Tuesday, 16 June 2020

Chinesium tool touch device disembowelled!

What's this? 
That effing idiot, The Stupid Fat Bloke was playing with the tool length offset measurement operation, trying to help me figure out how it works. I was trying to measure how much lost motion was happening due to the operation of the switches in the Renishaw probe (almost none) and the TT (apparently almost none either). The best I could measure suggested a total of around 20um, which is well within my expectation for accuracy and backlash.

I had the spindle close to the top surface of the vise jaws and asked for an automatic tool length measurement. I wasn't certain if the move would start with a tool check (full spindle retract) and thought the best way to find out would be to issue the command and see what happened. Before I could position myself next to the e-stop or grab the wireless MPG, The Stupid Fat Bloke pressed the start button and we were able to watch the solid reference tool (Tool #1) sail towards the TT. I couldn't stop the move in time to prevent the tool crashing into the TT. Fucking idiot.

The TT looks sort of OK, although the bell end bit seems to have gone a bit floppy. That might simply be an age-related issue in normal circumstances but on this occasion, something has snapped inside. Looks like time to go in!

For an idea of what to expect, here's one Marty did earlier:


4 little grub screws hold the bell end thing in place:


"It just came off in my hands!" Fnnaaaarrrr.


There's a nut hidden in that first layer of silicone.


But time to remove the white plastic disk at the bottom, as I'm not going to get any further from the top. I just stuck a small screwdriver through it and levered it off. It turns out it was epoxied in place, so there wouldn't have been much choice.

The gubbins look different to the one Marty dissected. There's the microswitch which is used to signal an "overtravel" - I assume this is supposed to be connected into the e-stop or limit switch circuits to kill the Z axis if the probe operation doesn't stop the tool. I chose to ignore that function when I connected it up. Instead, I am only using the main switch itself, which results in negligible lost movement. 

But there also seems to be an LED in there too, mounted on a small PCB. WTF??? Looks as if it would normally be connected in series with the contacts, so would extinguish when the contacts opened.


When I'm here, I will be able to repair the conduit which got stretched some time ago when it got caught during a park move. It unscrews, leaving a screwed adaptor.


Removing the moving yoke, we can see a switch contact at the end of the tunnel.


Here's the corresponding contact on the moving yoke. Each contact is mounted in an insulator so that it's floating. I hope the contact materials are "proper" choices for the application.


Here's the LED on its own dedicated PCB. Bizarrely, it is next to a hole in the housing but is pointing in the wrong direction. Furthermore, it's not even connected. WTF??? It can fuck right off - I'll be reassembling this device without any such unnecessary clutter.


This overtravel microswitch can also fuck off. I suppose it might have prevented the damage if I'd connected it up as an additional e-stop....



The shaft is actually quite nicely made - hardened and ground. And a good fit in the bore. Do I have to make another one or can I repair it? It snapped clean off and being hardened, the parts mate together perfectly. Ideal for adhesive perhaps? I have some Loctite and activator that is about 25 years old, so what better? Glue - you are having a laugh surely? No. Let's try it and see what happens. 

If it fails the finger test tomorrow (as I'm expecting), I'll probably just make one in brass. Time will tell - only a few hours of it.


EDIT:
Short answer - er, no - not quite. It might have been OK for most purposes but would still have been a weak spot. Took a bit of snapping off this morning but either way, it's in 2 pieces again. So now Fat Boy has the task of creating a replacement shaft for the yoke. Bollocks.

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