on 2 dowel pins. The screw heads are deeply counterbored into the part so we can machine most of the shape without hitting the screws. This type of fixturing is very robust and accurate. The same fixture we’re using here will also be used to hold the part when it is mounted on the rotary (4th) axis.
This shows the part after we’ve finished machining the second side. Now the basic “profile” is complete except for removing the holding tabs which will happen later. Notice that the capscrews are now visible just below the cut surface, so we removed quite a bit of the holding tab with these cuts. This was done with a high-helix 3-flute carbide endmill which roughs and finishes quickly, and a 1/2″ carbide ball endmill which created the cylinder shape and the floor area of the two pockets. We are using SmartCam Freeform Machining software to create the toolpath from a model made in Solidworks.
The first stage, after surfacing a block of aluminum flat and parallel on both sides and adding some hold down slots, was to clamp it down on a scrap piece and machine the first side profile, using standard carbide endmills for roughing and ball endmills for finishing. The picture shows after this first operation is complete. We’ve added four bosses outside the actual part profile for hold down capscrews and dowels. They will be cut off in a later operation.
This is a recently finished part that we did for Eric Raymond, who is currently building a 2-place solar powered airplane in Europe at www.solar-flight.com. The mechanical design of the nosegear was done by Rick McWilliams at Tangent Instruments, with assistance on the shock absorbing portion from Gil Vallaincourt of Works Performance Shocks. Eric is building an extremely light and efficient airframe from carbon fiber, and weight is a huge factor. So this unit has to incorporate oleo-pneumatic shock absorbing and damping, steering, and retraction- all in a very compact unit. Total weight with hydraulic fluid is about 4 pounds, and the design has been optimized with finite element analysis to meet the anticipated landing loads. The trunnion (main piece) is 6061T6 aluminum, machined from the solid, Type I anodized, and the stanchion (moving cylindrical piece) is 7075T6, because it sees the highest stresses. It is hard anodized and polished for wear resistance. Most of the fasteners are titanium. It has some fairly fancy internal bearings and seals. I will publish additional bits as I go along showing how some of the components were made. We can’t wait for his first flight.
Thanks for checking out Design Intent. This is a website describing my interests in design (mostly mechanical, aerodynamic, etc) and the manufacturing methods used to create things, primarily by machining.