106. Accuracy, repeatability – a case

CNC

Positioning accuracy, repeatability – achieving tolerance less than the accuracy

Here’s a question:
A CNC lathe has a positioning accuracy of 0.010mm and repeatability of 0.002mm. Is it possible to machine a diameter with a tolerance of +/- 0.006mm on this machine ?

The answer:
Yes.
With a pos. accuracy of 10 microns, a command of X100.0 will cause the tool to go to anywhere between X99.990 and X100.010. However it will repeat this error within 2 microns, every time.

Let’s assume it goes to X100.07 in the first part – out of our tolerance range. Subsequent parts will be in the range X100.05 to X100.09. So just check the dia. on the first part, and fine tune the offset so you get 100.00 dia. Because the repeatability is 2 microns, subsequent parts will come within 100 +/- 2 microns.

Essentially, this CNC lathe makes a mistake in positioning, but it makes the mistake consistently enough that it is a behaviour. The ‘mistake’ is the positioning accuracy. The ‘consistency’ is the repeatability. This machine is making its mistake with a high degree of consistency, high enough for us to achieve close tolerances on parts. This applies to any machine – lathes, machining centers, grinding, etc.

The idea for this post is from a question that Mr. Virendra Patil of Shree Spherotec Pvt Ltd asked in an earlier post. He also confirmed the logic through laser measurement.

 

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My favourite no-nonsense pub

Pecos in Bangalore is the place I go to when I want some good food and lots of draught beer. There are some 5 brands of beer, and a long food menu with unusual dishes. There’s music from the 70s playing. Big spread for carnivores, not so much for herbivores. The dishes have names that tell you exactly what they are, no fancy names. Nothing like Chicken 65, that’s ubiquitous all over India (and nobody knows what the 65 is – maybe the age of the chicken when it reached the Lotus feet of the Lord ?). You also have dosas and parotta to go with whatever you are eating. Next time I should trying pairing the parotta and the beer – dip the former in the latter, like dipping biscuits in tea 🙂 The parotta is the fluffy Malabar Parotta of Kerala, not the same as the Paratha in Punjab.

pecos2 pecos1

pecos3 pecos-menu

The older branch of Pecos is off Brigade Rd, and the newer on Church St. The one on Church St has an actual side panel of a minibus (I think it’s a Bajaj Matador) mounted on the wall, with wheels. Others may think this is corny, but I love it and think it adds character to the place.

pecos-food2 pecos-food

Everything about Pecos has been the same for decades, and the management couldn’t care less about changing trends in decor, music, food or booze. Their ‘This is it, people, take it leave it’ attitude works for them. It’s popular with people across generations, and often sees my 21 year old daughter and 57 year old Me hanging around (the former making full use of her newly acquired ‘007: License to booze’).

1 pint (your usual mug size) of draught beer has 180 calories, which means it took me 1.5 hours of cycling to work the next day to burn off the 4 mugs I had that day. I read somewhere that sleeping burns up 60 cal / hr. Next time I’ll just sleep 3 hours for every mug – 12 hours for 4 mugs. Why didn’t I think of this easy weight loss program earlier ?


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