3D Printed Threads Modelled in Fusion 360

I recently posted an idea for a 3D printed depth setting jig for use on my Myford Big Bore lathe. A couple of readers had run the STL files and struggled with the fit of the mounting boss thread (M35 x 1.5) that mates with the thread on the end of the lathe spindle. This is a known problem with 3D printed threads where the accuracy of the 3D printer and the size of the thread being printed can interact and have an impact.

Fusion 360 does not have a tolerance tweak in the thread creation tool. This is not a problem in that you can use the Face Offset tool to adjust the thread geometry. This does not take long to do. The process also allows you to add fillets to the thread peaks so they are less aggressively ‘sharp’ and therefore more likely to survive longer.

Select the Inspect/Section Analysis to view the cross section of the thread to be adjusted. Choose any axis for this. Manipulate the view so you can see the cross section face and the around to the side of the 3D model. Do the tweaks shown below by selecting the appropriate faces of the thread and making an Offset Face adjustment and then adding a chamfer. The difference is very minor but it makes the thread less ‘sharp’ and aggressive to its mating half which is likely to be a metal component. If you are working with a modelled threaded hole rather than a rod then the changes are the same. The values shown are nominal and will change with the modelled thread size. If you overdue the offset the thread will become very sloppy.

The only tricky part is Manipulating the view in Fusion to allow the appropriate face selection otherwise the Offset command is straightforward.

To a degree some of this could be achieved in your 3D slicer but adjustments would become global rather than specific to just the thread geometry.

If you want a more detailed explanation then I suggest you watch Kevin’s post on Product Design Online.

I have modified the geometry of the Depth Setting boss threads to give more tolerance and reposted the STL to match on the link below.

If you have a Myford Small Bore lathe and would like to send me the bore size and end thread I can create a new version of the depth setter to match.

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The “Modern Clock” by Goodrich

I have been working on a John Wilding ‘Scroll Frame Skeleton Clock’ which has been somewhat entertaining. Getting the clock in beat has caused some frustration. The pendulum crutch is elbow hinged to allow the beat to be adjusted but the adjuster sits behind the pendulum rod. It is easy to tweak the hinge angle but the pendulum rod then masks access to the tightening screw. In the end I resorted to getting the beat somewhere near and then fine adjusting it using the levelling adjuster feet on the base. It now seems to be running nicely, the beat sounds strong and timekeeping is looking good so far. As shown below I’ve got the light sensor of the Microset Timer on it at the moment so we’ll see how well it performs long term.

To revert back to the post title, John Wilding’s book on this particular skeleton clock makes reference to two books as being worthy of any clockie bookshelf – Britten’s ‘Clock and Watchmaker’s Handbook’ and ‘The Modern Clock’ by Goodrich. The latter is a bit of misnomer in that it was published in 1905 but it is a very detailed book and worth a read.

Not having either of these books I did a search. The Britten book is available from Amazon. The Goodrich book is available as a free download from the Gutenberg library. The download is possible in various digital formats, one of which is EPUB3 which is Kindle friendly.

To load third party documents into your Kindle you have to email the document as an attachment to your personal Kindle upload email address. This address can be found in the Settings section of your Kindle. I use this quite regularly to store interesting documents such as technical PDFs. I now have quite a collection of personal and third party techie documents stored alongside John Le Carre, Gerald Seymour and others. You never know when you are going to be bored while sitting on a beach and in need of some technical stimulation.

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Streaming camera video from an Arduino Giga

A close friend has been trying to get a video feed from an Arduino so he can make astronomical observations from a gizmo he has made that will sit in the garden and observe the video in the house over USB.

We tried various methods including using the Processing app but did not have any success. The release of the Arduino Giga with Display Shield and onboard plug in camera (OV7675) has changed things. Using these integrated modules with the OpenMV application produces good quality video over the USB connection. An image of the Giga from the Arduino website is shown below.

There is a write up on using OpenMV on the Arduino forum. This is easy to follow and works very well. If I understand it correctly OpenMV loads specific firmware into the Giga rather than an Arduino based code uploaded in the normal way.

There are a few minor things to watch out for. The Giga seems to like a double reset to clear out any existing code before loading the OpenMV code. Likewise when reverting the Giga back for Arduino use you must also do a double reset.

As mentioned in the article you need to load the display.py demo example code but the demos are not available for selection until OpenMV detects the board in use. Once the boards is detected you will get details on the bottom status bar.

The other minor thing that is not immediately obvious …. if the video and the histograms are not present on the right hand side of the screen you need to drag them into view using the side arrow.

Here is a screen shot showing the code on the left hand side and the video feed and histograms on the right hand side,

The Arduino Giga is a very sophisticated module and the various example sketches that are available to run on it are impressive. It will certainly stretch my ‘cut and paste’ coding….

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My bees have swarmed

Three warm sunny days and my bees got itchy feet and swarmed yesterday. The hive is what I call a passive hive in that it provides a home for the bees and I make no demand on their honey. Free lodgings are given provided they pollinate my various fruit and veg production.

The hive is shown below and is mounted high on the wall of an adjacent building, well above flight paths. The central column is an original commercial item from Gardeners Beehive. (Note that Kevin at Gardeners Beehive is very dyslexic so please be patient and understanding when reading his website). The side boxes are additions to give more comb space. The right hand side box has a hinged door over a Perspex viewing window which is opened by the lever.

The bees had been unnaturally grumpy this week and I picked up a couple of stings. The first one occurred while in the veggie plot and started as a serious buzzing in my right hand side hear aid. This did not abate and stupidly I brushed at it and in the process my hearing aid flew out among the veg. The buzzing then came round to the left hand side and the second hearing aid ended up also in the veg.

I had had enough and headed out onto the lawn running tighter and tighter circles to shake off my attacker. Revenge was a sting under my eye which was very painful and tender for a couple of days. I recruited my wife to help me to find both hearing aids, one of which now had a damaged transducer cable. A trip to the audiologist followed. (There is some discussion about whether hearing aids give off some form of acoustic signal that attracts insects).

The second sting was while I was on the roof of our bungalow cleaning the solar panels. I try to keep them as clean as possible to get every last kW of energy. Regular downpours containing Sahara red dust work against me. Anyway I’m on the roof minding my own business when I felt something land on my head. Next minute I felt the sting. I must have been right in the flight line and the little devil must have got out of bed at the wrong side.

Yesterday while heading down to the veggie plot I heard the tell tale buzz of activity and sure enough a huge cloud of bees was vacating the hive and whirling around. It is quite impressive to see them dramatically exiting in high volume. However to my surprise the swarming mass only moved about 10 metres and settled on our rhubarb plant where it has remained overnight. This morning the scout bees are rushing back and forth and no doubt will be voting for the best location for their new home before the swarm heads off.

My wife duly informed me, in no uncertain terms, that rhubarb crumble was off the menu for Sunday lunch. There’s always hope that Spotted Dick might be the substitute…

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Rosebud Fire Grate on a Silvercrest BR Class 4

I received a blog enquiry asking if I could make a fire grate for a Silvercrest 5″ BR Class 4. The owner had inherited the locomotive but it was missing the fire grate. Given the age of the model Silvercrest were unable to help. I gamely said I would ‘have a go’ but on the basis of making a rosebud rather than a conventional bar grate.

What I didn’t realise was that the grate on this model is made up of 3 separate long sections which are shuffled into place via the firebox door …

I asked the owner to first of all make a template of the grate size based on three equally wide sections of hardboard. This resulted in an overall grate size of 57mm x 209mm. From this I used my Excel spreadsheet to derive a grate with 15% hole area occupancy from a matrix of 25 x 6 holes each of 4mm diameter counterbored with a BS4 centre drill.

I thought it might be a good idea to have a draft angle on the inner edges of the grates. Don’t ask my why – it just seemed like it would make it easier when dropping the middle section into place and perhaps reduce air leakage through the joint .. and of course this makes the design more complicated than necessary and added to my machining woes. This can be seen on the image above.

The design was created as a single grate and then extrude cut into the three sections so the middle and outside grates could have their own CAM program. Here is the Fusion model view looking from the under side.

Having grasped the basis of the design I sent the owner a set of 3D printed grate sections which he cross checked and confirmed would fit correctly. (Could these be the ultimate chocolate fireguard I wonder ?)

The next step was to think about the CNC CAM operations and the stock holding. I find holding the stock and the order of doing things to be the most challenging part of the machining process. As the three sections of grate were all under 25mm I opted to use 6mm x 25mm BMS as the base stock material. I had the Fusion 360 model reference the stock at the centre point. This is easy to probe using the Tormach PathPilot inbuilt routines. I cut three pieces of the BMS to 215 mm lengths and mounted each in turn on parallels in the machine vice. I had a vice stop set up to make the process repeatable. Each section was then drilled for the 4mm through holes (50 off in each) and then counter bored with the BS4 centre drill such that the taper on the drill finished flush with the stock top. This just left the outer profile of the model to be cut on the residual stock.

This was accomplished by using a piece of 30mm wide BMS as the sacrificial backing jig mounted on parallels. This was centre referenced and a number of the 4mm hole locations were drilled at 3.2mm and then tapped 4mm. These holes were then used to bolt down the three grates in turn so they could be profiled to size. The profiled width had to be the maximum width ignoring the intended draft profile (i.e. 19.10mm + 20.916mm). Here is the end view (upside down) showing the three grates with their draft angle .

The machining process was then transferred to my Myford VMB manual mill. This was set up with a tilting vice set to 10 degrees. The two bright edges of the centre grate and one edge of each of the outside grates were then blackened with a Sharpie before each being placed in the tilted vice and skimmed at 10 degrees such as to just remove the blackened surface.

The resulting grates all butted up together nicely and the job was complete. Here is the bottom side view.

These were shipped off to the owner for trials and he later confirmed that they fitted nicely into the firebox. I am not sure if anyone has made a rosebud for a Silvercrest with split grates so it should be interesting to see how well this configuration steams out on the track.

I think the above should give you enough to think about but there is one other spin off that resulted. While drawing up the grate in Fusion 360 I decided it would be much more convenient if the rosebud grate design could be automated using Fusion’s parametric functionality.

This resulted in the Excel spreadsheet being modified and the Fusion model also being updated. The result is the ability to get all the factors needed for the Fusion parameters from the Excel sheet (but you have to manually copy them between Excel and Fusion).

Here is a ZIP file with both these files included.

Update : the user has reported back that the grate performs well. Like other installations of a Rosebud grate, he has found a need to keep the blower slightly open.

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