Author Topic: The basics  (Read 12272 times)

Offline awemawson

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Re: The basics
« Reply #50 on: August 27, 2016, 11:36:01 AM »
Tool Changer equipped machines John - you need home switches on such a machine so it knows where to grab the next tool  :lol:
Andrew Mawson
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Offline John Stevenson

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Re: The basics
« Reply #51 on: August 27, 2016, 11:37:44 AM »
Yer, North 40 as a table, rub it in.......................................
John Stevenson

Offline sparky961

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Re: The basics
« Reply #52 on: August 27, 2016, 12:14:58 PM »
I never fit limit switches to my machines or homing. Homing wastes far too much time. just set work co-ordinates and machine co-ordinates to the same point and then zero on the corner of the work or vise and you always know where you are.

I have to respectfully disagree with omitting limit switches.  While my project is still ongoing, I've already made a few things under computer control and the limits have come in very handy.  Initially when you're setting up drivers and other parameters, hitting the limit switches is pretty common.  Even under moderate speeds it's nice to have the machine hit a switch and error out rather than hitting mechanical end of travel <cringe>.  Perhaps if you're using under-powered motors it's ok because they'll just stall out, but when you ramp up the power or mechanical advantage you risk damage without working limit switches.

After finalizing your control/drive settings, the limit switches are very handy when you're jogging around and inadvertently go too far.  With my current configuration (LimuxCNC), I can repeatedly bump into the soft limit (just shy of the actual switch and safely away from travel limit) and it just stops jogging that direction and lets me reverse.  In order for this to work properly, you _do_ need to home the machine at least once at the start of your session using fixed switches.  Better yet, the switches plus the index pulse from an encoder.

Oh, and I don't know about anyone else but I'm never in so much of a hurry that I can't watch a machine home itself a few times in a session.  I like to know that the limits and motors are functioning correctly and this is a "quick" and easy way to do it.... assuming you have a system that can move faster than a snail.  With my previous iteration (under-powered steppers and cheap driver with Mach3) I'd have to go get a snack while waiting for it to travel the length of the machine.  Now with decent servos, it's only a few seconds.

-----

But I digress.  Habitually.

Back to Will's questions, one that wasn't addressed was the setting of distance traveled relative to steps.  The first thing to do is the math that involves the angle of rotation per step (commonly 1.8 degrees, or 200 steps per rotation), fractional driver steps (full, 1/2, 1/4, 1/8th, etc), the gear ratio between motor and axis (if not driven directly), and the pitch of the leadscrew.  You can work it out logically or do a search for some calculations you just plug the numbers into.  This should get you _very_ close. 

After that you'll want to tweak it as close as you want, within the mechanical limitations of the system.  You can do this with a 1" travel indicator, which is ok for hobby use but does have some "significant" error in full travel.  Or using a DTI and gauge blocks - a more accurate method, or if all you have is a set of digital calipers and you can figure out a clamping arrangement this can be pretty good too.

The idea is to determine the difference between what the machine "thinks" it's moving, and what it's actually moving.  Then you adjust one of your parameters by the percent error.  If it gets worse you're doing likely it backwards.  Keep doing iterations until you get as close as your hardware lets you.  Do it for each controlled axis.  For some really long axes it may be worthwhile to set up a "leadscrew map" to account for differences in pitch along the length, but for most machines this probably won't gain you much.

Offline John Stevenson

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Re: The basics
« Reply #53 on: August 27, 2016, 12:26:46 PM »
If you keep belting the end of the table, pay more attention.

Don't be lulled into a false sense of security, seen it happen all to often. Fit a 4th axis at one end of the table and that limit switch is now null and void because it's behind it. now if that switch is also the homing switch and you aren't paying attention........................................  :palm:
John Stevenson

Offline Joules

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Re: The basics
« Reply #54 on: August 27, 2016, 12:53:04 PM »
For starting out, you want underpowered machines, you also don't want to take your eyes off it whilst it's moving, no limit switch will save over confidence, you don't have a Z limit switch.  If you go for the controller I am currently using, it asks you for steps per mm which in my case is 533.333   YES, it handles to 3 decimal places and will have rounding errors in the final moves, but as it does 32bit maths I won't loose sleep over it.

Guy's TOOLCHANGERS !!!   Will is asking for basics here.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline sparky961

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Re: The basics
« Reply #55 on: August 27, 2016, 02:02:08 PM »
If you keep belting the end of the table, pay more attention.

Don't be lulled into a false sense of security, seen it happen all to often. Fit a 4th axis at one end of the table and that limit switch is now null and void because it's behind it. now if that switch is also the homing switch and you aren't paying attention........................................  :palm:

I'm not convinced you got the point.  To summarize: the significant effort required to select, mount, configure and test all of my limit switches was well worth it.

It's easy to be lulled into the trap of omission to save time and money, but if you want to move to the next step, they're essential.

Kinda like ballscrews...... for which I'm sure I'll eventually give into the purchase thereof.

Offline John Stevenson

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Re: The basics
« Reply #56 on: August 27, 2016, 02:09:36 PM »
No seriously do a rotab or dividing head.
It learns you do do the mechanical of connecting a stepper to the axis of the table and making sure it all revolves free.
Then you move on to the electrics with a simple breakout board like this.

http://www.ebay.co.uk/itm/361459921691

A power supply robbed from a computer, either laptop or desktop and a driver like this.

http://www.ebay.co.uk/itm/TB6560-3A-/301199977453

A simple stepper motor size depending on table but 180 oz/in for a 4" table is fine.

UK based one here.

http://www.ebay.co.uk/itm/Nema-23-Stepper-Motor-1-26Nm-2-8A-4-wires-6-35mm-Shaft-DIY-CNC-Robot-3D-Printer-/121651189123

then it's just a simple thing to wire up and that has taught you how to build a single CNC axis.
And instead of wasting time building a crap wooden router that will only suffice as part for the MKll this will last you all your life.

Every aspect is a learning curve and a keeper, nothing will be wasted.
Plenty of people on here to help you
John Stevenson

Offline John Stevenson

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Re: The basics
« Reply #57 on: August 27, 2016, 02:11:42 PM »

I'm not convinced you got the point.  To summarize: the significant effort required to select, mount, configure and test all of my limit switches was well worth it.

It's easy to be lulled into the trap of omission to save time and money, but if you want to move to the next step, they're essential.

Kinda like ballscrews...... for which I'm sure I'll eventually give into the purchase thereof.

.

You put limit switches before ball screws ???????????????
John Stevenson

Offline philf

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Re: The basics
« Reply #58 on: August 27, 2016, 02:36:59 PM »
My CNC has limit switches on all three axes and a dedicated home switch only in z. (Limit switches can be just that or can be a home switch.)

When I switch on I always do a "home all axes". This first moves the z axis down to my home position (to get to the bottom limit would take a lot of time - but would be the safest option) before homing x & y to the limit switches. It takes less than half a minute.

Mach3 is configured to home in the same z, y & z directions every time. I mount my 4th axis on the LH end of the table. The x-axis always homes to the left away from the spindle.

In Mach3 you can configure soft limits. When you home all axes it sets the limits accordingly and doesn't forget them even if you zero on the corner of a job. In theory you will never hit the limit switches whilst machining.

If you are machining a job which is towards the limit of the capacity of your machine Mach3 checks the GCode for the job and gives a soft limits warning if executing the GCode will exceed these limits. Better than machining an item and only discovering you have a problem when everything stops as you hit a limit switch (or even worse - if you haven't got a limit switch the control won't know and will try to continue machining - wrecking the job.)

Just my 2p's worth.

Phil.

Phil Fern
Location: Marple, Cheshire

Offline sparky961

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Re: The basics
« Reply #59 on: August 27, 2016, 03:25:40 PM »
You put limit switches before ball screws ???????????????

Indeed. 

On my vehicle, I'd fix a brake problem before I became concerned over a small amount of play in a ball joint or tie rod end.  Priorities.

Backlash can be dealt with to an extent in your machining strategy, though it does severely limit the type of cut that can be made based on the direction of travel and where those direction changes occur.  Want to drill an accurate hole pattern?  Just come in from the same direction (in all axes) after every move.  The fundamentals of machining still apply here.

While bordering on being moderately insulting, you're also demonstrating excessive faith in the human operator and control software (written by humans).  Do you omit the E-stop switches as well?  If you do include at least one E-stop in your retrofits, I bet it would get a lot less use if you included limit switches as well.

Nothing is a panacea but limit switches will remain on my list of CNC essentials.

Offline awemawson

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Re: The basics
« Reply #60 on: August 27, 2016, 04:16:49 PM »
I think the consideration is different depending on the power of the drive systems. With a a few kW or so of servo power behind the axis drives of my big CNC lathe or the Beaver Partsmaster CNC Mill it would (in my opinion) be irresponsible not to have limit switches as over travel can cause considerable expensive damage if things go awry.

However if the system is using relatively lower power systems where over travel just stalls the drives without mechanical damage it's another kettle of fish.

When I cocked up and drove the CNC Mill Z axis into the work once it sheared off bolts holding the soft jaws to the vice the sheer strength of the bolts I calculated as 9 tons  :bugeye: No limit switch would have saved that situation, but my point is that the forces can be very considerable.

http://madmodder.net/index.php/topic,9305.msg102982.html#msg102982
Andrew Mawson
East Sussex

Offline PK

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Re: The basics
« Reply #61 on: August 27, 2016, 05:03:29 PM »
I'll add my voice to the limit switch thing. I wouldn't fit them to anything with NEMA 23 or smaller steppers. Our full sheet router has them now, and you can still see the sheared off mechanical stop bolts from when it didn't!

Homing is only useful if you use fixtures to produce parts.

Offline John Stevenson

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Re: The basics
« Reply #62 on: August 27, 2016, 05:24:01 PM »
And the title of the thread is ? The basics.
John Stevenson

Offline PK

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Re: The basics
« Reply #63 on: August 27, 2016, 05:50:11 PM »
To rephrase:
Basically, you don't need limit/home switches on a basic machine.

Offline John Stevenson

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Re: The basics
« Reply #64 on: August 27, 2016, 05:51:30 PM »
Sorry PK, wasn't replying to you, only the post in general.
John Stevenson

Offline Will_D

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Re: The basics
« Reply #65 on: October 22, 2016, 06:10:06 PM »
Have managed to fit a stepper to the x-axis and am getting into the DDCNC controller and its parameters (There are a lot).

Have set up soft limits, backlash compensation, and am starting to speak "baby g-code"

Question about Z axis tool sensor:

Helps if you have one and know how it works!

Quick google finds the cheapest at about 15 and the most expensive is 150 plus.

Am I right in assuming that its just an insulated (from the table) block of metal with an exact, known height of say 30 mm that when touched by the tool closes an electrical circuit of the controller (+12v to Probe Gnd)?

Just checked that Tungsten Carbide conducts but what about exotic ceramics (not that I can afford any)

I will be simulating this and the hard limits/home limits by a simple switch (NO/NC) tomorrow.
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Offline awemawson

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Re: The basics
« Reply #66 on: October 23, 2016, 03:38:33 AM »
The Z height sensor doesn't NEED to be a precisely known height as it is used comparatively. Set your longest tool using it to zero off set, then all your other tools are measured and their 'shortness' is set as an offset in your tool table.

Touch off your workpiece top surface with the longest tool and this becomes Z Zero, and all the other tools will be correct when their offset value is read from the tool table.

Of course if the height of the sensor IS known precisely then it can be used on the workpiece instead of touching off Tool Zero
« Last Edit: October 23, 2016, 05:40:55 AM by awemawson »
Andrew Mawson
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Offline DMIOM

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Re: The basics
« Reply #67 on: October 23, 2016, 05:06:42 AM »
.....Question about Z axis tool sensor ...... Am I right in assuming that its just an insulated (from the table) block of metal with an exact, known height of say 30 mm that when touched by the tool closes an electrical circuit of the controller (+12v to Probe Gnd)?.........

Will,

The essence is that when the tool touches the sensor, a circuit is made (or broken) and the simplest form is, as you describe, a conductive plate or block insulated from the rest of the machine.  However, a solid block is unforgiving, and you can damage the tool tip or block unless you sneak up via a Rizla paper or similar. Better sensors have a platform with some 'give' - the contact pad can be driven down (against a spring) if you over-travel but once you back off the platform rises back to its datum until it contacts a fixed lip (and, just like referencing a machine's axis, its usually better to take the position when you back off as that eliminates any over-travel). The platform can be just mounted on a plunger, but there is a small danger of binding especially if the contact is off the centre of the platform or button, so even better is the arrangement like the three-legged (but not Manx!) innards of a Renishaw probe.

Dave

Offline PK

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Re: The basics
« Reply #68 on: October 23, 2016, 06:05:04 AM »
Here's the first one I made:
<a href="http://www.youtube.com/watch?v=HqEmL2C54_0" target="_blank">http://www.youtube.com/watch?v=HqEmL2C54_0</a>
Spent hours getting that sliding fit perfect..
All subsequent units have just been blocks of conductive things. The one that came with our 1325 router was just a 10mm thick disk of brass.
Others have used un-etched copper clad PCB material, and I've seen a commercial unit that did the same.

If you are bringing Z down at 30mm/s and it can accelerate at 3000mm/s/s then it's going to take about 1mS to stop.  Average speed (ignoring fancy acceleration curves) will be 15mm/s, so it will overshoot 0.025mm..Now that might be a lot on a big machine, but small machines flex an order of magnitude or two more than that...
PK

Offline Bee

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Re: The basics
« Reply #69 on: October 23, 2016, 03:06:12 PM »
Will  - what size motor have you used? nobody answered your original query about the 425ozin ebay advert.
John gave us
1)180 ozin for a 4in rotab.
so advice please on
2) 6in rotab
3) dividing heads of various sizes
These would always be direct drive but a mill could use 2:1 or 3:1 gearing with effect on speed too so next question is for the basic intro
4) what step rate is sensible for the stepper pulses and
5) what traverse speed in/min is adequate for a hobby mill
6) what speed for Z axis if different from above

having established speed, and assuming direct drive
7) what size stepper for each axis on say X2 and X3 mills.
John again gave us I think 325 ozin for a Z axis on a ?? mill moving the whole head without counterbalance what about
8) quill only and
9) raising knee
as opposite ends of the spectrum.

Offline philf

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Re: The basics
« Reply #70 on: October 23, 2016, 04:13:14 PM »
.....Question about Z axis tool sensor ...... Am I right in assuming that its just an insulated (from the table) block of metal with an exact, known height of say 30 mm that when touched by the tool closes an electrical circuit of the controller (+12v to Probe Gnd)?.........

Will,

The essence is that when the tool touches the sensor, a circuit is made (or broken) and the simplest form is, as you describe, a conductive plate or block insulated from the rest of the machine.  However, a solid block is unforgiving, and you can damage the tool tip or block unless you sneak up via a Rizla paper or similar. Better sensors have a platform with some 'give' - the contact pad can be driven down (against a spring) if you over-travel but once you back off the platform rises back to its datum until it contacts a fixed lip (and, just like referencing a machine's axis, its usually better to take the position when you back off as that eliminates any over-travel). The platform can be just mounted on a plunger, but there is a small danger of binding especially if the contact is off the centre of the platform or button, so even better is the arrangement like the three-legged (but not Manx!) innards of a Renishaw probe.

Dave

I think relying on continuity through the cutting tool to a conductive block (even with an over-travel allowance) is flawed from the start.

With a contact system, in the event of e.g. a wire breaking, the tool will continue driving until something breaks or stalls.

A system which breaks a circuit on the tool touching down (but ideally still with over-travel allowance unless you're prepared to run the tool down at a ridiculously slow speed) is inherently safer in that if a wire breaks or a plug falls out the control thinks the tool has already touched down and the tool won't be driven down at all.

Hope this is of help.

Phil.
Phil Fern
Location: Marple, Cheshire

Offline PK

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Re: The basics
« Reply #71 on: October 23, 2016, 04:42:05 PM »

A system which breaks a circuit on the tool touching down (but ideally still with over-travel allowance unless you're prepared to run the tool down at a ridiculously slow speed) is inherently safer in that if a wire breaks or a plug falls out the control thinks the tool has already touched down and the tool won't be driven down at all.
But you still have wires, if they break then you're diving for the stop. Yes the over travel helps here. But I've done a LOT of tool height setting with both systems; if I broke 3 tools tomorrow because of a dodgy wire, I'd still reckon I was way ahead of a non automatic tool setting machine.
Oh, and:
As a reliability engineer I offer that a single wire to a conductive block has half the failure modes of two wires going to a switch.
 :smart:

PK

Offline philf

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Re: The basics
« Reply #72 on: October 23, 2016, 05:22:19 PM »

A system which breaks a circuit on the tool touching down (but ideally still with over-travel allowance unless you're prepared to run the tool down at a ridiculously slow speed) is inherently safer in that if a wire breaks or a plug falls out the control thinks the tool has already touched down and the tool won't be driven down at all.
But you still have wires, if they break then you're diving for the stop. Yes the over travel helps here. But I've done a LOT of tool height setting with both systems; if I broke 3 tools tomorrow because of a dodgy wire, I'd still reckon I was way ahead of a non automatic tool setting machine.
Oh, and:
As a reliability engineer I offer that a single wire to a conductive block has half the failure modes of two wires going to a switch.
 :smart:

PK

Sorry PK,

My system is fail-safe and there's no diving for an E-Stop button

Yes I still have wires (and 2 at that) which connect to the control via a jack plug. If the plug gets pulled out or one or both wires breaks then Mach3 gives an error message (from memory "Tool Already Grounded") and the Z-axis doesn't move.  Try breaking a wire connected to a piece of pcb material used as a sensing pad and the control will keep driving the tool towards the table until .....

My system also works with e.g. a diamond drag engraving tool which is non-conductive.

Phil.
Phil Fern
Location: Marple, Cheshire

Offline PekkaNF

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Re: The basics
« Reply #73 on: October 24, 2016, 12:29:55 AM »
.....
Oh, and:
As a reliability engineer I offer that a single wire to a conductive block has half the failure modes of two wires going to a switch.
 :smart:

PK

How many well defined nodes you have on return leg? Your failure analysis is only half done :poke:

Pekka

Offline Will_D

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Re: The basics
« Reply #74 on: October 24, 2016, 05:29:39 AM »
Thanks for the replys and warnings about the Probe Overshoot! As I am used to manually touching off the tool didn't realise that the Probe function in the controller will drive the tool into the tool height sensor and so some flexability is required. If and when I motorise the Z axis then I am forewarned.

Am also using a simple switch to explore the Home function of the controller and the Hard limits.

Just waiting for the Y axis ball screw to arrive as I think I have found a simple (ie no milling of the machine) way to fix it in place.

Will
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