Abstract

To Conduct A Structural And Modal Analysis Of The

ShapeOko CNC Mill Frame

Student name : Eoin Robinson Supervisor name: Gerard Ryder

Department of Mechanical Engineering , Institute of Technology Tallaght Dublin , Tallaght , Dublin 24

Objective: To determine the natural frequencies of the structure.

3 x Accelerometers were attached at several locations on the X axis beam and Y axis beam. They are indicated above. The

Structure was struck with a variety of implements at different locations and different Axes . Amplitude vs Frequency data was

measured on Labview and tabulated. The structure was tested on 3 different surfaces , Air, Foam Mat which are indicated

below, and Floor to see if the natural frequencies changed, the results indicated ωn did change. Air proved to give the clearest

results because there was less vibration from the surrounding surfaces. The natural frequencies did change when the structure

was struck at different locations and different surfaces . However , 66 Hz and 76 Hz did occur the most frequently. They were

taken as the systems natural frequencies.

Conclusion: Dremmel Speeds of 66Hz and 76Hz are unstable and must be avoided .

FEA – Numerical Test

A structural analysis was carried out on the X axis gantry frame , a series of weights were applied

to the structure and deflections measured. The structure is expected to hold a load of max 8 kg ,

with a safety factor of 2 . Testing showed that under loading the structure deflected less than 1

micron, with greatest deflection occurring in the centre of the X axis gantry ( see image ). For this

application it is safe to conclude that the beams of the X Axis gantry will not deflect by any

significant amount . Focus was shifted to the XZ plate which would bear the XZ plate which

would hold the Z axis gantry. Max deflection was 0.00025 m , max stress 1 MPa, this is illustrated

below.

The objective of this project is to design, build, test and

mechanically analyse the structure of the ShapeOko CNC Mill

machine. ‘Structure’ in this report refers to the X and Y axis

gantries only. The Project will show how the X and Y axis

gantries behave when loaded with the Z axis Gantry (stress)

and the Dremmel (vibration). The cutter drill will be a

Dremmel (330g) and the Z axis gantry is estimated to be 3

.5kg. A safety factor of 2 will be used the maximum mass

applied upon the structure will be 8kg. The structure illustrated

below in CREO, must support 8kg with negligible deflection.

Firstly a Numerical test will be conducted on the CAD model

via Creo Parametric 2.0; Finite Element Analysis (FEA) will

indicate how the structure deforms when loaded with a

Dremmel. With the CAD model completed the components

were fabricated; Physical testing with actual forces and

vibrational testing can begin. The Physical tests show how the

structure deflects when and the Drill is mounted (Stress

deformation Test ) and when or at what speeds resonance will

occur when the Drill is turned on (Tap Test).

The tests will be conducted on the lightest and weakest parts of

the structure, the Mounting plates. The numerical test results

will be validated by the physical test for stress and deflection.

The tap test will find the natural frequencies of the system

creating an operating range of speeds for the drill.

Design Optimizations

Accelerometer Locations

Strike Locations

Foam Mat Suspended in Air

It was apparent upon visual inspection that the XZ

plate would deflect a great deal. It was held to the Y

axis gantry by 4 x M5 steel screws. They acted like 4

cantilever beams, as stated above in the Introduction

the structure must support the maximum load as well

as accommodating the safety factor of 2. Masses were

applied to a steel hook and increased in increments up

to 8kg. The results are shown below in a table. The

maximum deflection was 0.00035, which is illustrated

in the graph.

Tap Test

The maximum deflection for the physical test is 0.00035m,

0.00025m for the Numerical. Reduce deflection by redesigning

the XZ plate ,thicken the bolts, add another Guiderail on Y axis

gantry.

Natural frequencies are too close to the Dremmel Operating

range must increase the difference. Increase the mass and

thickness of the mounting plates from 1mm – 3mm, this will

reduce the natural frequency, and move the resonant frequencies

away from the Dremmel Operating Range as seen in the adjacent

graph.

Stress Deformation Test

Regenerative chatter

Regeneration occurs when the cut produced at time t leaves a wavy surface on the

material regenerated during subsequent passes of cut.

‘Self-excited chatter vibrations are caused by regeneration of waviness which

result in poor surface finish and reduced productivity in machining operations. It

is well known that the regeneration effect is related to the change between two

vibration waves during the subsequent cuts on a surface. For certain cutting speeds

this phase is minimized increasing stability of the system. ‘(A. Ertürka, 2005)

Stability charts have been derived to show the areas of stability and non stability.

Because of some perturbations in the tool starts to vibrate relative to the piece and

the surface becomes uneven or wavy. Because of the uncut material from the

previous cut the current cutting force will not only depend on the present cut but

the previous on as well.