Mill Z Axis gas lift

[picclock]

VM32B mill Z axis lift for future CNC conversion. I am not sure if this will pan out but the idea is to use two pneumatic air cylinders either side of the column with hangers attaching to the Z Axis These will extend around 450mm. They will be connected to a pressurised vessel, 3-400mm of 6" steel pipe, which will be pumped up such as to give a minimal load for the Z axis ballscrew which I will be fitting. Will probably use 20 or 25 mm cylinders with a working pressure of 80 or 50 psi.

The base of the cylinders will be connected to a plate secured to the base of the mill. Bearings at all three ends should keep everything sweet, and lifting from both sides keep it true.

I figure that if I can reduce the apparent weight to about 10lbs this will minimise wear on the ballscrew and allow fast(ish) movements of the z axis. Also possible to overdrive such that the head will 'fall' upward but can see no benefit in this.

Any ideas or thoughts much appreciated.

Best Regards

picclock

[philf]

picclock,

Why the pressurised vessel?

I used pneumatic cylinders to counterbalance safety guards on various bits of equipment but only used a relieving pressure regulator. I can't see the point of a pressure vessel. (Or are you thinking it will stay at pressure without running a compressor?)

Cheers.

Phil.

[picclock]

Hi PhilF

Pressure vessel is to maintain constant force from cylinders regardless of position.  Change of force over extension distance is ratio of displaced volume to total gas volume ~ 2.5% min to max extension.

Once pressurised only cylinder seal leaks need to be compensated for - probably top up pressure every3-6 months depending on how good the seals are.

Best Regards

picclock

[philf]

OK - I understand your plan now relying on minimal leakage.

Having now seen your drawing what I don't understand is the linkage between the end of the cylinder rod and the vertical slide! Unless I'm missing something obvious the link will always end up in line with the cylinder rod if all three points are supported by bearings allowing free rotation.

Phil.

[picclock]

Hi Philf

The linkage is because the shortest height of the cylinder is larger than the attachment point distance to base. The overall length of the cylinder without extension is about 4" greater than the top of the z axis column slide at its lowest position. This is the part I have some concerns about as I don't want any bending stresses on the cylinder.

Best Regards

picclock

[philf]

In which case why not make the linkage fixed solid to your z-axis and offset so that the cylinder is always parallel with the slide?

[picclock]

Hi Philf
That was my first thought, just a hanger off of the z axis head plate. But I wasn't sure how the turning moment would affect the head movement as it is applying a clockwise moment exacerbating the large turning moment caused by the head. It may be that I can align the base of the cylinder close enough that the effect will be of no consequence. Using the linkage the force will exert a slight counterclockwise moment.

In many ways the over the column with a pulley trick is superior as the head could be supported at its centre of mass. The snag with that is that it makes it impossible to rotate the mill head, consequently the z axis head plate is the only place to attach to.

As an alternative I suppose it would be possible to use a single cylinder with a pulley and a steel cable over the top of the column. The only reason for using a cylinder instead of weights is to reduce the  acceleration mass for the Z axis.   

I have not yet received the mill as I failed to mention I needed R8  , consequently it will take another couple of weeks. The Z axis will be the first thing I alter. As std the machine has a gap of 150mm at the lowest head position to the table, which is too large for stuff mounted directly to the table. I will increase the size of the column cut out to give an extra 100 mm of z axis travel, and that combined with a ballscrew and stepper will give me easy movement of the axis. The next thing which I will do is motorise the X and Y, which I will probably do both together. It seems to take about 150 turns end to end so I think that will get old very early on    - battery powered drill to the rescue I hope.

Best Regards

picclock

[picclock]

Having thought about it a bit more I think the following ticks all of the boxes. The bending forces on the column are minimised and there is little additional mass to accelerate. A single 32mm cylinder should do the job. It looks a bit crude but the functionality should be excellent.

Best Regards

picclock

[philf]

Picclock,

Looks better but...... the Z-axis on my CNC (which is based on an Alexander engraving machine) lifts the knee and X-Y table via a 2505 ball screw and I don't have anything to take any weight off. Are you sure you'll need to do anything? Much heavier machines (e.g Bridgeport size) lift the knee and table without cylinders, counterbalances or gas struts. The other thing to consider is that, unless you spend a fortune on zero backlash ball screws, balancing the weight of the head will mean that you may see more problems with a counterbalance than without. Just my opinion - I'm happy to be proved wrong.

Phil.

[Brass_Machine]

Hi Picclock...

You second drawing is similar to a design I saw that I wanted to emulate for my mill. He had hid his inside the column.

I will see if I can find the link.

Eric

[BillTodd]

The mass of the Z axis is necessary to counter the verticle cutting forces  and add preload to bearings and screw. None of  cnc machines I've worked on have any kind of counter balance - all have braking on the z motor to prevent the head falling if the drive fails.

I wonder if you should lash something up as an experiment before you spend too much time on it?

Bill

[picclock]

@billtodd
The mill head weighs around 40kg. Downforce needed for drilling/milling is a lot less than this 10kg tops and likely zero or -ve for side milling I would guess. By counterbalancing the head the bending forces on either side of the column are nearly equal. This should result in reduced flexing for different head heights. I'm quite tempted to balance the head such that it is driven down by the motor. Because of the simplicity of the setup and ease of adjustment it will be easy to see if any benefits accrue from this.

@brass_machine
It crossed my mind to do that but I'm happy to leave it outside like this. Its  easy to belt drive the ballscrew from a motor behind the head.

@Philf
The issue with these "benchtop" mills is their lack of rigidity. With larger industrial machines you are not going to see much deflection when the knee is in a different position. With a column mill the bending forces and flexing which occur due to the head weight are significant. This will go a long way to eliminating this.

Best Regards

picclock