Metrology-7-Roundness Flashcards
How is roundness meaured?
A part is said to be round in a specific cross-section, if there exists within
that section a point (I.e. the centre) from which all other points on the
periphery are equidistant.
* If the cross-section is not a perfect circle, the out-of-roundness is
specified as the difference in distance of the points on the periphery from
the centre.
What is the simplest method of measuring roundness?
Using a V-block and dial gauge with the component rotated slowly and carefully.Circular components will not alter the dial gauge reading.
What is the V-block form of roundness measurement limited by?
The vee angle and spacing of the irregularities.False readings can be generated by
regularly spaced features.
* A more accurate would be to rotate
the component between centres.
What must a machine made to measure out of roundness supposed to have?
A precision bearing which will provide an accurate axis of rotation to be
used as the datum for measurements.
Ø A means of very precisely aligning the axes of the work piece surface
and the bearing.
Ø A measuring transducer capable of providing an amplified signal of the
indicated errors of roundness.
Ø A suitable means of recording such errors in a convenient form.
Ø A means of making quantitative assessments of the errors measured.
What are the two types of roundness measuring machines?
One type of machine has a stationary measuring head and a rotating work
piece, whilst the second type has a rotating measuring head and a
stationary work piece
What are the advantages of the rotating measuring head design?
Ø The precision spindle carries a light load
(measuring head) therefore allowing high
accuracy measurements without excessive
cost.
Ø The work table (stationary) can be large
thereby accommodating large work pieces.
Ø Computer aided centring and automatic
centring is available.
Ø Internal and external measurements can be
made.
What are the advantages of the stationary measuring head design?
Ø The measurement pick-up is more easily
adapted to measurements of concentricity
and alignment.
Ø The measurement head is more free to move
into constricted measurement positions.
Ø Straightness measurement can be made.
Ø The precision spindle of the rotating turn
table limits the mass and size of the work
piece being measured.
Ø Computer aided centring and automatic
centring is available.
How are the roundness results recorded?
- Displacements of the stylus are movements relative to the axis of rotation of
the spindle, in both designs of instrument. - A perfectly centred cylindrical component will generate a perfect circle on the
polar chart. - With an irregular component, the measurement stylus will detect departures
of roundness relative to the centre of the component.
How do you analyse roundness results?
All methods are based on establishing a centre and then using that
centre to draw concentric circles which contain the trace.
* The radial separation of of peaks and/or valleys with respect to the
circles, divided by the magnification will give the deviation from
roundness.
* Reference should be made to ISO 1101:2013 which details all definitions
of design tolerancing for geometric features
What are the minimum zone reference circles?
- The Minimum Zone Reference Circles are two concentric circles which
just enclose the profile and have minimal radial separation. - The maximum radial distance between the two circles is the value of the
out-of-roundness. The centre is termed as the Minimum Zone Centre.
What is the maximum inscribed reference circles?
- The Maximum Inscribed Reference Circle (Plug Gauge Centre) is the
largest circle which is just contained by the trace. - The out-of-roundness is specified as the height of the largest peak
above the circle.
What is the minimum circumscribed reference circle?
- The Minimum Circumscribed Reference Circle (Ring Gauge Centre) is
the smallest circle which will just contain the trace. - The out-of-roundness is specified as the depth of the lowest valley
below the circle.
What are the preferred British methods of evaluating out-of-roundness?
The Least Squares Reference Circle and the Minimum Zone Reference Circle methods
What is the least squares reference circle?
- The Least Squares Reference Circle can be regarded as representing the
average of all the peaks and valleys (minimum departure calculation). - The out-of-roundness is a function of the radial distance of the maximum
peak (P) from the circle and/or the distance of the maximum valley (V)
from the circle.
What are the other roundness parameters?
Ø Eccentricity is the term used to describe the position of the centre
of the profile relative to some datum point. Denoted as a distance
(mm) and an angle (degrees).
Ø Concentricity is similar to eccentricity and is defined as the
diameter of the circle described by the profile centre when rotated
about the datum point.
Ø Using additional attachments, squareness can be measured,
which is defined as the minimum axial separation of two parallel
planes normal to the reference axis, and which totally enclose the
LS reference plane
