CNC mill oiling system – concepts
Monday, 25 June 2012
As I’m preparing to dismantle the milling machine for its major ball screw overhaul, I’m thinking about what else I want to do while it’s apart. One thing that most commercial machines have is a semi-automated way of oiling the sliding parts of the machine. Like most of the awesome mods that can be done to this particular mill, Hoss has already figured it out, so I’ll base much of what I do on his existing work.
The biggest component of this project involves modifying the saddle in such a way that it can deliver oil to each of the four X and Y-axis ways. A series of tubes (shown in blue below) deliver oil into holes drilled into the saddle (shown in red), which in turn come up in the middle of each way. The surface of the ways are then milled with a shallow “S” curve groove to distribute the oil across the surface. Additionally, the saddle holds the ball nut mounts so it can conveniently distribute a squirt of oil to each of the ball screws as well.
The Z axis ways will be oiled from the head, so it will have its own plumbing. But the two assemblies will have to meet on the column via some flexible tubing, then connect to the oil pump and reservoir.
I have some thoughts on implementing my oiling system in a more compact way than Hoss. There are a lot of tight spaces in which this will operate, and other things I may want to integrate into the same real estate like limit switches, way covers, scales, etc. Furthermore I feel that clear plastic tubes are useful for seeing that oil is present in the system but they seem a little too flexible for my taste, probably requiring that they be tacked down periodically to prevent them getting in the way. I’m thinking about using rigid copper or aluminum tubes that are bent into position, which seems to be the way most commercial machines do it.
I took a look at McMaster’s selection of fittings and decided they are too bulky for my application, and probably way overkill for the relatively low pressures my system will see (up to 7 psi). I started thinking about how small a fitting could get, and figured that soldered connections are about the best you can do. The trouble with fully soldering all the connections is that the plumbing becomes permanent to the machine, which is probably bad. So then I thought maybe the plumbing could all be soldered into a semi-flexible assembly that is then pressed onto fittings in the saddle (see the above sketch).
These fittings were starting to look a little fussy to machine, and in a high vibration environment like a milling machine I don’t like the idea of friction alone making a pressure connection (albeit a low-pressure one).
So next I thought about a hollow screw solution that would also serve to attach the fitting to the saddle. There would need to be a nice soft seal (like an o-ring) that could compress enough to allow aligning the hole in the screw (shown in green) to the hole in the sleeve (medium blue). Then I realized that if the screw had a shoulder that reasonably sealed to the inside of the sleeve, it could also serve as a flow control valve– useful for getting the flow consistent between the varying oiling points. The idea can also be adapted to a tee configuration, where there is free flow past the junction:
So I was feeling pretty good about myself when a coworker pointed out that I had basically re-invented a banjo fitting, which is a low-profile, high-pressure connection commonly used in brake lines. A banjo fitting has a donut-shaped reservoir around the bolt, designed to provide free flow at any orientation. So my design provides the flow adjustment that a banjo fitting intentionally avoids, which is a useful feature in my low-pressure system.
Oh well. I realized a long time ago that coming up with something that has already been invented is just means you’re on the right track.