Mini Mill Upgrades

After using my Central Machinery 44991 mill on a few projects, I decided to upgrade it with a belt drive system and air spring from Little Machine Shop (LittleMachineShop.com, along with digital readout (DRO) displays and a tachometer from Amazon. Since I was installing the belt drive and air spring at the same time, I reinstalled the motor on the headstock after I had drilled and tapped the holes for the air spring installation. Having the motor out of the way made it much easier.

Belt Drive Installation This mini mill uses a two speed gearbox to get the appropriate torque and speed from the PWM (Pulse Width Modulation) motor. I described the differences in the various models of mini mills in my previous article, "Mini Mill Overview" (August 2023). The gearbox uses plastic gears which are very noisy (I wear hearing protection when using the mill as is) and area known weak spot. The belt drive system uses stepped pulleys to get the Lo/Hi speed ranges and bypasses the internal gears.

The smaller pulley is installed on the motor and the larger one is installed on the spindle. The motor mount has an upper and lower plate; the upper plate, which the motor bolts to, pivots on the lower plate and has a locking bolt to adjust belt tension. The parts are well made and the installation instructions are easy to follow. There are videos available as well. I’ll cover the basics in this article.

The installation starts with unplugging the mill and inserting the spindle lock pin into the spindle so it doesn’t rotate. The set screw in the spindle nut is removed and the nut is turned clockwise to loosen; it’s a left hand thread. The set screw damages the spindle threads making it a little difficult to rotate the nut initially, so I cleaned up the threads with a file before reinstalling the nut.

The four socket head bolts that hold the motor mount to the headstock are removed and the motor and mount are removed from the headstock. I set the motor on top of the headstock and secured it with a bungee cord. Note the orientation of the motor on the mount relative to the power cable. The motor will need to be installed in the same orientation on the new plate. Remove the four screws that hold the motor on the mount. The retaining ring and small gear are removed from the motor shaft.

The gear may need a little persuasion to slide off. Keep track of the key in the shaft as it will be reused. Remove the screw from the plastic gear on the headstock and gently pry the gear off the shaft. Remove the key and install the plastic sleeve that comes with the kit on the shaft. The sleeve is a very tight fit and

Plastic sleeve installed on small shaft. Above center: New components installed. Motor has not been installed to provide clearance for drilling and tapping the holes. The motor is secured with a bungee cord. the instructions say to soften it in warm—not hot—water. I waved a heat gun (low setting) at the sleeve until it was pliable, and it slid on without much effort. The sleeve is intended to prevent the shaft from dropping into the headstock.

The shaft is retained by the press fit in the bearings and I doubt it will fall out, but you never know. Remove the metal sleeve from the spindle, keeping the key in place. Install the new lower plate on the headstock with the supplied bolts and install the pulley on the spindle with the smaller diameter groove on top. Align the slot in the pulley with the key in the spindle. Push the pulley down as far as it will go. Install the spindle nut, tighten it counterclockwise and install, then tighten the set screw.

The gearbox needs to be in the neutral position after the belt drive is installed. The instructions state that lever should be pushed against the plate to be in neutral. I found the neutral position by turning the spindle and watching the shaft with the plastic sleeve to see if it turned. Moving the lever until the shaft did not turn indicated the neutral position; the lever was about 0.3" from the plate on my mill.

When the motor pulley is installed, align the slot with the shaft key, which is the smaller diameter groove facing away from the motor. The key on the motor shaft was a little too tall and I had to file it down so that the pulley would slide onto the shaft. Install and tighten the set screw. Bolt the upper plate onto the motor making sure the power cable orientation is correct. I had to enlarge one of the holes with a 17/64" drill because the bolt was binding against the side of the hole.

Install the belt on the pulleys before setting the upper plate on the lower plate. Install the pivot bolt and tensioning bolt and set the belt tension. The hole for the spindle lock pin on the new plate is smaller than the original and the original pin will not fit. Instead, I am using the largest Allen key that fits through the hole. The plastic spindle cover is flimsy and had to be spaced out from the headstock to prevent the belt from rubbing on it. I will fabricate a better one in aluminum.

Air Spring Installation Most mini mills use a torsion spring to offset the weight of the headstock when raising and lowering it. The issues with the torsion spring are that it doesn’t do a great job in holding the headstock in position and it limits how close the spindle can get to the table. It also has a retaining collar on the right side of the mill column that prevents the installation of a DRO for the Z-axis. The

Above center: Belt drive assembly competed. The hole for the spindle lock pin in the lower plate is smaller than the original, requiring anew pin. air spring deletes the entire torsion spring assembly. The installation instructions are pretty good and there is a video linked to the Little Machine Shop website. I located and marked the hole positions on the top of the headstock, then drilled and tapped them. The bolts only have 0.25" thread engagement so I drilled the holes 0.75" deep and tapped them 0.5".

The diagram in the instructions is a little misleading as it shows the mounting holes centered on the slot in the headstock. The 60 mm dimension centerline should be located from the outside edges of the headstock, not the slot for the rack. The motor is normally in the way when doing this, so I did this task before I reinstalled the motor when doing the belt drive conversion. I marked the location for the hole in the rear of the column and drilled it. Anew rack for the Zaxis is included and replaces the original.

The new rack and relocated upper bump stock increases the maximum spindle height by about 0.6" and the air spring allows the spindle to get 1.5" closer to the table. Overall spindle travel increased from 7.4" to 9.5". A T30 Torx bit is required to remove and install the rack screws. Relocating the upper bump stop required drilling and tapping another hole on the front of the column. The upper bump stock was shortened to clear the air spring bracket on the top of the headstock.

Again, having the motor removed from Below center: The torsion spring collar is removed from the right side of the column and the spring assembly is removed from the left side. the headstock makes this much easier. The air spring is dropped down through the column and the nut is installed on the lower bolt, but not tightened. The air spring is installed on the headstock with the rod and brackets. When all the parts are in place, the bolts and nuts are tightened.

The torsion spring and mounting hardware are then removed. The collar on the right side of the column is removed first. The bolt assembly that rides in the slot on the left side is loosened and the spring assembly is pulled from the column. The cup that contained the spring is removed from the column. Note, the cross bar for the torsion spring interferes with the air spring rod and the nut on the back of the column cannot be fully tightened until the crossbar is removed.

The air spring does a much better job in holding the headstock in place. When you move the headstock, it stays in the new location.

DRO Installation I decided to install digital readouts on the three axis to make machining parts easier and more precise. There are several videos on You- Tube, and I picked up a few ideas on how to do it. I found some inexpensive magnetic-type DROs on Amazon that had the needed travel length. The brand I used is Shahe and they are available in 6, 8, and 12-inch lengths. I used the 12 inch for the Xand Z-axis and the 6 inch for the Y-axis.

The overall length of the DRO is about five inches longer than the effective measurement length, including the mounting brackets on the ends. The slider is 3.1" long and has tapped holes on the back to attach mounting hardware. Each DRO has a readout screen with on/off, inch or millimeter selection, and zeroing buttons. These are basic readouts and they don’t have the offset and other functions found on more expensive models.

These DRO units use two CR2032 batteries (supplied) and have magnets on the The slider shown in the middle of the bar is attached to the base with an aluminum bracket. back. I stacked the three readouts on a thin piece of steel attached to the wall behind the mill. I installed the Xand Y-axis DROs before writing this article so I can only show them as completed. The main task when installing the DRO is to ensure the magnetic bar is parallel to the axis travel so the slider does not bind on the bar.

The mounting brackets are slotted to help do this. On the X-axis, I installed apiece of one-inch aluminum angle to the rear of the table using existing tapped holes and at- Far tached the bar to the aluminum angle. The slider was attached to the mill base with a bracket made from thin aluminum sheet, again using existing tapped holes. The existing holes made this part of the installation much easier than having to remove the table to drill and tap them.

The aluminum angle covers the magnetic bar and slider which helps keep the chips off them. The hardest part was getting the accordion baffle reinstalled. The Y-axis DRO installation required drilling and tapping holes on the left end of the mill, in the base for the bar, and in the table for the slider bracket. The left side is readily accessible so this wasn’t too difficult. The bolt at the left rear corner of the base had to be permanently removed to make room for the bar.

Removing the torsion spring collar opened the right side of the column for the Z-axis DRO. After marking the new upper and lower

Far I placed them on a thin steel sheet in a readily visible location. travel limits on the column, the mounting hole locations for the bar and the required shape of the slider bracket were established. I used a gib adjustment screw and nut to secure the slider bracket to the headstock. With the slider bracket in place and the screws holding the bar to the column loosely installed, the headstock was moved through its full travel to align the bar with the travel and the screws tightened.

The tapped holes on the back of the slider are through holes and depending on the thickness of the bracket, the screws can bottom out on the magnetic bar, locking the slider. This type of DRO required spacers behind the bar mounting brackets to provide clearance for the slider mounting bracket and screw heads. Each installation is different, so some fitting is required.

I’m happy with these DRO units as they’ve proven to be accurate and they make life easier when moving the table and spindle to the desired position. The 12" DROs cost $40 each and the 6" one was $20. The sensor and magnet are on the bottom. Below center: The sensor, shown on the right at the spindle, is mounted on a bracket that is bolted to the headstock with the threaded bar from the torsion spring assembly. Twelve volt power supply plugs into the back.

Tach Installation The belt drive reportedly increases the max speed in the Lo range from 1,200 rpm to 1,700 rpm and from 2,500 to 4,300 in Hi range. I typically use the Lo range when milling. I decided to try a cheap digital tach from Amazon to measure the spindle speed. The tach uses a Hall effect sensor and a small magnet that attaches to the spindle. I made a bracket to hold the sensor at the spindle and bolted the other end to the headstock using the large, tapped hole and threaded bar from the torsion spring.

The magnet pole orientation is critical so I checked it for function before gluing it to the spindle. I mounted the readout in a small plastic electrical box and mounted it to the headstock using the screw and nut that held the pointer for the Z-axis scale. The tach requires 8 to 18v so I used a small 12v power supply that plugs into the rear of the box. The tach worked fine, the max speed in Lo range measured 1,865 rpm.

The readout doesn’t drop to zero immediately after stopping the spindle; it takes several seconds to zero. Starting the spindle again displays the correct rpm. The tach only cost $19 so I really can’t complain. The mill is considerably quieter with the belt drive (I don’t have to wear hearing protection when using this mill anymore) and it should be more durable. The extended Z-axis travel is very useful and makes setups easier. The DROs are well worth the time and cost to install them.

These upgrades were relatively easy, not overly expensive, and made a marked improvement in this mini mill. These upgrades will fit many brands of mini mill of this size. The LittleMachineShop.com website has compatibility lists for the belt drive and air spring. AG