Emco F3 Z Axis Gas Strut

[fastbike]

While I'm waiting for some parts to arrive for the CNC conversion of this machine, I have added a gas strut to the Z Axis. As the design is a knee mill, where the whole table rides up and down on the column, there is a far amount of weight to lift so I decided some assistance would be a good idea.

The attached photo shows the lead screw for the Z Axis with the shrouding removed. The nut is stationary, being bolted to the bottom of the frame. The top of the screw is secured by a thrust bearing into the casting that supports the X axis and the table. A crossed helical gear set is driven by a large handwheel to raise and lower the table.



You can see at the bottom of the picture two round tubes (one partially obscured) that are set into the base. These tubes prevent coolant from entering the base of the machine. The visible one allows the lead screw to move as the table is lowered. The partially obscured tube is unused.

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[fastbike]

My thinking is to mount a gas strut to the underside of the table and support the other end with a bracket.

Initial rough calculations indicate approx 100kg of force will be needed (approx 1/4 of the 450kg mass of the machine is contained in the table/X axis).
The Z axis range is 350mm.

I have found a suitable gas strut online at https://www.comac.co.nz/modules/SP_w....php?cat=1546?
The strut has 120kg of force with 360mm stroke and 840mm length, 23mm diameter.

Fastening to the underside of the table can be secured with an angle bracket and balljoint. There are existing holes tapped M8 that are in approximately the right position.
The base of the strut will be supported via a clevis that is mounted inside a large threaded tube which fits within the spare hole into the base. This is required to fit the length of the strut.
The threading on the tube will allow preload to be applied to the strut to compensate for the extra room required for fitting.

[fastbike]

The tube in the base has an ID of 48mm. I have some medium wall tubing left over from another project: 42.4mm OD with 3.3mm WT.
Consulting standard metric threads shows M42 is the nearest. A standard coarse thread is 42x4.5mm (pitch), but the threads will be too deep for the wall thickness of the tubing I have.
Fine threads for M42 are 42x2 and 42x3. I can source nuts for M42x2, although they are expensive (US$27 each) and will take a while to arrive.
Time to have another think about how to do this.

[fastbike]

I have cut a M42x2 thread onto the pipe. As I do not have a pipe centre, I held it in the 3 jaw chuck and indicated it to get it as true as possible and then fitted a lathe steady near the end to support it while I took a skim cut off the outboard end. I re-centred it and refitted the steady to run on the section I had skim cut.
I turned the outside diameter to 41.8mm which is within the bounds allowed for the major diameter.
I used the technique demonstrated by Joe Pie to cut a thread starting from the chuck and working out. This required two grooves to be cut for the start and end of the thread, these cut back to 39.6mm diameter which is just under the larger tolerance for the minor diameter.
The actual thread was cut using the thread cutting tool fitted upside down, cutting on the front of the pipe, with the lathe running in reverse. Both the chuck and the threading lead screw rotate in the same direction so a right hand thread results. My lathe has a threading indicator dial, which the manual says to engage on the 16T pickup for a 2mm pitch thread.

[fastbike]

The nut was cut from a short piece of 100x12mm flat bar. This was held in the mill, vertically against a right angle fixture, and the top cut with a face mill to provide a reference surface. A series of cuts followed where the reference surface was indexed against a large external hex tool (used to install/remove bike bottom brackets) to create the 60 degree cuts required. The finished nut blank was 72mm across the flats. The blank was mounted in the three jaw chuck in the lathe and drilled / bored to get the minor diameter required. An internal thread was then cut into the nut.

After cutting the pipe to length and with a small amount of cleanup, here is the finished result.

[fastbike]

Fitting the strut to the bracket and the mill was relatively straightforward. The angle bracket at the top required the 6mm clearance holes to be enlarged to 8mm.
The threaded pipe has a 10mm hole drilled through near the base for the clevis pin to rotate in. I used a short length of M10 threaded rod, with the clevis bracket centred between two nuts.

Here is the gas strut attached to the clevis at the base of the threaded tube.


Here is the gas strut fitted, showing the mill table fully lowered


and with the mill table fully raised.

[fastbike]

One small hitch ... I cannot refit the sheet metal that shrouds the mill's column and prevents dust and swarf from contaminating the Z axis lead screw. The hex nut I made is too big.

I need to either make a new circular nut slightly bigger than the spigot in the mill base, or turn the hex nut down to have a smaller round outside profile. Given the amount of work cutting the hex I am tempted to make a new nut

Regardless, the gas strut makes a huge difference in the effort required to lift the table. This should allow a smaller stepper motor to be used when I convert the Z Axis for CNC.

[fastbike]

I hit another snag yesterday - I broke the carbide tip for cutting the internal thread in the nut. So it is time to move back to the Y Axis mod on the mill - the 20mm round bar that I needed has arrived.

[fastbike]

With the Z carriage removed from the machine for milling the X-axis ballnut housing, it is the perfect time to drill the correctly aligned holes for the upper strut mount. The centre of the strut needs to be 75mm from the face of the Z dovetail way and 40mm inboard from the left side. Adjusting for the offset from the ball joint attachment bolt, this becomes 60/40mm. Drilling cast iron is very different from steel, and tapping the internal thread proceeds very easily.

Here's the bracket in place, waiting for reassembly of the Z Axis ways.


Compare the placement of the bracket to the second photo in post #6 above.