Luthier CNC

[Kitwn]

I love the control box! It's so much easier to fix everything to MDF than mild steel or aluminium and makes future changes a lot easier. It also allows you to concentrate the funds where they make a difference rather than just spending money on something that looks good. I've never quite seen the point of making one-off home constructed machines mimic professional, mass produced factory built versions. The priorities are completely different.
The only possible problem is electrical noise, a discussion going on elsewhere on the forum. This is something you can address with additional screening and appropriately placed capacitors if and when it shows up during testing.

Kit

[bluesking]

I love the control box! It's so much easier to fix everything to MDF than mild steel or aluminium and makes future changes a lot easier. It also allows you to concentrate the funds where they make a difference rather than just spending money on something that looks good. I've never quite seen the point of making one-off home constructed machines mimic professional, mass produced factory built versions. The priorities are completely different.
The only possible problem is electrical noise, a discussion going on elsewhere on the forum. This is something you can address with additional screening and appropriately placed capacitors if and when it shows up during testing.

Kit

Thanks Kit. I couldn't care less what it looks like, as long as it functions well!

Everything seems to be ok so far - no interference yet, but it is very early days. I have implemented a single point star ground and shielded cable throughout, even on the limit switches - I am more worried about interference on all that cable than I am into the control box. Also, almost every EMI noise source in the room is actually sitting in that box already, so I'm not really convinced a metal enclosure will help anything for me! We shall see!

Also, I've been meticulous in avoiding ground loops. Despite this, I did knowingly introduce one ground loop into the system - this loop goes between the control box and the VFD, and back through the mains circuit. I wanted to avoid this, but I also wanted to have a Z probe which worked without a crocodile clip. There were a few options I could see:

1) lift the mains earth in the VFD - this would probably be the best way but the VFD is very high power and I was worried that earthing it through the signal cable shield could potentially dump a huge amount of current into the control box if the VFD failed. Then I would have to replace not only the VFD, but also debug and fix the whole control box!

2) rely on a common earth between the control box and VFD - earth both to the mains, and assume that is adequate to close the loop on my Z probe. This would certainly have worked in my workshop - because I plug both devices into the same mains circuit so they share a common earth. However, if I ever moved the machine somewhere else and the two devices no longer shared a common earth, my Z probe would fail! This seemed like a really unpleasant easter-egg for future me (or another owner) to have to deal with!

3) Knowingly introduce a ground loop. This may or may not cause an interference issue - but is safest option. No issues noted so far!

[cropwell]

I also wanted to have a Z probe which worked without a crocodile clip.

My tool tip has earth continuity, so this type of touch plate works for me.

Even though your spindle body may be earthed, that may not continue to the tool if you have ceramic bearings, so a switch type would have to be used.

Have you metered the resistance between collet and spindle body ?

[Kitwn]

As I've just mentione elswhere, an RCD in your mains supply offers no protection against shocks from the output of your VFD. Don't take risks with the safety earthing of your spindle.

[bluesking]

Don't take risks with the safety earthing of your spindle.

I haven't taken any risks: the spindle is earthed. Thats the reason why I had to introduce a ground loop to make my z probe work how I wanted and still be safe - at the cost of some potential interference...

It's a basic catch 22 - and the reason why many valve amps have a ground lift switch. Always seems a bit dangerous to me to leave it in the hands of the guitarist to decide what is safe! But it's there when you use multiple amps and you know they are all on the same mains circuit yet the ground loop is causing noise to get in the system.

[Kitwn]

We should probaly start a new thread to start talking about multiple amps, Brian May, whether Russian ECC83s are better than the Chinese, using CNC machines to build guitars.....................

Kit

[bluesking]

We should probaly start a new thread to start talking about multiple amps, Brian May, whether Russian ECC83s are better than the Chinese, using CNC machines to build guitars.....................

Kit

For sure. And by the way, the best ECC83s are dutch: amperex bugle boys. They might not be as well known as mullards but they are cracking. You can find them for under £10 each - if you know where to look :)

[bluesking]

My tool tip has earth continuity, so this type of touch plate works for me.

Have you metered the resistance between collet and spindle body ?

I've metered between collet all the way to the BOB, thats what made me think it would work. I then tested the z probe (same idea as the one in your pic), thats how I know it works!

Weirdly my spindle is quoted as having ceramic bearings though so not quite sure why it does work. But I'm happy so far.