Thanks. If I had some residual weight on the rack and pinion, the worm drive fine feed should pretty much eliminate any downward free play. Trouble is I’ve currently got the opposite, which I think is making life very difficult for any work involving z-feeds.

Thanks all.

To answer a few questions:

* The gas strut is rated at 150N, this is the same one used by Andrew Whale on his SX2P in his “Learning Turning” series on YouTube. Obviously the vertical component of that changes slightly with the position of the head.

* There’s no point lowering the head and using a DTI to see if there’s any movement – as I mentioned: I can see and hear the movement – it’s basically all of the backlash.

* On the last occasion the slot drill was in a drill chuck, but it’s happened plenty of times before when using a collet chuck.

* I’m not using modified tools, they are standard mills, drills or slot drills. I wouldn’t know how to modify them anyway.

Having considered all the options, a lower rated, or adjustable strut would be the best way forward. Adding weight to the head seems like the wrong way to solve the issue.

As it stands, it’s just wrong to have the head forced up by default. It must be detrimental to accurate work – if you have a hand feed lever, you intuitively assume it’s limiting the tool position downwards. Currently, it’s basically moving the tool to its highest limit – all bets are off in terms of where it could end up in the workpiece. I’m surprised nobody else has picked up on this potential issue. Perhaps I’ve just got the wrong strut? I wonder if I could use some scales and a bottle jack to get the actual vertical force required to balance the head, resolve it for the angle and knock 5kg off?

Thanks Eric.

What I meant by "intuitively assume" is that if I move a lever of handwheel in a certain direction, I assume that I'm moving the tool in a corresponding direction and setting a hard stop wherever I leave it. I'm assuming that it's not an arbitary position that can then move further under load. It has nothing to do with how I approach precision engineering (not that I pretend to be much good at it at present anyway), but more how I expect a tool to work. I agree that it's not possible to do consistent precision work like this, which is why I'm trying to solve the problem.

I don't think adding yet more upward load is the best solution, for a couple of reasons:

1) The magnitude of the extra load required to resist all likely downward loads generated by cutting is difficult to determine, and would effectively have to be a guess, erring on the high side – which is definitely not a phrase I'd associated with precision engineering!

2) It adds unecessary load on all the mechanisms resisting the weight of the head, and makes the hand lever and fine feed wheel stiffer and more difficult to use – Instead of using gravity to advantge, I'd be fighting against artificial load.

The backlash is annoying in term of feel, but I have a DRO, and use that for getting positions. So if the z-position was to a hard stop downwards, backlash would be irrelevant. It's also not relevant for any operation where I can lock the head.

A further assumption in all this, albeit one that seems valid from the simple checks I've done, is that the fine feed worm wheel drive is not back-driveable (further external down-load won't turn the worm gear). Any further downward movement of the head once set would be due to deflections (bending) within the machine.

Here's a video, which will hopefully work if you click on the image:

There must be about 5mm -10mm of movement after setting the depth.