Fusion 360 – Page 3 – Woody's Workshop

January 7, 2025January 8, 2025

Drawing a parabola in Fusion

Using the sketch conic curve function

This is an interesting one and I admit to not fully understanding the result.

The conic curve function in Fusion allows you to set the two end points of a conic curve and then ‘pull back’ the curve to its apex and enter the rho value. When the rho value is entered as 0.5 this will create a parabola. (If you enter rho as >0.5 it will create a hyperbola and if <0.5 it will be an elipse).

With a rho value of 0.5 the focus point is the same distance from the curve vertex as the curve vertex point is to the directrix. The directrix is a mirror line behind the curve that at the parabola vertex is the same distance from the back of the curve as the focus is to the front, hence the 0.5 rho factor. (Best to see the sketches below …..).

If you draw a perpendicular line from the directrix to the curve and then from the curve to the focus, then in a true parabola the length of these two lines should be equal. (This is why parabolic shaped dishes are used for radio communication links. In simple terms, any parallel radio waves incident on the dish will have the same reflected propagation distance to travel to arrive at the focus point where the detector device will be located. There is a cumulative addition of the incident waves at the focus. This is the power gain factor achieved by the dish in amplifing the signal.

All good so far ?

The problem I encountered was I could create a random conic parabolic curve using the Fusion function (two end points and vertex using a rho of 0.5) but the paths from the directrix to the curve and the curve to the focus did not match.

Until quite by chance … I had drawn a parabola conic (0.5 rho) where the diameter of the curve (curve peak to curve peak) was twice the vertex peak to the directrix line. Using this ratio always gave me equal path lengths between focus and curve and the curve to directrix.

Conclusion – the conic curve function in Fusion is not necessarily producing the result you might expect of a true parabola shape. I think it is almost certainly to do with the equation of a parabolic curve which the Fusion function is maybe not fully addressing.

Update : Looking back to my amateur radio microwave activity I remember the true focus of a dish was derived from the formula f=D²/16d where D is the dish diameter and d is the depth of the dish. From this you can calculate the focal ratio f/D. For efficient illumination of a centre fed dish the f/D ratio had to lie between 0.3 and 0.45.

By chance my choice of 120mm diameter and 30mm dish depth provides a 30mm focal length and the Fusion conic curve graphing is correct and gives an f/D 0.25. I think for any other f/D ratio the Fusion conic curve will not be an exact match to a true parabola curve (that is the focus to curve and curve to directrix path lengths will not be equal). I think this is because Fusion assumes the focus point is on the same axis as the end points of the conic curve. On this basis any conic curve created in Fusion with an expected rho value of 0.5 (a parabola) must have the directrix positioned at a distance equal to half the distance between the two end points. The conic curve is then ‘pulled back’ to the directrix line and the rho value of 0.5 entered. This could be automated using parametric functions. If the focus is not on the same axis as the curve end points some head scratching will be necessary and any resulting conic curve will not be a true parabola.

A construction related comment.

Never trust the accuracy of a floating Point placed based on just a selected grid position in Fusion. You must dimension lock the Point back to the sketch reference otherwise any resulting sketch using the Point will not be constrained. The parabola sketch is an example. I placed the two end Points for the curve, the curve centre and the directrix centre as my initial sketch postions. Then using the conic curve function clicked on the two curve ends and pulled the vertex back to the directrix point and entered the rho value of 0.5. The curve line immediately came up in black to indicate it was fully constrained. If any of the Points had not been dimension locked, the curve will appear blue – unconstrained.

To conclude this waffle and complete a solid version of the parabola, use the offset command to draw a parallel line behind the parabola curve and then ‘seal’ the end of the two curves with a short line and also add a short centre dividing line. You can now select half of the solid in extrude mode and rotate around the centre line axis to give a complete solid parabola. Here is a step by step process.

Here is the resulting solid parabola. This might be an interesting 3D print …..

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July 10, 2024July 10, 2024

Creating Customised Threads in Fusion 360

An attempt to describe the process

As ever this started off with a need and from the need came some learning. In my experience such needs are always welcome for the resulting learning benefit but inevitably lead to a few hours of frustration.

We have a small Jacuzzi spa at our home in France. It has two cartridge filters that are screw mounted into the sump of the spa. The threads on the cartridges are plastic and are loosely defined as 2” SAE spec. (I think SAE is a fine pitch thread (?) and as the filter threads are around 5mm thread to thread pitch, they don’t seem to me to be fine pitch).

When filling the spa from empty, Jacuzzi recommended that the filters are removed and the filler hosepipe nozzle is wedged into the outer vacated of the two filter holes. The nozzle has to be jammed in place by packing a cloth or sponge around it. Filling via the filter mounting ensures the spa fills from the bottom up with minimal potential for an airlock in the pipework.

The problem with this is that the filler hose tends to have a mind of its own and when your back is turned it will liberate itself from the filter hole and whiplash round like a demented cobra and give you an unexpected bath.

After one such soaking I resolved to stop this happening. What was needed was an adapter plug to fit into the “2” SAE” socket that would accept a standard hose push fit connector. This would hopefully keep the rampant serpent retained during the filling process.

I opted to use a standard commercial male hose connector for the interface to the filler hosepipe. These have a DIN Pipe thread specification (G26.441 x 1.814 mm). This left me with the need to model the 2” SAE from scratch which on inspection appeared to take the form of a pseudo Acme profile but with an asymmetric thread to valley ratio.

Having failed to find anything helpful on the Internet I set about creating a custom thread in Fusion 360. New experience ….

Here is the resulting adapter.

The attached ZIP file below has the full write up, the STL file and the source Fusion file.

Modelling Weird Threads in Plastic using Fusion 360 Download

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August 13, 2023August 14, 2023

Automated 3D printed collet storage using Fusion 360 parameters

How sad is this ?

I recently bought an Imperial set of ER25 collets to compliment my metric versions. I also have a set of metric ER20 and ER11. They have all been delivered in little yellow boxes which is good from a shipping point of view but a faff when needing to use them. In the past I have made storage blocks using odd pieces of wood with tapered holes cut using a cone cut drill bit. The cone cut has a 6 degree taper and ER collets all have 8 degrees so it sort of works but not quite ideal for my perfectionist mind.

As ever, the 3D printer sits in the corner begging to be used and Fusion 360 challenges me to do something a bit more professional. The scene is set for an overengineered collet storage tray.

Now I could just model and print something to hold the twelve new Imperial size delivery but what if I add to them at a later date? What if I decide to get some ER20 Imperial versions also ? ……….. Could I create a Fusion based automated model that will cope with any sized matrix of holes and any size of collet?

The first exploratory step was to create an Excel spreadsheet (another of my fascinations) as a means of identifying the steps that might be involved in automating the design and to tabulate the different parameters of the various ER series collets. Here is a screen shot.

A little bit more detail will be needed for you to understand what this all means.

The top table just details the published specification dimensions of the ER range of collets. I needed the height of the tapered section and this is calculated. Note that not all collets have the parallel section at the top of the taper. If you extrapolate the taper it then coincides with the dimension D1 and this is one I have used in the calculations.

Each collet holder hole in the block will be an 8 degree tapered hole that the collet sits in such that it does not sit all the way through the block (I opted for a 1mm bottom gap) and such that a percentage of its height protrudes from the top of the hole (I opted for 30%). The middle right hand sketch shows the various parameters and the red outline represents the collet.

The lower table shows the set up for the model parameters with spacing and borders defined. This section allows entry of the all important ‘how many holes do I want’ and the aspect ratio of the matrix. The final two red lines shows how big the resulting storage block will be.

All with it so far ? This is so embarrassingly sad … should I even be thinking of posting it…

These various parameters are now mirrored and entered in Fusion 360’s Fx parameter set up. I have tried to be logical in the naming convention and the blue stars indicate a function that is defined and entered by the user and will also appear in parallel in the Fx Favourites listing. This allows a focussed entry of just the variable parameters without all the other background calculation clutter.

I wanted to round the resulting block size to a nearest whole number. My preferred function in Excel is “mRound” where you can define the rounding to an increment value. There is no such equivalent in Fusion. In the end I used the “Ceil” in Fusion and “Ceiling” in Excel but Excel’s “Ceiling” requires a ’rounding to value’ hence the entry line (R) in the Excel listing which is set to ‘1’ to make it match the Fusion calculations.

By setting the length and width hole count to ‘1’ a single holder can be printed and checked to dimension before doing a full matrix as needed. I found that on my first single hole print the collet sat slightly higher in the hole than designed but this was traced to a print artefact on the taper wall. This was was easily rubbed down. On similar tack, I have allowed a fudge factor for print shrinkage should this be needed. Any errors in this respect will cause the collet to sit higher on the block. This will be most apparent as the collet size reduces. The value entered (say 0.1mm) adds a linear amount to the top and bottom hole sizes and therefore the taper.

Once you are happy with the single hole print you can define the number and aspect ratio of how you want the holes to be printed on the finished block. A 6 x 2 print will match the normal 12 piece ER25 collet set. On the Qidi ifast printer this took around 4 hours at 0.25mm layer height. Note that I used the ‘Shell’ command on the block lower surface of the print to reduce material use. This means you need to place on the printer bed ‘bottom side up’.

Well I did say this was going to be a sad nerdy post but as ever I learned a bit more about Fusion, Excel, refreshed my school geometry and made the workshop even more tidy and organised. What’s not to like ? …..

While this is an almost facetious waste of time and effort, the principles used in the Fx programming has many other applications.

The planning spreadsheet has been added to my spreadsheet compendium which along with the Fusion file (for ER25) is attached on the following ZIP file link.

My Engineering Spreadsheets plus Fusion ER series Download

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July 17, 2023July 17, 2023

Fusion 360 Keyboard Shortcuts

Fusion 360 Shortcuts Lookup table and keyboard overlay

In an idle moment I have updated my collection of spreadsheets to include the Fusion 360 shortcuts listing. The workbook tab also includes an edited version of the Fusion keyboard overlay.

You can download this and other eclectic tabs as part of my Engineering Spreadsheets workbook.

My-Engineering-Spreadsheets-0723 Download

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June 7, 2023June 7, 2023

Creating a worm drive in Fusion 360

The hard way and the easy way

A colleague asked if I could model a worm drive in Fusion 360 such that the result could be used to 3D print the worm and the associated wheel.

My first attempt was to create the assembly from first principles. This was painful and took a number of versions before I got a process that I could repeat and appeared to work. I was not happy that the result correctly reflected the dynamic shape needed on the wheel teeth.

Somewhat appropriately I then went down an internet worm hole and discovered there is a scripting plug in for Fusion 360 that allows the creation of these components. Not only that but the author Shwivel has also created a number of similar script routines for other gear related constructions.

The routines are chargeable (USD20 each) but given the time I spent try to do it the hard way, this seemed like a good price for what looked like a better result.

The script once downloaded appears as an icon in the Fusion top menu bar under the Shwivel tab.

Once opened it needs various parameters entering and allows entry of centre holes in each component. Once you’ve entered the parameter the script goes off and crutches the result. This can take a few minutes so patience is needed. The Dimensions section gives you all the values you have entered and the resulting parameters created.

I had a weird initial experience whereby the script entry window kept giving me red error messages no matter what measurements I entered. This was resolved by doing a RESET in the software FILE menu.

Here is my resulting design for a 60 tooth wheel.

While the routine allows the bore hole dimensions to be entered, I have found it easier to keep these to a small reference diameter hole and then revert to normal extrusion routines once the two main component shapes have been created.

Unlike a normal model, once you have created a model you cannot ‘right click’ in the timeline to edit it. Instead you have to click once again on the Shwivel top menu item and this gives you the entry screen for editing.

All in all I have been impressed by the results achieved but as yet I have not run any 3D printed models.

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