In the last project (Sterling Helicopter) all of the parts were created and assembled in one file. Although convenient in many respects it also gets to be quite cumbersome when the number of parts exceeds about twenty or so since the tree is longer than the screen is tall. An alternative to this is to divide the parts up into sub-assemblies in their own file and then assemble them into a top-assembly file. Fortunately, Assembly4 accommodates this quite nicely while still allowing design to be done in-context. You can work either in the top-assembly or sub-assembly files.
The work process starts with creating the top-assembly and sub-assembly files using "Create a new Assembly4 model" for each and then assembling the sub-assembly in the top-assembly Model using "Insert a link to a part" before creating any body or feature. Each of the sub-assembly files have to be saved and opened at this point. In this manner, you can begin modelling in either the sub-assembly files or the top-assembly file links in-context as the assembly design evolves. I use the default LCS in the Assembly4 body to locate it in the sub-assembly. That way you can either work on the body in the Part folder of the sub-assembly at the global axis or in-context in the sub-assembly link. Fantastic!
All the parts in the assembly shown in the first image can be found in the restoration video:
https://www.youtube.com/watch?v=e5vCJz3mK6w
- These are all of the parts to be created for the push mower assembly which I am sure all you greybeards will recognize.
The size of the cutting reel is about 24 in. long and the wheel diameter is about 10 in. to get an idea of scale.
The first thing to do in the project is to decide which parts, limited to no more than 20 or so in one file, should be in each sub-assembly. The sub-assemblies I chose are primarily divided between rotating and stationary parts: the cutting reel including the blade assembly, bearing races with preload springs, and the pinion gears; the height adjusting roller assembly; the wheel and tire assembly; the frame including the end pieces and the cutting bar assembly; and the handle assembly. - Capture_AllParts.JPG (127.05 KiB) Viewed 3086 times
- This image shows all of the parts created thus far. I am currently working on the end pieces for the frame sub-assembly, not shown in this image.
The top-assembly file shown in the Combo View above includes the cutting reel and wheel sub-assemblies assembled in their proper place. They were located using the Model LCS_Origin for the reel and an LCS_1_wheel that was added for the wheel sub-assembly. The latter was attached to the master sketch of the gear layout shown in the image below.
The Tree View in the right side panel shows all of the parts in the cutting reel sub-assembly which nicely fits within the panel dimensions. - Capture_top-assy.JPG (267.29 KiB) Viewed 3086 times
- This is a view of the cutting reel sub-assembly file. Note that each body of the assembly is located in the Assembly4 folder and assembled in the Model. If any of the parts need to be saved for reuse in other files, they could have been just as easily created in their own individual file and assembled in the sub-assembly as required.
I began with the cutting blades first using a helix additive sweep and assembling them with polar link arrays. The spacers for the blades turned out to be a bit tricky because the mounting tab and rivets are attached to the helical surface of the blade. I did this by adding a blade helix sweep to the spacer and then trimming it to proper shape with the groove tool as shown in the next image.
The hole sketches for the blade rivet was attached to the blade helix object using the FrenetTN mode and spaced along the helix using the Map Path Parameter property. This insured that the holes were perpendicular to the helix surface.
The pinion gear tooth profile pad was created with the PartDesign > InvoluteGear wire object. The gear module and pitch diameter were graphically derived from the gear layout master sketch discussed in the following image. - Capture_reel-subassy.JPG (229.54 KiB) Viewed 3086 times
- The detail features of the blade spacers are shown in this image.
An additive sweep of the blade profile using the same sketch and helix shapebinder as the blade, offset one blade thickness, were used to create the mounting tab. The blade is shown in-context highlighted in green.
The inset shows the groove sketch used to trim the tab to size after the polar pattern of the offset blade and rivet hole features. The rivet hole was added in the same manner as the blade to insure an exact match with the blade. - Capture_spacer2.JPG (68.68 KiB) Viewed 3086 times
- A master sketch of the gear layout was used to set all the proportions of the gears and attachment points for all of the parts in the frame sub-assembly.
I first estimated the pitch diameter of the ring gear based on the known dimension of the wheel tire outer diameter (254 mm). The tooth numbers (pinion 12, ring gear 56) are also known from the video which then sets the center distance of the gears (shown as reference dimension of 77 mm).
The rest of the proportions for the frame attachments can be finalized as the design of the frame components evolves.
This master sketch will also be used to attached all of the mounting LCS 'connectors' for the frame sub-assembly and a carbon copy for the end pieces currently in work. - Capture_gear-mastersketch2.JPG (270.55 KiB) Viewed 3086 times
One convenient aspect of putting all of the rotating parts into one of the sub-assembly pays off if you want to animate the moving parts to check for proper operation and clearances as shown in the above video. Variables for the cutting reel and the wheel sub-assemblies were defined in the top-assembly Model. The independent variable is the rotation angle of the cutting reel about its axis and the dependent variable is the wheel rotation which is 12/56 of the reel rotation. You can probably argue that the wheel angle should be the independent variable and the reel, dependent, since the wheel turns the reel not vice versa.
The attachment offset angle of the reel sub-assembly link about its X-axis is defined with an Expression and the variable rotation_angle. The LCS_1_wheel attachment offset angle was used to animate the wheel sub-assembly with an Expression and the dependent variable ring_gear.
I will continue to work on the bodies for the frame sub-assemblies and report progress in additional posts if there seems to be continuing interest. Hopefully, by now you can already see how the use of sub-assembly files simplifies the top-assembly and adds more flexibility to the in-context modelling process and animation.
OS: Windows 10 (10.0)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.22894 (Git)
Build type: Release
Branch: master
Hash: 9eb080488d970d313c538473e7272117ea0a7cd1
Python version: 3.6.8
Qt version: 5.12.1
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/United States (en_US)