Tuesday, February 28, 2006

X1 Mill Head Counterweight

It is somewhat commonly accepted that the head of the Micro Mill needs to be counter weighted so that the Z-axis screw isn't doing all the "heavy lifting." One thing that I found interesting though is the amount of folks on the 'net that will simply quote a McMaster-Carr part number for a gas spring or state that they are using a pulley system of some kind. I have yet to find someone that backs up their counterweight system with some empirical research. Also, I've seen folks using gas springs that are significantly longer than the mill's Z-axis. What's that about? I've also seen some very elaborate mounting systems to compensate for "out of square mounting surfaces" and the like. There's basically some pretty poor engineering going on out there. Scratch that! Some of this stuff should just be common sense.

So that other folks can benefit from my research and draw their own conclusions rather than simply accept mine, here's my counterweight narrative:

I just took a rough measurement of the head weight. This method won't work for everyone, and the "why" will become evident quickly! In order to measure the head weight after it was mounted on my bench, I had to get creative. Yes, I could have just yanked the whole head off and placed it on a scale, but there are a few problems with that. First, it is awkward and hard to get an accurate measure on bathroom scale. Also, it has a light covering of oil on various surfaces and I hesitate to slime the scale. Most importantly, I'm lazy and would rather not have to try to remount the head after removing it!!

The solution was to start by locking down the Z-axis using the gib screw. I then unbolted the Z bearing block. After placing a bathroom scale on the floor, I had my wife climb on and I took the reading. Not everyone's wife would be that accepting as being used as an instrument of measure, but one uses the tools at his disposal! Kudos to my wife for being a good sport! Anyway, I unlocked the gib screw and, after making sure the head moved up and down freely, took another reading. We moved it a few times and took further readings just to verify. After some cowboy-style statistical sampling, I figured it was a good enough data set. I locked the gib screw and sent my wife off to wash her hands!

The total weight of the head is 19.5 lbs. Frankly, I was really hoping it would be closer to 25 lbs so that I could use a 30 lb gas spring and get 5 lbs of upward preload to deal with some of the lash in the Z screw. I think I'm going to go for the 20 lb gas spring, thereby achieving almost a neutral weight and deal with the backlash in a more appropriate fashion at a later date. But wait: "surely you can find a 25 lb spring" you say. And to that I would answer, "yes, I can" but more important than further pursuing this inappropriate backlash fix, I have other reason for choosing a 20 lb spring.

I'm sourcing the majority of the hardware for this project from McMaster-Carr. Mainly due to the fact my local hardware stores suck. The ones that are open past 5pm just don't have a good selection (and I refuse to shop somewhere that closes their doors before most folks get home from work). McMaster is also cheaper in many cases. They are also excellent folks to deal with, ship quickly and provide one stop hardware shopping. These and other reasons have directed me to choose to limit myself to their selection. Of the gas springs they offer, the smallest available in a 25 lb size is 18 inches long fully extended and over 10 inches compressed. Not a truly good fit for what I'm going to do with regards to mounting.

This brings me to another design upgrade this project offers. Without having the Z gib very tight, the front of the head sags slightly. Not ideal when trying to plunge and end mill or do precision boring. To help with this the ideal solution is to get the counterweight force as close to directly below the center of gravity as possible. Anyone know where that is? I can certainly tell you where it isn't! At the lead screw, screw bracket/nut, or gib! Right where most gas springs are mounted! This is where the folks using pulley systems really have the design advantage. They can choose a mount point in a number of locations. I would do a pulley system, but they look gangly. :-)

The final solution (all part numbers are from McMaster-Carr):

Part Number
20lb M6 threaded end gas spring
9416K74 (2) 10mm steel ball sockets for M6 threads
9512K73 (2) 10mm ball stud 5/16-18 threaded

The idea will be to drill and tap two 5/16-18 holes. One in the base casting and the other in the Z axis bracket/nut. Once it is mounted I'll post pictures for that part of the project.

Regarding the Z axis lash, I had a thought: I think I'll cut the Z "nut" horizontally and add a couple screws to space it away from the other half. That would serve the function of putting a bit of preload on the nut/screw assembly. I'll follow-up with details when/if I do this.