Swarf Guarding - Part II, Y-axis Rear and Skirts

5 Bears Home Homebrew CNC bench mill

With the front Y-axis bellows completed, the rear of the Y needed similar attention. This is a fairly critical area, as the addition of bulky components back there can bind and/or further limit an already small Y-axis stroke. I was fortunate in that this bellows compacts so effectively, meaning a full 6 to 8 inches of bellows will collapse down to perhaps 3/4". The aluminum mounting brackets and flanges also needed sound engineering to keep them compact, yet effective.

My original and very crude rubber swarf guard was one cannibalized from a Harbor Freight Mini-Mill. To mount it, I machined a simple 6061 aluminum bracket which mates with the vertical column between the column's side plates, and flush against the column's face. This bracket carried a pair of 1/4" SHCS which make use of removable T-nuts for secure attachment to the column face.

Fortunately, I was able to make use of this bracket with the new Gortite bellows section. Like the front bellows, the inside edges of this section were carefully shaped with a Dremel sanding drum so as to fit the shapes of the THK HSR-25 rails.

In this picture, the bellows is secured to the bracket, which is currently not secured to the vertical column. Note the gap between the rear bracket and the column section... this will vanish as the bracket's T-nuts are tightened.

The forward sheet aluminum mounting bracket has been cut and drilled for a series of 8-32 stainless BHCS (Button Head Cap Screws). The tapped holes for the BHCS were formed in the aft edge of the saddle sandwich.

With the rear, heavier mounting bracket still not securely attached, I was able to check the fit and function by simply grabbing the forward bracket, and holding it against the lower saddle sandwich plate. I could confirm that the bellows would do what was needed!

The rearmost bracket was positioned vertically in the correct location, and secured. The front of this bellows section was then mounted, via its aluminum bracket, to the saddle plate.

Note that the bottom edge of the bellows section resides just slightly above the Bosch 90 X 45 structural aluminum members which carry the HSR-25 rails.

With both the front and rear Y-axis bellows sections installed, I quickly realized that a large portion of the Y-axis was still exposed! Note here the exposed pair of HSR-25 trucks... while the X-axis would protect this somewhat, I knew from my experiences with the fine swarf that this machine generates, it would soon become polluted, rendering the bellows meaningless. I needed to protect this area on both sides of the mill.

My choice of HSR-25 wide trucks early in the mill's construction forced me into an unusual situation. Take note of the edge of this saddle plate, with its four rather large SHCS. These screws secure the plate to the trucks, and to make it all work, the counterbores I cut for these screws extend over the edge. When the plate is secured to the trucks, the SHCS are exposed a small amount, and they protrude perhaps 1.5mm. I point this out as this protrusion greatly increased the complexity of what I ultimately called the Y-axis saddle skirts.

In retrospect, having four HSR-25 trucks for this mill's Y-axis was total overkill! HSR-20 or even HSR-15 would be entirely appropriate. Oh well. I'll take overbuilt to underbuilt any day!

The right-hand skirt was constructed of sheet aluminum of 0.064" thickness. If the HSR-25 truck mounting cap screws hadn't protruded, the skirt would be as simple as securing the sheet to the saddle plate. But since they did protrude, I was forced to elevate the skirt off of the sandwich plate.

Both right and left skirts were engineered so as to be attached to the upper saddle sandwich plate. A simple strip of 0.125" aluminum was mounted using a series of 4-40 BHCS to the raw skirt. This assembly was milled and cleaned up as a unit, and two further holes were drilled for 8-32 clear. These holes ultimately secure the skirt assembly to the upper (X-axis) sandwich plate.

The notch in the upper edge of this right-hand skirt was cut to clear the X-axis ballnut. The left-hand skirt did not require this notch.

Jumping forward somewhat in sequence, after the swarf guarding was essentially completed, I snapped this picture to show the left-hand skirt in its final form. You are looking upwards at the bottom of the X-axis from low and slightly left of the mill.

Visible is the simple support end of the X-axis ballscrew. The split bearing clamp was machined from aluminum. Given a choice, a split clamp like this is always preferable to a set screw or any other method of securing the simple-support bearing, as it avoids marring the OD of the bearing. It's also more elegant!

One additional set of notches was cut in both skirts, seen here lower right. These notches clear the mill's front plate when the Y-axis is furthest forward. I intentionally left these skirts a bit tall, as that would provide better protection than if I had trimmed the bottom to match the bellows.

Both skirts were finished with 400 wet-dry paper and 0000 steel wool after this picture was taken. The combination gives the aluminum a nice, slightly brushed satin finish.

To execute the Y-axis bellows, both front and rear, the X-axis was earlier removed from the mill in its entirety. With this particular design, that process is quite simple, taking all of five minutes. The X axis, with its sandwich plate, is secured to the lower plate using six precision 5/16" shoulder bolts. Once these are removed, the entire X axis, c/w upper plate and servo, can be lifted off of the mill.

Inverted on my workbench, I inspected the X-axis linear motion hardware for any problems. I was interested to see if swarf had managed to migrate into this sensitive area, which houses two SHS-15 rails, four trucks, the THK class 3 ground ballscrew, and the ballscrew's associated bearings.

I was pleased to see that the rails and ballscrew were as clean as they day I finished the mill. The lube was still in place. Both rails and ballscrew were pristine.


You can see the light grease still thoroughly coating the rails. The blob of grease in the right of this photo is actually the grease zerk on one of the SHS-15 trucks.

Beyond a light cleanup and additional grease, nothing further needed to be done with the X-axis. I did take some time to refine the X-axis limit switches to deliver another 1/2" of travel... the original installation was intentionally conservative.

So far, these modifications have taken more work than I had anticipated, but it will be well worth the effort. The next installments will focus on the Z-axis, which was by far the most labor-intensive.