ABS Plastic Machining Precision

[cs8]

Hi Every one,
I have an machining operation where a robot has to machine holes in abs plastic and machine square holes to accurate measurements say for instance machining the keyboard pockets in Atm machine front fascia. I have faced problems earlier with ABB robots where in the robots shows signs of overshoot at corners of the sqaure, which we had resolved by tunning PID values of the servos.
Does staubli show such problems as well, while doing machining operation to an accuracy of + or - 0.3mm. Please do recommend a model of staubli robot that do these operations with out any problem. I have attached a picture of a ABB robot doing the operation with an overshoot shown on its corner.

Cheers,
Bharat
Software engineer

 

[tygerdawg]

All robots have this problem. Staubli by its design is probably one of the best for path accuracy & corners, if not the best. But don't take my word for it, send your stuff to Staubli's Applications Engineering department and let them program a demo for you to show the true results.

You give no details on the payload or reach required, much less speed of travel etc. You must spec that info to determine robot model.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[cs8]

HI TygerDawg,

The robot that was used previously was IRB2400 robot from ABB. it could take a payload of 16 kilos and we had a spindle at the ned of the robot which weighs about 7kg. The speed it was run at was based on the chip load and the speed was kept at 40mm/s max. The way i got around that was going in and coming out of the corners at 5mm/s. That way it overcame the problem, but didnt resolve it completly. DO you have any contact or emial id of the Staublis application engineering dept.

 

[tygerdawg]

http://www.staubli.com/en/robotics/contacts/

The last I heard, Application Engineering centers are at the Headquarters in Faverges, FRANCE; Bayreuth GERMANY; and North American HQ in Duncan, South Carolina USA

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[EdDanzer]

What you may be seeing is deflection of the arm components from the cutting tool generated forces. Reducing the feed rate, reducing the cutter diameter or changing the type of cutting tool may help.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[cs8]

We had tried reducing the cutter diameter and the feed rate and even that didnt help.
It was like the robot did a jerk as it started the move and when ever it changed direction.
Thats why I reduced the speed of the robot movement to say 5mm/s , for first 5mm of the distance and then ramped the speed up to 40mm/s.

 

[EdDanzer]

The jerk is from a lack of rigidity in the arm, spindle and drives. Slowing the acceleration and deceleration rates when changing direction is the simple solution to your problem. If you need higher production rates a larger capacity shorter arm unit would be required. To test the stiffness of the system attach a load cell to a tool in the spindle and mount a dial indicator to the tool by where the load cell is attached and another indicator to the spindle housing just above where the load cell is attached and start applying a load parallel to the dial indicator movement. As you apply the load you will see the indicator move. If 20 lbs. causes less than .001” of movement on either indicator then changing the acceleration and deceleration rates should help. Use the shortest solid carbide tool possible may also help. If possible use 3 times the diameter stick out on the cutting tool. If 20 lbs. causes more than .001” of movement a more rigid robot or spindle is required.

If you go to this page it will provide some additional information:

http://www.danzcoinc.com/html/spindle_tester.html

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[cs8]

Hi Ed Danzer,
The problem was resolved by doing the following,
1. changing the PID values for robot axis servo.

2. Changing the speeds as we approach the corners and move out from the corner.

3. The tool angle and orientation is changed outside the material and then the drill bit is moved into the robot.

Now the machine is best sent to the client and he is happy with it.

Since we are looking at a similar application again, I would want to use a ABB robot as I had this bad experience with it. I was wondering if there is any other industrial robots which doesnt show this overshoot problem.

 

[EdDanzer]

The things you did to tune the arm are because of a lack of rigidity in the arm structure. I would suggest you do a stiffness test on several similar sized machines from different manufacture to find the most rigid unit. Few people want to do this type of test and many manufactures don’t want you to do this type of test because the results requires judgment and action based on the results, not specification, price, reputation or perception.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[cs8]

But it doesn't seem practical to do a rigidity test. If manufacturers dont allow this test, how will we be able to perform this test. we cant buy different robots, from different manufacturers and do this test on them.

 

[EdDanzer]

With current market conditions you should be able to convince the manufactures to do the test as it is required before you can purchase a machine. Explain how it effects the customer perception and satisfaction of your product and that it should expand your market into other areas.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[cs8]

HI ED Danzer,

Will the manufacturers do the stiffness test on their own and give us the results.

Cousl you explain the stiffness test, how should it be performed?

 

[EdDanzer]

I don’t know if having the manufacture do the test is the best choice. The same measuring device should be used for every machine and the same person should do the test so the results are a direct comparison. The test should be done two different distances from the center to see if the results are linear.

The objective of the test is to measure deflection when a load is applied similar to the cutting tool forces. The cutting tool force is tangential to the flute so you need to measure the deflection in a 360° rotation. The test should be done two different distances from the center to see if the results are linear.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com