Milling Machine Quill DRO and Backlash Eliminator!

This mill is heading slowly toward digitisation, eventually I want to motorise the X,Y & Z axes but to make this a workable option I need to address the issue of backlash. The most essential one in my opinion is the Z axis, the quill and spindle.

On My machine, even with the spindle bearings correctly adjusted, there is around a millimeter (40 thou in imperial) of backlash, I assume this slack must be in the rack and pinion, the effect of this is that the quill clamp always needs to be slightly tightened puting additional pressure on the rack and pinion making it harder to move the quill.

Failure to clamp the quill can result in a chattering cutter that can drift up and down when machining.

I have seen people fit springs to hold the quill rack up against the pinion but spring rates change as the spring is stretched so the lift force gets stronger as the spring gets longer, again causing more pressure, hence wear, on the rack and pinion.

What is required is a device that will give a constant force at any length, in other words a gas strut!

Most of you will have come across gas struts, they hold up the tailgate on a car, they adjust and damp your computer chair under the seat, they seem to be very simple items but few people seem to know or care exactly how they work, they are just happy that they do!. For those that don't know but want to learn read on :)

Gas struts are very cleverly designed devices that consist of:

A piston with an output rod surrounded by a sealed cylinder
There are rubber seals on the piston and where the piston rod emerges from the cylinder
There is an amount of hydraulic oil inside the cylinder.

All pretty standard stuff so far but this is where it gets clever, there is a hole in the piston! When gas, under pressure, is introduced to the cylinder, and sealed in, that gas will be both sides of the piston, the force required to move the piston up inside the cylinder is controlled by the pressure of the gas.

A bit of maths may be in order here, I will use pounds per square inch (PSI) in my example as I was trained in that regime :)

let us say that the area of the cylinder is 4 square inches

The piston rod is 1 square inch

The gas pressure is 10PSI

This means that the top of the piston has a force of 40 lb acting upon it (Area of piston X pressure), the underside of the piston has a force of 30 lb acting upon it (area of piston minus the area of piston rod X pressure) = a force of greater than 10 lb will be required to push the piston up the cylinder. This is described as its differential pressure.

Once released the piston will be pushed back down the cylinder with a force of 10 lb, when the piston nears the end of the cylinder the oil will damp the last few millimeters of stroke, it is also there to lubricate the seals.

It is worth noting that gas struts will last longer and work better if mounted as drawn, with the piston rod downward, this keeps the oil on the seals so that they do not leak off the pressure. (have you ever wondered why the gas strut under your desk chair tends fail before anything else? Now you know, as they are frequently mounted upside down with the piston rod at the top the pressure leaks away).

The mill has a threaded depth stop, which I have never used, this is where I want to mount the gas strut and a DRO (Digital Read Out)

The first drawing is a schematic of the existing depth stop

The second drawing is a schematic of how the gas strut will need to be mounted to lift the quill and remove the backlash.

I don't appear to have taken any photo's of how I worked out the force the gas strut would have to impart to lift the quill against the rack, the drawing shows a schematic of the method.

The basics are that a spring balance is pulling up a lever that is pivoted beyond the quill, the force used to lift the quill is noted and multiplied by the lever ratio which in the drawing is 5 to 1.

Once the figure is calculated you will also need to measure how long the stroke needs to be, quill up to quill down.

Armed with these figures get on the website of a gas strut company and find out what the nearest standard strut is to your figures. You need to get one that has a bit more force than you calculated to ensure it can overcome any stiction in the mechanism of the quill. I found a standard strut for mine that was 3lb above the calculated rate but if you cannot get anywhere close these companies will make one to your spec at a surprisingly reasonable cost.

I have not put any dimensioned drawings up here as I did not do any, this was all made up as I went along, if you are thinking of copying this idea the following may be of use

To make it fit nicely I filed the top of the casting flat where the DRO and gas strut block mounts as it was very rough and at an angle. (see first picture)

The length of the DRO and its mounting plates gives you the length of the slide rod tube, (which replaces the threaded depth stop) as they both connect the same two brackets.

The slide rod is a piece of stainless tube.. mine is in two pieces with a join near the top, no reason for this I just cut it to the wrong length :( this tube goes over a long piece of M10 studding that clamps everything together.

You will need to drill a hole in the belt cover for the gas strut to go up through, ensure it misses the cone pulley and drive belt! (See second and third pictures).

You can always cut the gas strut piston rod down in length if your strut is a bit longer than required as you probably won't need to use the full stroke. When you assemble the parts ensure that the gas strut is within a couple of mm's of full extension when the quill is as high as it will go to ensure that the damper and lubrication work effectively.

Other than that the only other modification I needed to do to the mill was to move the forward/reverse switch backwards to clear all the bracketry.

The DRO I chose came from Machine DRO.

The display is mounted on the same bracket that the piston rod of the gas strut connects too. To ease assembly the gas strut rod can be pushed into a bush as far as a shoulder, this goes into a hole bored right up through the DRO bracket that is large enough to clear the body of the strut to allow it to be pushed up through the mount on assembly, the bush is held in place with a couple of screws through the side of the bracket.

There are two further brackets to go with the guide rod studding and tube, plus the end bush for the gas strut piston rod to complete the kit.

The first bracket (picture 2) bolts to the bottom of the Quill at the bottom of the guide rod, it also takes the bottom end of the DRO scale via a tapped hole in the front face.

The top bracket (picture 3) connects to the top end of the guide rod, clamps the cylinder of the gas strut in the correct position and holds the top of the DRO scale.

Not much to say here, all you need to do is put it together, the GA drawing shows the basic setup and I will just put a number of pictures of the assembly to give a good idea of what it looks like. The first photo shows me pulling the gas strut up into place.

Pictures three and four are just general views of the assembly.

The final picture shows me using a piece of wood as a lever to check if the backlash is gone....it is! Success!