Motor Mount Plate
5 Bears Home Homebrew CNC bench mill
Once again, we tackle a boring item that is actually fairly important to the longevity and speed of the motor/leadscrew interface... an adequate motor mount. All of the motors that I have collected so far have been NEMA 23-framed. Three of these came with the Flashcut system, and are rated at 110 oz/in stall, 330 oz/in continuous torque. I am not sure of the make - they are not labeled. The Flashcut servos come with a 1000 line Renco encoder, model RM-15. The five Parker compumotor servos are rated at 110 stall, 334 continuous torque, and also have a 1000 line quadrature encoder. These are all brushless DC motors. Three other motors I snagged from eBay are MCG brushed, Renco 1000 line encoder equipped, unknown torque but they appear beefy. I don't expect the NEMA23 motors to have any problems with this mill, as the torque requirements of the ways are low. All of the motors I have are among the most powerful NEMA 23's available, but if I were to start this project again, I would set up the end plates to handle both NEMA 23 and NEMA 34 motors. I could still adapt NEMA 34 motors if the need arises, but the installation wouldn't be as neat, and the X-axis, for example, would protrude above the table plane, while the Y-axis mounting would poke a bit above the front plate. The Flashcut servo drive is the larger 20 amp momentary, 12 amp continuous box, and could probably power a BP-sized setup, so the limiting factor now is the motors themselves. The other option that I have if I need more torque is to use either timing belts, or a planetary gearbox like those offered by Bayside, but for now I'd like to avoid that and instead attempt direct drive. The reason I machined these motor mount plates is simple; I couldn't find any off the shelf that I liked! If anyone out there knows of a company that sells off the shelf, quality mounts, ready for Nema 23 motors, please email me. The ones I did find either were too light, difficult to mount, or had no register for the motor boss.

The idea with any motor mount is to create one that is both rigid and accurate. I will be using Oldham couplers, which will absorb some misalignment, but with a theoretically ideal inline mounting, the motors will run better, and there will be no wasted energy, or wear on the bearings and leadscrews. The Oldham couplers are nice... they allow one to remove just the motor from the mount, without disconnecting either the ballscrew or the motor portion of the coupling.

The print here is my simple design for a strong, rigid mounting. It will allow any length of coupling by simply changing the lengths of the standoffs.

Since I am eventually building 2 mills, I elected to make four plates, 1ea. for the X and Y axes. The Z-axes will probably use a different method, so for now I am ignoring the Z. Four sections of 6061 aluminum were face milled and generally trued to 84mm long by 60mm wide. Care was taken to be sure the plates were truly flat.

The first step was to bore for the motor mount boss, which is 1.500" in diameter. Two plates at a time were mounted to save setup effort.

A 1/2" hole was drilled through the two plates to start...
... followed by plunging a 3/4" end mill through. I have no drills larger than 1/2", so this at least got me started.
The boring began with a manual Criterion-style head and a heavy bar. A cut of 0.120" per pass is easy in aluminum. The surface "roughness" here is simply oil and swarf. The faces of the plates were previously milled to a nice finish with the Lovejoy face cutter.

Approaching 1.5", the telescoping bore guages came into play. I was looking for 1.501", +0.001", -0.000", for a nice fit to the motor mounting boss.

With the boss hole bored to spec, and without disturbing the setup, the various holes were started. For 90% of my work, I use as an origin a 0,0 located dead center in the work. For a DRO (Digital ReadOut) mill, this is definitely the easiest way to do symmetrical drillings like those here.

There are no layout lines, blue, or center pops. The mill DRO takes care of it all.

The four holes closest to the boring are for 10-24 cap screws to secure the motor to the mount. The four holes at the corner were drilled through and counterbored for 1/4" X 20 SHCS, which will ultimately secure the entire mount, plate + standoffs, to the mill. The remaining four holes, here already drilled, are for 10-24 cap screws to hold the mounting plate to the standoffs, creating a single unit from the three separate pieces.

Four 10-24 holes were tapped for the servo to plate mating, 23.57 mm symmetrically about center, which is standard for NEMA 23.

Tapping is easily done in the mill with a spring-loaded center in the quill. Using a tap wrench which has been center-drilled, the stubby tap handle is replaced with any handy rod offering leverage, here a ball-nosed hex wrench.

The pre-drilled hole is located X and Y, and the spring-loaded plunger inserted into the tap wrench to provide pressure. Started in this fashion, the plate can then be removed from the vise and the tapping concluded. The result is perfectly straight threads.

Of course if you have a tapping head, that will really speed things up!

With the holes appropriately tapped, drilled, and counterbored. the finishing touch was a 45 degree bevel along the long edges. This is a face mill with a 45 degree flank doing the honors. The proximity of the conterboring prevents a deep bevel, but even a small one really makes the plate look nice!
Cleaned up, deburred, coddled. The mill marks will be removed later, but this is optional, of course.
One of the Parker motors is inserted into the boss hole. Everything lines up and looks correct. The 1.500" boring also provides clearance to install and tighten any imaginable coupler.

From the other side. The plate is just large enough to mount, no larger. The thickness is perhaps 0.485" after milling, down from 0.500". 3/8" plate would probably do, but the 1/2" plate is nice and rigid.