Author Topic: A Beginner's First Engine (Single-acting Oscillator)  (Read 23608 times)

Offline gary.a.ayres

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #150 on: March 16, 2019, 06:46:24 PM »
Hi Peter -

Yep... hoping to have it finished tomorrow (apart from the wooden base, which will need at least three coats of Danish oil).

Am planning to put a bit of copper pipe in as the exhaust which in due course will be extended and routed up the chimney of the boiler.

Offline gary.a.ayres

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #151 on: March 17, 2019, 10:17:50 PM »
Yep... hoping to have it finished tomorrow (apart from the wooden base, which will need at least three coats of Danish oil).

I knew I shouldn't have tempted providence like that.

The day started well as I began work on a composite pivot as suggested by Tug on page 2 above. The main section was turned from 8mm precision ground mild steel (tip from Jason), and the studs from 4mm stainless threaded bar. The nut also went well. I have only tried knurling twice before, without much success, but this time it went ok (youtube is my friend). Here is the nut half way through being made:



I also opened out the last few millimetres of the exhaust port at the back of the standard and added a curved copper pipe which will eventually be extended and routed up the boiler chimney (no photo).

Then a minor disaster befell me. There is only about 5mm between the port face of the cylinder and the bore. This only allows for a very short tapped hole for the pivot. The plans call for a 5BA thread but I don't have any BA taps so I just went with M4 - and stripped the thread by giving the wrench one twist too many   :facepalm:
This left me with a shallow, threadless hole with a rounded bottom. I decided to resort to JB weld:



I ground a point on the threaded stud to maximise the depth of penetration into the hole. The wider section of 8mm steel (which fits the bearing) is drilled, tapped and faced at both ends and sits flush with the port face so should stabilise the stud and hold it perpendicular to the face. Only the 4mm stud reaches down into the hole. There is JB weld between these two components for added stability (though this version loses Tug's composite pivot in which a loosely tapped thread allows wiggle room for the cylinder to bed against the port block).

The JBW is curing now, and I did clean off the excess that can be seen in the pic.

The problem is that only about 3mm of the stud, ground to a point, goes down into the hole and bottoms out. I know JB weld is good, but is it that good? In effect, this is little more than a butt joint in a location that receives a lot of torque.

I was wondering what to do if (when?) it shears. I'd rather not remake the cylinder (which might also mean another piston), so I came up with the idea of counterboring the hole down to about 3.5 mm with an 8mm endmill to create a flat-bottomed seat for a section of the 8mm bar, which would be pre-drilled and tapped for the 4mm stud. This assembly could then be fixed into place. I suspect the JB weld would do better on a joint of that kind.... or would I need to silver solder it... or could it be soft soldered...? Or perhaps Loctite...?

And of course, maybe I'm wrong and the JB weld fix that I am trying now might do the job.

I'd value your thoughts on any of this, and of course you may have ideas for a completely different way of doing it.  All suggestions welcome!

Offline Gas_mantle

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #152 on: March 17, 2019, 10:43:23 PM »
Hi Gary, that's a pain that the threading didn't go to plan.  :(

I guess everyone will have a different solution but I'd be tempted to drill out the damaged thread to form a flat bottomed hole slightly larger (and as deep as you dare go), if you then turn a close fitting plug and secure it with Loctite 638 you ought to be able to redrill it.


Offline gary.a.ayres

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #153 on: March 17, 2019, 10:58:05 PM »
Thanks Peter.

I take it the Loctite 638 would be ok at steam temperature?

Do you think that if Loctite and a plug would do, then maybe Loctite and the 8mm shaft straight into an 8mm diameter flat-bottomed hole would do too? It would save me the horror of drilling twice and tapping once...

There really isn't much metal there to work with and even my M4 plug tap doesn't really get going until it's in deeper than that (which contributed to the issue in the first place).

Offline Gas_mantle

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #154 on: March 17, 2019, 11:09:33 PM »
To be honest I'm not sure what the specs say about Loctite 638 but the pivot on my biggish wobbler is secured with it and I've steamed it many times and it is still solid. I guess on a real working application it may not be ideal but for a small ornamental engine that will only see light use I think it will be strong enough.

Do you mean have the 8mm cylinder secured solid to the cylinder and have it oscillating? I thought the 8mm was a fixed bush to be secured in the upright?

Offline gary.a.ayres

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #155 on: March 17, 2019, 11:16:58 PM »
I just looked - it's good for 180 celcius.

I have some 603 which does for 150 degrees. Do you think that will do?

Yeah, I mean the 8mm fixed to the cylinder and oscillating. The bush in the upright is 8mm ID and 12 mm OD. The 8mm bar makes a nice running fit in it.

Offline gary.a.ayres

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #156 on: March 17, 2019, 11:21:38 PM »
638 seems to be better at filling gaps than 603 is though. 603 is more of an anaerobic thing.

Offline Gas_mantle

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #157 on: March 17, 2019, 11:27:07 PM »
I don't know how hot the metal gets but as I say 638 seems to hold up without any problem. I've never used 603 but 638 cures in a lack of air (oxygen?) and is incredibly strong if there is a small space between the components for enough of the liquid to spread.

Offline gary.a.ayres

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #158 on: March 17, 2019, 11:36:48 PM »
The main difference seems to be that 603 is close-fitting but 638 will fill gaps up to about 0.25mm.

You'd need to be Alan Turing to crunch the numbers of all the different Loctites and know what each one does.

Meanwhile, my JB weld repair is beginning to cure and I'm still hoping against hope. JB weld is good, but it's not a promising joint. You never know though...

Offline MJM460

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #159 on: March 17, 2019, 11:56:42 PM »
Hi Gary, there is actually not much torque on that joint, it can be a nicely sliding pinhole a bearing, as it is in the standard.  The main force is the axial spring force which is required to hold the cylinder against the standard.  The only torque is from the friction as the cylinder oscillates by rotating around that pin.  I have even seen designs where the cylinder is held against the port face by a spring pushing on the opposite side, so no axial force on the pin.

I would see no issue with using your milling cutter to enlarge the hole, providing you are very careful to set it up square in both directions.  (This is the critical step in which ever method you use to recover, take plenty of time and double check.)  But instead of using an 8 mm pivot, I would turn down the pivot section of the pin, perhaps to your original intended diameter, as the side forces on a smaller diameter pin will result in less torque resistance due to friction.  And you can then use your spring and nut without modification.  (Well done on that knurling, by the way).  The larger diameter insert in the cylinder gives the loctite more surface area to develop its shear strength.

Going well and not far to go,

MJM460

P S - For another time, I have found that an extra bottom tap, ground down on the end to minimise the lost thread, used after your normal plug or bottom tap helps with tapping those holes where the depth is limited.  But it is always a tricky job for those of us with less experience.  Maybe for the others too!

The more I learn, the more I find that I still have to learn!

Offline gary.a.ayres

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #160 on: March 18, 2019, 12:29:55 AM »
Hi MJM -

I initially made the pivot 8mm wide to make a composite pivot as suggested by Ramon on page 2.

However, due to today's events that has (at least temporarily) been shelved.

If I go back to a narrower pivot, I'll probably screw it into a plug before Loctiting the plug into a larger hole in the cylinder. That way I could avoid having to tap that thin wall and risk the same thing happening again. I'll also need to rebush the standard for a smaller pivot but no big deal there.

First, though, I'll wait and see if the present repair attempt works, and if it does I'll probably just keep the 8mm pivot. That of course remains to be seen.

On keeping it square - I nearly drove myself crazy obsessing over that today, and it looks like I may have to do so again!

Great suggestion about grinding the tap!

Thanks for your help,

gary

Offline Jasonb

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #161 on: March 18, 2019, 07:02:23 AM »
The JB Weld will hold, its only a weedy little spring that will be pulling against it, if in doubt look back at Ramon's tests on the stuff.

I've gone over to using Spiral Flute taps for almost everything metric now, just need the one and goes just about as deep as a plug tap

Offline gary.a.ayres

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #162 on: March 18, 2019, 08:20:46 AM »
Jason -

having just checked it this morning I had pretty much come to the same conclusion. It seems to be pretty solid (and perpendicular  :))

Will definitely check out spiral flute taps.

Cheers   :ThumbsUp:

Offline gary.a.ayres

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #163 on: March 18, 2019, 05:14:29 PM »
Yay!

A great day for me   8)

My first engine, and it certainly won't be my last.

Here she is, 'Wasp', running on live steam:



Video to follow over the next day or so.

Just some fettling left to do and I still have to make the walnut base.

Thanks for all your support, guys. I wouldn't have been able to do it without you.

gary

Offline Gas_mantle

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Re: A Beginner's First Engine (Single-acting Oscillator)
« Reply #164 on: March 18, 2019, 05:30:41 PM »
Yay, well done - it looks great  :)

Good to see we have lift off, I look forward to seeing a video.
« Last Edit: March 18, 2019, 05:35:19 PM by Gas_mantle »

 

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