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Thursday, 30 June 2022

Excel files to help you design Gear Trains for wooden clocks


Spreadsheet to calculate the length of clock pendulums

If you have decided to design and build your own wooden clock then the following charts should give you some idea of how to tackle the tricky mathematical bits that are needed to make sure that the clock can run accurately throughout the day. Some of my earlier posts should have introduced you to some of these formulae that are used in this process. The idea here though is that we try to automate the process using Excel to do the heavy lifting and at the same time give you the opportunity to try out different combinations needed to me the constraints of the design you have in your head.

This first chart is used to calculate the length of the clock's pendulum which in turn uses various combinations of gears to achieve this, with you inputting tooth numbers into the appropriate box.

Basic concepts
  • Minute shaft makes 1 revolution every 3600 seconds

  • You are using a Graham Escapement that stops the rotation 2x for each completed swing(Tic Toc)

  • metric units

  • g= 9.8m/s/s

  • T= pendulum period of time for a Tick and A Tock

  • pi= 3.14159

  • length of pendulum = g(T/(2 *pi))2

  • If you aim to have a second hand then the Escapement wheel should rotate once in 60 secs.

The chart provides inputs for a maximum of 3-wheeled Gear Train and if you only use 2 then fill the appropriate boxes with the number 1, it also requires you to provide a pair of gears with a Wheel and a Pinion for each pairing. You must only input figures into the Green coloured boxes as the orange ones contain the Formulae to calculate the value.
If you want to add other combinations to the green area either clear an unwanted row or use the normal Excel technique to copy the whole Green and Orange bottom row of boxes and paste them to the row below.
Click on the picture below to download the Excel File.





Spreadsheet to calculate Gear tooth Profile.

I have always used a modified version of the standard gear tooth profile so as to give me more clearance between teeth and a bit more leeway when cutting the teeth by hand or on a scroll saw. The most important thing with regards to tooth profile is consistency in the pitching of the teeth. The charts below reflect this and the details are given for both the large gear and the Pinions in the most used tooth sizes.

Again if you want to add other combinations to the green area either clear an unwanted row or use the normal Excel technique to copy the whole Green and Orange bottom row of boxes and paste them to the row below.
Click on the picture below to download the Excel File.

PCD is the Number of TeethX Mod
Outside Diameter is (Number of teeth + 2)X Mod
Root Diameter PCD-(ModX3)









Spreadsheet to calculate centre distance between a Pair of Gears

This is probably the simplest of the charts as it simply calculates the distance between the two mating gears by adding the two Pitch Circle Diameters (Ø50 mm in this case) together and dividing by 2. If you experience difficulty with distortion of the gears that causes problems with assembly then you might want to increase this by 0.5 mm.

























Tuesday, 28 June 2022

Useful File types for wooden clocks builders - Step File

I have included STP or STEP files with most of the clocks that you can purchase as these files provide you with exact 3-D models of the clock that you can use in your 3D CAD program to modify any of the parts.

That is useful if you own and use 3D CAD software but if you don't then the files can be used in conjunction with an STP file viewer program to provide you with a great deal of extra information about the parts of the clock and the way that they fit together.

There is a considerable number of these available on the internet, some free and some not so much. I have only looked at the free ones as, to be honest, you should be able to get all you need from them. The programs fall into two groups, On-line where you upload the files you want to view and PC or mobile Android or iOS based.

What I was looking for in a viewer was first and foremost how easy and intuitive it was to use and manipulate the parts or assemblies when loaded. Secondly was whether it would produce a list of parts in an assembly and whether it would highlight a part within an assembly when clicked either in the model or the parts list. This is important as it is sometimes difficult to find parts and identify them when you click on either the part or the part in the list. 

Autodesk Viewer

Starting with the Online STP viewer my clear favourite here is the viewer from Autodesk, simply called the Autodesk Viewer which will open over 80 file types including STP,  STL and Solidworks, etc.


 















Once you have registered and signed in, use the Model Browser to find the part of the assembly you want to view. once loaded as shown below the assembly can be opened up in the Model browser where you can click on individual parts or groups to make them either visible or invisible. It would benefit you at this stage to either read the instructions or set up some of the settings, I advise you to do
this as it makes it easier, later on, to manipulate the model and interrogate it for more information on the parts.












Now you are ready to use the viewer, one of the rather nice things you can do with the loaded assembly is to explode so that the parts can be more easily identified, this is simple simply click on the Explode symbol at the bottom and then slid the blue ball to the right to see all the parts fly apart. 












To see individual parts without the clutter of the other parts around it simply click on the part in the parts list and the other parts disappear, to bring them back right-click on the same part and click on the Show all message to bring them back. finally, on this I would recommend you go into Configuration - Display settings and turn off 'Ghost hidden objects' as this makes the visible part more difficult to see clearly.



e








eDrawings

This next one can be downloaded to run on your PC or Mobile no Online option here. This is similar in many ways to the Autodesk Viewer but can load Solidworks files as well if you need that functionality. It is also slightly faster to load larger files as there is no internet connection needed.


Functionality on this one is pretty much the same as the Autodesk viewer with a Parts tree showing all the parts in their sub-assemblies with the same abilities to hide and show individual parts or sub-assemblies. Identification of those parts is easily done as highlighting in the parts tree also highlights the part on the model. 















Exploding of the assemblies is also similar to the Autodesk Viewer. Not much to choose between these first two and may come down to a preference for either PC or Online-based.

CADassistant

CAD assistant has similar functionality to the above and is either PC or mobile-based which may well be useful to iPad owners. It lacks the ability to explode assemblies but does have the functionality to change or add colour to a part which may well be the function that you are looking for.

I found it slightly more difficult to use than the others but to be fair that may be just me and certainly the ability to add colour could well make it worth looking at.
















So those are the ones that I consider worth using when you are trying to find more information about the wooden clock design you are going to build. Each has there merits and no reason for you not to try each one before you decide.

Monday, 25 April 2022

Pendulum Construction

 Some time ago I published a couple of posts about the clocks Pendulum, firstly a Simple Pendulum and then another post about the more complex Compound pendulum.
This time however it's time to look at the construction of the Pendulum and how it interacts with the rest of the clock. The following are all based on the construction of the simple Pendulum but they can also be read as applying to the Compound pendulum as well.

The view above here shows the back of the Clock with the pendulum pivot fitted to the same shaft as the Escapement Arm at the top, and at the bottom, the pendulum Bob is attached. It is these two extremes that I shall concentrate on through the rest of this article but just a word concerning the Pendulum Rod itself.
I always like to use Carbon Fibre tubing for its stiffness and lightness but other materials such as wooden dowels or Aluminium tubing can work just as well.



Now looking at the top of the pendulum first I have shown two design routes that I have used in the past. The first on the left shows the conventional layout with the Pendulum pivot resting in a groove on the protruding stub, this is really the minimum friction approach as the sharp edges on both the Pivot and the groove give the minimal surface area of contact. To get the clock working the swinging Pendulum needs to be connected to the Escapement arm on the other side of the Back frame to do this the Yoke is used which lightly grips either side of the pendulum rod carrying the Yoke and the Escapement Arm with it. This works quite well but it does generate some friction as the Yoke slides up and down the Pendulum Rod as it swings because the two items are not hanging from the same pivots, the Yoke pivoting a couple of inches below the Pendulum itself.
However depending on the materials used this will end up wearing quite quickly so some harder material than wood should ideally be used, for instance, Brass works quite well. 

As we are not aiming for the ultimate accuracy in the clock, there is another route shown on the right which is a little easier to build and avoids the friction losses between the Yolk and the Pendulum rod. in this case the Pendulum head is connected directly to the Escapement Arm by having both swings on the same pivot and using the Connector pin fixed directly into both parts through a slot in the Back Frame. This is the method I have chosen in many of my later clock designs.

Moving to the Bottom of the Pendulum there are quite a few variations on the construction that I have developed over the years. The first of these shown here to the left is the use of an adjusting nut. You can see that at the very bottom of the wooden rod the material has changed to Brass and a threaded section added to the end. The Pendulum Bob is slid onto the bottom of the pendulum and then followed by an adjusting nut, the effective length of the pendulum can now be adjusted by turning the nut so that it moves upward to make the clock run quicker or downwards to make it slower. The reasoning behind these adjustments is discussed in the earlier article on the Simple Pendulum. The use of the adjusting nut makes it a lot easier to accurately adjust the rate of the clock because very small movements can be accurately made to the effective length of the pendulum. The disadvantage is that the construction is more difficult because of the joining of the two parts of the Rod together although drilling the ends of the parts and pinning them together can make it easier.

I have not mentioned the shape of the pendulum Bob but to all intents and purposes it doesn't matter as you can have any shape you like. I generally either use a Brass rod cut to a length, the same as in the illustration above showing the back of the clock. Alternatly a disc shape like shown above left.

Moving on, the first of the alternative methods of constructing the Pendulum Bob is shown below. This method requires that a rectangular slot is cut through the centre of the Bob and then a Lock component fitted into it. This Lock component has a hole at its inner end that is slightly offset from the centre of the Pendulum Rod passing through it. The outside end of the Lock is shaped like a bow, so by pressing in the centre of the bow the hole in the other end can be aligned with the Pendulum Rod to allow it to pass through. When pressure is released from the bow the hole is pulled against the rod locking it in place. In this way, the Bob can be slid up or down by pressing the bow as you move the Bob. Adjustment is not so accurate as with the threaded nut but it is easier to construct and gives a lot more adjustment movement. The weights added to this Pendulum Bob design give the Pendulum a little more momentum to overcome any small glitches in the gear train driving the movement of the clock.


Another version of this concept is shown below it has the Lock moving from the side and the actual locking is made using a small metal spring. The advantage of this is that adjustments can be made more easily without putting your hand behind the pendulum.


A completely different approach shown next adds a little fun, I used this on a Clock built for my Great Grand son and has a face built up from the 3 layers of the pendulum, the centre layer containing a couple of spring elements shown in blue, they press on the sides of the Pendulum Rod to provide the friction to hold it in place, adjustment is therefore quite easy simply sliding the Bob up or down. Again not as accurate as the screw method but simpler and visually far more iinteresting.


Another way that we can use to add more interest to the rather bland appearance of the wooden disc is to Laser engrave or 'Vee carve' a pattern into the wood itself. Three examples of what is possible are shown here. By 'Vee carve' I mean to use a vee shaped bit for CNC machining where the cutter uses the 3rd dimensions for producing the thick and thin sections of the profile being cut. If your CAM software does not have this feature you could try 'Carbide Create' it is available as a free download for either Windows or Mac.



Whether you laser engrave or V carve you will need a pattern to start with so I have attached some example DXF files for you to try.




Thursday, 24 February 2022

Daisy wheel motion for clocks Hour Hand


 A clock's Minute hand rotates once for each hour and the Hour hand rotates once for every 12 hours, so a ratio of 12:1 reduction is required from the gear train that connects the two hands. To do this I normally use two pairs of gears with 3:1 and 4:1 ratios giving the total reduction of 12:1 required.




I have used this design on nearly all of my clocks in the past. but there has always been another design that I could have incorporated. This is a concept devised by a Clockmaker called Aaron Dodd Crane around 1830 and uses a variation on the Hypocycloidal Speed Reducer.


The hypocycloidal Speed Reducer shown above is used to produce a very high torque output in a compact space. In this form, it has a couple of disadvantages. The first is that it requires extreme accuracy so as to avoid high friction and scraping, and as if this was not enough, it also has an output that runs in reverse of the input. This, of course, is of no use to the clock builder, who needs both Minute and Hour hands to run in the same direction.


The Daisy Wheel concept gets around both of these problems and is called the Daisy Wheel motion. 
It has an inner ring of 11 teeth and an outer ring of 6 teeth, the teeth being none standard, as shown in the video here. The inner ring is constrained by the brass rod sliding in a rocking sleeve and adopts a rocking action to move the outer ring by rotating about an eccentric pivot. The motion is complex and almost mystical in the way that it works. If you watch the video below and concentrate on the Left-Hand side, you will see that each of the 4 teeth on the outer ring is given a little nudge by the top of the nearest Daisy petal, and this is happening at each of the teeth on the Outer Ring in turn causing almost continuous motion of the Outer Ring. You may also note that even though there are only 4 teeth on the Outer Ring and not 12, you still get a 12:1 reduction required for the Hour Hand. The silver rod hanging down from the Daisy engages in a two-fingered slot below so that the Daisy itself is restrained from rotating and simply rocks in place to provide the nudge needed to move the outer ring.
I have adopted this to use in a new clock project that incorporates the Daisy in this more literal sense and opens up the workings to be more visible on the front of the clock.



The main workings are mounted directly onto the front of the clock frame on the Minute shaft and are held in place by the Minute Hand when it is fitted to its shaft. I was a little unsure about mounting the Hour hand around the outside of the Outer Ring and it could have been mounted directly to the eccentric but then its ribs would have obscured part of the Daisy mechanism. However, seeing this arrangement with the Dial-in place convinced me that it would look and work OK.








The diagram below shows all the parts exploded so you can get a better view of each of the ports and how they relate to each other.





I shall be incorporating this design into my next clock coming in the spring, but if you would like to try using it in one of your own designs you can try it out with the files here

If you are interested in more information on Aaron Dodd Crane, extracts from the book 'Aaron Dodd Crane.An American original ' are shown below.








Friday, 14 May 2021

The woodenclocks weight drive - Alternative designs.

 Wooden clocks are driven, in the main by a single drive weight hung from a cord wrapped around the Drum, and it is this type of drive that we are going to look at in this article.

In the first clock, I ever built I used a piece of granite with a hook screwed into the top, it worked well but did not sit well with the traditional image of a clock. For a long time, my plans just included a simple Brass weight which looked the part but, to be honest, wasn't necessarily very easy for woodworkers to construct, so began a series of designs using fabricated parts and some proprietary items.

Design 1 uses a Plastic Tube with wooden endcaps and a threaded rod with nuts at the top and bottom to hold the whole together. I wanted the weight to be attached to the cord simply but a screw-in hook was not going to work with the threaded rod sticking out the top of the weight. This led to the design shown below that uses a Bowline knot on the end of the cord and a wooden hoop holding the nut inside.




With this Tubular construction, the weight is added to the Tube before it is fully assembled. My favourite weight type is of course Lead as it is the densest material that you can practically use and it comes either as Lead shot or as cylindrical weights of the type generally used for Sash type windows.

An alternative to Lead is to use Ball bearings, if bought in the form of 'Catapult shot' they will be much cheaper.





I struggled for a while to find the correct type of Knot to use to hook up the weight to the clock. The problem was that all the knots I was trying would slip until I came across the Bowline knot which is shown here, a bit more complicated to tie but once done it works a treat with no slippage.


Design 2 simplifies the design so that the construction can use a simple hook to connect to the cord hanging from the Drum.



The same weight type can be used as in the previous design but the construction is slightly different in that the top and bottom end plugs are screwed to the Tube around the periphery as shown above.
To make this design even more flexible you can cut a large hole in the Top End plug so that Lead shot can be poured in or out to adjust the weight.

I have the tube shown with a Brass colouring in the above illustrations, you can achieve this by spraying the colour to the inside of the tube before assembly or wrap a self-adhesive Gold film around the outside to achieve the effect.

Design 3 goes for a really simple solution using a standard bottle to hold either Lead shot or Ball bearings or even water depending on your weight requirements.


The illustration above shows how this works using a 500 ml bottle with a Ø28 mm neck, the Hanger showed here cut from plywood sits over the neck and beneath the Cap and the Loop in the Cord formed using the Bowline Knot slips over the bottleneck and then around and to the front of the hanger to engage in the two notches. This simple attachment allows the bottle to be hung on the cord simply and neatly.

I have used this little part numerous times when building prototypes as it allows for the easiest way to adjust the weight to get the clock running regularly without overloading it and putting too much strain on the parts.

Design 4 uses a similar approach with cans, if you own a 3D printer then you can print this Cap adapter to be fitted to the top of a can so that you can use it as a weight on your clock. Click here for a link to the STL file




Design 5 uses a very simple brass weight which consists of a section of Ø24mm brass bar with a length calculated to give you the weight that you need. In the end, I liked this solution best as it was so simple and the small diameter bar give extra clearance from the pendulum Bob which is necessary for some of the clock designs.




To attach the weight to the cord, form a loop at the end of the cord using a Bowline knot and then push the loop in the short hole at the end of the weight. Fit the short weight pin through the cross hole and through the loop and then let the cord take the weight.




The calculation for the length of the bar is as follows:-

Length L = Weight W/π x Radius r ²  x 8.5   where 8.5 is the density of Brass in grams per cubic centre meter.

L = 500/π x 1.2²x 8.5 =12.9 cms ( I rounded it up to 13.5cm to allow for the hole drilled in the top.


Design 6 This is a variation of Design 2 where a single Pulley is incorporate to double the running time. The cord is passed beneath a Pully fixed to the top of the weight and the end anchored to the hook fixed into the clock frame. The only disadvantage to this is that you have to double the size of the weight as half of the weight is effectively lost as it goes into pulling on the Clocks framework.






Monday, 21 September 2020

A design for a Minute engine

 I have tried on several occasions to design a wooden clock with a Swiss Lever Escapement, but so far none has been really successful. The problem is that the lever requires a Balance wheel and Spring for it to work and I can not get a wooden spring to work successfully. 

I developed Clock 29 as a 3D printed clock a couple of years ago and this used a plastic spring but in association with a gravity escapement, so as a development project, the design of the Minute engine is to look at some of the variables that would need to be resolved for a clock utilising a Swiss lever movement to function

Typical Swiss Lever Escapement

Using the arrangement of the parts in the illustration above, which is standard for a typical Swiss lever escapement, the next decision to make was the type of drive train to use and what the power source would be. The power source was straight forward as I wanted a small compact design so it would have to be a steel clock spring that would allow the engine to run for several hours. 
From this point it the design of the gear train would hinge on the requirement to have as an output an Indicator Hand that would revolve once every minute. Initial testing of the oscillating frequency of the Balance wheel and spring suggested a gear pairing of around 10:1 for the first stage and then followed by a 64:1 would give a relatively simple 3 stage gear train with 10:1, 8:1, 8:1 using three large wheels with 64 teeth and two pinions with 8 Teeth and 1 Pinion with six teeth.


Gear train



This is the arrangement of gears that I finally settled on leaving only the connection to the Spring used to power the device and a frame to hold it all together.




The two images above show the completed Minute Engine and how the drive power is fed to the gears through the back frame.
The Engine, as designed here will run for around 4 hours on a complete wind is accurate to around 1 minute per hour. To improve on that accuracy more development will be needed on the Spring and the Balance wheel. The design of the balance wheel includes the provision for the fitting of up to 12 Ø4 diameter steel ball bearings around the periphery and in this case, I adjusted the number used to just 2 balls, 1 positioned at the 12 o'clock position and one positioned at the bottom in the 6 o'clock position.
This ran to within 45secs per hour, accuracy wise this was as good as I could achieve with this current arrangement
To improve the accuracy further, work is needed on the Balance Spring to speed up the running slightly to get to a more reasonable 10 seconds/hour which for a plastic 3D printed clock is probably as good as you could ever expect.

It was never my intention to create an actual clock from this exercise, only to use it as a means develop a working Swiss Lever Escapement and then use that as the engine to drive a complete 3D design for making by 3D printing. As a consequence of this, I inadvertently made it rather difficult to incorporate the Minute hand and Dial that would be essential to gauge the accuracy of the running clock, the subsequent compromise was the introduction of a Minute hand at the back of the gears and a short section of Dial mounted on the Back Frame, not ideal but I was able to use it to measure the accuracy of the running clock.

Building the prototype

Should you wish to make your own you can download the STL files from any of the sites listed below.


I would start with the main gears and make sure that the 3D prints are clean with no strings or bumps that could prevent the smooth running of the gear train.





You may have to drill the holes in the frames to size so all the holes indicated as being a Tight fit should be Ø3 mm diameter and the holes designated as loose should be Ø2.2 mm. Now mount the gears onto the shafts to the dimensions indicated above and lightly oil the shafts to ensure smooth running.


There are 5 pins to be fitted to the frame as shown above before attempting to fit and of the gear sub-assemblies, these will all be a tight fit in the frame. Again lightly oil the shafts prior to fitting the gears to them.


Start by fitting the 3 large 64 toothed gears and the small 6 toothed gear into the front frame.



The spring I used in this engine is a Loose Loop Triflex Clock Spring 6mm wide x 0.4mm thick x 1000 mm long, purchased from Cousins UK part number. The ends will have to be modified as shown above by heating the ends to red heat and allowing to cool slowly. This will soften the ends and enable them to be reworked so that they can fit onto the Cas and drive printed parts.



With the spring-loaded into its case and the drive component fitted into the centre of the spring, offer up the Spring case and slide the drive over the end of the shaft and push it home. You may have to slowly rotate the Spring Case Anti-clockwise to get the sprockets on the drive to engage and allow the assembly to be pushed right home flush with the back case.




Now for the tricky bit, whilst holding the spring case in place fit the Retaining ring over the Spring case, whilst swapping fingers so you never let go of the spring case, and then having got the retaining ring fully over the Spring case make sure the window in the side is near the bottom rotate the ring clockwise to the stop to finally have the spring locked in place. The last step is to fit the Sprig loaded Pawl to the bottom of the ratchet using the two pins on the frame beneath the spring. To fit the spring locate the left hand hole over the left hand pin and squeeze together the other ends so that the very end of the Pawl pokes through the window in the Spring Retainer and engages the Ratchet.


Start by fitting the Escape wheel to the end of the shaft that contains the 6 tooth gear on the other side of the frame, this should be a tight fit on the shaft. Follow this by sliding on the Lever and adding one of the endstops to retain it in place, this to be a loose fit on the shaft.
The Spring shown here in purple should be fitted along with the Spacer shown in blue to the back of the Balance wheel and orientated to the Balance wheel using the short pin. The outside end of the spring has a boss that should be fitted onto the short pin in the Frame and held in place with another Endstop.
Now fit the two ball bearings to the Top and Bottom positions on the balance wheel and finally fit the Balance wheel to its shaft securing with another Endstop. This Balance wheel / Spring sub-assembly should be a loose fit on the shaft.



That's it the Minute engine is complete and should run for a good 4 hours between winds.

Parts and materials required

1 - I used ABS for all of the parts and printed all the parts with 0.2 layer thickness and 50% fill, the exception to that was all of the smallest parts printed solid.

2 - The Mainspring was a Loose Loop Triflex Clock Spring 6mm wide x 0.4mm thick x 1000 mm long, purchased from Cousins UK Part number 6401000LL. Hopefully, the description is enough for you to source the same spring locally.

3 - All the shafts were made from Ø2 diameter Silver Steel or Drill Rod as it is termed in the US. A 300mm or 12" length should be sufficient.

4 - A couple of Ball bearings to weight the Balance wheel Ø4 mm diameter or Ø3/16"

5 - You can use this JPG files for the Dial insert that runs in an arc around the top of the back frame.

6 - STL files for you to build your own Minute Engine can be downloaded here.

7 - If you use Solidworks you can download the Spiral spring file here