Final Assy - Part I

5 Bears Home Homebrew CNC bench mill

The mill is now a large gathering of parts, motors, assemblies, subassemblies, carefully arranged in a corner of my shop. Different sections were previously assembled, tested, disassembled, refined, in a continuing cycle. By now, I was ready to put the mill together, aligned, in a somewhat permanent sense, meaning I no longer wanted to tear anything down if it could absolutely be avoided.

The box which forms the base of the mill, and the Y axis, has been together and aligned for quite some time, all the way back to the milling of the Bosch 4590 members, and the mounting of the rails. There was never a need to disassemble the box up to this point, so I was fortunate in not having to realign the Y-axis. The Y ballscrew assembly can be removed from the base with ease if the fixed support end is disassembled first. Note that if the Y-axis nut carrier had not been removeable from the ballscrew, this would not be possible.

Rear view; column to backplate
The Z-axis column was lifted and inserted like a fence post into the nest created by the base. The column was complete... the rails were mounted and aligned, and the ballscrew and its backplate were already installed. Despite being made of aluminum, the entire column, being 800mm in length, was heavy and quite stout.

This view is of the back side of the mill. Most obvious is the backplate. The 4 heavy cap screws secure the backplate to the two Bosch structural members, and are 12mm metric. The six smaller screws are 5/16" SHCS. These provide the bulk of the securing power for the column, being routed to two steel T-nut strips which are inserted into the column's t-slots. The Bosch structural members have a nice friction fit, pinching the column on either side, while the column's 1" thick aluminum sideplates also bear on the Bosch members. The whole setup exceeded my expectations for being both rigid, and self aligning.

Front view; base, Y axis saddle plate, column
The mill is turned around and the assembly continues.

The Z column with its two NSK heavy rails is obvious, as is the longer, heavier 16mm Z-axis ballscrew. Just below the simple support end of the Z ballscrew is the aluminum block and ball bearing for the Y-axis screw simple end. It had to be installed before the column went in, as there is no way to install it afterwards. It is free to slide up and down the column within its immediate area, as it too is secured with T-nuts. This is a huge advantage in using these Bosch members, as final alignments can be made with ease.

The Y-axis sandwich plate has been secured to the Y-axis blocks, which are four ea. THK HSR-25 blocks; hugely rigid and very effective. I could with ease stand on that saddle and dance a hornpipe without damage, except I'd be kicked off as the saddle moved like a puck on ice!

It looks as if the entire mill is unstable and possibly tip to the left or right. I was concerned about that, and each front and back plate has heavy tapped holes to potentially extend the base plates with some type of "outrigger" member, but so far there is not the slightest tendency to tip, even with the X-axis table fully left or right.. I think it'll be fine as it is.

The Y-axis ballscrew, nut carrier blocks, and limit switches go in. The Y axis system uses two aluminum millings to join the ballnut to the saddle plate. One of them is already attached to the nut, and can be seen inside the base, canted to the left. The other, in my hand, is a simple but heavy block which joins the previous to the saddle. Note the hole pattern on the face of the block which matches the ballnut plate inside the base. Four 1/4" SHCS do the honors. Probably overkill, but the finished junction (ballnut-plate-block-saddle) is very rigid. It needs to be, so as to position a heavy assembly (the entire X-axis table, vise, etc), with repeatability, to within 0.0002".

All of the untapped holes which accept cap screws are intentionally large, to allow adjustment of the entire system in all directions.

Both ends of the Y-axis ballscrew for now have no bearings installed.

The Y ballscrew was temporarily and loosely secured to the saddle via the two carrier blocks. The cap screws were tightened just enough to keep the screw from flopping about, but the system could be shifted left and right, or up and down, within the limits of the previously mentioned loose mounting holes.
A better view shows the limit switch and the round button which will trip it, secured to the nut carrier. A grease zerk allows lubrication of the ballscrew. Note the plastic ballnut wiper inside the ballscrew. Both the Z and Y ballscrews will need protection from swarf.
It was time to assemble the angular contact bearings and the flange block to the Y-axis ballscrew. From the front of the ballscrew shaft, we have the locking nut, an angular contact bearing, a steel spacer, the second angular contact bearing, and the housing. (details)

Not shown is the cover for the housing. This was secured after the body was installed onto the front plate, but before the hex locking nut was tightened.

The bearing stack was lubed, assembled, hand tightened, and inserted into the bearing housing. The ballscrew was then "injected" into the mill, carrying the saddle plate rearward, so that the bearing housing could be mounted with 4 cap screws into the front plate. Finally, the housing cover was installed. The ballscrew was now fixed longitudinally.

During the insertion of the Y ballscrew, the simple support ball race block was loosened, and sat ready to receive the ballscrew journal.

The other end of the Y axis limit switch assembly can be seen here to the lower right, along with its cable.

The simple support block and race is self-aligning, meaning that once the rest of the Y-axis was tightened, and the saddle moved towards the column, the block shifted slightly on its own into the correct position. All that remained was to tighten the two mounting cap screws which secure the block to the column.


With the Y axis ballscrew inserted into the mill base, and the far end secure within its journal block, the various cap screws were sequentially tightened and the axis aligned. An Oldham coupler was then installed.

Now came the fun part - mounting the servo. It was presecured to its motor mount, and the mating Oldham coupler fixed to the servo shaft. Four long 1/4" X 20 cap screws secure the motor mount assembly to the front plate.

The Y axis is now essentially complete!

The easiest part remained... the entire X-axis assembly was inverted, and the two sandwich halves were joined with a series of six 5/16" precision shoulder cap screws. It took all of two minutes to do, since the X-axis had been assembled and tested seperately from the Y. The precision screws automatically align the X axis perpendicular to the Y.

In Part II, we continue with the final assembly of the mill.