Author Topic: Burrell Single Crank Compound Traction Engines - 4" Scale  (Read 16138 times)

Offline yogi

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #90 on: September 23, 2016, 12:34:26 AM »
Excellent work Andrew!  :ThumbsUp: :ThumbsUp: :ThumbsUp:
I always enjoy seeing you techniques. Thanks for sharing!

Offline scc

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #91 on: September 30, 2016, 08:37:21 AM »
Hi,  Andrew,      Re your oilers on TT I attach some pics of our 12" scale stuff.  1st pic is 1904 SCC roller, then a 1902 "Devonshire" SCC, and finally a1919 Gold Medal DCC.  The latter does a fair bit of road work, hence big oil pots.                   

Good work as usual.......................Regards             Terry

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #92 on: September 30, 2016, 09:22:46 PM »
Thanks Terry, looks like I've been over-cautious putting a 1/32" hole in my oilers, compared to the holes in your pictures! I've also got my square head bolts fitted the wrong way round. :embarassed: Nor do I have a split pin, but locknut and split pin does seem a bit OTT. Particularly as they're not fitted properly. I wouldn't sign off a split pin like that on a glider; the legs should be bent round the shaft. Makes them a PITA to get out though.

Andrew

Offline scc

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #93 on: September 30, 2016, 09:59:29 PM »
Hi,   The split pins are fitted that way for ease of removal in case some fettling is required on the road.  They do not lock the nuts but merely prevent them from dissapearing after you have heard the knocking and stopped to investigate.  The nuts will be loose but still on the stud!   I know that sounds odd but bearing caps sometimes need to be backed off on the move if it gets too warm.  If all is set up properly at the build / rebuild stage none of this should happen, but we are dealing with 100yr old iron here with unknown histories.

When I used to restore cars, etc. split pins were always bent right around as you describe. Horses for courses I suppose.

I've just started on all the studs for the cylinders :'(    I covet your "production" machinery.........I could be some time........

Regards               Terry

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #94 on: October 07, 2016, 07:05:34 PM »
Terry: So if I understand it the split pins are a later addition rather than original fit?

Andrew

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #95 on: October 07, 2016, 08:33:54 PM »
It appears that I have posted some of the pictures included here elsewhere on the forum but not, I hope, in  this thread.

The steering gear drawings are rather sketchy. The OD of the worm is given along with a lead of 1" and pitch of 1/2", ie, two start. The worm wheel is stated as having 22 teeth and an OD is given, but no other information. The worm wheel is clearly a helical gear rather than a worm wheel. Starting with the worm I assessed the options for machining. My lathe gearbox  will cut a 2 tpi thread but not 1 tpi. I suspect that even 2 tpi would be pretty scary.  :o  Another option was to use the 4th axis on the CNC mill. In theory this is fairly simple, the code:

G01 X100 A720 F200

should advance 100mm in X at the same time as rotating 720°, ie, two turns, around the A axis. The feedrate should be interpreted in mm/minute. Experiments with Mach3 showed that this was simply not the case for the feedrate. Then I discovered the 'radius compensation' option, which I though would compensate for changes of the diameter being machined. For the same numbers in the G-code the distance actually travelled increases as the diameter increases. Wrong again, experiments gave numbers that made no sense. Then I found some information on a forum implying that the radius compensation was for the axis of rotation being different to the axis of the machine. But in my case the axes are co-incident; if you set the value to zero it disables the function, so you need to set it to a small value, say 0.0001. :facepalm: After more experimentation I thought I'd come up with some feedrate numbers that were sensible.

It was now pointed out to me that a two start worm is not prototypical, and it will backdrive, in other words the worm wheel will drive the worm if torque is applied. Back to the drawing board. A worm wheel will backdrive if the tangent of the helix angle is greater than the coefficient of friction between the parts. That's the theory anyway, but is highly dependent upon surface finish, lubrication and the amount of vibration present. I changed the design to a single start worm with a lead and pitch of 0.5". Flushed with success I re-wrote the G-code.  I drew up the thread form 20 times size on graph paper and plotted the positions needed for roughing with a 6mm and then 4mm endmill, and finally a tapered endmill with an included angle of 30°. Of course normal Acme threads are 29° included, but no cutter was available for that value, and since I am making both gears, it doesn't matter. Good luck to any rivet counter who spots it.  :) I then machined a trial worm in Delrin. It looked great, except that it was the wrong hand thread.  :'( Still, simple enough to change, just a minus sign in the G-code. Then I cut a couple of worms in steel. Oddly there was some chatter using the HSS tapered endmill and the finish wasn't as good as I expected. Slowly it dawned that the rotary axis hadn't slowed down for each successive tool as coded, it was just running flat out. Cue more experimentation.

After some hours of trials I contacted the manufacturer of my CNC mill. They said that Mach3 was bleep-bleep in this area and I should use G93, inverse time feedrate. This is an odd one. The time taken to execute the line of code is the inverse of the value given, in minutes. So a value of 1 will execute in 1 minute, but a value of 0.2 will take 5 minutes. More experiments showed that this worked, with a small but consistent time offset, a few seconds. Having sorted all these issues out I could then machine the real worms. Here's a general shot of the setup with a 6mm cutter taking its roughing cuts:



And the finished worms with keyways cut:



Having made the worms I moved onto the worm wheel. I wanted this to be single enveloping, ie, the worm wheel wraps partly around the worm. Normally these are cut using a hob with the blank geared to the rotation of the hob to produce the correct number of teeth. I don't have facilities for this, so I resorted to free hobbing. This is where the worm wheel is gashed with embryo teeth using a standard involute gear cutter and the hob is then used to clean out and form the teeth. Since the worm wheel has been pre-gashed the hob can drive the worm wheel itself, at least that's the theory. Making the hob was simple, just a longer version of the worm. Here is the embryo hob before the teeth were cut. Material was silver steel:



The thread at the right is an odd one, 25mm OD and 20 tpi Whitworth to fit a Clarkson style milling chuck. As a precursor to machining the worm wheels from the supplied castings I made one in steel to test the process. This highlighted a number of areas where I needed to up my game. On the steel worm wheel I machined the curved throat on the OD by eye. This wasn't sufficient as there was some rubbing of the hob at maximum depth. For the proper worm wheels I filed a template from sheet steel and used this as a gauge to get a more accurate throat shape. To get the maximum depth I could gash the teeth to while allowing the hob to clean up I used CAD. I can't remember the value but it worked well. Except on the prototype steel worm wheel. I traced this to lack of care in centring the hob transversely, so it cut more on one side than the other. I took more care with the proper worm wheels. Of course I was keen to see if the technique worked or if the hob would disintegrate so I rushed the setup with the steel worm wheel. Starting the hobbing process is binary machining, you can't sneak up on using the hob, you just have to drop the clutch on the mill and stand back!

For gashing the worm wheel blank is set over by the helix angle of the worm, although it is not obvious in this picture:



For hobbing the worm wheel blank is set back to be perpendicular to the hob. Here is a picture of the hobbing of the worm wheel, notice the significant difference in tooth shape:



The material is cart iron so machining is done dry. After I had machined the steel worm wheel I noticed some faceting on the tooth faces. I eventually twigged that this is a function of the number of teeth per revolution of the hob. Essentially the hob cuts the tooth form as a series of flats. For the proper worm wheels I improved this faceting by moving the hob axially after full depth was achieved. The hob was moved one pitch of the worm, 0.5", in steps of 1/16" and at each step the worm wheel was allowed to make a complete revolution, driven by the hob. Although slight there was a definite change in the sound of the cut as the worm wheel completed a revolution.

Finally here are the worm and worm wheel installed on the engine:




Andrew
« Last Edit: August 17, 2017, 10:51:32 PM by jadge »

Offline scc

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #96 on: October 07, 2016, 09:37:30 PM »
Terry: So if I understand it the split pins are a later addition rather than original fit?

Andrew                 Made that way from day one as far as I know.



                            Steering bits look good............                  Terry

Offline Kim

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #97 on: October 08, 2016, 06:58:26 AM »
Those are some mighty fine looking gears you cut there Andrew!  And I KNOW it isn't as easy as you made it look!
Kim

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #98 on: October 08, 2016, 11:40:19 AM »
Terry: That's interesting, I'll have to look more carefully at some full size engines when I get the chance. However, it's probably a step too far to add them to my engines. I have enough diversions as it is.

Kim: You obviously speak from experience! In retrospect the worm wheel was reasonably straightforward. For the worm the biggest issue was faffing about with feedrates in Mach3. The outline given is the sanitised version. :o After my CNC controller had a wobbly last year (turned out to be an iffy HDD connector) I took the opportunity to buy a new controller and upgrade to Tormach PathPilot. I haven't played with the 4th axis yet, but I live in hope that the feedrates will be interpreted correctly.

Andrew
« Last Edit: August 17, 2017, 10:51:56 PM by jadge »

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #99 on: January 24, 2017, 08:55:42 PM »
The governor on my traction engines uses two 1:1 bevel gears to transfer motion from the horizontal to the vertical. They are specified on the drawing as 18 teeth and 18DP, so a PCD of 1". I designed these in 3D CAD some while ago, while looking at the overall design of the governor. Having recently fitted a high speed milling spindle to the CNC mill I thought I'd try it out by machining the governor bevel gears. When looking at cutter sizes it became clear that I needed a 0.8mm diameter cutter to get into the tooth root at the inner end of the bevels. A 0.8mm cutter came out at £32 - I don't think so! Of course I could have used a (cheaper) 1/32" cutter. But I'd already gone ahead and bought some 1mm diameter carbide ballnose cutters. So I did a quick redesign on the gears to be 16 teeth and 16DP; still a PCD of 1" and I could use the 1mm diameter cutter. I used a pressure angle of 20 degrees. Here are the two gears in a 3D assembly:



Deriving the CAM program for the gears proved rather problematic. I spent ages adding circles on the gear to act as limits for the machining. The toolpath drawn by the CAM program looked good. But plotting out the G-code generated revealed some odd arcs that seemed to run the cutter through solid metal. After a lot of faffing about I simply added some radial and axial limits, by typing into boxes in the setup pages and was rewarded with a sensible looking toolpath and G-code.  :ThumbsUp: Here are the combined toolpaths:



There are three steps. A roughing step with a coarse stepover, and stepdown, to remove most of the metal. This was followed by a prefinish profiling cut, leaving 0.2mm of stock, to clean up any nasties left from roughing. I've been bitten in the past where odd areas were not machined away as expected and consequently the cutter broke while profiling. The final operation was a profile cut with a fine stepover. Here's the high speed spindle in action on the CNC mill:



The spindle was running at 24000rpm with a feedrate of 350mm/min. For roughing stepover and stepdown was 0.3mm. For the prefinish profiling stepover was 0.1mm and for the final profiling stepover was 0.04mm. This is finer than needed for the tops of the teeth, but is dictated by the relatively steep flanks of the teeth. Too large a stepover leaves a 'steppy' finish on the tooth flanks. I also had to tweak the G-code to remove tool length setting via the tool table, as the spindle doesn't have a quick change collet. So the Z axis has to be referenced every time a tool is changed. That was one factor in deciding to do everything with a 1mm cutter, rather than be a smartypants and rough out with a bigger cutter before profiling. Total machining time was around 3-3/4 hours. But of course one doesn't have to baby sit the mill. For the first time ever I actually left the house with the CNC mill running to walk up the road to the local garage for emergency chocolate rations.

Here are the four gears (two engines) and the aluminium test gear; aluminium being somewhat cheaper than bronze:



Not that it really matters, but these are true bevel gears. I was very pleased with the way the Tormach behaved, these are pretty small gears and all the movements were only a few millimetres, but everything worked fine. Well almost everything. PathPilot had a wobbly after I'd left the mill on overnight so as to not lose work references. It ran the G-code fine, but I noticed that the axis displays were not being updated, although the DTG numbers were changing. I resisted the temptation to fiddle with it while the program was running. After it finished none of the screen buttons or the jog controller worked. I had to reboot the controller. I probably ought to upgrade PathPilot to a newer version.

These are the last gears on the traction engine to make. :'( So now I'm going to have to knuckle down and get on with assembling the wheels.

Andrew
« Last Edit: August 17, 2017, 10:54:57 PM by jadge »

Offline Admiral_dk

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #100 on: January 24, 2017, 09:22:35 PM »
I was going to point out that you can get some nice Chinese carbide mills down to 0.8mm at a very good price when you search for those to do PCB work on AliExpress ...!... and then I realized that you needed ball nose mills ...  :slap:

I still like the gears - they look good  :ThumbsUp:

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #101 on: July 22, 2017, 09:59:05 AM »
Obviously I've been caught up in the Photobucket farrago. I only use them for hosting pictures on this site, so they can go to hell on a hand cart before I pay. I have exact copies of the hosted pictures on my computer, so it's no concern if my account gets closed.

In due course I'll decide whether to edit this thread and reinstate the pictures by other means, or just leave it and move on. It'll probably come down to the time involved.

Andrew

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #102 on: August 11, 2017, 09:52:56 PM »
Decision made! I have transferred all my pictures to a new host, and will slowly update the pictures in this thread. In some fantasy world where I have unlimited time  :ROFL:  I might try and do the same to some of my other posts.

Andrew

Offline Kim

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #103 on: August 12, 2017, 05:11:24 AM »
Where are you hosting, Anderw?  I'm using Ade's coppermine site he setup and that's working well. And the price is right!
I'll be glad to have your pictures back!
Kim

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #104 on: August 12, 2017, 09:24:05 AM »
I'm using Ade's coppermine site he setup and that's working well. And the price is right!

Me too.  :ThumbsUp:

Andrew