Engines > Your Own Design

James Coombes - Not As You Know Him

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Having done a bit of  :pinkelephant: :cartwheel: earlier today which as you know can be induced by that happy feeling you get when an engine runs first time you waft a few psi of air towards it I think it is safe to start a new build thread.

Having enjoyed the "reinvented" Stuart Real project last year and seeing Jo's current bulid I thought that their James Coombes could do with a similar treatment to bring him into the 21st century. So I set to with a copy of Andrew Smith's book which is the cheapest way to get a set of the drawings and redrew it in Alibre to use mostly barstock and to metric standards. Some of the main points I wanted to address were:

-Do away with the cast box bed and outrigger bearing support and mount the engine on a "stone" plinth

- The sole plate and table on the stuart are just bits of flat 3/16" plate so these to be made to look a lot more like castings and generally more appealing to the eye.

- Similar with the plain columns to be replaced by ones with classic bases and capitals

- Do away with the soldered conrod which may put people off and not easy to solder it up true

-Do away with the clumsy bolt together eccentric rod and replace with a single straight piece

- Add decorative bands to the cylinder as I was not going to timber clad this one and do something with the inlet as I don't like that going in via the valve chest cover. And while I'm at it get rid of the chunky exhaust flange.

= Generally add a bit more detail but not go as far as things like fluted columns, wedged and cottered straps etc otherwise I may as well just build a Waller Table Engine.

Starting with the sole plate and outrigger bearing base two pieces of 12mm plate were squared up on the manual mill to overall size. Then the soleplate was clamped to the CNC's bed with some packing below so that the four hole sthat will hold it to the base could be drilled and I also thinned down an area where the crankshaft clearance slot will be cut so that you don't see a thick edge around this hole as it would not have been cast full thickness.

I find mounting the clamp bars the wrong way round is a good way to lower their profile so you can get in closer without too much tool sticking out of the collet holder.

Using the four previously drilled holes the sole plate was screwed to my well used machining plate to have 1.5mm taken off most of it's thickness just leaving raised bosses under the screws and a raised rectangular pad for the bearing pedestal using a cutter with 1mm corner radius to leave a fillet for that cast look. I also cut out the clearance for the crank, drilled holes for screws to fix the columns and counterbored these to accurately locate spigots on the column ends.

Just to show you don't need a CNC to do these things the outrigger base was done on the manual mill, the counterbored holes with have thivk "washers" bonded in to create raised bosses for teh hold down bolts

The quickest way to get the decorative moulding around the edges was to hold the plate at a slight angle to replicate the draft angle and then a couple of deep passes of the side of a ball nose cutter produced the required profile unfortunately the photo was out of focus for that

Next four squares of steel were JB Welded into place and once set were milled down to an even 5mm high. These will form the bases of the columns.

The table was done in much the same way from 10mm material so I'll just post a couple of photos of the underside and then the top.

It would be interesting to know how long it takes for the CNC to mill these pieces out of solid so we could compare it with machining castings  :)


F360 gives 44mins for the table and 107 for the sole plate.

In reality I would say add 10% or so to that to allow for some loss of rapids due to using the free version and tool changing. Still considerably faster that doing it by hand as you would require several setups on a rotary table to do the round bosses as solid or like the outrigger bond in pieces and then fillet with JB Weld.

I have got to the stage where I'm happy to leave the machine to get on with it so I can be doing other things while it is cutting which you can't do when using a manual machine so maybe only 20mins spent at the machine for each. Computer tells me how long each machining operation is going to take so I know when to go back and be ready to change a tool if I'm not actually working in that workshop while it is running.

There is also the initial machining to size and the moulded detail around the edges to add in which were done on the manual machine.

These were reasonably quick for their size, had they been more 3 dimensional with lots of vertically curving surfaces and draft angles then that would put the time up as the stepover between each pass needs to be smaller so runtimes increase. For example the soleplate for the next project which will be a long out of production Stuart engine is going to be just over 4hrs for a smaller engine, can you tell what it is yet?

If these were castings and we will assume they are soft and without major imperfections then it's just a case of fettling, finding a datum, milling top and bottom flat and drilling some holes which with a DRO is fairly straight forward.

However as castings are not available for these items do you want to compare total time including making the patterns and then machining them assuming you get the casting done elsewhere? We can discount design time as that would be the same for either

john mills:
the machining time could be less but the time doing the drawing and then generating the program is were the time is spent .
the time machining will also depend on the machine lower powered small machines that can't use carbide modern tooling to advantage will take much longer.
when i was working i had a job on a smaller CNC milling machine .it was a lightly built but with ample power and higher speeds .
The owner soon could see were i spent time doing the program  and setting the machine the time for machining was quicker for small numbers.while the man on the big powerful manual mill got straight to machining which took the time.


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