RF-45 Repair – Reinforcing the bed

The RF-45 mill I purchased on craigslist a few months ago had some issues… In an attempt to convert the mill to CNC using an extremely oversized ballscrew, a previous owner had milled a pocket out of the bottom of the bed, leaving the material below the bottom of the center T slot uncomfortably thin. With the goal of restoring some of the bed’s original stiffness, I’ve fit a steel plate into the pocket. This post documents the process of machining the plate to fit the bed.

Here’s the bed as it stood before adding the plate:

View of bottom of bed showing material removed, and thin wall condition at bottom of T slot

I ordered the nearest size of 3/8″ thick 1018 steel from McMaster, which ended up being about 0.100″ too wide to fit in the slot, and several inches longer than necessary. After bandsawing the stock to the proper length, I needed to reduce the width. The best method I came up with for that was to do it in several passes using my mini mill. The workholding was a little sketchy, with two small vises supporting a piece of steel a little over twice as long as the mini mill’s bed, but it ending up working ok.

Reducing the width of the reinforcing plate

I had to re-position the plate 4 times to machine the full length due to the limited travel of the mini mill. I really can’t wait to get the RF-45 up and running!

After finishing the last pass on the mill, I still had a couple tight spots when fitting the plate into the bed. A quick touch with an angle grinder fixed that. The plate now fits the milled pocket pretty well:

Notice the mounting holes and dowel holes for the leadscrew support have been machined away. This will have to be addressed later…

Now I had to work on attaching the plate to the bed. I ended up using a bunch of M6 socket head cap screws laid out on either side of the T slot. I figure this should do a decent job of connecting one side of the bed to the other, instead of relying on the flimsy base of the T slot.

The first step was to transfer the location of the T slot to the plate. Precision isn’t really required here, so I just took some caliper measurements from the top of the bed, then transferred them to the bottom with layout die and a scribe:

I then layed out and center punched all the screw locations:

With all the soon-to-be holes marked, it was time to head back to the mini mill, operating as a drill press this time. I rigged up a cart to support one end of the bed/plate assembly, and rested the other end on the mill. I set the depth stop on the mill to the appropriate level, then drilled through both the plate and the bed with the tap drill size in order to ensure alignment between the plate and the bed:

All done:

After completing the hole pattern with the tap drill size, I enlarged the holes in the plate with the appropriate clearance drill:

It was now time to tap the holes in the bed to accept the cap screws. That’s a lot of tapping (34 holes in all):

After a couple holes, I began to wonder if there was a better way… I ended up putting the tap in a cordless drill, and adjusting the torque limiter to let the drill slip when the tap bottomed out. I was surprised, but it ended up working pretty well! You definitely have to be careful about breaking a tap with the extra weight of the drill, but the M6 tap held up fine this time at least:

The plate needed one more modification. I needed to provide clearance for the leadscrew support alignment dowels. The pocket in the bed had cut partway through the dowel holes, so I just slotted the plate to clear the pins. This required another funny looking work holding setup on the mini mill:

All that’s left now is installing all the screws. I ran them in with the cordless drill, then torqued them to spec:

I’m now finished with the first phase of the repair. Next up is figuring out a way to mount the left X axis lead screw mount. The mounting holes in the bed were machined away when the pocket was cut in the bed as mentioned above.