It is possible you have seen mention in some of my posts about our shop-made barfeeds (shop-made sounds much more professional than home-made
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We have three units that are hydraulic powered; actually the working fluid is the coolant with one unit using compressed air over coolant and two using electric driven hydraulic pumps that are supplied from the machine coolant pump through a filter. The bar that is being fed acts as the hydraulic piston inside a long carrier tube that is the cylinder. Different bar sizes are accomodated by different sizes of liner bushings inside the carrier tube and the liner at the chuck end is just made a close fit on the bar and has an O-ring seal between it and the cylinder. The hydraulic pump has sufficient flow that the leakage between the bar and liner is overcome and pressure builds up to advance the bar when the chuck is opened; it is the cross section of the bar that the pressure is working on, there is not any piston head. For example a 1/4" dia bar has a cross section of about 0.05 in^2 and at a fluid pressure of 400 psi is advanced with a force of 20 lbs.
The carrier tube is about 7 feet long with chuck end screwed into a flange which mounts inside the chuck; most power chucks have a liner piece that fastens in the front of the chuck to stop chips getting into the works, we just made the carrier mounting flange to take its place. The opposite end of the carrier tube is mounted in a pillow block with a second pillow block close to the end of the spindle; actually it is close to the actuator for the hydraulic chuck and because this actuator moves in and out to open the chuck nothing can be attached to it. Hence the pillow blocks for the carrier tube. The pillow blocks are mounted on a large frame attached to the machine and are adjustable so the carrier tube can be kept concentric and parallel to the spindle within about 0.005" over its length. The 7 feet long accommodates a 6 feet long bar; most bar stock comes in 10 or 12 feet lengths so they are simply cut in half. A 12 feet long bar feed takes up too much floor space.
The fluid supply to the carrier/cylinder is through a rotating seal at the far end. On the air powered unit which feeds an air operated chuck a simple lip seal is used because the chuck has to be stopped and the barfeed is only under pressure when it is stationary. The other two units both feed hydraulic chucks which can be opened while running so they have face seals that can take several hundred psi while running at over 1000 rpm. All three barfeeds can run at a speed of 4000 rpm and the full hydraulic units feed at 1000 rpm.
I do not have much in the way of drawings for these units; they were built directly out of my head with a few rough sketches. I have attached a couple of photographs showing the end support where the rotating seal is. If you cannot build a good enough image in your mind from my description I can make a few sketches but I have the feeling this is serious overkill for what you want. Have you ever seen any pictures of what was used on manual machines for barfeeding? This could be more along thre line of what you need.