plasma table idea

[simonr]

Some quite cool stuff there! You should post it in a new thread - a lot of people will ignore this thread if they've not been following it from the start as it's quite long!

Si

[robertspark]

Don't know if this is worth a shot to someone?

http://www.metalwebnews.com/howto/plasma/plasnator.pdf

[robertspark]

Si thanks v much for the credit for inspiration comment it was very much appreciated and unexpected

Did not want to taint your build thread with my thoughts and problems which are nothing to do with your good designed table (in case anyone else is reading this fir the first time) mine uses a totally different principle for the z

Basically my z sticks and is not smooth when it ascends and descends and the way that it works with the floating head means the switch can stick sometimes

I've been having a go at several iterations and variants for a better z in inventor (grooved bearings on a v rail, smooth bearings on a shs (50x50), thk rail, circular sliding bearing on circular rail (sc10uu) and circ sliding bearing on supported rail

Note I am a perfectionist and I've put a trapezoidal tr10x2 leadscrew (metric acme) as my z and I'm not happy with the travel speed. It is set at 10 microstep mode (programable stepper), std stepper, 200 steps per revolution, 2mm pitch because it now gives me a metric pitch resolution of 0.001mm per step but that limits my theoretical z travel speed to 20mm / second (20khz - stepper maz step speed before it starts to loose steps) but because the leadscrew is doing theoretically 600 rpm with a Delrin nut it induces a (harmonic??) vibration at a much much lower speed which jerks and vibrates the z (with carp!!!) plastic bearings 28mmODx6mmIDx~8mm U groove bearing running on a piece of lazer cut steel edge filed (or angle ground) with a partial chamfer so the v on the plate rides inside the plastic U groove of 3 bearings with a lot of play in the bearings inherently.

Also note I have mission creep!

Now I have a very fast smooth powerful X and Y axis and fully appreciate the benefits of 2.5d design and sheetcam I was wondering if I could add the following onto my Y-carrige:

- Sheet metal scribe (Chicago cp9361) been done before as a load on YouTube and looks like a nice addon for a finishing touch plus and centre point marking using a thk rail or two circular rails and a double acting pneumatic cylinder two regulators (ar2000) for cylinder up & down (including plate pressure) and a second for the scribe set at 6bar as manuf req

- pneumatic 3/8 (10mm) chuck drill used for small holes plasma can't cut well less than 1.5x whatever plate thickness you are cutting say up to a max of 5 mm because of the cutting forces required. Seen a few on YouTube using a 42mm clamp to hold the drill sitting on either a dual circular rail or THK guided rail and pleumatic cylinder to apply cutting force to the plate (set with an ar2000 simple regulator following adding an stop code into the post processor for toolchange (regulator setting))

- potential for a spindle addition for mdf cutting still in the back of my mind (yup I know that the rest of the assembly is not designed for it but I've seen drawer slide routers doing diy bits (thinking kids artwork using vcarve not production for resale plus occasional furniture). Weight is a problem as a 2kw water cooled spindle is over 5kg but I thought that anything I do should at least consider if it could be latter added and therefore try to keep the z as close to the y-rail and minimise any overhang) only considering the use of small cutters to either carve or straight cutters to cut objects out not edge profile (I have a hand router and a decent selection of bits plus feather edge for board jointing etc) backlash under cutting would be a huge consideration given the backlash at the moment is very small but it's not under cutting load with belt stretch pull our tensioner springs (so the spindle way not be a goer but like Edison said he'd found a thousand ways not to make an incandescent lamp or something to that effect). What's the worst that can happen... Provided it doesn't come loose it will just be a carp cut ... And offer someone a discount of ebay resale

My thought for a better z was...

- 5m htd belt (same as my x and y) running on the 20T pullys with the carrige being clamped to the belt

- direct drive stepper (same as x&y) set at 50 step micro resolution able to give 12m/min no problems (good enough for THC) and smooth, 0.01mm step resolution and backlash no worse than 0.2mm (8thou) (in a non force cutting application) which is what I get on my x and y

- drill one of the pullys out flom 8mm to 1/2" (12.7mm) and press in a few 13mm od 6mm I'd bearings

- install the flats of the pulley flanges in the same plane as the y carrige rail on a 5mm backplate (cutout to save weight) and mount the stepper directly to this on a sliding mount for belt tension

- use either thk 15 guide rails or sc10uu circular rails for the z on 4 carriages to spread any loads and maintain squareness

- install a 5mm z tool backplate with quick tool change alignment holes

For the plasma z tool holder, reuse my existing mount but mount it onto a single thk 9 rail with twin carrige or twin 6mm circular rails with 4 carrige sc6uu bearings on a backplate / tool mount to provide a small floating head using inductive proximity sensors so there is no switch resistance add a few small ball bearings into the tool alignment holes and some n35 5mm magnets to provide a magnetic plasma tool holder so that the tool holder disconnects if there is a tool / plate clash could use another inductive prox sensor to provide an e-stop it the tool becomes detached using mach3 brains and a spare input on the smoothstepper

For the scribe using either a single thk rail with two carriages (more likely for weight) or dual circular rail and 4 carriages scu10uu or a dual rail pneumatic cylinder (high possibility) to drive the scribe or drill directly down tithe plate. Thought with the drill is to use the cylinder regulator to set the cutting pressure make sure that the holes never clash with a plate supporting rail and use a magnetic adjustable stop with a prox sensor to set the cut depth marker or just use the bottom of cylinder travel with a cylinder mount prox switch

Very sorry for the long post but hopefully the thought process is clear for anyone interested

As always any comments are appreciated

Thanks

Rob

[simonr]

Robert - what about mounting an automatic centre punch to the Z axis but not on the slide. Sharpen the punch so it will act as a scribe and make a cap to cover the tip of your torch when scribing.

To centre punch you just drive Z a few mm below where the torch is touching the surface (but it will lift the slide rather than breaking anything). For scribing you do the same, just don't drive down as far.

Why do you need to move Z that fast? After all, it is only moving a max of 10mm between cutting & traversing and only 2.2mm between pierce & cut. If you want to move fast, you need to accelerate & decelerate the axis - and I think the acceleration / deceleration will take longer than the movement. Even cutting a sheet of parts with say 100 piercings, you will only save 9 sec going from 20mm/s (1200mm/min) to 200mm/sec (12m/min).

Maybe this is one of those Perfectionist things I don't understand?!

Could you post a photo of your Z axis?

Si

[robertspark]

Si,

Thanks for the reply, the automatic centre punch is a clever idea and never considered.

Travel speed, there is no requirement for 12m/min specifically, given my x and y are 12m/sec, with acceleration and deceleration at 200mm/sec. so to accelerate to 12m/min would take 0.5 seconds, and the axis would have travelled 100mm. But the belt driven axis are very smooth.

My z has only a single end supported leadscrew which seems to suffer from eccentricity issues..... and it has been a right PIA to get the thing eccentrically aligned in the motor coupling and the nut so that it does not wobble when traversing up / dn. (8mm stepper shaft, `10mm leadscrew, 8/10mm rigid coupling (the rubber plum coupling did not work... as the z was pulling down on it (school boy error!))

Also those plastic bearings have a degree of slack in them.

Hence a bit of a more rigid assembly may allow for better touch probing accuracy too (given it won't suffer from a degree of wobble....)

Earlier picture here (I'll take another of my setup a little later)

[simonr]

I've used a flexible coupling: http://www.ebay.co.uk/itm/6-35x12-8x10-8x12-8x8mm-CNC-Motor-Jaw-Shaft-Coupler-Flexible-Coupling-OD-25x31mm-/151610337389?pt=LH_DefaultDomain_3&var=&hash=item234cae286d

Which works fine! The bit of M12 studding I've used, as it turns out even has a slight bend in the middle!

Similarly, the nut is loosely coupled (by the bolts not being that tight) so any eccentricity is easily absorbed.

I did find the plastic V rollers were useless! I bought metal ones (which were cheaper!) and 10x better!

Your's looks much more solid than mine which I quite like - but I adopted the same principal as for 3D printers where you want to minimise the moving mass to give you the best acceleration and least (or at least, highest frequency) vibration possible.

I don't think you'll have to change much to make it run fine.

Si

[robertspark]

Thanks Si I've got it cutting very well now

Spent a bit of time aligning the leadscrew and with a bit of fine grade wet & dry paper smoothing off the components of my floating head mechanism

I've also realised that my thc is not working ... Well it is working but it's not moving the z axis accordingly ... The reason though blatently obvious to someone else (maybe) was not clear to me at the time

I'm using a usb smooth stepper (why because I bought it about 5 years ago when I started collecting bits for my cnc), and the bit that I did not realise was how the smooth stepper handles gcode and what functions that are done by Mach without gcode modification via software algorithm before pulses are output to the parrallel port driver that now don't work so well because the smooth stepper is basically just processing the gcode commands and outputting pulses to the ports unmodified by things such as torch height control and backlash compensation (the ones that I'm aware of with my small limited knowlege of how Mach applies to plasma (threading may be another one given there is a fair few posts on the ss and threading but I've not considered that process)

I'm aware that the Ethernet ss has a thc checkbox and can apparently do backlash compensation but I thought (more wondered) why no one else seems to have pointed out this flaw ... I knew it was reliant on a small band of programmers (one or two) so development progress would be slow to implement changes (source code does not appear to be available for anyone to have a go at development , bit closed shop maybe I'm just getting too use to collaborative working for mutual gain life's too short and all that)

I like the ss because of its size and I have a bob for it now too so could buy the ess and flog the usb ss for little loss (no loss I've learnt a lot!)

I have a plan.... Or idea more likely and I thought you had a proma THC

Upon setting up the Proma Thc there are only 3 settings: target voltage, delay (before arc ok ... Presume delay after the pilot arc drops off) and high voltage ( to indicate the presence of a pilot )

My plasma has a pilot arc of 255v as listed in the manual and a target cutting voltage of about 90v (varies with current setting as you know)

Looking at how some of the other thcs work an antidive setting seems quite common, and one way this is done is basically if the voltage rises by more than say 4% the torch down signal is inhibited (given it's probably found a gap and there is now a voltage spike)

I was thinking... (I've given it a go too and it seems like it would function well) Given the arc ok signal is required before Mach responds to the up and down signals if instead of setting the pilot voltage at 255v I set it at say 100v, so the arc ok signal would be extinguished if the plasma found a gap and the voltage spiked (mine spikes to about 140 when cutting circles or ending a cut)

Then setting the voltage band quite close say 4v (plus or minus 2v) if that would provide better control

The next bit may improve things as far as the proma thc, you may know they have the SD version which drives the stepper motor directly but as I have the standard version was wondering if the other criticism of thc could be sorted which is the anti dive function under slow moves which is done a bit crudely in my opinion via Mach to turn off thc if the speed of the torch moving across the table drops below a arbitrary fixed percentage...

Given I'm a bit of an arduino man and the stepper step and direction signals work at 5v it should be easy to piggy back an arduino micro or mini onto the stepper drive signals to employ some proportional z axis drive based upon a torch enabled signal, thc ok signal, and a thc up or down signal in connection with calculating the velocity of the combined x and y axis using Pythagoras theorem and the PulseIn arduino function to then directly control the z axis stepper drive whilst the torch enabled and arc ok signals are being received

It may be possible to add a few optoisolators to the proma thc given one of the criticisms that it uses relays and they are slow to respond at high torch cutting speeds and also maybe tap into the optoisolated torch voltage to provide pid control directly to the stepper

What do you think?

Complex solution to a simple problem (swap a usb ss for a ess or proma thc with a proma SD thc)

The thing that does not impress me with the proma sd thc is the only extra setting you get is speed which appears to be the z axis speed setting no acceleration or deceleration settings

Shooting the breeze question maybe

The thing that got me thinking was (besides the usbss appearing not to do thc or me understanding how and where thc was controlled ) was the " neuron thc " and how that appears to work and wondering if through the addition of a small arduino you could add some of that functionality at little extra cost plus I'm intrigued at improving my hole cut quality and how hypertherm true hole may be emmulated using a double pass by raising the cut height for the first pass for more of a bottom inward bevel and then lowering the cut height for a second pass to remove the bevel and square the hole.

Only been test cutting 2mm ms plate hoping to get some 5mm tomorrow, I've got some 6mm but not tried / don't need that yet

Cuts a 50mm square bang on (slight edge ripple from the transformer plasma (non inverter or igbt)) but the circles are 0.5mm undersize on all holes (20mm, 10mm and 5mm dia) so they could be very easily drilled correct size

No distortion can hardly see a heat affected zone (HAZ) and no top and only the smallest bottom dross and very small divots on the circles with arc leadin and out (3mm) on the 10mm and 20mm holes and a 2.5mm perpendicular leadin and out on the 5mm dia hole

Still a bit buzzing with how easy it is for me to now go from cad to holding the finished item in my hand in seconds

(sorry long post hopefully someone will find something interesting in there if even a bit off topic for a landrover forum but now at least I can make and copy sheet metal parts to a degree keen to get the lathe and mill done at some point using the same driver box)

Rob

[Blanco]

Don't worry Robert, I find lots of interest in it,.... if I am completely honest it also makes me a bit anxious about the control side of things, my kit from Si is still in its box and will remain so until the present house move is done and dusted (France to Ireland,.. all change!). Still I'll be very happy if I can realise a reasonable quality cut. This is going to give a whole order of magnitude improvement in the quality of little fabrication projects. Thanks go to you guys for posting all this. Regards, Rob

[robertspark]

No problem

Don't be anxious about any of it just ask questions before you turn anything on if your not 100%. Take photos and post and we will all help

Nothing worse than buying something and then never putting it together (I'm guilty of that) (I know that's not the case with your move, but when you get into it)

At least people answer questions on this forum .... And don't hold anything back (some of the other dedicated forums I'd be happy for some to just say it won't do whatever I need it too and go look at another product or just respond with something (70 views and not a comment at all is not good)

Anyone else built theirs or need help just post your questions

Rob

[robertspark]

Bit off topic, but may be of interest to anyone considering the intricacies of torch height control with a plasma cutter on a cnc table, and what the box that controls it has to deal with.

It's interesting to note that the guidance (many manuals from CandCNC) seem to provide the guidance that a 1 volt difference in target torch height is equivalent to around 0.025" or 0.015" (most ref's on the "greater than 0.015" statement) [0.635mm to 0.381mm].

Now at first this may appear somewhat insignificant, but consider that the following following torches have the following recommended cutting heights:

all hypertherm (easy to find very comprehensive info online).

Powermax 350 ~ 90V cutting, open circuit 255V, torch to workpiece 1.6mm @ 25A

Powermax 600 ~140V cutting, open circuit 300V, torch to workpiece 1.6mm @ 40A

Powermax 800 ~120V cutting, open circuit 300V, torch to workpiece 1.6mm @ 50A

Powermax 1000 ~140V cutting, open circuit 300V, torch to workpiece 3.0mm @ 60A

Powermax 30 AIR + XP ~ 83V cutting, open circuit 256V, torch to workpiece ? (drag cut!) mm @ 30A

Powermax 45 ~132V cutting, open circuit 275V, torch to workpiece 1.5mm @ 45A

Powermax 65 ~139V cutting, open circuit 270V, torch to workpiece 1.5mm @ 65A

Powermax 85 ~143V cutting, open circuit 270V, torch to workpiece 1.5mm @ 85A

I had a look on the R-tech website and could not see any specific cutting voltage or cutting height information (these seem like fair value machines available on long term drip which makes them cost effective IMHO [like the 50HF machine myself])

The key item in common is the ~ 1.5 to say 1.6mm (1/8") optimum cut height.

recapping the guidance that seems to be 0.635mm to 0.381mm (say ave 0.5mm / 1V torch deviation), we have a band of roughly 3 - 4V (at best) before the torch could dive into the work piece.

This brings up another interesting point which is micro-processor resolution.... the vast majority of arduino boards are 10 bit resolution (i.e 1024 counts), so if you were to measure the say 256 V open circuit voltage (using a voltage divider or voltage sensing chip) to lower the full scale voltage to 5V (i.e. 256V = 5V), the lowest resolution you could get would be ~ 0.25 volts. Fairly accurate one may say at this point, but consider that the actual band that we are trying to control the torch within is 3 to 4 volts, the 0.25V only allows for 12 to 16 counts of sensitivity.

The 0.25V "accuracy" seems to be quite a regular number quoted in specs and probably aligns with the 10bits of resolution for voltage readings.

to improve the sensitivity, we could employ a voltage divider that is offset.... think of 3 resistors, 1/4 resistance, 1/2 resistance and 1/4 resistance... so that supply is ~256V, 3/4 scale is 192V, and the 1/4 scale is 64V, so the band being measured is 128V full scale reading, but offset by 64V. Now the resolution would be 0.125V (similar to having 11bit resolution...)

This THC is quite an interesting offering (bit expensive for my needs), but the specifications are interesting:

http://www.neuroncnc.com/

The Neuron system uses advanced DSP (digital signal processing) software servo control loops to control speed, position and arc voltage with unparalleled precision, so you maintain the selected arc voltage with the highest degree of accuracy: ± 0.25 volts!

The Neuron gives you a torch height position control with a set point resolution of +/-0.125 mm (.005") and an arc voltage control with a set point resolution of 0.25 volt.

0.25V accuracy, and a resolution of ~ 0.125mm, so for 1V of THC, the adjusted height would be 0.5mm which seems to align with the average of the 0.025" or 0.015" [0.635mm to 0.381mm].

I've put a controls flowchart together more out of curiosity than programming point of view at present, I'll have a go at uploading it at some point (when it's been smoothed over [looks a bit too rough at the moment], but it's quite interesting how close the tolerances are.

It's also worthwhile to note that given the accuracy of the microprocessors (Arduino ones for example), being 2 to 3 bits, the number being measured with a 10 bit resolution over 256V could be ~ 0.5 to 0.75 volt out, so it's probably best to get a good cut without THC and have a look at the cutting voltage and then set up your THC to track that voltage and not focus on the actual number so much given the THC's are more than likely not voltage calibrated. and over time (like any electronic instrument) will drift and loose their accuracy. So, tracking the same number is more important than the number.

For those interested and not aware, a big word of caution, if you are using Mach3 to do your motion controlling via the standard parallel port, no problem, otherwise it's best to make sure that your motion controller (smooth stepper, KFlop, Purelogic, etc) actually does THC, as I've found out that the torch height correction is handled directly before it's output to the steppers, nothing is actually done within Mach3.... hence the reason why apparently a number of motion controllers do not offer THC or backlash compensation.

From my observation, using backlash compensation in Sheetcam, via the post processor just breaks the g-code down into smaller chunks in order to compensate for the backlash. Think of a circle being cut in one smooth motion, this can be done with one g-code ("G02" & "G03"). Sheetcam (when you select the Mach3 + backlash comp. post-processor) will break that circle down into smaller moves to accommodate the backlash, nothing wrong with it, but if you're on the sheetcam demo licence (hobby use) [i.e. it's free!], you will be limited to I think 500 lines of g-code.... may be worth thinking about .... whereas using mach3 + the printer port (or a backlash compatible motion controller!) will do the corrections internally without the use of breaking the code down and using more lines to do the same task.

Hope this is of benefit to someone (if not, at least I know where to read it later....)

Rob

[Lewis]

The vast majority of this thread goes way over my head, but I enjoy reading it nonetheless

Thanks rob and si for your many informative posts, I'm certain it'll all mean more to me when I get this project started

[garymorris]

I'm with Lewis , this has gone straight over my head the worst thing is I could really do with one of these (I'm a steel fabricator)

I could manage the assembly bit , it's the computer/controller wizardry that got me beat

[robertspark]

Sorry... I apologise if it was a bit too much over the top (I was trying to denote some of the thought process behind torch height control, or at least how I saw it, I am very mindful that this was probably / most definitely the wrong forum, and I may be leading someone down the wrong path given if it was posted on a cnc forum it would be given better peer (or is it armchair?) review of the content.

Not expecting anyone to comment on the content.

In simple terms I have a problem in the selection of one of the pieces of hardware that I've chosen (it will not be a problem if you follow Simon's bill of materials for his design). I've posted on the manufacturers forum for an answer and got nothing, not a response, and to be fair I've got a bit P'd off given a simple "it don't and won't ever do that!, you need to buy this piece of hardware." would have been great.

Hence I've been thinking about a way to overcome that problem and get two items of hardware to do what they are suppose to (torch height control and a smooth stepper USB), and in the process [i'm known for mission creep!] I've tried to figure out in my own mind how a torch height controller may work, and whether I could build a simple circuit that could piggy-back onto the bits of kit that I've got to get it to solve my problem (instead of buying an Ethernet smooth stepper for ~£140 and selling my USB smooth stepper for whatever I can get).

Rob

edit: - the forum (and this thread) has acted as a great searchable resource for other things that I've wanted to go back and change or improve later (such as the bearings that Si used are listed on something like page 5 or 6)... the info is all there if you want to improve something later.

It's actually surprising how much information is actually on this forum that can be adapted to other non-landrover applications (just look at the wiring and switching bits in the megasquirt section).

[garymorris]

No need to apologise Robert , not your fault that I'm daft

[simonr]

A couple of things worth mentioning....

I couldn't get the Proma THC to work on material thinner than 6mm. Less than that and the torch just rises & rises! I don't think much of the user interface either. I've actually ended up just using the touch-down torch height control (winding z down until the micro-switch opens and using that to reset the Z=0 position). That works well for all thicknesses.

I have also been thinking about building / writing an Arduino THC but using one of these http://www.adafruit.com/products/1085 in differential mode and using a variable resistor to set the reference for the differential. It has a 16x amplifier for differential - so your resolution can increase by 256x compared to a standard Arduino.

You only need to measure under / over the target voltage with any precision.

My USB / Parallel adaptor is THC compatible: UC100 USB Motion Controller and not withstanding the shortcomings of the Proma, it works fine!

I'm surprised you need Backlash Compensation! I have so close to zero backlash that it's not worth worrying about.

Anyone worried about the wiring / software / setup - have a look here: http://www.instructables.com/id/CNC-Plasma-Table/ where I've written it up.

There are still two of the kits available - just in case!

Si

[robertspark]

I've actually ended up just using the touch-down torch height control (winding z down until the micro-switch opens and using that to reset the Z=0 position). That works well for all thicknesses.

I use G28.1 in the sheetcam post processor (mach3 + backlash + scribe, just edit the file for the custom settings as you may need to change them to you preference) to reference before every cut and that is working well (but I'm only cutting short / small pieces at present)

I have also been thinking about building / writing an Arduino THC but using one of these http://www.adafruit.com/products/1085 in differential mode and using a variable resistor to set the reference for the differential. It has a 16x amplifier for differential - so your resolution can increase by 256x compared to a standard Arduino.

That A/D board is very interesting

My USB / Parallel adaptor is THC compatible: UC100 USB Motion Controller and not withstanding the shortcomings of the Proma, it works fine!

I like all my spare ports (I've put limit switches on it + the THC (not that it's working!!!)) --- plus maybe the pneumatics for the engraver etc. (circuit boards.... the options are endless).

I'm surprised you need Backlash Compensation! I have so close to zero backlash that it's not worth worrying about.

I'm not really worried about the backlash (0.20mm), it's more of a curious thing (it cuts the outside vernier perfect!, but my circles are 0.5mm undersize ??) If it's got a feature, may as well use it if it improves the cut / accuracy.

Rob

[robertspark]

With regards to that 16bit A/D chip, the only negative thing that I can see for me and the way I was looking at the control loop was I was looking at using the interrupt pins on the arduino boards to pick up on z axis movement (i.e. if mach [or the SS]) outputs a signal (PUL / DIR) on the z-axis line, the THC will be inhibited, and the chip would act as a "passthrough" for the signal.

The ADS1115 is I2C so on the arduino (std chips), it uses pins 2 & 3 (which are also the interrupt pins unfortunately).

Hence I was wondering about the use of one of these:

http://www.ebay.co.uk/itm/SPI-AD7705-Dual-16-bit-ADC-Data-Acquisition-Module-Programmable-Input-Gain-TM770-/251932274810?pt=LH_DefaultDomain_3&hash=item3aa855707a

Below is a link to the flowchart I was working on..... it is a whole long way from finished, but consider it bear bones... page 4 and 5 is really the loop bit that is how I was thinking about controlling it, the other pages are just fillers at the moment (consider them as ideas).

https://drive.google.com/folderview?id=0B9eqGqWQVBbWfk94NlNZbUF3b29UcmtQQ3l5cUhUNDNyYTVwQkRjc00wbnFPcHpfWDd0R28&usp=sharing

Rob

[Blanco]

Just on the theme of the technical depth, I think this thread has become a great resource and should stay exactly where it is, and added to with all the nuances of everyone's builds, particularly from Si's kits. I am going to become a Supporter to the forum just to guarantee I can always find it, there I have said it now.

[robertspark]

I was worried it was becoming a little off topic and not landrover related specifically (but you could cut your own sheet metal parts or replacement chassis parts....)

Rob

[simonr]

Robert - pins 2 & 3 are used if you want to detect a rising or falling edge but you can use pretty much any other input for a 'pin change' interrupt. 2 & 3 are only needed if you are using the Arduino to time signals - which you're not.

Any of the pin change interrupts will detect the transition of Pulse or Direction inputs. You then read the pin (in the case of Direction) to see which has been selected.

I've used AD7705's before but found they had poor linearity and noise immunity. Neither of these may be an issue for you and it is cheaper so maybe worth a go!

Mine also cuts circles a little under-size. I had assumed this was deliberate as the hole will likely be used for a shaft or bearing to pass through and the cut surface is nowhere near good enough for that & needs drilling / reaming anyway.

It's easy to remember to add a little clearance when you are drawing the part.

It's my feeling that all of this IS Land Rover related. Most of us will use the Plasma for making Land Rover bits. The rest of the thread is about getting the best out of it so analogous to things like how to sharpen a drill bit!

[landroversforever]

Certainly is on topic..... Perfect for making LR stuff!

Quite a bit of it goes over my head on the control side of things but I'm still finding it interesting!

[robertspark]

.... here's another reason why I2C is probably not a good idea...

the use of pins 2 and 3 can also be used for timer interrupts... so if for example we wanted out output x number of pulses to the z-axis to make it rise or lower, would this not be best accomplished by a function such as tone() which uses the timer associated with pin 3.

tone(pin, frequency, duration)

say you wanted to correct for a 3V deviation (at 0.5v / mm) = 1.5mm correction, z axis as a 2mm pitch, stepper with 200 steps / rev, and 10u step driver setting (i,e. 1000u step per revolution), , so 1500 step correction, you could output

tone (pin, 6666 Hz [400mm/min @ 1000ustep/mm], 225 miliseconds)

which should not delay the loop cycle, before the PID loop can have another iteration, and update the correction?

Rob

[simonr]

Or alternatively:

Use a relay or bilateral switch to connect the Mach3 Z output to the stepper driver while THC is disabled. Then the microcontroller does not have to worry about it.

When in THC mode.

if(ADCValue > Threshold+DEADBAND){
  digitalwrite(ZDirection,MOVEZUP);
  digitalwrite(ZPulse,HIGH);
  delayMicroseconds(10);
  digitalwrite(ZPulse,LOW);
  delayMicroseconds(10);
} else {
  if(ADCValue < (Threshold-DEADBAND)){
    digitalwrite(ZDirection,MOVEZDOWN);
    digitalwrite(ZPulse,HIGH);
    delayMicroseconds(10);
    digitalwrite(ZPulse,LOW);
    delayMicroseconds(10);
  }
}

Well, something like that. Don't worry about making a precise correction because by the time you've moved, you'll need to move again!

If you find it hunts - increase the size of the dead-band or the delay between pulses or both.

Si

[robertspark]

Ideas....(torch voltage sensing)

http://www.analog.com/media/en/technical-documentation/data-sheets/AD202_204.pdf

http://www.ebay.co.uk/itm/HYPERTHERM-VOLTAGE-DIVIDER-ISOLATED-for-Arc-Voltage-Height-Control-Pt-No-041274-/131184598033?pt=LH_DefaultDomain_15&hash=item1e8b361c11 (zoom in to picture)

http://www.analog.com/media/en/training-seminars/tutorials/MT-071.pdf

http://www.avagotech.com/docs/AV02-1896EN

http://www.lem.com/docs/products/lv%2025-p.pdf (quite possible)

http://media.digikey.com/pdf/Data%20Sheets/LEM%20USA%20PDFs/LV%2020-P.pdf

[simonr]

I've used Isolating voltage transducers a few times - most notably in the high voltage (up to 300v) charger for my electric car. They work very well and are quite linear.

Si