Motor Standoffs, Couplings
5 Bears Home Homebrew CNC bench mill

With the more difficult motor mount plates complete, we need to create some fairly precise standoffs. The standoffs are milled from 1/2" plate, and must be matched as pairs in height so as to mount the servo plate perfectly parallel to the X or Y axis end plates as needed. While all servo couplers will absorb a certain amount of misalignment, why waste torque and slow things down with a sloppy installation? The only thing that I could have done differently with this particular style of mount is to use dowel pins between the standoffs, servo plate, and mill end plate. This would have required 4 minimum (8 optimum) dowel pins per assembly, and was rejected as overkill.

The couplers I chose were these Oldham style zero-backlash couplers from MSC. I use MSC for most everything; they cost a bit more, but their stocking is phenomenal, and their customer service is excellent. In a pinch, I can have stuff at my door in less than 24 hours, and even normal UPS ground tends to find its way to me in 2 to 3 days.

These couplers have an imperial bore measurement. For this mill, I have a variety of shaft sizes to adapt, including 1/4", 8mm, and 3/8". To deal with the 8mm shafts (the THK ballscrews) I decided to bore out a few 5/16" (7.93mm) couplers.

These were chucked, and the original bore indicated for zero runout. A tiny, sharp indexible boring tool opened the coupler to 8mm ID. All of the couplers chosen were of the clamp style, meaning there are no set screws to mar the shafts. The entire hub is pinched down to firmly grip the shaft.

The boring tool is a Circle-T micro boring bar. Having a miniscule indexible tool like this on hand is very handy indeed. Go for the carbide shank, it is worth the money, especially in this size where the tiny 1/4" shank is otherwise prone to flex and chatter.

Oldham couplers can be used with a variety of inserts, such as hard rubber, acetal (shown here), nylon, etc. Each has its own characteristics. Acetal inserts exhibit stiffness and zero backlash. The aspect of oldham couplers I really like is the fact that motors can be removed instantly, swapped with other motors, etc, as the dimensions of the inserts are common throughout a given series of shaft couplers. The entire assembly is shown here, consisting of 2 couplers and one insert.

The ballscrew coupler remains in place. If desired, the motors may then be swapped with ease, even if the motors have different output shaft diameters, as the motors carry their own coupler. It is the middle insert that makes this possible. Great for prototypes and experiments.

Some measurement revealed the lengths of standoffs required. I made the standoffs long enough to handle both the Flashcut-supplied servos and the Parker servos. Careful measurement here is important so as to make the standoffs long enough to handle any motor you wish to mount.

First, a long "sandwich" (2 plates) was milled to dimension. Sandwiching saves work, setup time, and produces dimensionally identical parts. Once again, the excellent Lovejoy face mill does the honors.

This stock was cut to the required lengths, drilled through for the necessary 1/4-20 main hold-down bolts, and drilled/tapped for the 10-24 SHCS which will secure the standoffs to the motor mount plate, creating a single unit.

In place on the Y-axis end plate. The mount is accurate and will minimize misalignment between the servo and the ballscrew.

Odd, the photo looks crooked or warped, like the assembly has been slightly melted.

Likewise, the entire X-axis ballscrew assembly becomes a single unit. From left to right, we have the servo mount, the X-axis endplate which bolts to the table, the black bearing housing, and the ballscrew and nut with carrier.