So far this has been very gentlemanly. The CNC conversion of the Bridgeport clone appears to have gone together nicely and be working as you might hope. But is it? At this stage, most of the Pootube Warriors would be bragging about "holding tenths" and other bollocks. This is a fairly old machine fitted with Chinesium ballscrews, so accuracy and backlash aren't going to be anything to write home about. However, there's nothing like measuring what you've got in your hands and asking yourself if it's what you'd expect - and be happy with. If not, it's time to either swallow your pride / adjust expectations - or go back and have another go.
How to do it?
Using MDI commands:
X0 ; Start at zero
X10 ; Move to +10mm
X0 ; Return to zero from above. (Set DRO to zero here).
X-10 ; Move to -10mm
X0 ; Return to zero from below. (Read DRO measurement here).
Like this:
Then edit / repeat for Y and Z axes etc.
When I say "Set / Read DRO", I'm referring to the independent (HXX) Chinesium DRO which has its own glass scales on the X and Y axes, not the display on the Newker controller. I won't be able to use the DRO for the Z axis until I refit the scale there so for the time being I will have to use a DTI for that instead.
This example should give an idea of the unloaded backlash in the X axis. Of course, a loaded machine will see bigger backlash but for starters, this should give an idea of what we are dealing with here. I can then look at how much more horrific it is when an actual load is applied (gulp!).
To run this on the Newker controller, select "Single" mode from the front panel keyboard. This allows you to step through the MDI commands one line at a time each time you press the green "Start" button. The title bar on the display should say "MAN INC 1.000" which sort of makes some sense - when you know what it's doing.
Results:
X axis - total 30um. Gave a very consistent reading over 5 runs. And the positional accuracy in the same direction is bang on over the first 100mm or so. TBH, 30um seems about what I might expect. It will be interesting to see what it increases to when I present it with some sort of preload to simulate a cutting force that is trying to return the tool to the zero position. That won't be so pretty....
Y axis - total 390um. Oooof. Yes - clearly something amiss there. Time to check out the assembly. I doubt that can be down to the ballscrew alone. More likely to be due to the ballnut being securely fastened in place or (preferably) due to the bearing / pulley / ballscrew not being correctly tightened up.
Z axis - total 100um. Actually 0.004" using an imperial DTI. Oooof again. That's even shitter than I'd expected. This requres further investigation. I could live with ~1/4 of that ie closer to that of the X axis.
(note the HXX DRO only reads 5um increments).
Oooof.
Wasn't expecting nano scale accuracy but even if I get the Y axis sorted, the Z axis is still nothing short of abysmal. The most likely culprit is going to be the ballscrew and ballnut but a methodical investigation is going to be required before I start looking at messy corrective actions such as fitting larger ball bearings etc.
At this stage, I could hear The Stupid Fat Bloke muttering something about not having finished setting up the machine yet. I'm not sure that's the way to look at this but it's true to say that I haven't really done much more than functional testing so far, whereas testing it against "reasonable expectations" is something you'd do before signing something like this off.
Next steps:
Bring the DTI to bear on the Y and Z axes with a view to figuring out the breakdown of the total backlash measurements. They won't be totally down to one component although I'm expecting to find something dominating in each case eg assembly cockup (Y) and ballscrew (Z). Time will tell - but hopefully not much of it.
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