Basic Tube Mill Speed / Cut Off

[69FIREBIRD]

Hi All, I've been asked to design a CHEAP and “accurate” cut off system for a tube mill. Thought I’d put this out there for some feedback – perhaps save reinventing the wheel so to speak.



Basic principle is the length is predefined by down stream detector once activated a cut off saw accelerates alongside on a parallel track aiming to match the tube mill production speed and then cuts the tube. Then the cut off saw returns home for next cut. Mill production rate is slow at approx 6m per min.



Based on the cheep nature of the job I’m looking at the AB Micrologix range. Using an encoder or tacho to measure the production speed of the tube mill and get analog or Modbus feedback from a VSD to confirm speed match before cutting. It has been suggested that the cut is accompanied by a prior clamp to tube and electric / pneumatic drive disengage of the cut off saw drive mech. to avoid any issue with precise synchronisation.



· Has anyone achieved this sort of thing before?

· How do I accuracy and continuously calc line speed using an encoder?

· Any other points of potential failure of accurate measurement / speed matching?



Thanks in advance.

 

[Manglemender]

Hi,

I have seen a few flying saw applications and worked on a few flying shears. They normally work on a length (encoder count)with a position controller determining the synchronisation; this normally involves a cam controller too to govern the accelleration to syncronisation. Once synchronised, trigger the clamps, dissengage the drive then activate the cut.

In my experience, "Cheap" and "Accurate" do not come together otherwise the expensive ones wouldn't exist.

A cheaper option would be to run to a set lenght then stop and use a static saw that you could do quite easily in a PLC.

Nick

 

[Peter Nachtwey]

· Has anyone achieved this sort of thing before?
Yes

· How do I accuracy and continuously calc line speed using an encoder?
You buy a high speed counter card that can calculate the velocity for you. A high resolution encoder is also required to do this. My customers often use encoders in the 500,000 count per revolution range. That exceeds the counting frequency of most PLCs.

· Any other points of potential failure of accurate measurement / speed matching?
Calculating the line or reference speed is the most difficult part. The reference encoder should have very high resolution so the line speed can be calculated accurately. BTW, what is your definition of accurately.

The reason more 'expensive' exist is because they have a lot of the features you need already done like how to calculate the speed, the gearing, cam tables, and clutching in and out which is the IEC term for ramping the gearing up and down. The motion controller people already have done these types of systems many time and have canned example programs where just a few parameters need to changed to make the system run. In the end the 'more expensive' systems will be cheaper.

 

[kamenges]

Once you start talking about clamping to the pipe and disengaging the drive you add enough mechanical cost to justify more expensive controls, even if this is a multiple machine job. If it's a single machine it is an absolute no-brainer to go with the motion controller, clamp or not, unless you consider development time free. But that's creative accounting.

Throw your money at a good motion controller and good traverse drive components and you will be much better off.

Keith

 

[leitmotif]

Once you start talking about clamping to the pipe and disengaging the drive you add enough mechanical cost to justify more expensive controls, even if this is a multiple machine job. If it's a single machine it is an absolute no-brainer to go with the motion controller, clamp or not, unless you consider development time free. But that's creative accounting.

Throw your money at a good motion controller and good traverse drive components and you will be much better off.Keith

To repeat what several have already said.
There are numerous installations of flying cutoff.
You can have cheap or good not both. It is a binary choice.
I worked on flying cutoff systems that cut plastic extrusion to length. We had almost all permutations of methods to do it - some were a royal pain.

As Kamenges said put the money into the drive components and do not worry about PLC too much. From my experience stay away from using pneumatic pistons to do saw traverse. The best system we had used a clutch off the "main drive" to activate the saw traverse which was chain driven. Encoder sensed main drive rotation thus part length. Worked like a charm and very low maintenance. Saw traversse return was on a pneumatic cylinder - that worked well. Pneumatic for saw lower and raise worked well.

Dan Bentler

 

[parky]

Done this at british steel, only the feed rate was 40 mtrs a minute
this required some special plc control & were in fact purpose built based on a little known dc drive electronics modified into a plc (well programmed in ladder of a sorts).
The scada system calculated the metal removal through 4 points in the tube (this could be round or square tube).
The calculation was very complicated that took a HP programmable calculator 11 seconds to reach an answer, we converted the calculation into C as a function in the scada, this calculation was important as to control the feed speed of the "very large travelling circular saw" s as to reduce the wear rate of the blade.
The blade is very expensive to have re-cut & sharpened it is essential.
The saw would match the speed of the tube, travel & cut it was held in a large protective "Building" as a blade smash at these speeds would slice through anybody within the main factory.
This was in the late 80's so plc speeds have increased & dc drives are far better, however we acheived accuracy of +- 1.5mm on tube lengths of 6 mtrs & dia of 220mm.

 

[69FIREBIRD]

Hi All, Thanks for your valuable feedback thus far. I'll advise more detail after site meeting in the coming days.

 

[widelto]

I used to work for a pipe mill for 15 years in a row, my boss did not want to spend money a new flying cutoff controls, he wanted to keep on using same controls based on relays, no plc´s.

This is what we did, we installed a hydraulic cylinder ( move the carriage) which we knew its speed, we make the speed of the line always the same and equal to the speed of the hydraulic cylinder, we use an encoder 1024 PPR conected to an external counter connected to the mill ( Durant from Cutler hammer), we set the counter to trigger to a value that correspond to the desired length, when the counter trigger the cylinder start moving ahead then used a delay timer that triggers when the speed of the cylynder is the same as the mill line, at this point we turn on two solenoids, one to catch the pipe and travel with it, and the other one to lower down rotary knives, when a limit switch was touched by the knives (depending on pipe diameter size ), the knives went back and when they touched another limit switch then we released the pipe from the flying cutoff ( it was held by a chuck ), then the main hyd cylinder went back and a fast conveyor was on in order to send cut pipe out of the line.

Problems:
No change on speed line, the boss wanted it that way
Slow speed no more than 80 FPM
Cuts were +/- 1 inch in a 6 or 12 feet pipe ( withing API limits )
Every time pipe diameter is changed, limit switches need adjustment.

Advantage:
No change on controls, it´s been the same for the last 18 years.
It works almost flawlesssly.

 

[69FIREBIRD]

Hi All, After a site meeting I can confirm the line speed is going to range between 7 and 16 feet per minute (quite slow by some of the numbers I seen discussed) perhaps now the line speed is known some of you may have a different opinion on it needing to be a servo drive?

Other initial specs (need refinement) are -

• Estimated that a 0.55kw motor will drive the carriage (flying shear)
• Estimated gearbox / gear ratio is 100:1
• Rack and pinion system for driving / positioning the carriage (flying shear)
• Existing clamp arrangement included at this stage with option to include clutch to avoid over/under drive of VSD if speed matching an issue.
• AB PLC with encoder input for line speed / length determination and either analogue or comms (DeviceNet to enable fast access to drive parameters - speed ref, accel, decel, etc) to control VSD (Telemecanique Altivar 71)

Thanks again for your input.

 

[Peter Nachtwey]

do let us know how long it took to implement it and what the precision is.

 

[69FIREBIRD]

Hi Peter, with your obvious experience in this field what your gut feel with regard to this config working at this sort of production rate?



Thanks Mike.

 

[ATU]

I am not familiar with this type of process, but when you talk about Clamping , do you mean to affix the saw carriage to the material as it moving?

 

[69FIREBIRD]

Yes that is correct – 2x 1/2 cylindrical clamps sized to suit the tube diameter close (via pneumatics) when the carriage is up to speed to fix the position of the saw in relation to the tube. The tube then draws ate carriage along while the cut is being made.

Ps the current method has an error factor of around +/- 15mm on a 6+m length cut – this is currently improved on in a process downstream to get a clean finished edge. I understand the lengths are sold slightly over length as they are further cleaned up to a perfect finish and square cut buy the purchaser.

It is my understanding that the aim of the exercise is to improve on the error and improve the sample cut function to reduce wastage in both instances.

 

[protocolpacket]

Hi 69FIREBIRD

Haven seen in any of your post the most important question raised by Peter for your requirement:

exercise is to improve on the error

BTW, what is your definition of accurately.

...and what the precision is.

Regards

 

[ATU]

I do a lot of work around stamping of material of high tolerances +/- .0001 inch or better. They use a piloting feature to this. Why couldn't you do the same type of thing. Get close and clamp to the material, then put a flag on the carriage that would be picked up by the encoder card's registration input and latch in that reading. That gives you an exact reading of the realtionship of the saw to the end of the last cut. Add a small adjustment to the saw in realtionship to the carriage. You wouldn't need much , only enough to cover the initial error. You could use an eccentric tied to a stepper motor or a ball screw slide. Make the final +/- adjustment and then cut. Or always clamp short and then the move is always in one direction. I would also measure the cut piece and then feed that information back to adjust the clamp on the next piece.