Author Topic: James Coombes - Not As You Know Him  (Read 5718 times)

Offline Jasonb

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James Coombes - Not As You Know Him
« on: January 29, 2023, 06:34:04 PM »
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.


« Last Edit: January 29, 2023, 06:37:42 PM by Jasonb »

Offline Jasonb

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Re: James Coombes - Not As You Know Him
« Reply #1 on: January 29, 2023, 07:02:40 PM »
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.




Online Jo

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Re: James Coombes - Not As You Know Him
« Reply #2 on: January 29, 2023, 08:02:49 PM »
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  :)

Jo
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Offline Jasonb

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Re: James Coombes - Not As You Know Him
« Reply #3 on: January 29, 2023, 08:56:11 PM »
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


Offline john mills

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Re: James Coombes - Not As You Know Him
« Reply #4 on: January 29, 2023, 08:57:43 PM »
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.
John

Offline Michael S.

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Re: James Coombes - Not As You Know Him
« Reply #5 on: January 29, 2023, 09:00:10 PM »
A nice, interesting start.
Like it a lot.

Michael

Online Jo

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Re: James Coombes - Not As You Know Him
« Reply #6 on: January 29, 2023, 09:12:09 PM »

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

I think it might be fairer to compare CNC machining from solid with making the pieces out of layers of steel plates of various thicknesses cut to profile for a one off. If you are trying to encourage others to build one. I suspect more members are likely to have plates of steel rather than a CNC mill.

Jo
Enjoyment is more important than achievement.

Offline Dave Otto

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Re: James Coombes - Not As You Know Him
« Reply #7 on: January 29, 2023, 09:52:39 PM »
Hi Jason

I will enjoy watching you carve out another beautiful engine. Maybe someone should start another thread on the pros and cons of home shop CNC?
There are a number of us who fall into that category and might make for some interesting discussion, but not to muck up your build thread here.

Dave
 

Online Kim

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Re: James Coombes - Not As You Know Him
« Reply #8 on: January 29, 2023, 10:25:10 PM »
Yes, this will be an interesting build to watch come together!  :popcorn:

Kim

Offline TimB

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Re: James Coombes - Not As You Know Him
« Reply #9 on: January 29, 2023, 11:08:54 PM »
I like the look of that!!!

Tim

Offline vtsteam

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Re: James Coombes - Not As You Know Him
« Reply #10 on: January 29, 2023, 11:54:22 PM »
Looks good! :ThumbsUp: :ThumbsUp: :popcorn: :popcorn: :cheers:
Steve

Offline Jasonb

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Re: James Coombes - Not As You Know Him
« Reply #11 on: January 30, 2023, 07:26:19 AM »

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

I think it might be fairer to compare CNC machining from solid with making the pieces out of layers of steel plates of various thicknesses cut to profile for a one off. If you are trying to encourage others to build one. I suspect more members are likely to have plates of steel rather than a CNC mill.

Jo

Yes that is why I also showed the smaller outrigger base being done manually. These are not really suited to making up from plates apart from adding the couple of rectangular pads or the round bosses. Though I have used that method in the past, really just need to assess each individual part and use the best approach based on what materials and machines you have.

Dave, good idea. I suspect Jo's question may have been prompted by someone taking 3 days to CNC machine a small part as she often raises the point about how long CNC takes but in reality it is often quicker than making scratch parts manually though possibly longer than from castings if they are available and don't need reworking.

John, your points about the machines ability can be covered in a separate thread and I'll detail why that job took someone 3 days and I could have done it in maybe 1/4 of that time or less.

Other thread will have to wait until later as some of us go out to work, god knows how I manage to keep completing these models in such little spare time :lolb:

Online Jo

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Re: James Coombes - Not As You Know Him
« Reply #12 on: January 30, 2023, 07:49:21 AM »
Other thread will have to wait until later as some of us go out to work, god knows how I manage to keep completing these models in such little spare time :lolb:

It is very important for someone to go out to work otherwise who is going to pay the taxes to cover for our pensions  :old:  ;D  You wait until you retire then you will really know how hard time is to come by  ;) .  Can't stand around chatting I've got to go on the village monday morning walk 

Jo
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Offline RayW

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Re: James Coombes - Not As You Know Him
« Reply #13 on: January 30, 2023, 11:31:22 AM »
Looks like an interesting project jason. I will be following progress with interest.
Ray

Offline GWRdriver

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Re: James Coombes - Not As You Know Him
« Reply #14 on: January 30, 2023, 05:44:15 PM »
I will also be following along with GREAT interest as a few years ago two sets (one complete, one incomplete) of orphaned James Coombs castings came begging for a new home and I took them both in.  This build is a ways down my List, but my intent has been to build a Double and enhance their looks in just the ways you've outlined, without having to admit I should have built something more highly detailed.
Cheers,
Harry

 

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