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Author Topic: Purposly Making a Weak Spot  (Read 1299 times)

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Offline Katoh

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Purposly Making a Weak Spot
« on: May 12, 2017, 07:41:46 PM »
Hi Friends

This is more a machine mechanical question then a Mach question.

On my lathe I have removed the induction motor VFD and Digispeed controlled and replaced it with an AC Servo. The Servo is 2Kw and drives
the spindle with a toothed belt on a 1:0.8 ratio. The induction motor drove a first a stepped V-belt pulley with a timing belt also but from the first pulley
to the spindle was V-belt driven. I have found if things go wrong for some overseen reason and the cutter suddenly ploughs itself into the work piece that
spindle would lock-up as the v-belt would always have a degree of slip in it. Certainly the job would be ruined but at least minimal damage to the lathe.

With the servo tooth belt driven to the spindle that degree of slip I had with the V-belt is gone, If things go astray I can see some major
damage being done to the lathe, I need to build in a shear point, weak joint to give way first. My thoughts are to machine a plastic keyway on either the
motor or spindle pulley or both to replace the metal one, If things jam up that point might give first.
What are your thoughts?


Cheers
Katoh

Offline RICH

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Re: Purposly Making a Weak Spot
« Reply #1 on: May 13, 2017, 07:06:11 AM »
Katoh,

Shearing pins / devices are designed and require a basis. I would say that % functional accuracy would be
in the +- 15 to 20% range. Trying to get to say 5% requires a "lot" testing to have repeatability in that range
but even then 10% may be considered practical.

Before you even try to design a shear device for protection one needs to define what they are trying to protect.
Cost and degree of protection is evaluated based on hazard potential.

Electronic control can be done and in industry they have redundant protection.

FWIW,

RICH
« Last Edit: May 13, 2017, 07:09:52 AM by RICH »

Offline Katoh

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Re: Purposly Making a Weak Spot
« Reply #2 on: May 13, 2017, 08:42:27 AM »
G'day Rich
This is why I asked the question, Maybe somebody could steer me into a different direction. The shear pins is what I came up with from two sources, basically its based around a concept that works on firstly my Rotary-hoe, on the tines is an aluminium shear pin that drives them round, if you try to push the hoe to hard or deep in one action the pin would shear and major damage to the machine reverted.
I don’t know enough about Servos or the programming to put in something that stops when a certain figure is reached, amp draw ect, nothing in the manuals or setup relate to it.
I look at it as simple insurance on the machine, a given sacrificial part is way easier and cheaper to replace than a complete spindle or damaged to a quick change tool post  ECT.
Maybe there is no such thing. I do know, another thing I noticed was the Bridgeport mill uses a plastic key-way on the spindle and this I assume is for the same reason.
Thanks for the reply.
Cheers
Katoh
Re: Purposly Making a Weak Spot
« Reply #3 on: May 13, 2017, 01:59:05 PM »
You don't say what servo drive you have but it would be unusual for it not to have overload trips. The best would be if it offered an instantanous trip output at some very high load, and a time delay trip for short term overload that doesn't break things. The best reason for using a drive output is you can E-stop Mach 3 to prevent continuing motion driving the tool when the spindle stops. A mechanical device won't easily do that.
Re: Purposly Making a Weak Spot
« Reply #4 on: May 13, 2017, 06:22:44 PM »
Hi,
I go with garylucas on this, the only AC servo I've had any experience with allows me to program a maximum current
ie maximum torque. If I decide 2Nm is the max torque my machine can handle then max current  equals max torque divided
by the torque coefficient, .8A/Nm in the case of my servo spindle.

This method will limit the torque developed by the servo but not the forces that occur if you bury a tool into a rotating
workpiece suddenly. In fact I don't see any 'shear pin ' helping you either.

Craig
My wife left with my best friend...
     and I miss him!

Offline Katoh

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Re: Purposly Making a Weak Spot
« Reply #5 on: May 13, 2017, 11:22:35 PM »
Thanks for the replies, I definitely can see your point about e-Stopping Mach as well. I will read through the manual again and try and find the parameter on the current draw, with the VFD was easy but the literature on these units is quite vague.
The servo used is a Mige 2kw driven by a SD 220v AC driver produces 7.7Nm which is close to the same spindle torque as what the original VFD driven motor with gearing would of had at 500-600rpm. I will try and find the torque coefficient and work from that, might even have to experiment a little and bring it up in steps to find the correct setting.
Definitely gives me somewhere to look, thanks to all.
Cheers
Katoh
Re: Purposly Making a Weak Spot
« Reply #6 on: May 16, 2017, 02:04:30 AM »
Hi,
just about all the servos specs I've read, and I've looked at quite a few, comparing various units for the purposes of making a high torque
spindle have both a voltage and current coefficients. While the voltage coefficients are commonly quoted in different units and so can be
somewhat confusing when comparing units the current coefficients are almost always in Nm/A. As a matter of principle the higher the coefficient
the more torque can be achieved for a given amount of current, very useful to glean differences between models and manufacturers.

Craig
My wife left with my best friend...
     and I miss him!

Offline Katoh

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Re: Purposly Making a Weak Spot
« Reply #7 on: May 16, 2017, 10:45:26 AM »
Hi Craig
A bit more research and basic maths , I worked out my motor is 0.85a/1Nm = 6.5A at run speed not start-up which is 16.25A and in the manual has a figure for rotor inertia of 21.7Kg/cm2.
In the Drive Manual I managed to find exactly what I am looking for its Alarm No.13 (excess load) "Servo output torque over allowed value" In saying this I still can not find the correct parameter to put the figures in. I was thinking when I find the correct parameter I would start of with 2 amps as max  and turn a piece of scrap at maxim cut depth/speed for the lathe if it stalls keep going up till I find its point where it just wont trip but any more will.
I have added the Driver manual if you get a chance look at page 27 you might tell me what I’m looking for. One thing with all this is I plan to use this with swapaxis too, so not only will it be a spindle but a "C" axis degree rotational around Z. I wonder if lowering these figures will cause troubles with this?
Thanks!
Cheers
Katoh
Re: Purposly Making a Weak Spot
« Reply #8 on: May 16, 2017, 04:33:56 PM »
Hi,
looks like parameter 61 is motor rated torque. Set that to the nameplate torque.
Parameters 45 to 48 are percentage of rated max torques.
The 'internal torque' limits the maximum current within the torque control loop while the 'external torque'
limits the torque output of the torque control loop. Note it is possible that the output torque be limited to less than the
internal current command, it allows more dynamic control at the potential risk of loop instability.

Craig
My wife left with my best friend...
     and I miss him!

Offline Katoh

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Re: Purposly Making a Weak Spot
« Reply #9 on: May 17, 2017, 12:12:13 AM »
Hi Craig
You sir are a champion!, I did look at those parameters 45-48 but wasn't certain exactly what they meant. I guess I would have to now jut play around with those figures to get a good outcome, I think a good starting point maybe at 50% then work from there.
Again thanks so much, cant wait to see how it all performs.
Cheers
Katoh