Messages

271

General Mach Discussion / Re: THC moves on the Z axis...

« on: February 13, 2016, 02:15:49 PM »

Yup, 200 steps at 4usteps.

4mm pitch / 200 full steps / 4 usteps = 0.005mm/step

1/0.005 = 200 steps per mm.

Make sure its all off electrically before flicking dip switches.

First try it at 3000mm/s/s acceleration...then try increasing it to see if the stall point has gone up (or down).

If down, your fallback is just to go back to what you had.

Nothing lost but a bit of playing around time.

You can then try to find your THC up / dn sweetspot.

Try thcspeed at 30% if Ok try at 40%, if fail, try 35% if Ok, try 37%, if Ok try 39%......sort of split the difference ( must have a name for it ), then split again etc.

You at least know the maximum setting and can adjust down to prevent overshoot (if you end up with that)

272

General Mach Discussion / Re: THC moves on the Z axis...

« on: February 13, 2016, 12:32:28 PM »

Dave, what stepper drives are you using?

At the moment you are using 10ustep resolution...

500 steps per mm... 1u step = 0.002mm

Suggestion to try (if you are up for an experiment and its easy for you to try...)

Change you z axis usteps to 4

This will give you 200 steps /mm (0.005mm per step)

If you can still command 3000mm/s/s acceleration before stalling

Using the above acceleration formula

(3000 / 200 x 60 ) / 3000 (if that is what you want for you rapid velocity), means your target THC % or rapid speed will be 30%

That way you get THC rapid % range (30%)... And Z axis rapid motion speed 3000mm/min


I'll work out the time difference later dropping from say 3mm pierce height to 1.5mm cut height... But the difference should be very small

What is your retract height before touch off (25mm?)

273

General Mach Discussion / Re: THC moves on the Z axis...

« on: February 13, 2016, 12:11:50 PM »

Dave, yup, that's what I was getting at.

Terry, as always, thanks for the tips and guidance.

274

General Mach Discussion / Re: THC moves on the Z axis...

« on: February 13, 2016, 11:50:37 AM »

After reading Terry's post above....

if the maximum acceleration you can get before stall is 3000mm/sec/sec.... and you are running 500 steps / mm

Should you not be better setting your Z axis rapid velocity at say 500mm/min.... and your acceleration at 3000mm/s/sec

That way your THC Speed setting for THC Up & Dn motion would be ~72% of rapid z axis motion...

(3000 mm/sec x 1/500 ) * 60 / 500 mm/min

Hence you can always tune your setting a little more (72% is an easier number to adjust by 1% up or down than 12% is as a 1% change at 12% is 6 times greater than a 1% change at 72%)

...... Sorry I hope the maths logic makes sense it may take a few times to read and understand what I'm getting at .....

275

General Mach Discussion / Re: THC moves on the Z axis...

« on: February 13, 2016, 11:40:24 AM »

Dave,

Remember the THC Up and Dn Acceleration is near infinate (it is shunted / slapped) to whatever your percentage of rapid velocity you have set for the THC

If you look at post #3 on page one...

You listed your maximum (rapid velocity as 1500mm/min) and the THC setting as ~20 to ~30 %...

Say its 20%.

this means your THC slap and go velocity is 300mm/min (5mm/sec) .....

Given acceleration = (Velocity initial - Velocity final ) / Time

In post 6 you gave the steps / mm as 500

hence the time to accelerate from 0 to 5mm/ sec = 1/500 of a second...


You acceleration is 2500mm/s/sec.... for THC


(providing your z axis does not miss a step or two in the process (note 1 step is 1/500 of a mm..... if you can measure that I'll be impressed .... but if it was to miss a step or few the error would be cumulative.... hence if you had a way to toggle the z axis up and down to match the THC UP and THC DN signals.... you shunted it up and down 1000 times (and it missed a step on every cycle) you would accumulate an error of 2mm... which can be a little more visible


But I've read that steppers don't tend to loose one step at a time but the step loss tends to be greater or the motor just stalls.
(can't find the post now,  but it was on here regarding encoders, closed loop and steppers missing steps about 5 years ago)


Also you will note that the 2500mm/s/sec acceleration correlates with your Mach3 target smooth curve acceleration nearly of 3000mm/s/sec..

Hence you could probably up the THC % or rapid speed to ~24%   (if your rapid speed is still the same as post #3 at 1500mm/min)...... however at 3000mm/min.... your THC speed setting will be at 12%


Try it see if your THC works at 15% / 20% etc if your rapid speed is 3000mm/min.... but according to the calc above it may not.



______________________________________________________



If you had an arduino ..... you could always programme it to toggle two outputs and interface it with two pins on your motion controller, swap your THC pins around and try it.... (I use an ESS so have a load of pins but dod thing about this as a potential THC UP and THC DN testing philosophy)

276

General Mach Discussion / Re: THC moves on the Z axis...

« on: February 13, 2016, 10:40:32 AM »

Aghhh, thanks!

277

General Mach Discussion / Re: THC moves on the Z axis...

« on: February 13, 2016, 10:32:17 AM »

I don't know about backlash being irrelevant, as acceleration is very high under THC up and dn control, but the degree of slop may be small.

One of those time will tell moments as it probably will have little effect until wear becomes a big issue... Talking 1mm sort of movement not hairs thickness stuff (m8 nut + studding backlash)

How much travel does your z have endstop to endstop?

I've got about 135mm (5.3"), can't make my mind up if I need more with my redesign (175mm / 7")

Plan on putting a 4th axis below my table... Underslung ... So I can take the slats out and cut tube below ( about 1" to 4").  I have converted a 6" rotary table, but in hindsight... At 90:1 reduction ratio... It may not have been a good idea....  As getting 3m/min cutting speed will be hard (0.75 rev / sec on 1" 1.5mm thick stuff)... Anyone got suggestions on z axis travel or a good idea for a plasma 4th axis?

278

General Mach Discussion / Re: THC moves on the Z axis...

« on: February 13, 2016, 09:56:22 AM »

delrin coefficient of friction ~0.33
http://www2.dupont.com/Plastics/en_US/assets/downloads/design/DELDGe.pdf (page 33)

brass on steel (no lubrication 0.35.... add lubrication 0.19)

not sure what you would lubricate delrin with.

given it is a polymer, it obviously has creep and also some elasticity + a higher coefficient of expansion etc....

there is brass and then there is brass too.... phosphor bronze is a very "slippery" material.... then there is sintered bronze (I suspect oilite goes into this category... but not sure how good it would be at making "bronze nuts"...)

In short it will probably be better than delrin for slippiness .... but may not last as long before showing backlash (where as the delrin will probably mask this)


I have been looking using one of these for my new Z although probably the delrin nut and not the brass one.
http://www.engineeringtoolbox.com/friction-coefficients-d_778.html


If anyone has any experience with either I'd be interested to know a comparison in use / longevity.

279

General Mach Discussion / Re: THC moves on the Z axis...

« on: February 13, 2016, 08:20:59 AM »

Oh..forgot to say.. I sort of  give it a motion of say 1000 lines  (500 one way or the other).

If nothing trips out or stalls (my drives fault for a few different reasons and give an error output which I have linked to my estop) then I know I'm good to go.

Its quicker that using the MDI, one line at a time (you can use "teach" too.... but excel is quicker for me [drag, copy paste, save etc]

I'd do G02 / G03 as well .... when I feel confident doing MDI circles [one day get my head around doing them] .... after that there is not much else you could throw at a plasma cutter is there given the cutting force should be zero.   (bit different on a mill with different feedrates and tools and materials being cut etc).

280

General Mach Discussion / Re: THC moves on the Z axis...

« on: February 13, 2016, 07:57:19 AM »

Have you tried a z axis test script?

Say your travel is 5" (125mm)

set the z at 10mm from the bottom of travel, zeroing the z axis, and give it the following gcode (best done in excel by dragging cells down and then copying and pasting to text file and then saving as a *.tap file)

G00 Z100
G00 Z0
G00 Z100
G00 Z0
etc.

Then change it to
G01 Z100 F3000  (or something of your choosing)
G01 Z0
G01 Z100
etc

Then change it to:
G00 X100 Y100 Z100 (make sure you obviously have the travel distance, and all axis were zero'd at the start)
G00 X0 Y0 Y0
G00 X100 Y100 Z100
etc

This last one will test your power supply by moving all three axis at the same time as well as how well your drives cope....

You obviously want to make sure your machine is setup in its normal cut mode (i.e. torch on torch holder etc.... so the effects of shunting an axis about are correct).