Building Blocks to the MPAS (Bicycle frame assembly)

In this short illustration I would like to do a very brief explanation on how the MPAS can be used by engineers to ensure that their designs are correctly assembled when using welding as an assembly process.
I will use a bicycle frame for demonstration purposes.
While there are many fantastic bicycle frame jigs available from companies like Anvil, the great advantage offered by the MPAS is its rigidity.
This demonstration is less about bike frame building though and more about how the tools I am providing here on GrabCAD can be used to create a general layout of the fixturing tooling and how a template can be created to make sure that future setups is correct and accurate.

  1. Step 1: Download some Step files

    Go to my GrabCAB THOR Welding Innovations and brows through the various step files.

    • Start by importing into your design a table surface large enough to accommodate your assembly. Leave enough space around the sides for fixturing devices and clamps.
    • Look at the various fixturing tools to get yourself familiarized with their shape and uses and download a few of the more common ones.
    • Make use of the color coded files to help identify different fixturing devices or download the grey items as you prefer.

    To learn more about the MPAS read the THOR page.

  2. Step 2: Identify a starting point

    • Inside your own design look for a clear and identifiable point with a fixed and measurable surface. The rule of thumb is that this would usually be the first component that will be placed in the jig when starting the assembly. In my example the BB (Bottom Bracket) would be a perfect component to locate 1st. (For those that do not ride the Bottom Bracket is the cylinder through which the crank and pedals gets mounted and it houses some form of bearing arrangement)

    BB or Bottom Bracket mounted with cone and shoulder bolt.

    • To keep enough clearance between the table surface and the assembly to aid the welder in reaching behind the assembly I used the 100mm Extension Block Series-100 (MAEB10S10) as the base to which I mount the BB with a 100mm Shoulder Bolt and a 55mm cone.

    • To set the height at a fixed measurable distance it is important to aim to stay in the 50mm grid pattern. In this case I used a spacer just below the BB to ensure that the center of the BB will be exactly 150mm. The reason for this will be apparent soon

    • The cone ensures that the BB is 100% in the center. This is very important as the hole spacing will provide a very accurate way of measuring. This is a great feature when working with round material as accurate measuring of especially center lines is near impossible.
  3. Step 3: Joining the MPAS to your design.

    • To join the MPAS to your design one must make use of the joint command. In order to do this with the least amount of effort one must decide to add the MPAS to your design or to add the design to the MPAS. In my bicycle design I used the latter and joined the bicycle frame to the MPAS but here it is important to make sure that you first create a ridged group of all the components in your design. If there are too many components to do this I suggest using the first method and rather join the MPAS to your design.

    • In the Bicycle design it means adding the green Extension Block first then add the table and position it so that the entire assembly will fit to the table but allow for enough clearance to the sides so that more MPAS fixturing devices can be added on the outside of your design.
  4. Step 4: Add more Fixturing Devices

    With the center of the BB at 150mm from the table surface it means that the center line of the tubing will also be at 150mm making it very simple to add more fixturing devices to hold all the different tubes in place.

    Take special note of the steering head positioner. The head tube is held by 2 cones between 2 adjustable positioners attached to the angle positioner. These slide positioners is mounted to a set of holes exactly 150mm from the table surface.

    More supports can be added to hold the top tube, down tube and seat tube in place. Here I used dedicated tubing holders designed so that they will hold the exact diameter tubing with the center line of the tubing being 50mm from the bottom of the block.

    This makes aligning very easy as one green 100mm Extension Block together with the red tube holder will provide a perfect 150mm position from the table surface to the tube center line.

    (Tubing for bicycle frames comes in a variety of sizes. THOR Welding Innovations provide these inexpensive tubing holder blocks in all possible sizes.)

    Finally special inserta clamps from Stronghand Tools can be added to hold the components securely in place to minimize distortion during welding.

    Inserta clamp from StrongHand Tools

  5. Step 5: Creating a template

    If many of the same assemblies will be made at various times, creating a template will be a big time saver.

    Creating a DXF file for laser cutting should be a simple procedure not taking more than half an hour.

    Make sure to leave a tolerance of about 0.075mm around all fixturing components.

    The template can be laser cut from 0.9mm sheet metal or even thinner material.

    Making a template will save time for every subsequent setup for assembly.