Let me preface this by saying that I am still fairly new to CNC machines. I have been in the process of designing my own using Open builds parts for the last couple of months. Can't wait to get building it! The Ox youtubes really helped! Now here is the question, a friend of mine runs two cnc machines and they both use gravity on the Z axis, to pull the bit/end mill in to the stock. What are the advantages and disatvantages of using this method vs having a fixed acme nut block? For those of you who aren't following, the lead nut is placed under a block that is attached to the z, so in effect it is free to travel down the rod, but when it goes up the rod it meets the block attached to the z and therefor starts to raise the z. Hope this makes sense.