Computer-Controlled Machining

This weeks individual assignment:

• Make (design+mill+assemble) something big

• See this weeks Group Assignment

I'm currently still waiting for the final painting of my parts, which I want to do before assembly to ensure the tolerances aren't distorted during assembly and to prevent the joints from becoming worn out. Accordingly, everything except the assembly is documented so far; the rest will follow. I'd probably postpone the welding of the metal structure and the laser cutting of the synthetic leather and PU foam to Wildcard Week; I'd welcome feedback on this :)

This weeks learnings:

This week I learned above all that two long days surrounded by wood dust and noisy machines would have been enough for me personally. The idea of ​​working with wood is perhaps somewhat romanticized. I also looked into many areas in more detail and learned a lot this week. You will find out exactly what below.

Design

Because Fusion360 maps the entire path to production and at the same time offers more than just 2D and a full 3D preview, I chose this for the design. A fair amount of material will be used up this week, so I don't want to make anything that will no longer be of any use just because I have to build something big. Unfortunately, I have seen this happen with a few projects in recent years. While I was preparing for the performance test for my football coaching license, I got out my old training plan and made the most of the home gym. Because I don't train with machines, I have a lot of small training utensils that aren't well organized at the moment, and at the same time I was thinking about replacing the broken weight bench. With a bit of searching, I found this product here, for example, which gave me the decisive inspiration. At the same time, I was groundlessly optimistic that I could do better. So here we go.

In order to accommodate all my training equipment, I measured it beforehand in terms of its possible packing size to ensure the best possible dimensions for the weight bench. This resulted in the internal proportions for me. The height of the bench was determined by the fact that I wanted to have a knee angle of around 90 degrees at best when lying on the bench. A height of around 40 cm works very well for me. In order to have maximum freedom in the downward movement when bench pressing, I also measured my back and came to the conclusion that a width of 30 cm seemed reasonable. The length of the seat has also been measured to suit me and is approx. 37 cm. I also measured the length of the backrest on the basis of my upper body and came to the conclusion that it is around 70 cm. I checked the plausibility of all the values using my old weight bench, because I was still puzzled by some of them. For classification: I am about 1.9 m tall. With all these boundary conditions and the inspiration shown above, I quickly had enough information to be able to design.
Overall, the design is somewhat more complex in my view, which unfortunately means that I cannot go into detail without going beyond the scope of this article. But I'll try to highlight a few points, either you do it based on your measurements anyway, or you use my existing files. For a better understanding, we might try the reverse engineering approach here. So here is the result.

Above in brown you can see the seat and reclining surface, which I am building on the basis of a 12 mm wooden panel and 30 mm PU foam and will cover with artificial leather. However, the whole thing will take place outside of this week and will be documented afterwards. In any case, there is nothing special about it yet. In order to support the 12 mm board, I have constructed wooden supports for the seat, and a steel substructure made of 20x20 square tube is indicated in gray for the lying surface - in order to be able to set it up - which ends with a 17 mm steel tube and thus fits into the two recesses in the wood depending on the position. Both the seat and backrest can be folded up to make optimum use of the storage space. As with the original idea, the larger surface is also accessible via the side cut-out. The box that is designed into this is intended to be used both as a box for storage space and, conversely, as a mini elevation for various exercises, but is also not part of this week or was not manufactured.

The body is completely pluggable, but due to the high loads to be expected and for the sake of my sense of safety, I considered more than just plug-in connections.

In order to protect and stabilize the body from twisting under load and slight lateral movement, I have provided a 4 mm screw hole at the top and bottom of each side and middle section. Because I will be milling from the other side, I cannot do more in the design. Here I will set 4.5 mm stainless steel screws from the outside, for which I will drill out the hole by hand beforehand with metal drills in order to countersink them and, at best, close the hole again with wood.

I think the picture shows quite clearly what the backrest is resting on and what the heavy load will be. Because I liked the 5cm thickness of the material at the top as a bridge from a design perspective, I decided to double this part up to absorb and distribute the load. On the one hand, I will glue the doubling on with wood glue; this flat arrangement ensures good adhesion. In addition, and also to secure the position, I have distributed or provided five 8mm holes along the length, which are drilled into both the doubling part and the side part, into which I will later insert 8mm thick wooden dowels.

As you might have seen in the first picture or from the hidden connection of the doubling to the short right side, I want continuous fronts and not typical views of the plugged parts. So I left 3 mm of material everywhere and at all times to ensure an intact and clean surface. So far, this has worked out well, but we'll wait to give a prognosis until after assembly. Just to show that the mentioned connectors exist, here is a picture without the large side plate.

After I had carried out a few tests beforehand with Julian Baßler , we decided to use T-bones with a diameter of 4 mm on the connections and to hide them as best as possible. There is just over 25 mm² on the entire weight bench that could not be hidden. If you find them, please let me know. You can see what my T-bones look like here.

You can see that the T-bones can be arranged in such a way that they disappear when inserted and that is exactly what I wanted.

From other tests we did, I figured out that I needed to add a tolerance to fit the parts together. The easiest way to do this was on all the negative connectors. The tolerances are clearer in pictures, so they are included here.


The design process may be a little easier to understand using Fusion itself, so here is a brief video of the process. I included a cut in the video because many of the operations in between were just for setting the tolerances, so there is a small jump. The part on the right is already arranged for the following process of preparing the .nc file. But for now, enjoy the video.

Preparing the milling operation

In the design I used two different plate thicknesses, here in Germany 12 and 18 mm are common. With a few exceptions, all parts are milled from 18 mm. In total I created three files or jobs for milling. I milled all 12 mm parts in one process and divided the parts made of 18 mm thick material once for time reasons. All three setups can be found in the Fusion file, which can be downloaded below - as well as the three associated .nc files.

At the beginning of this week, Julian Baßler and I came up with a general milling strategy. We both knew the jobs would be time-consuming, but at the same time, we didn't want to compromise on some of the more aesthetic aspects. We decided to create the T-bones with a maximum diameter of 4 mm, which meant that we could only finish our parts with a 4 mm cutter in some places. Generally speaking, thicker cutters allow us to penetrate deeper into the material and thus remove more material in less time. Therefore, I would always recommend choosing a cutter that is as thick as possible and as fine as necessary. In all our operations, we used the following cutters. For example, T1 means that the cutter is positioned at position 1 in the milling machine. This information is essential for automatic tool changing.

• T1: 8 mm roughing cutter, CrN prism coated
• T2: 6 mm up- and downcut milling cutter, CrN prism coated
• T3: 6 mm finishing cutter, CrN prism coated
• T4: 4 mm roughing cutter, CrN prism coated
• T5: 3 mm finishing cutter, CrN prism coated
• T6: 4 mm drill bit
• T7: 4 mm finishing cutter

Here are the CrN-coated cutters (left to right, T1 to T5) that I installed in the machine specifically for this week. Typically, we work with other milling strategies and uncoated cutters in Bottrop. The coating hardens the cutters and, in case of my milling cutters, allows us to achieve about 25% higher feed rates, while also increasing their service life.

I received a .tools file from the milling cutter manufacturer that could be imported into Fusion360, allowing us to work with the manufacturer's milling parameters. For me, this was definitely a selling point when purchasing, especially since a lot can go wrong during milling. But that doesn't necessarily have to be the case for you, of course; you can probably approximate some milling cutters quite well. If you can get a .tools file, I would recommend importing and using it.

It may seem unusual that we included a drill bit in our setup. By deciding on T-bones with a 4 mm diameter, we originally assumed we would finish the T-bones with a 3 or 4 mm cutter. Considering the cutting depths we could achieve with this—at this point, we were still assuming a depth of half the cutter diameter—we decided to drill the T-bones with a 4 mm drill bit. Even at the end of this week, I'm still convinced that this was a very good idea. We also added the 4 mm cutter on the T7 to our setup later when we realized how slow our progress was with a 3 mm cutter. There were plenty of 4 mm cutters, so we could experiment with the depth during finishing without risking not being able to replace a cutter later. Therefore, both cutters on the T6 and T7 are not coated with a CrN prism; they came from the FabLab Bottrop toolbox. Good news: We were able to gain new insights into the finishing depth, and more importantly, the cutter withstood this new insight very well. More on that later.

Operation 01

Let's get to the actual preparation. First, all the bodies or components that we want to mill simultaneously must be arranged horizontally next to each other. For clarity, it's best to hide all other bodies and components. Now we'll jump from the "Design" tab in the top left to "Manufacture" mode.


Here we'll initially create a new setup, which we'll later use to create our milling job. We do this in the top left via Setup > New Setup. This will open the following menu.

The following settings can be easily adopted, but it is particularly important to work correctly with the Work Coordinate System. Here we have worked with the Model Orientation and set the origin to the stock bottom because our machine has the zero point on the sacrificial plate. It is important to be familiar with this. If this height or reference is incorrect, there is a risk later on during milling that the milling cutter will plunge too deeply and break. Under Model we select our three components, and the yellow box will automatically be drawn around them to symbolise the stock. In our case the fixture is controlled by screwing it on and a vacuum plate, accordingly nothing sticks out or protrudes upwards, so we do not need to change or add anything in the Fixture section.

You may need to set the mode in the next menu item. Here, I'm using the Relative Size box. I haven't changed anything in the post-processing. This completes the setup, and we can confirm it with OK.

Drilling

Next, we create the individual operations. As described above, our strategy is to drill the T-bones first; this is easier and cleaner when there is still solid material around them. To do this, we select Drilling > Drill from the button above. I performed the operation with the following settings.


I highly recommend the two check boxes "Select Same Diameter" and "Only Same Hole Depth," as this saves you from having to select individual contours. The "Drill Tip Through Bottom" option is equally important, ensuring that not only the tip of the drill bit is driven to our intended depth. In future jobs, I will also use an offset to drill into the sacrificial plate; I expect this will result in fewer tears on the underside of the plate. Depending on how your plate is clamped, pay close attention to the retraction heights you need to set to avoid collisions!

This is a quick simulation of the operation we created.

2D-Contour

Next, I created a roughing pass for the upper part, using the largest possible 8 mm cutter. The settings for this can be found in the slide show.


The tabs might also be sufficient at a greater distance. It's worth experimenting with the "Tab Positioning," "Positioning Method," and "Tab Distance" settings, and checking each one in the live preview. The "Multiple Depths" field is extremely important; it should be checked in each case. The Maximum Roughing Depth should be set to half the cutter diameter whenever you plunge into solid material, as in this operation. For the subsequent finishing, we activate "Stock to leave," whereby I decided not to leave anything axially (in the Z direction). The "Radial Stock to Leave" can be set to 1/4 of the diameter of the final finishing cutter, according to specifications confirmed by Stepcraft, among others. In my case, it's 4 mm, so 1 mm is set here. Feed Optimization protects the cutter and increases its service life by traversing curves where high loads act on the cutter somewhat slower.

Here, too, a short simulation of the operation follows.


Overall, many more contour operations followed in this setup. Here is a complete overview of all jobs within the setup.



I roughed with the T1 cutter and finished with the T7 cutter. It's important for both operations that the tabs match, so that we don't potentially break tabs during the finishing operation that we deliberately left in place during the roughing. This could cause our workpiece to come loose.

Otherwise, just in brief: There are few differences in my finishing operations. Despite the thinner cutter, I went deeper in each of the "Multiple Depths" operations, to 1.5 times the cutter diameter, because we only remove a small amount of material in this operation. We tried it once without Multiple Depths at all, but found that this resulted in vibrations in the cutter, which made our finishing result unsightly. The factor of 1.5 consistently produced good results for all cutters used for finishing. The second, different setting is that we deselect "Stock to leave". Here is what my Setup looks like in the simulation.

Operation 02

The second operation is the first to use 18 mm thick wood. However, this doesn't change our approach in general; we just require a greater number of depth cuts. So first I arranged my components again.

For this job, we need another operation that I haven't introduced yet: the 2D pocket, for the areas marked in red. Because the cuts don't go through here, in order to maintain an unbroken surface on the other side, we need to completely remove the material here. For this, we use the pocket operation. As before, here is a quick slide show with my settings.


It's important to adjust the bottom height to the height of the selected contour without any offset. For this, it's important to select the bottom or inner line when selecting the contour – if you make mistakes here, they'll likely show up in the simulation. Generally speaking, we recommend activating the "Keep Tool Down" checkbox for all 2D contour operations; otherwise, the milling cutter would return straight back to the retraction height before each new plunge. That's a real waste of time. You can set all of your settings as user defaults using the three dots next to them. This saves a lot of time in all future operations.

This is the sequence of my operations for my second setup. The simulation follows.

Operation 03

There's not much to add about the third operation. Here, too, I'm working solely with Drill, 2D Contour, and 2D Pockets according to the self-imposed framework outlined above. Here's the sequence of the different operations, followed by the simulation of the entire job.

milling

The milling machine in Bottrop is the following. I will only go into my setup here. In order to be able to react quickly at any time, the machine has a remote control, which was placed next to my laptop in front of my large and far too loud TV for the entire milling time.

With every tool change, I looked at the first few milling paths to check the milling depth and ensure safe operation of the machine, among other things. At these times, I was even closer to the milling machine, holding the remote control.

Earlier this week, we did some experimenting, the results of which can be found on the Group website. From this, we derived the offset of 0.3 mm, the depth we go beyond the material thickness, the dimensions of the T-bones, and the tolerances I incorporated into the design above. We derived this from the following test, which is available for download as a file on our Group website.

Operation 01

In the first step, I milled the base plate for the seat and backrest, as well as the center section, from 12 mm multiplex. This is the basic material available in Bottrop, although it wasn't absolutely necessary in many places. This milling job took about 20 minutes, but the geometries are and were comparatively simple. Here you can see the result, still screwed onto the sacrificial plate.

To remove my panels, I used a chisel and a hammer to remove the retaining bars. You can still clearly see the remains of the retaining bars here.

In the next step, I therefore reworked the edges with a multi-router.

The result is as follows. The strategy certainly worked, thanks to the mobile milling machine, even with more complex geometries. I reworked all the following parts in the same way, and I also rounded some edges with a different milling cutter.

Operation 02

The milling process is similar, but the operations differ, as described above. Therefore, I will refrain from describing the two subsequent operations. A picture follows at the very bottom of each paragraph.
All the experience gained so far in Bottrop during milling has not been able to say or estimate how long the jobs will take. Personally, I think that this should be a relatively central piece of information - especially as a guideline like our projects this week. All that is known so far is that Fusion360's forecast is wrong. To get closer to the truth, I stopped all tool changes and the associated operations for operations 02 and 03 and compared them with Fusion's forecasts. The result follows here in table form and is intended to be used to derive rough real times.

Here, Fusion360 is about 130% off overall. When analyzed separately, it's clear that drilling operations are calculated 193% too short on our machine. Contours are about 75% longer in reality than calculated in the simulation, and the pocket took 146% longer in this setup.

Now that the individual milling steps are known, I am providing a picture of the result. All parts were removed from the plate and reworked in the same way as the 12 mm parts.

Operation 03

For the third operation, I first removed the parts from the second one so that I could rework them while milling the other parts, keeping an eye on the current job. Important: With every tool change and thus every new operation, I checked very carefully to see if everything was actually running as planned or if any errors had slipped through. Fortunately, the latter was no longer the case thanks to the meticulous preparation.

As in the previous operation, I stopped the times and listed them below.

Here, as above, we'll just differentiate between the operations. Compared to the simulation or calculated values, the entire job took a whopping 217% longer. The contour operations took an average of 98% longer, the pockets 425% longer, and the top performer was the drill operation, which took a whopping 455% longer than calculated. At this point, an urgent warning: If, given the times in Fusion, you think you can just mill something, do yourself a favor and forget about it. This is fake news!!

This is the result of me spending 2 hours and 30 minutes in front of the milling machine.

Just for the record: Here are all my milled parts in their temporary natural habitat.

assembly

Before assembly, I still had to do some finishing because I didn't want to leave the wood untreated. Since my backrest will later be covered with foam and faux leather, offering a wonderfully large surface, I tested various pigmented wood oils, each of which I could get in 20-ml containers. To do this, I sanded the surface with a 220-grit sanding sponge, and later with 320-grit sanding sponge, in the direction of the grain, and taped it off as shown in the picture. In the top row, I applied the oil once and after 30 minutes, wiped off the excess oil with a nonwoven fabric. In the bottom row, I applied a second coat of oil instead. After consulting with various designers (thanks again!), I chose the color with the beautiful name "Asphaltgrau" perhaps a good first word for learning German. It works without any context.


Before oiling the surfaces, I took a closer look at the wood and discovered that the 18 mm thick panels had unfortunately been sanded in two directions at the factory, meaning against the grain. The effect of sanding against the grain would be significantly enhanced by oiling, because the pigments are deposited primarily in these recesses. Accordingly, I sanded all surfaces in the direction of the grain until the cross-grain sanding disappeared. To illustrate this, here are two pictures: before and after sanding. I did the whole process with a disc sander, followed by finishing with the sanding sponges mentioned above.


The assembly is a bit more complex, I'm currently still waiting for a few parts and the appropriate hard wax oil to finish the weight bench - so there will only be step-by-step updates here.

Download this weeks files

Fusion360-File

.nc-Code Operation 01

.nc-Code Operation 02

.nc-Code Operation 03