Triac ball screw pitch

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sweep
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Triac ball screw pitch

Post by sweep » Mon 24 May , 2010 15:28 pm

I thought the pitch of the ballscrews on my 1988 Triac were 5mm but after doing some sample measurements using Mach3 it seems that this might not be the case.
With my Imperial DTI on the X axis and programming a move of G0 x12.7 (i.e. 1/2") I get a discrepancy of around 0.003", that is I measure 0.497" on the DTI.
If i change the steps/unit from 200 to 201.18**** then I get the DTI to read the true 1/2".
The only thing I can think of is that the ballscrew pitch isn't 5mm but some Imperial alternative
Anybody know for sure??

...Sweep

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Re: Triac ball screw pitch

Post by Denford Admin » Mon 24 May , 2010 16:54 pm

I can't imagine it being so close and am sure we wouldn't have been able to enter 201.18 Steps/mm on the old controls
What's the reading for 12" (or full) movement ?
If it were out 3 thous out on 1/2" then over 12" you should be able to easily see the difference (nearly 2mm) by marking the slides and using a ruler.

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Re: Triac ball screw pitch

Post by sweep » Mon 24 May , 2010 19:00 pm

I'll do some more measurements tonight and also count the number of teeth on the timing pulleys. I had taken the ratio as 2.5:1 by turning the z axis pulley as that is easiest to get to. mabe an extra tooth on there may have caused my error.
I agree with your assumption that 201.8 steps/mm would never have been adopted.
By the way, there is no slack in the screws and the DTI needle doesn't move at all when I apply force too and fro to the table.

...Sweep

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Re: Triac ball screw pitch

Post by Denford Admin » Tue 25 May , 2010 8:38 am

The other thing it could be then, is loosing step pulse somewhere - set a high accel/decel rate in the software and make sure you move the 1/2" very slowly.
Does this make a difference ?

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Re: Triac ball screw pitch

Post by sweep » Tue 25 May , 2010 10:24 am

No luck.
I have dropped the acceration right down, increased the step and direction pulse width from 5us to 20us, besides trying other settings in-between, I've tried changing from active high to active low on the step, but still i'm coming up short everytime by exactly the same amount. I even increased the acceleration to see if I could lose more distance but it performed the same.
Last night I stripped off the X motor and counted the teeth on the timing pulleys and they are as expected 2.5:1 (12x30).
I measured the ballscrews with a rule and they are 5mm pitch and identified the ballscrew nuts as Warner RM1605 UF.

what now, anyone?

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Re: Triac ball screw pitch

Post by Denford Admin » Tue 25 May , 2010 10:39 am

Dodgy clock / base / measuring point ?

What about a full length measurement ? Is that as far out as it should be ?

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Re: Triac ball screw pitch

Post by Steve » Tue 25 May , 2010 14:03 pm

What drives are you using with the machine? Are they still steppers?

Are you getting the same error when you move 10mm as you are when you move 100mm or do you get 10 times the error?

Now repeat the test with 10 moves of 10mm. what is the error this time?


If you are using steppers and you are jogging at slow speeds the Acel and Decel should not be an issue as you are running within the start stop rate of the motors.

Have you been running the spindle at the same time?

I would clock the slide in a stationary position and see if starting the spindle or coolant pump makes any difference, it may be electrical noise interfering on the step or direction lines.

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Re: Triac ball screw pitch

Post by sweep » Tue 25 May , 2010 14:43 pm

Hi Steve and thanks for your input.

The stepper drives are still the original PKS Digiplan SD2 items driving the original motors. They are interfaced to Mach3 by a simple transistor array (see attachment) though I will be changing this to an opto-isolated creation if I can sort out this distance loss issue.

I have only tried the measurements over 12.7mm as my Imperial DTI is limited to around 1" range. I could experiment with larger distances but would have no way to accuratley measure them. For example at 127mm I wouldn't be able to get 0.030" accuracy. I suppose I could fasten my digital caliper to the table and have a go with that.

Spindle is currently disconnected and is not wired into either supply, though the Lynx08 motor controller still energises when the panel is powered up.
As the Lynx08 comes on with the main contactor it would be difficult to do what you sugest, though I could do the opposite and remove the supply from the Lynx so it cannot have an effect.

...Sweep
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Re: Triac ball screw pitch

Post by angel-tech » Tue 25 May , 2010 23:11 pm

add 100nf caps on all inputs, i had problems with added and lost steps before adding them.
I use the same transistor buffers and have had no problems since adding the caps.

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Re: Triac ball screw pitch

Post by sweep » Wed 26 May , 2010 9:00 am

Thanks for your thoughts Angel-tech.
As i'm a bit of an electronics novice can you just eloaborate where exactly to introduce the capacitors. If you could print off the drawing, sketch on your mods then re-scan it that would be fantastic.

thanks ...Sweep

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Re: Triac ball screw pitch

Post by Steve » Wed 26 May , 2010 9:06 am

The capacitors would be a ceramic bead type so it does not matter which way round they go.

Fit one end to the ground wire the other to the input of the PK2 unit.

What is probably happening is that when the transistors are switching on and off the voltage is not a clean switching level and the change could be registering twice. This could cause stalling or the addition of steps.

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Re: Triac ball screw pitch

Post by angel-tech » Wed 26 May , 2010 10:07 am

on my machine, the caps are used to stop interference on the INPUTS rather than the step outputs. I have placed the caps on the x y and z home connections and the e-stop connection. I found that the e-stop would trigger randomly and the axis would trigger also, but with the caps fitted this stopped.
As Steve has said they go from ground to the connection, and can be fitted anyway round.

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Re: Triac ball screw pitch

Post by Denford Admin » Wed 26 May , 2010 10:57 am

I found that the e-stop would trigger randomly
Do you think that a very short emergency stop signal, triggered by noise, is the cause of steps being inhibited then ? (or even the direction signal being switched off)

I agree noise could well cause such issues - We once had a spindle drive wiring issue that generated enough noise to make the axes move on their own (due to falsely generated step signals)

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Re: Triac ball screw pitch

Post by angel-tech » Wed 26 May , 2010 11:43 am

Denford Admin wrote:
I found that the e-stop would trigger randomly
Do you think that a very short emergency stop signal, triggered by noise, is the cause of steps being inhibited then ? (or even the direction signal being switched off)

I agree noise could well cause such issues - We once had a spindle drive wiring issue that generated enough noise to make the axes move on their own (due to falsely generated step signals)
I'm not saying the e-stop false triggers would cause the added or lost steps, but without the cap added to the e-stop input, i would get false e-stop triggers. Adding the caps to the home inputs stopped false triggers on the x y and z axis.

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Re: Triac ball screw pitch

Post by angel-tech » Wed 26 May , 2010 11:51 am

One example on my triac, as i traversed the x axis the y axis would add steps randomly. This didn't happen when i first converted the triac, but appeared after a few days use.

Needless to say the caps cured the fault.

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Re: Triac ball screw pitch

Post by Steve » Wed 26 May , 2010 12:00 pm

The clock and direction wires should be as short as possible and be screened. They should also not run parallel to any cables carrying large currents (mains supply or Spindle) as these can cause interferance.

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Re: Triac ball screw pitch

Post by sweep » Thu 27 May , 2010 18:23 pm

As far as the capacitor modded circuit is concerned, is this what you mean?
Also, considering the old age and delicate nature of the PKS Digiplan SD2 units, are we pretty safe installing these capacitors?

Since posting last someone pointed out the error in my maths as I have lost around 20 pulses togive the error recorded. Is there still a chance noise could lose this many or is it looking more like mecanical backlash?

...Sweep
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Re: Triac ball screw pitch

Post by Denford Admin » Fri 28 May , 2010 8:35 am

Is there still a chance noise could lose this many
I think there's always a chance of strange things happening with noise/interference - the key is to just eliminate things as best you can.

If noise is affecting the pulses, then cutting a shape should show it up - Doing something like a square raster X clearance cycle will show up an issue if the sides of the square "drift" one way (or even side to side depending on the cycle)

If its backlash then the cutting results could be better, as the lost movement is only limited to the backlash amount

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Re: Triac ball screw pitch

Post by Steve » Fri 28 May , 2010 9:14 am

Hi,

Your diagram shows where I would try placing caps. along with short well screened cable this must help.

Angel-tech was suggesting adding caps in the same way yo the inputs of the e-stop and home inputs of the controller.

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Re: Triac ball screw pitch

Post by Denford Admin » Fri 28 May , 2010 11:01 am

Not knowing your system, but thinking about it logically:

Loosing steps could be a symptom of the estop signal being made intermittently due to noise - this may then inhibit the step pulse output for brief periods
Adding steps is more likely to be a symptom of noise on the step/direction wires- ie, phantom step pulses being generated by interference in the wires.

It's also just reminded me of a strange problem we had once when some dodgy control firmware would intermittently change the stepper's direction signal out.
eg, during a single direction feed move of the X axis, the direction pin changed very briefly a couple of times - the axis still seemed to run ok, but sounded a little bit rough and, obviously, had positional accuracy problems.

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