Digitising Probe

[Leadhead]

Could anyone recommend mid price digitising probe please. - Such as the Polish models on ebay at about £130. Are they any good?

[m_c]

Both of mine I got from US sellers on ebay.
The first one (used on my digitiser) was from the seller Deepgrove1, as he was the only one who actually answered my questions about using them on 24V inputs, however he has since stopped making and selling them.

The second one I got from Imsrv - http://www.ebaystores.co.uk/imsrv/Pr...sub=5194280011
I've got one of their adjustable ones, but I see they've stopped selling them, in preference for selling a pre-calibrated version of their fixed probe.

The calibration depends on what you plan on doing with the probe. If you're using it for digitising, then the calibration doesn't matter, however if you're using it for edge locating, then you really need it calibrated. The calibration can either be done in software, or mechanically.

[Leadhead]

m-c - Thanks for info.
I actually found an old one of that type which I had abandoned due to eccentricity some years ago. It has three small screws on the 8mm shaft side to fix it to the main body. There are similarly three small screws on the internal tri pivot plate. Would you know please which should be used to re-calibrate the probe spindle. Both are effective and both a p.i.a to adjust if truing in a chuck. I managed to get it to about .004" runout but not convinced it will stay there.

As an aside I may use the above for Z digitising and revert to "semi solid" probe for edges. By semi solid, I have been experimenting with an old rotary edge finder, the eccentric sprung head type. -- Beginning by trueing the end in the moving m/c collet chuck by eye. Stop and connect the circuit with a crocodile clip. Then perform the edge finding routine. This can be done more safely since the sprung rod end can give some radial movement if required. (Assuming of course one keeps the split line of the sprung head above the top surface of the plate being measured.) Have adjusted the spring to maximum tension and the approach speed low.
Still playing with this, but early signs look cautiously promising.

[m_c]

I'd personally use the 3 screws on the main shaft to adjust it.

Physically centring a probe is something I've never done, as I always calibrate them in software. On the digitiser, that is done simply to allow for any inconsistencies in the tip/switching, where as on the mill is done to allow for any inconsistencies and to ensure they are known relative to the spindle centreline.


If you've got an old style edge finder, I'd just use it like it was meant. Jog up until it wanders, then subtract/add half the diameter from the position, and you're done. Saves having to centre the edge finder and connecting a crocodile clip.

[Muzzer]

The method for centring a probe in a mill is to use a conventional DTI touching the probe ball and rotate the spindle manually. Then adjust the 3 screws so you end up with no runout on the DTI when you rotate the spindle. Which 3 screws to adjust depends on the probe. On my Renishaw (ebay purchase), they cause the body to pivot about a ball bearing. It's trial and error and a bit of skill, rather like setting up a 4-jaw chuck but not difficult at all.

If you plan to rely on software settings, you will need to ensure the spindle is always at the same angular position each time. Setting up the probe as described is pretty straightforward and I wouldn't go back to an edge finder now.

[Leadhead]

OK so been trying the static edge finder in the angled edge software and as carefully as I can measure. (digital angle finder) It is pretty much spot on. Edge finder does not visibly move when the approach speed and thus inertia, is reduced. Software instantly does the calculation.

Muzzer - Unfortunately none of my DTI`s have a sufficiently weak spring to effectively clock the stylus.

[Muzzer]

That's a pity - it's how these probes are supposed to be clocked. If you think about it, there aren't many other ways to do it sensibly. Have you actually tried it? And how does the manufacturer suggest you do it?

[m_c]

And how does the manufacturer suggest you do it?

Method I've seen mentioned, is centre the spindle over a known bore (a big bearing secured to the bed is ideal - you don't need any fancy gauge ring), then carry out probe centring routines to find out how far off centre the probe is, adjust, and repeat.

And valid point about If relying on software, you need to maintain orientation.

[Leadhead]

m-c - the orientation issue is probably the reason I am coming to prefer the semi fixed method. I just made a reamed 8mm dia. half shell to align the centering device quickly. Works effectively by pressing on and carefully removing. Even added a little handle to it.