I'm building a trailer for my HF mill and found 2 sticks of 3x4x7/16" x 20' rail for $250 total. Needed a wider track groove on my rollers. When I chucked the HF wheels onto my Clausing lathe, I discovered the thickness of the HF wheel is inconsistent. One end is 0.012" thicker than the other -- they wobble at RPM. The groove OD and the bearing are concentric, but the roller thickness is wonky. The steel is pretty soft as well.
HF wheel / roller info:
- The axle is a M20x100mm bolt
- Overall size is 3.5" OD x 3" track OD with an H-type groove to accommodate the 5/16" (8mm) rail supplied with the mill.
- Width of wheel is 1" (25mm).
If you're looking to replace your HF sawmill gantry roller/wheels, there isn't much out there. Hud-son makes a $25-$30 roller (p/n 976-019) that's 3.5" OD and has a 0.33" groove (would fit HF's 5/16" rails), but their bearing is for a 3/4" shoulder bolt (19mm).
This looks like it will work for the original HF sawmill, but would require to swap in a non-standard (but available) 20x40x12mm bearing to fit the HF metric bolt. They've been making this same roller for 20yr.

Cooks sells some that are small. Also some on ebay that are small. "Track rollers" are overkill and priced as such.
Best place to find other alternatives is searching Amazon for a "sliding gate wheel" or "sliding gate roller". These have a pretty decent load capacity and are typically equipped with standard sized sealed roller bearings. Most of these wheels are V-groove. Look for items that list or show the bearing # to make sure either your M20 bolt will fit, or if you can swap the bearing out to a larger ID. Most bearings with a 20mm ID are either 35mm OD or 52mm OD.
-Dan