[joeavaerage]

Hi peteeng,
Fu******king Oath its a problem!!! How do you suppose a coil in a car works. A current is feed through a coil and then when the points open, ie the current is suddenly open circuited
the voltage spike is sufficient to throw a multi-kilovolt spark at the spark plug when its under compression. That is how Kettering ignition works.

When the coil current of the electromagnetic brake is suddenly open circuited the voltage spike could literally be kilovolts and there will be an arc between the opening contacts.
Lenz's Law:
V=L.dI/dt

So effectively an open circuit causes a very large if not infinite rate of change of current, and therefore even with a low inductance the voltage will be high....kilovolt high!

Craig

[peteeng]

Hi Craig - Thats with a primary and secondary coil so no inductance? This is only one coil. Peter

[joeavaerage]

Hi peeteng,
the coil of a car is an autotransformer. The primary is inductive. When current flows through that coil it generates a magnetic field, and that field has energy. When the current is
stopped suddenly that energy has to go somewhere. The primary has a voltage spike of somewhere between 500v and 1500V, limited only by the condensor and the arc-over voltage of the points.
That voltage spike is transformed up to 15kV to 25kV by the autotransformer

A coil has inductance, and therefore it will store energy when a current flows through it. If that current is interrupted the 'voltage excursion of the inductor will oppose the change in current'. This principle
was discovered by Faraday in the early 1800's. Maxwell took Faradays experimental observations and generated a universal theory of electromagnetism in the 1840's. These remain the basis of electromagnetic theory
to this day.

Craig

[GeckoSub]

Hi,



Cant imagine what's going on. My brake locks on within a few milliseconds, I've never had the servo rotate at all between when the servo Enable signal goes inactive and the brake locking on.

Craig

I will have to read the rest a few times to make heads or tails of it. But the reason I have a delay is that I am running the 24V totally dumb. There's no relay, no connection to an "enable" or anything like that. It's just hooked up directly to mains like the servo drive. The delay comes from the ~ 2 secs it takes for the servo drive to power on and hold the servo whilst the 24V PSU turns on way faster after they both get fed mains power.

As for the rest of the posts on this, I gather you think that's an unhealthy idea?

[joeavaerage]

Hi,

But the reason I have a delay is that I am running the 24V totally dumb. There's no relay, no connection to an "enable" or anything like that.

Well that explains it. there is s delay between when the mains supply is applied to the power supply module and when its output is useful. In that period of time the
brake status and the servo status are not in sync.

You need a relay or switch so the the brake is released at the same time as the servo is enabled, and going the other way the brake is de-energised, ie locked on when the servo is disabled.
Use a relay or a switch. The millisecond delays have not proven problematic for me, but I can well imagine half second and longer delays that you are experiencing would screw my machine as well.

If you do use a relay, and want it to last, then fit a catch diode. The relay will work OK without it but the contacts will suffer arc damage and likely at some point in
the future weld themselves together.

Craig

[GeckoSub]

Heya guys,
This build took another turn a while back. Apologies for not updating
It's no longer a build - I ended up buying a used, smallish machine to get started on.
Same reason as last time I was trying to buy one: Realizing if/when I design my own, it would be smart to actually have some real experience plus, I really need to start making parts.

I lost out on a very good Taiwanese machine frame at a steal of a price, which really hurt and probably prompted this buy. Maybe a bit hurried, but the machine was cheap enough that it's OK and it will be an OK learner. The Taiwanese one was pro level and I would never have grown out of it. This one is possibly more of a stepping stone. But it comes more or less ready to run with a controller and all. Stepper, rolled ballscrews and no idea of the accuracy or stiffness of it, yet. But it will cut alu and get me started. Then in half a year or so I will know if I need to sell this one and upgrade.

The machine just arrived on my island but is still in storage at the freight people's warehouse until I settle a deal on a small workshop space.

Anyways, here are some snaps from the seller and it being sent out:


So, while this has been making its way here from China, I have been busy shopping for vises, clamps, (a lot of) parts for an MQL system, a bit of precut 6061 alu. A lot of stuff to sink my teeth into.
I think I will swap out the controller at some point failry soon as it's a bit limited in features but if the frame is not too terrible, I might also put the servos on it. It's +220kg so I hope it's not as flimsy as it might look.

Not sure if it makes sense to keep posting here. I do think the MQL system I have in mind will be pretty interesting. Ideally, it will be an improved version of a Fogbuster as I will drop the needle valves completely and use a peristaltic pump for dosing and I have some really nice low pressure regulators, too. Also, I will test some Luer Lock dispensing needles as nozzles.

I might post about that here or in a standalone build thread.

Again, thanks so much for the help gents!

[G-Spot]

Was tempted to do the same myself, but a manual machine that could do steel.
If finances are tight and you just need to machine parts and not join a CNC cult then this is the smart thing to do.
You should have waited until you had it all clean and polished and really make us jealous.

[GeckoSub]

Was tempted to do the same myself, but a manual machine that could do steel.
If finances are tight and you just need to machine parts and not join a CNC cult then this is the smart thing to do.
You should have waited until you had it all clean and polished and really make us jealous.

Not sure I can make anyone jealous here. There are some kick ass builds out there. But yeah, I can't wait to get to learn how to run this thing. Honestly, I haven't even slept well for the past week because my mind keeps racing with little projects I wanna make on this thing

But I think you are spot on, this machine had an attractive price and I really, really had to get going on this. Also, that realization that I didn't have any experience to base my design objectives on in terms of a DIY build.

Will post a bit more soon and get some pics uploaded, but yesterday I got it into the workshop. Yes, I found a space. But... it's sitting on the floor. 5 guys were not enough to get it lifted onto the base, haha. But that's alright, I'll spend some time cleaning it up and will try to find an engine hoist to rent for a day. Worst case, I'll get two long pieces of pipe and 8 guys to come lift it.

First impressions is that it's heavy for it's size. But whether it's accurate, we have yet to see. Also, while it's dirty, I cleaned it off in a few places and it cleans up pretty nice. A bit of spot rust but nothing on the ball screws and rails. Strangely enough, I have yet to find a metal chip anywhere. Mouse or rat **** aplenty, though.

Either it was cleaned up really nicely - or it was never used much - before being left for the rodents to use as a toilet

Whether this machine will be a starter, stepping stone or a keeper, I am far from knowing. Only time will tell.

[G-Spot]

But... it's sitting on the floor. 5 guys were not enough to get it lifted onto the base, haha. But that's alright, I'll spend some time cleaning it up and will try to find an engine hoist to rent for a day. Worst case, I'll get two long pieces of pipe and 8 guys to come lift it.

Be careful if you get an engine hoist and have not used one before, they are stable when the load is over the long feet that stick out, but if you try to use them to put something on a bench then those long feet wont fit underneath. You will think "Simple" and position the load sideways to the bench then just swing the load over the bench, if you do so the hoist will become, in the blink of an eye, unstable, the base because it is on wheels that rotate, will fly off away from the bench and the load will come crashing down. Don't swing the load sideways and if you have to then you will need to put chocks under the wheels to stop them from moving. Just blocking the wheels with your feet will not work, the weight of the load is now working like a trebuchet.

[GeckoSub]

Great advice/reminder!
I will go measure the gap under the base right now and then check whether I can fit the hoist feet under it.
If not, I will block it up a little bit higher as the engine hoist really is my best bet here.

[G-Spot]

You will be fine, you will be able to extend the boom fully because you are lifting less than 500kg. This means you wont have to get the feet far under the bench if at all.
What I should have said last time is the danger with these hoists is their wheels auto steer(I said rotate), they can be pointing in one direction when you start to lift then midlift they turn in another direction.
Makes them great for getting an engine or gearbox perfectly aligned because wherever you push the swinging load the base will follow but for lifting other loads in other situations they can be really dangerous.
I have my own, I modified the wheels so I get to do the steering and not the load.

[GeckoSub]

You will be fine, you will be able to extend the boom fully because you are lifting less than 500kg. This means you wont have to get the feet far under the bench if at all.
What I should have said last time is the danger with these hoists is their wheels auto steer(I said rotate), they can be pointing in one direction when you start to lift then midlift they turn in another direction.
Makes them great for getting an engine or gearbox perfectly aligned because wherever you push the swinging load the base will follow but for lifting other loads in other situations they can be really dangerous.
I have my own, I modified the wheels so I get to do the steering and not the load.

Yeah, I was supposed to get the it done today but the car I was gonna borrow to go pick the crane up is broken. It's OK, I put some more time into cleaning the machine up instead.

It's a mystery machine of sorts.
It has some really nice parts on it and some weird ones, haha. E.g. it has Japanese NSK linear guides and blocks and I think it may have ground ballscrews, too. On the other hand, the limit/home switches are USD 1 inductive switches...
Also, on the floating end of the ballscrews they didn't bother to turn down the ends to get rid of the grooves. The ends are just mounted straight into a ball bearing, grooves and all. Maybe they used second hand motion parts and just lopped the ends off.

It has cheap non-closed loop steppers but those are easily upgraded to closed loop or with new motor brackets to the servos I already bought when I thought I was gonna build my own machine.

Speaking of brackets, besides some stripped screws on the sheet metal the only thing that was broken on this machine was the motor bracket for the Y. Either it took a hit in transport or this happened long time ago and was why hte machine was left aside for the rats to **** in
Now, sadly the has some non standard motor brackets that I haven't seen to be able to find anywhere. They have a center height 5mm higher than normal. So, I may just print a 5mm spacer plate and order the standard bracket and then machine my own riser plate as the very first thing.

The sheet metal is fairly thick and the cast iron frame seems very solid, so if they mounting flanges were machined precisely and the machine put together properly trammed and square and all that, I think this will serve me well. I will order a granite triangle soon to check this at some point.