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Join Date: Nov 2013
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How to Calculate Rotary Table Position Accuracy

11/30/2013 6:03 PM

I want to know how do i calculate exact position accuracy of Rotary table which has a accuracy of lets say 1 arc min & i want to use dial plate of 700 mm diameter then

How much position accuracy should i get at an distance of 700mm.

Please advice

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#1

Re: How to Calculate Rotary Table Position Accuracy

11/30/2013 6:15 PM
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#2

Re: How to Calculate Rotary Table Position Accuracy

11/30/2013 6:22 PM

d = πrθ/180, where θ is expressed in decimal degrees.

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#3

Re: How to Calculate Rotary Table Position Accuracy

12/01/2013 10:29 PM

Rotary tables, used, for example, to drill hole patterns are not as accurate as doing the calculations to generate XY coordinates for the holes.

Machinerys Handbook has the tables for this process which is called Jig Boring.

Sorry if this isn't useful to you.

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#4

Re: How to Calculate Rotary Table Position Accuracy

12/01/2013 11:37 PM

I would use a PLC stepper motor for this and let it determine the accuracy you need for this application.

I have further solutions if needed.

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#5

Re: How to Calculate Rotary Table Position Accuracy

12/01/2013 11:54 PM

First, Welcome to CR4!

For any rotary device, the center of rotation is the origin, and you must specify some direction as zero for Ø (angle). You have specified a dial plate of 700mm DIAMETER, and a DISTANCE of 700mm. Presumably, you are referring to a position 700mm from the center of the table, or twice the dial radius. Since that position is twice as far from the origin as the dial, the position error will be twice as large as the inherent error of the dial.

You have NOT specified a tolerance for either of those numbers. Does 700 mean a value between 650 and 750, a value between 695 and 705, a value between 699 and 701, a value between 699.9 and 700.1, etc?

What is the meaning of "an accuracy of 1 arc min"? Does that mean ± 1 arc min, or ±0.5 arc min? Can you read this dial with a precision ±1 arc min of error? Most rotary tables have a worm gear that connects the drive handle or motor to a set of teeth on the periphery of the table. It may well be that the 1 arc min means that those teeth have been fabricated with an accuracy of ±1 arc min of error.

Then there is backlash; for maximum precision, the operator or the motor must always rotate the same direction when arriving at the specified angle. A careful, knowledgeable operator can achieve significantly higher precision than a sloppy and/or uneducated one, whether the table is manual or motor driven.

Until you specify the precision of ALL factors involved, there can be no definitive answer to your question.

Finally, study the correct meanings of 'exact', 'accurate', and 'precise'. No measurement is ever exact, unless it is a count of discrete items. ALL measurements other than counts have inherent errors, which vary according to the quality of the instruments, the care in controlling conditions such as temperature, and the knowledge and care of the person doing the measurement. It is an accurate statement to say that a person who is 5'0" tall is 2 meters tall, since that person is closer to 2 meters tall than to 1 meter tall. That statement is definitely NOT a precise statement. ALL calculated values MUST be rounded to an appropriate number of significant digits, in order to avoid implying unnecessary precision. High precision measurements ALWAYS cost more.

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#6
In reply to #5

Re: How to Calculate Rotary Table Position Accuracy

12/02/2013 12:09 AM

One of the fun games is attempting to locate the center of the rotary table. Any inaccuracy in locating that center will make all your other measurements moot.

Frankly, I don't think you can get there with an off-the-shelf Rotary Table.

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#7

Re: How to Calculate Rotary Table Position Accuracy

12/02/2013 3:17 PM

The rotary table manufacturer (Camco, Ferguson, etc.) will normally specify a radius or diameter for their table's positional accuracy. It's a straight line extrapolation from that to your application based on the radius to your tooling. ex. 1 arc min specified at R=250mm would give 1 arc min*350/250 at R=350.

Here's a tip: mount your 700mm diameter plate to the table with a timing mark and machine your plate while on the rotary table to avoid compounding tolerances. Use the table to index the plate positions under your mill head.

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