Emco F3 mill conversion Z Axis

The existing Z axis acme leadscrew is retained into the Z axis carriage by a pair of 42 mm OD angular contact bearings, with the acme nut being fastened by a bracket bolted to the base of the machine frame. There is a set of crossed pinions that connect to the handwheel to rotate the screw and raise/lower the carriage. Because there is some backlash in the screw and pinions, and the fact that the efficiency of the acme screw is low, I plan to replace the screw and nut with a ballscrew.

While I have the machine dismantled I have measured the layout of the lower face of the Z Axis carriage, this is easier to do bottom side up, rather than having to measure under the machine. These measurements may be required when I design and fabricate the parts to hold and drive the ballscrew.
Attachment 405746

I have two ideas in mind:
- mount a pulley on the shaft inboard of the bearings at the top of the screw where it attaches to the carriage. The pulley will drive the screw. This will require some machining to the screw to cut off some of the threads and extend the bearing area, and a new motor mount plate will have to be fabricated. The motor will move up and down with the carriage.
- fix the ballscrew to the carriage to prevent rotation, and fabricate a bracket for a rotating ball-nut. This bracket will fix to the machine base along with the motor and the ball-nut will be driven rather than the screw. This requires a lot more machining so is the second choice.

However these are just preliminary ideas that are parked here while I finish the X axis modifications.

I've worked out the layout for the Z Axis ballscrew, using option 1 above.
This will require making a new threaded fastening collar, as the existing one is too hard to access once the mounting plate and ballscrew pulley are attached. The thread is metric with 1mm pitch and ~43mm on the major diameter. It is easier to cut the thread on the lathe and check while the mill is still dissembled so this is next of the list of jobs before I clean and reassemble the Z carriage.
The 45mm diam 1214 easy machining steel round bar arrived today so this is the first job for the weekend.

I found a bit of shop time yesterday and machined the retaining collar/bolt from a 45mm 1214 steel round bar in the lathe. It has a thread to fit into the Z carriage, a 22mm bore to clear the ballscrew and a shoulder so it applies pressure to the outer race of the angular contact bearing.
Attachment 405934

And another view showing it screwed into the carriage
Attachment 405936

Once the mill is back together I can mill a hex for a thin 32mm wrench to tighten it.

I've completed more design work for the Z axis conversion. This requires a mounting plate for the NEMA34 stepper motor, which is bolted to the base of the Z carriage (this is a moving knee style mill).
The motor is then attached (standing vertically with pulley at top) to this plate using 16mm diameter stand offs to provide clearance for the drive belt between the plate and the motor head. I have completed the plan view of this, but need to do more work on the elevation/sections, and also the new ballnut housing that bolts under the machine column.
Attachment 474694
The ballscrew is fitted with a 28T pulley (5mm GT2) and the motor is fitted with 25T. A 66T tooth belt provides enough space to fit between the front of the dovetail and the rear of the chiptray, while allowing the belt to pass between the two stand off posts closest to the ballscrew.

This is a trickier piece to make so I'm waiting to get the X/Y conversion run G Code, then this piece will be easier to make on the partially converted (2D mill)
Attachment 474696

I can source a good quality 20mm x 5mm ballscrew and nut.
The stepper will need to be reasonably grunty as it is moving the whole knee up and down.

Calculations of the force required to move in the Z direction are as follows:
- the Z carriage is cast iron and is approx 20kg.
- the X axis carriage which mounts to it is another 50kg
- table mill table is approx 30kg
- chip trays etc are approx 10kg
- max permitted workpiece weight incl fixtures etc is 50kg
That is a total minimum of 110kg and maximum 160kg. I have added a gas strut to assist with the lifting, but I would not think this provides uniform force across its range (will provide more force when most compressed). As a lot of the machining will occur when the table is near the top of the Z movement, and the gas spring is extended, I will ignore any lift it provides when sizing the motor.
So 160kg becomes 1600N.
There is also the force required to overcome the friction in the dovetails, a reasonable estimate will be 100N.
That's a total of 1700N, and adjusting for ballscrew efficiency (say 80%) that will require max 2100N to be supplied to lift the table. Actual force will be less depending on what is on the table, and the position of the table.

There is a quick and dirty method at https://www.cnczone.com/forums/linea...lculation.html


Torque = Force x Lead
......... -------------------
......... 2 x Pi x Efficiency

Solved for
2100 x 0.005
------------------
2 x 3.14 x 0.8


Using the figures above this provides a size of 2.1Nm. However this does not account for the losses incurred by the drive belt, nor any safety factor.

I found a good online calculator at https://www.orientalmotor.com/motor-...ml#QuickReport
which gives more input parameters including the cutting force, belt drive and a range of safety factors.

This provides a basic motor size of 3.0Nm. However to get better acceleration will need a bigger motor.

I've done some prelim sketches for the bracket to hold the ballnut to the mill chassis.

First off, this is what the existing leadscrew, nut and bracket look like (sitting on my bench).
Attachment 474732

Then this is what the replacement part looks like, first off a side view. The green (top plate), orange (upstand) and blue (nut holder) are the three parts of the bracket, the other colours depict the ballscrew,nut, pulley and bearings.
Attachment 474734
And a front view of the bracket
Attachment 474736
And finally a plan view showing how it fits in with the Z carriage and the dovetail ways.
Attachment 474738

I still need to figure out how to fabricate this and join the 3 parts. I could TIG weld, although the heat will likely distort the parts so I'll need to partially machine, weld, then finish machine. This could be a bit tricky.
Or I could drill for 3x M12 cap head screws (bolts) and tap the top plate and run the bolts through from the nut holder plate, in which case I will recess the upstand into the two plates by 2mm for better alignment.

I just noticed that post #5 had included the efficiency of the ball screw, so the calculations in #6 do not need to include it (or use the lower force number). Reworking it then becomes

Torque = Force x Lead
......... -------------------
......... 2 x Pi x Efficiency

Solved for
1700 x 0.005
------------------
2 x 3.14 x 0.8

Gives a minimum torque of 1.7Nm, before safety factor, belt ratio/efficiency etc.

This now makes more sense when compared to the 3.0Nm provided by the online calculator at https://www.orientalmotor.com/motor-...ml#QuickReport

Following on from post #7, I decided to weld the 3 parts of the bracket together. I started by machining the plate that will hold the ballnut flat and square. I then cut a square edge onto the spine, bevelled the edges back (for the weld) and TIG welded the two pieces together. Being 25mm thick plate I set my welder to 200Amps, with a 1 second pulse, did a root pass and then added a fillet bead on the second pass. Some machining followed to true up the surfaces (a small amount of distortion had happened, and then welded the top plate onto the spine. I've cut the top plate to size and left the other oversize until the hole for the ball nut is bored. I can then shape that plate, and drill and counterbore the 4x mounting holes.
Functional rather than pretty !
Attachment 474820