Y axis ballscrew

5 Bears HomeHomebrew CNC bench mill

Due to the open nature of the extruded aluminum base structure, installing ballscrews is a snap, at least relative to a retrofit of a mill drill. When I had begun the retrofit of the HF mini-mill, I was stumped at the lack of room; only the tightest of ballscrews and nuts were going to fit. I came very close to ordering a set of Techno-Isel rolled ballscrews, but the ebay mill inspired me in this direction - a more open, completely shop-built mill.

Flange bearing housing of 7075-T6 c/w/ lid.

The THK ground ballscrews, as purchased from ebay, did not include the important fixed bearing support at the drive end of the shaft. THK sells blocks for these screws, but the cost is very high. I figured I could do almost as good by scratch-building some.

Steel would be best, but I executed these (3 ea) of FORTAL aluminum, which has a strength very close to mild steel. Fortal is essentially 7075; machines great, takes a high finish. I highly recommend the Fortal guy for selling a good product at a nice price.

A delrin ring provides protection from contamination; the bore of the ring is ~0.010" beyond the diameter of the ballscrew.

These blocks are a cosmetic copy of what are called flange bearing blocks. THK and NSK both offer them. I have 3 excellent NSK flange bearing blocks on hand, but I am saving them for the Mark 2 Bench mill. Thus, I was able to copy them easily.

The block is bored not quite through for the selected bearings. In the top picture is one of the angular contact bearings selected. I bought 8 of these from ebay (of course!), at a ridiculous price, something like $20 for $160 worth of bearings.

Angular contact bearings take loads both radially and axially. The latter is important to prevent any movement of the ballscrew axially under load. The bearings are situated face to face, and are "pinched" or preloaded between the ball screw shoulder and the hex lock nut. Again in the upper picture, it is hard to see but the bearing stands "proud" of the housing by perhaps 0.005". When the lid is secured, it clamps the outer races of both bearings firmly in position. Since the inner races are loaded by the ballscrew, the ballscrew is perfectly fixed into place.

This drawing shows the concept clearly enough... it was executed by QuickCAD, a $40 Autodesk product. The angular contact bearings and their races illustrate how the ballscrew itself is loaded through the inner races, with the races themselves supported by the balls against the stacked outer races. These in turn are fixed by the housing, being clamped between the lid and the bottom of the bearing well. Deep-groove radial bearings may be used instead of angular contact bearings, but can only handle lighter loads.

As the hex nut on the shaft is tightened, the inner races begin to press on the balls, which are forced against the sloped inner edge of the outer races. Note the spacer between the outer races; this permits the inner races to protrude slightly, if needed, from the nominal width of the bearing. More expensive matched duplex bearings do not require this spacer.

Slop is good!

The mating of the flange to the front plate. This is one area when you might be tempted to bore the plate for a nice fit to the flange - don't! What you want to do is have the ability to shift the flange on the plate a bit. I bored the flange hole in the plate oversized by 0.010". The mounting bolt holes in the flange likewise have slop. During final installation, this adjustment ability will allow for a stress-free mounting.

One thing I have noticed with these shop-made blocks vs. the commercial blocks... the commercial blocks may be loaded much more highly than this block, as the commercial blocks make use of angular contact bearings with a much steeper contact angle. Most ballscrew-purpose bearings have 30 to 40 degrees at the contact between ball and race, while these cheaper bearings have perhaps 10 to 15. This makes my assembly a bit stiffer in torque than the commercial blocks - not good, but acceptable.

Note the grease ports on the blocks... BenchMill MK1 will make use of these seperate grease zerks, while MK2 will have a central grease port for "one-shot" lubrication.

With the ballscrew mounted, I designed a nut carrier with consideration also given to final assembly alignment issues. The ballnut mounts to the plate seen here with 4 ea.8-32 screws. In turn, the plate mounts onto a heavy block with the 4 holes seen on the plate... 1/4"-20. The nut carrier plate is 1/2" thick, very stiff, and the block (not shown) is a heavy cube of aluminum which mounts onto the Y saddle plate. All are adjustable relative to each other, and no flex at all is evident.

Closer to the nut/saddle plate interface; contamination protection of the rails and the ballscrew will be an issue with which I must deal with at the close of the project. The heavy block connects the nut plate and the saddle plate in an adjustable manner.

Y-axis stroke is ~ 8 inches... not huge, but adequate since the table is only 6" X 18"

Standing back a bit... The base is assembled, the Y-axis ballscrew mounted (not secured yet), the 2 rails are in place with 4 blocks, and the lower saddle plate is set into position but not secured. In the foreground right is the heavy nut block previously mentioned. This entire assembly forms one precision axis; the X-axis has its own saddle plate. The two plates are sandwiched with 6 dowel screws, dowel pins, etc, to form a solid structure of accuracy. I will expand upon the "saddle sandwich" later!