Hovering over "Buy"... care to sanity check my thinking?!

[brumster]

It would help if you decided on the physical size so that you can get some advise on the frame.

Line in the sand, let's say 600x400 working area, give or take 100mm. For this iteration.

No offense taken. It's common for people to dismiss Experienced advice and seek other opinions when they don't hear what they want to hear.!
Truth is they sooner or later realise the advise was good, unfortunatly it's usually cost them money to realise this. . . . Called the price of learning.!

Sad thing is it needn't have done if they'd only listen.

To be honest it's a double edge sword advising people because those that do listen never really fully realise the bullet they dodged by listening so you never really hear from them because things tend to go fine and without issues.
Those that don't are often embarresed so never really say anything publicly and just go with what was suggested first time. (Although do often get PM or email thanking me and asking again.!)

Yeah, sure, I understand. And I am listening, don't worry. The choice for me is whether writing off £500-1000 on a crappy Chinese thing is a valuable learning experience versus investing the time and money into a £1500 DIY job first time - accepting that I might get things wrong, learn the hard expensive way.... I'm going to do some more stewing/thinking/reading.... mainly on the DIY side, get comfortable with the concept that I will have access to sub out any necessary work to those with the right tools.

As per my intro thread, I'm not averse to some fabrication, some welding; I've built numerous cars over the years (I know that's not totally relevant but I'm just trying to convey that I know the right end of a spanner, that's all!)... but likewise it's that knowledge of what needs to be done that's making me double-think whether I can really do it DIY without any mills, lathes, etc... just a bench pillar drill and a hacksaw... :D

[JAZZCNC]

The choice for me is whether writing off £500-1000 on a crappy Chinese thing is a valuable learning experience versus investing the time and money into a £1500 DIY job first time - accepting that I might get things wrong, learn the hard expensive way

The metal work is only a small portion of the expense. The real expense and BIG chance of wasting money is buying the wrong stuff. Cut corners here and you will without doubt waste money and cost more than buying decent gear in first place.! . . . It happens time and time again.!

Like wise you don't need massive tool collection either. Many great machines have been built with with just Drill press and hand tools. My first machine was built using just Drill press and hand held router. It wasn't pretty but was very accurate and cut nothing but Aluminium including this.


Infact Joe on this forum is doing a very nice job with Minimal tools, he's also cutting alumnium with hand held router. http://www.mycncuk.com/threads/4513-...ghlight=router

[brumster]

Very nice :)

I have an offer from a forum member to go and take a look-see at his machines, and a general chat, so I'm going to take him up on his offer. Think I will feel much better once I've seen a DIY-constructed device, then make a decision.

Breaking out onto a separate topic a little.... on the electronics side of things... the stepper drivers and the motion controllers. Just want to check my understanding; is the following right?

Motor Drives

These are really nothing more than stepper driver ICs, of which obviously there are a plethora out there with various ratings in terms of voltage, current, stepping capability and safety features. Bolted on to that are the relevant amount of heatsinking, protection/filtering but ultimately they take a low-voltage signal from a microcontroller or parallel port - with appropriate optical isolation - and move the motors as requested. One tick = one unit of movement (in the appropriate direction, of course).

There are differences in terms of matching them to the motor (number of wires for unipolar, bipolar) and you can get analogue and digital ones, digital ones being better all-round but presumably more expensive. There are trade-offs with how you drive the motor in terms of microstepping, torque, heat generation and so forth. So you need to know your motors in conjunction with your drivers, and also what you're going to use them for (what's most important - torque or speed).

You can get ones with direct parallel port drive (so no motion control; driven straight from PC) but then the honus is on the PC as the controller, hence all the requirements in terms of it being able to keep up. Each pin on the parallel port controls a function, which limits you somewhat to the number of axes/functions you can make use of.

Other alternatively you can get drives controlled from a separate motion controller in between the drivers and the PC.

Motion Controllers
These do the interfacing between your PC (via parallel port, USB or ethernet port) and the motor drives, but instead take a proprietary protocol and convert that to the native signals for each axis drive (they don't take G-code). The benefit of these is they can layer on all sorts of fancy stuff like inertia concepts, but the big gain here is really offloading the requirements from the PC to keep up with the frequency of signals needed to instruct the drivers directly. The motion controller interprets the movement commands (move X from 0 to 100) to how many steps that needs on which axis, at what speed, and then takes on the job of reliably hitting the driver with the requisite clock signal and appropriate-timed control signal.

You potentially gain some speed/reliability on your machine and generally it's the way forward unless you just want to have a play at slow speeds direct via parallel.

When you guys refer to BOBs (Breakout Boards) you're generally talking about motion controllers, although you can get BOBs that combine both of the above.

It strikes me that keeping them separate gives more flexibility as the drivers need to match the motors; if you get a combo BOB you'd need to be sure that the drivers side of it matched your requirements and they weren't going to change.

Point, laugh, poke the shit out of the above where I've got it miles wrong :)

[Clive S]

Motor drives say like the AM882 (Typical) are digital and have stall detection etc. are used after the BOB (and can be connected direct to the PP BUT not recommended) also they are used after the motion controller.

Motion controllers are generally fed from either USB or Ethernet port and they produces the pulses much better than the computer as they generate their own. Some motion controllers also have the BOB built in.


The Gcode is controlled from the machine controller ie Mach3 or Linuxcnc etc and fed to the PP or the motion controller

[brumster]

Does Mach3/LinuxCNC send actual gcode to a motion controller though? I thought not? It interprets the gcode on the PC, then some plugin relates that to wire signals that are specific to your motion controller (via a plugin on Mach3 for example) and then the motion controller converts that to the relevant pulses/etc...?

[Clive S]

Does Mach3/LinuxCNC send actual gcode to a motion controller though? I thought not? It interprets the gcode on the PC, then some plugin relates that to wire signals that are specific to your motion controller (via a plugin on Mach3 for example) and then the motion controller converts that to the relevant pulses/etc...?

No I was over simplifying it the Mach3 converts the gcode to pulses which then goes to the PP or motion controllor

[brumster]

Understood, thanks

Contemplating whether to make my own drivers and motion controller, but sounds like the latter is out! Too much coding effort anyway, I see there's an open source one out there and the code alone took 4 years of development... so no, I won't be going that route :)

[Clive S]

Understood, thanks

Contemplating whether to make my own drivers and motion controller, but sounds like the latter is out! Too much coding effort anyway, I see there's an open source one out there and the code alone took 4 years of development... so no, I won't be going that route :)

Good drivers are essential as they also take account of resonance, current control etc If you buy decent drive they can of course be used again if you decide on a bigger machine

[brumster]

Most I've seen seem to be configurable via jumpers or DIP switches so, presumably, as long as they can match the voltage and current ranges of the motors, presumably I can overspec on them now and re-use... makes sense. Ta

[Clive S]

Most I've seen seem to be configurable via jumpers or DIP switches so, presumably, as long as they can match the voltage and current ranges of the motors, presumably I can overspec on them now and re-use... makes sense. Ta

Motors are generally run at much higher voltages than the data sheets typical nema23 (low inductance) motors are run at about 68V with AM882 drives setting the current with the dip switches