Author Topic: James Coombes - Not As You Know Him  (Read 5419 times)

Offline Jasonb

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Re: James Coombes - Not As You Know Him
« Reply #15 on: January 30, 2023, 06:50:09 PM »
Thanks for the interest

While it would be nice if people decided to build there own from scratch I've done this and the earlier Real build to show people what is possible and even if they just feel upto incorporating one or two of the features then the threads will have been some use. It also shows that if you pick up a part set of castings or are unlucky enough to muck up or get an unusable casting then there is still hope as just about anything can be made if you put your mind to it and that you don't have to just buy another casting and build another engine the same as everyone before you.

It is also a good way to ease your way into designing completely from scratch as methods learnt modifying existing designs can be put to use when and if a modeller wants to move on to a subject that was never made available or has long gone out of production. Also lets not forget that without those that went down this route in the past we would not have the designs being built today, I'm sure some peoples casting stashes would be a lot smaller if the likes of Clarkson, Mount, Westbury etc had not designed engines from scratch and then shared their work with other.

Offline Jasonb

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Re: James Coombes - Not As You Know Him
« Reply #16 on: February 01, 2023, 06:46:04 PM »
The columns seemed the next logical item to make. Starting with four pieces of 12mm steel bar The ends were centre drilled, quite deeply at the bottom so that a ctr would still engage once it was drilled and tapped M5 for the CSK screws that will attach them to the bed and I also turned a short 8mm spigot to locate the columns in the bed. The tops were turned down to 4mm and threaded.

After that the embryo columns were mounted between ctrs with the tailstock one being held in my boring head so it could be set over for taper turning. The next couple of photos are a bit blurred but it's a similar method to what I used on the Real

https://www.modelenginemaker.com/index.php/topic,10858.msg251528.html#msg251528



Some round "washers" were parted off and then held on a simple arbor to round over each corner to give them a half round edge. Some plain edges square ones were also made to go at the tops of the columns.



After gluing the "washers" on with Araldite and allowing to cure they were held in the 3-jaw using the jaws as a stop and all faced down with the cross slide locked which ensures they will all be the same height.



On the Victoria and Real I more or less copied the Stuart design for the bearing pedestals but a third pair was going to be a bit boring so I decided to make these with square brasses and curved caps. Starting with the main pedestals two pieces of 10mm aluminium ware machined to overall size and then a pocket for the bearing milled centrally.



Followed by machining the sides to leave the feet, these were done in 17.9mm high x 2mm stepover passes using a 2-flute cutter designed for non-ferrous cutting. The last cut having the cutter lowered to the full 18mm depth to avoid any stepover marks on the foot.



The last couple of jobs were to drill and tap for the caps using an M3 spiral flute tap which lifts the swarf out of the blind hole and then two 4mm holes in the feet.



Two more bits of 10mm material were sized up for the caps, drilled and counterbored for some bosses to be bonded in, the central hole is for the oiler also counterbored for a threaded boss.



The underside was milled out 2mm deep (1mm overlap and 1mm gap) so that the cap would slip over the pedestal



A couple of holes were quickly drilled and tapped into a well used holding block and the CNC made quick work of roughing and then profiling the cap seamlessly blending the three arcs into one. but it can also be done on a rotary table without too much effort.



The bearings where soldered together, milled to size then drilled and reamed 12mm in the mill



Last job on these was to mill a 1mm deep groove all round just over 10mm wide to allow for the paint on the pedestals



Here is a shot of the assembled pedestal and the bosses have been bonded into the cap, I just need to add some fillets around them with body filler (bondo) and knock the corners off so they look like castings.






Offline TimB

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Re: James Coombes - Not As You Know Him
« Reply #17 on: February 01, 2023, 06:58:33 PM »
Using the boring head for an off-set centre, now why didnt I think of that!!  :noidea:

Some excellent work there Jason, loving the bearing caps, well done!

Tim

Offline springcrocus

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Re: James Coombes - Not As You Know Him
« Reply #18 on: February 01, 2023, 10:01:01 PM »
You CAN teach an old dog new tricks! Will be following this with much interest.  :ThumbsUp: :popcorn:
Regards, Steve


Member of a local model engineers society
www.stevesbritannia.co.uk

Offline Jasonb

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Re: James Coombes - Not As You Know Him
« Reply #19 on: February 02, 2023, 08:13:38 AM »
It's not a new trick, maybe you dogs have been digging in the wrong place :Lol:

Biggest advantage is that you don't upset the main tailstock position and you get easy fine adjustment if you need to tweak the taper. You can just turn up a plain shank ctr  to suit, I'm using the revolving one off my old Unimat 3 with a reducing bush.

You can also go up and down not just sideways, I took this video to illustrate the fact you get a concave "cut" if the tailstock is high by over emphasizing the tailstock height to 20mm above headstock. So next time you need a concave rod rather than a fishbellied one you know how to do it without a CNC lathe

<a href="https://www.youtube.com/watch?v=cYZQHedLqvc" target="_blank">http://www.youtube.com/watch?v=cYZQHedLqvc</a>

Offline simplyloco

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Re: James Coombes - Not As You Know Him
« Reply #20 on: February 02, 2023, 09:19:23 AM »
You CAN teach an old dog new tricks! Will be following this with much interest.  :ThumbsUp: :popcorn:
Regards, Steve

Agreed. I hadn't thought of that, but there again, I'd never owned a boring head until I retired!
John
Strong minds discuss ideas, average minds discuss events, weak minds discuss people.” ― Socrates

Offline Jasonb

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Re: James Coombes - Not As You Know Him
« Reply #21 on: February 06, 2023, 07:15:58 PM »
I did most of the crank on the CNC - drilling, reaming and shaping. In this photo I have just completed the milling of a web between the two bosses which was done with a 4MM dia cutter with 1mm corner radius to leave a fillet for that cast look.



After machining it was loctited onto a 10mm spigot that had been turned on the end of the 12mm PGMS crankshaft and once set was faced off ensuring the face was at right angles to the axis of rotation.



Stuarts show a soldered construction for the tuning fork shaped conrod but I decided on a bolt together one similar to that used on the Waller table engine. I roughed out the stock on the manual mill and then thinned the ends and reamed the holes on that machine too.



Then over to the CNC where a scrap of aluminium was drilled and tapped so the job could be screwed to that for machining, I added a couple of spacers under the ends so that the work was meld down firmly.



This is it straight off the CNC, a little draw filing with emery wrapped around the file should clean it up nicely. Machining time was 24mins with most of that being the fine stepover used to do the sloping top surfaces and fillets where they meet the round bosses.



The short leg of the conrod that has the big end was turned from some 16mm dia stock, first doing the 4mm dia section before pulling out some more material for the half fish bellied rod.



This was then transferred to the spin indexer on the mill and the rectangular end formed



And while still held in the indexer the main hole was reamed for the bearing and two holes drilled for the dummy strap retaining bolts



Progress so far including the big end bearing and a couple of square headed bolts, again from round stock and the milling done with the spin indexer.




Offline Jasonb

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Re: James Coombes - Not As You Know Him
« Reply #22 on: February 06, 2023, 07:36:56 PM »
The two long rods that pass up either side of the cylinder terminating at the little ends again started life as round stock rather than rectangular that Stuarts provide, like the shorter rod the ends were first reduced to 4mm and threaded then a small length reduced in diameter from the 12mm stock which will be the flange that bears against the cross piece.



Then with more stock drawn out of the collet and using tailstock support the rod was roughed down to 8mm diameter, this being the thickest part of what will be the fishbelly



It was then back over to the mill and indexer to form the rectangular end, drill dummy holes and ream for the bearings





Then once more back to the lathe to have the end 1/3rds turned to a taper with an insert that had a 0.8mm radius to leave a small fillet, hand blending of the three facets followed



After sawing off from the parent bar the little end was mounted on an arbor to be rounded over using the rotary table. I seldom use filing buttons for this type of job these days.



A couple of bearings were turned and reamed from Colphos, here I am checking the size which was a nice push fit, bearings will be loctited into place.



Four square head bolts for the dummy straps finish that part of the engine.







« Last Edit: February 06, 2023, 07:41:17 PM by Jasonb »

Offline RReid

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Re: James Coombes - Not As You Know Him
« Reply #23 on: February 06, 2023, 08:27:01 PM »
That's all looking very nice indeed. Following along. :ThumbsUp: :popcorn:
Regards,
Ron

Online Kim

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Re: James Coombes - Not As You Know Him
« Reply #24 on: February 06, 2023, 11:05:36 PM »
That looks great, Jason!  :popcorn:

Just curious; are the pairs of little bolts on the ends just for looks?  Or do they actually do something?

Thanks,
Kim

Offline Dave Otto

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Re: James Coombes - Not As You Know Him
« Reply #25 on: February 07, 2023, 01:18:11 AM »
Nice looking parts Jason!

Dave

Offline Jasonb

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Re: James Coombes - Not As You Know Him
« Reply #26 on: February 07, 2023, 07:21:30 AM »
Kim, they are just for looks and are meant to represent bolts that would have retained a strap around the bearing, one would possibly have been a wedge to draw the strap tight. Bearings also look thick but that is so they look like a flanged bearing which would have been used with a strap. It has not come out in the photo but I have scribed lines with a height gauge to give a dummy joint between rod and strap.

It's apart at the moment for final detailing so I'll take another photo close up.

This conrod for my Fowler is the sort of thing the dummy is meant to represent, you can see I have used a separate strap, one screw drives a wedge and the second is just additional fixing strength.

« Last Edit: February 07, 2023, 11:27:25 AM by Jasonb »

Online Kim

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Re: James Coombes - Not As You Know Him
« Reply #27 on: February 07, 2023, 04:58:44 PM »
Thanks for the very clear explanation, Jason!   :cheers:

Kim

Offline vtsteam

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Re: James Coombes - Not As You Know Him
« Reply #28 on: February 07, 2023, 09:25:05 PM »
Beautiful work, following along. :ThumbsUp: :ThumbsUp: :popcorn: :popcorn: :cheers:
Steve

Offline Jasonb

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Re: James Coombes - Not As You Know Him
« Reply #29 on: February 09, 2023, 08:29:12 PM »
The eccentric strap started life as two pieces cut from some 2 x 1/4" flat brass bar that were machined to their overall depth relative to the split line and then soft soldered together. Next they were machined to width over the joining lugs and a skim taken off each side to bring the blank to the desired 6mm thickness and finally a ctr drill was used to mark the ctr of the hole to be.



The ctr hole was clocked true in the 4-jaw using some parallels behind the work to make sure it was sitting flat and then then drilled then bored out to 26mm dia. A carbide grooving tool was then used to cut the groove that keeps the strap located on the eccentric



A top hat bush was quickly knocked up to clamp the strap to a tooling block so the outer profile could be cut on the CNC, You can see that I have also cut the recess for the eccentric rod and am using one of the rivit holes to stop the strap from rotating. I decided to use a couple of rivits to hold the strap on this time as the last two have been done with a csk screw.



After removal from the tooling plate it was back to the manual mill to drill for the bolts and add an oil hole, I also milled away part of the internal fillets on the upper bolting faces to leave flats for what will be square headed bolts to bear against so they won't rotate.



Some 30mm stock was then turned down to 28mm and then further reduced to 26mm leaving a location ridge in the middle, final sizing was done by test fitting the two halves of the strap until the lugs could be pinched together and the strap rotated with out binding or unwanted play.



I then offset the 30mm stub to the required throw and bored to a close fit on the crankshaft before reducing the two sides down, one for just 1mm to give some clearance and the other 6mm to allow for a grub screw to retain the eccentric. Holding in a 5C collet block by the longer side the CNC was used to cut the decorative weight reduction hole that you often see on full size engines. Roughed out in two 3mm deep adaptive cuts and then two full depth finishing passes with the 3mm cutter



That's a couple more bits ticked off the list





And complete with the 5mm x 1.5mm eccentric rod rivited into place





 

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