After much procrastinating, I've finally decided to physically do something about getting some CNC functionality in my workshop.

The first candidate, is a Harrison Knee Mill, that I've had stood doing nothing for the past couple of years. It's originally an Auto Cycle machine, which means it already has a ballscrew fitted to the X-axis. I don't yet know if this will be useable or not, as I do know that there is play in the X-axis, but I'm not sure if it's in the screw, or the nut mounting.
The mill is essentially a horizontal knee mill, which came with the option of a vertical milling attachment. I've only got the one vertical head to share between two mills, so it'll be remaining on the working mill until this mill has been cnc'd.

At the current stage, I've removed the table, and knee. The knee/table assembly will be getting weighed, and a suitable air cylinder + regulator fitted to act as a counterbalance. The knee itself is going to be the Z-axis.

At the moment, the current plan is to use servo's, but this may change depending on what motors I find.

I've attached some pictures of the mill. The first is the mill before any work commenced. The secon and third are the mill in it's current state.

I'll post up a exploded view of the z-axis nut design, with explanation later.

that looks like a very nice size mill for conversion.

how does the air cylinder counter balance work, does it keep a slight upwards pressure on the screw, and if so since air is compressible, what keeps it rigid? i'm afraid the ball is slow to drop on this one for me.

Mcgyver - The air cylinder counterbalance works by pushing the knee upwards, reducing the load placed on the ball screw. The knee height will still be precisely controlled by a ballscrew.


Here's the cad drawings of the z-axis set-up. Due to the design of the knee, and the lack of room to mount a motor/belt, I've had to opt to have the ballscrew fixed to the knee, and the motor spinning the nut.
Blue = ballscrew + nut (No thread shown as I couldn't figure that feature of Alibre Design out - anybody care to tell me in simplified terms???)
Red = Toothed belt pulley (dimensions are for a 80t)
Yellow = Bearings (in this case 7210 singular angular contact bearings)
Grey = parts to be machined (except the bolts obviously)

The mounting plate (which will bolt to the machine and also have the motor mounted on it), will be clamped between the two bearings. The adjustment of the bearings will be done via shims between the spacer and the machined tube that the ball screw nut is mounted into.

This retrofit is off to a bad start :(

Went out this afternoon to put the knee back on, so I could measure it up and get the ball screw and nut ordered. However, after putting the knee back on, I realised there wasn't going to be enough room to mount the ball nut the way I wanted.
I had been looking at my good mill, where there is plenty room when the knee is down, and thought this one would of been the same. I knew the two mills had different bases (good one is a newer welded steel base version, whereas the other one is an older one with a cast iron base), and the old mill had an inch less knee travel, but I didn't think they were that drastically different.

I could of hacked the cast iron boss of the base, and then bolted a steel plate on to mount the nut, but I've decided I'm going to build the old mill up into a useable manual mill, and then retrofit the newer mill. This gives me the benefits of a steel base which will be easier to modify/add bits to if need be, and the cnc mill will have the extra inch of clearance under the spindle.

I'll post up some new photos tomorrow to highlight the differences.

Here's the photos showing the difference in the bases.

The old cast iron base has two raised bosses, one where the mill body bolts on, and the other where the z-axis nut bolts on. When fully lowered the knee comes down below these bosses.

The new welded steel base has only the one plate steel boss (looks about an inch thick), which the mill body and z-axis nut bolts onto, and the knee rests on the same plate when fully lowered. Also the mill body is considerably higher, which gives over an inch extra clearance under the spindle.

I'm now of to rebuild various bits and get the old mill fully functional, so I can start retrofitting the newer mill.

Not had much time to play with the milling machines the past week, but I have got the old mill partly re-assembled.

I did get a pnuematic cylinder for the counterbalance of ebay, and have been looking at ways of fitting it in. Due to the size, I think it's going to have to go on the front of the knee, with a section cut out the front of the base for it to fit in close enough.