3D Modeling & CAD for Motorsport: Step 3 - Analysis
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Step 3 - Analysis
11.15
| 00:00 | - We now have a basic model of our battery box and the visual aid has already gone a long way in terms of analysis, showing us how our design will fit and work. |
| 00:10 | The analysis process really goes hand in hand with the CAD modelling phase as we tend to analyse and make changes to the model as we go. |
| 00:20 | If the part will function or not is fairly obvious with a simple example like this but there could still be areas we could improve and we also need to think about any issues that could be found during manufacturing. |
| 00:33 | The first and most basic thing we could could do is check the dimensions of the geometry using the measure tool under the inspect tab. |
| 00:41 | With this, we can measure the critical dimensions we're interested in which is the spacing of the fixture holes, the internal height, width an depth of the box plus the resulting clearance gap this gives to our battery. |
| 00:57 | By right clicking on the sheet metal bottles and ensuring the physical material is set to aluminium, so the density is correct, we can then check the properties of the bodies to find mass. |
| 01:09 | In this case, the combined mass of the top and bottom of the box is about 580 grams which isn't terrible since we're using sheet metal aluminium but there's still plenty of room for improvement. |
| 01:22 | So let's edit our model and see if we can remove some of this mass. |
| 01:27 | We'll start by sketching on the top face of the base of the box, making a centre point rectangle in the centre of the part. |
| 01:35 | Just be aware that this isn't the origin point, even though it appears that way. |
| 01:40 | An easy way to achieve this is by dimensioning the top and bottom of the rectangle 10 mm from the top and bottom of the parts and the sides 45 mm from each side which also sizes the rectangle appropriately. |
| 01:56 | We might need to use the project/include tool here to include the outer edges of the part in the sketch to allow us to reference them. |
| 02:06 | Let's also add another smaller centre point rectangle centred in the same location with the diagonals in line but with an 80 mm width. |
| 02:16 | This is an area to suit our etched logo where the material will be retained, but this will make more sense soon. |
| 02:24 | Then we can add eight lines that run parallel to the diagonals spaced 8 mm off each side and use the trim tool to remove the sections of these inside the inner rectangle. |
| 02:35 | We'll need to use the co linear constraint after this to make sure the sketch is fully defined. |
| 02:42 | To complete the sketch, we'll use the fillet tool to create a 5 mm radius between the lines of our rectangles and the diagonal lines and then click finish sketch. |
| 02:53 | We can now create an extruded cut, selecting these four profiles, which might require hiding the battery with the direction set to symmetric and extent type as all. |
| 03:05 | It's also helpful to use the objects to cut drop down in our extrude window to make sure we're only cutting the top and base of the box and not the battery. |
| 03:15 | Having the weight reduction holes in this layout gives us a nice cross bracing structure which helps retain some of the torsional stiffness. |
| 03:24 | Next let's remove some more weight from the side flanges by sketching on either of the outer faces. |
| 03:31 | The idea will be similar for this sketch but there'll be some slight differences as we'll aim for two cutouts rather than four. |
| 03:39 | We start by projecting the flat outer face of the box with the line type set to construction just so we can easily reference the edges. |
| 03:48 | With the line type still set to construction, we'll make another centrepoint rectangle 10 mm from the sides and 5 mm from the top and bottom and again another smaller rectangle with the same centre and diagonals but this time only 30 mm wide. |
| 04:06 | Then we need to select our four vertical lines and two longer diagonal lines and deselect the construction line type preference so they'll just be normal lines that form a profile. |
| 04:19 | Now we use the trim tool to remove the diagonal lines inside the inner rectangle. |
| 04:25 | As a final step again, add a 5 mm radius to the internal corners of the profile and finish the sketch. |
| 04:33 | The method is the same from here, using an extruded cut. |
| 04:37 | This time in one direction through both sides of the top part of the box, being sure not to also cut the battery component. |
| 04:45 | Checking the properties again, we can see that we've reduced the mass by about 30%. |
| 04:51 | That completes the design but we now need to consider design for manufacture or DFM. |
| 04:58 | Sheet metal rules get us most of the way there in terms of modelling something that shouldn't distort or fracture when formed. |
| 05:06 | The first thing we should consider is tolerances to make sure our manufactured parts will fit together and in the right mounting location. |
| 05:15 | For the base of the box, there's no bends between the mounting holes and any reasonable CNC machine should be able to accurately cut the holes which are 7 mm clearance holes for M6 hardware. |
| 05:28 | However, if we were drilling holes to suit our design, or struggling to take accurate measurements of the mounting locations, which is the case for our Honda, there's a good chance of error creeping in. |
| 05:41 | The top section of the box on the other hand features four bends between the mounting holes that fix through the box base and into the floor pan. |
| 05:49 | This is a concern because although the holes will be made by a CNC machine, the bends will be made by hand which presents a lot of room for error. |
| 06:00 | As a simple mitigation, we could slot two of the mounting holes on one side or upper section of the box and the base as well. |
| 06:08 | All we need to do is change the 7 mm circular hole to a 7 mm diameter slot with a 4 mm centre to centre distance. |
| 06:18 | The next thing we need to think about is the limitations of our potential manufacturing processes. |
| 06:24 | In terms of laser or waterjet cutting limitations, the thickness of our sheet metal at 2 mm is well within the limits of any reasonable machinery. |
| 06:34 | So nothing to worry about here. |
| 06:36 | Another concern is cutting holes that are very small. |
| 06:40 | The minimum hole size we can cut depends on the thickness and the type of material. |
| 06:45 | A good rule of thumb here is that the hole diameter should be at least 50% of the thickness of the material for laster or water jet cutting. |
| 06:54 | Since our minimum hole diameter of 7 mm is much larger than the thickness, this won't be an issue. |
| 07:01 | Also positioning the holes too close to the edge, although completely achievable for a laser or water jet cutter, often results in a part that cracks or splits when flexed, again there should be no issues with our design. |
| 07:16 | After the 2D cutting operation is complete and we have our flat part, the next process will be making the bends. |
| 07:23 | Again, our sheet metal rules have helped us throughout our modelling process and setting the bend radius and including bend reliefs so our part won't deform or fracture when bent. |
| 07:35 | But this doesn't cover all the DFM considerations around bends. |
| 07:39 | On the topic of bend radius, the general rule here is that it should be at least the thickness of the material. |
| 07:47 | However, for some manual methods of bending we won't have good control over the bend radius. |
| 07:53 | This is just something we need to keep in mind when it comes to manufacturing and remember that our geometry of our design, as well as the strength, depends on this radius being correct. |
| 08:04 | Another consideration around the bending process is if the bends are actually possible with the equipment we have access to. |
| 08:12 | What I mean here is, will the part contact the sheet metal folder while making the bends, preventing us from achieving the desired bend angle? A good way to check this is using a prototype cardboard part that we'll be discussing in the next stage of this worked example, all we need to do is use the cardboard prototype and whatever folder we have access to as a trial run to ensure there'll be no issues when it comes time to bend. |
| 08:40 | The order we make the bends in is also important here. |
| 08:44 | For the top section of this battery box in particular, it's possible to make the bends by first completing the middle front flange and then making the inner most bends on the sides of the box, followed by the tabs for the mounting points. |
| 08:59 | Lastly, on the topic of bends and also related to tolerances, it's always helpful where possible to use features to help locate the bend lines. |
| 09:10 | This minimises the chances of errors creeping in from measuring to bend lines. |
| 09:15 | This is only necessary for the top of our box as the bend line on the base will be easy to locate. |
| 09:22 | So let's activate the component for the top of the box and then make a flat pattern with the top face as the stationary side. |
| 09:31 | With the bend lines visible, we can sketch on he top face, making a 4 mm diameter circle with the centre on the intersection of the each bend line and the edge of the part. |
| 09:43 | This is just a matter of using these to create an extruded cut so we'll be able to use these small indents on the final part to locate the bend lines without measuring. |
| 09:54 | There are two things to keep in mind with this process, the first is that these cuts will reduce the strength of the design by creating stress risers that could lead to cracking which is why they're circular shapes, rather than triangular cuts to try to minimise this impact. |
| 10:11 | And the strength of this design isn't a primary concern, so the risk is low anyway. |
| 10:16 | The other thing to note here is since we made these cuts on the flat pattern so we could make use of the bend line, we won't be able to see them in the normal model for the part. |
| 10:26 | This also isn't a concern since the DXF to laser cut the parts will be made from the flat pattern anyway. |
| 10:34 | The last point we want to consider is design for assembly and service, making sure we'll have access to the hardware to mount the box, remove it and the battery if needed as well as access the terminals to connect and disconnect the battery, ensuring they will never contact the box or other components and short. |
| 10:54 | In our case, everything looks promising. |
| 10:57 | We could also use the finite element analysis or FEA method, to determine the stiffness of the battery box but this is a simple part that forms an enclosure and is fixed to the floor pan so we can be confident it's stiff enough. |
