Author Topic: Large Wobbler  (Read 6133 times)

Offline bwal

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Large Wobbler
« on: July 14, 2017, 01:00:10 PM »
Hi,

I'm in the process of making a large (hopefully) double acting wobbler from scrap I have laying around and no real plans.

The large pieces of aluminium I picked up as a part of deal for a Douglas shaper I purchased several years ago. The bar stock I've collected along the way.

The flywheel I found at work in the scrap bin. I think its cast metal disc that's 25 mm thick (could be 20 mm) and 100 mm Diam.  It had a large hole in the centre which I filled with a metal slug and peinned over hence the hammer marks on the face.  I layed out the 6 hole PCD at work. Unfortunately I was distracted when drilling out one of the holes (talking to one of the blokes) and missed the mark! I'm kinda disappointed because when I found the disc I knew straight away I could use it for something (I've held onto it for about two years).

Anyway I'm pushing on.  I finished the the cylinder today. I had to bore it out to 5/8".  I thought I bought 9/16" bar stock of a ebay seller but he sent me 5/8".

Cheers Ben.

Offline Thor

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Re: Large Wobbler
« Reply #1 on: July 14, 2017, 03:32:08 PM »
Hi Ben,

 Good luck with your wobbler. I have made double acting wobblers with up to 16mm bore and got them running, I used a smaller flywheel - about 50 to 60mm diameter.
 You can find a description of the first one I made with a 13mm bore here.

Thor

Offline b.lindsey

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Re: Large Wobbler
« Reply #2 on: July 14, 2017, 06:35:21 PM »
Good start Ben. Looking forward to following along.

Bill

Offline bwal

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Re: Large Wobbler
« Reply #3 on: July 21, 2017, 09:41:57 AM »
Hi,

Thanks for the replies and link.  It'll come in handy.

I've made some progress but not much. I tapped the flywheel for a grub screw (everyone laughs at the flywheel PCD holes - especially the bloke who I was talking to when I drilled the wrong hole!). I made a brass bearing and bearing hole in the pedestal - I think I put a grub screw in as well.  I turned up a crank wheel but I think I'll make one up with a larger diameter- maybe about 40 mm instead of 25 mm. I made up a really cool cylinder cap but decided not to use it (due to technical problems) and made up a rectangle one instead.  And finally I fixed the pedestal to the base.

Ben.

Offline Thor

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Re: Large Wobbler
« Reply #4 on: July 21, 2017, 12:27:12 PM »
Hi Ben,

  Pedestal and base are coming along nicely. I don't think the slightly displaced hole in the flywheel will prevent it from running. Is there any vibration when you spin the flywheel.

Thor

Offline bwal

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Re: Large Wobbler
« Reply #5 on: July 21, 2017, 10:52:58 PM »
Hi,

I haven't noticed any vibration yet but I've only hand spun it.  It probably won't be too bad as I'll only run it on a small air compressor.  I also ordered some 6 mm diam stainless steel rod to use a axle.

Gotta go.

Ben

Offline bwal

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Re: Large Wobbler
« Reply #6 on: August 01, 2017, 11:44:02 AM »
Well I'm still going.  I've pretty much got most of it done.

I just started the ports today.  I'm using the same method as Mr Pete when he made his wobblers, drill through the cylinder and transfer punch the ports.  Only problem is I'm not sure if I'm doing it right.

Attached is a picture of the sliding face.  I've drilled the top ports and punched the bottom ones, as you can see the bottom ports are more wider apart than the top ones... Is that right? I had the same situation in the smaller one I made but it runs well.

Anyway I'll keep plodding along.

Ben

Offline Thor

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Re: Large Wobbler
« Reply #7 on: August 01, 2017, 02:23:47 PM »
Hi Ben,

 I am no expert but on the wobblers I have made the top and bottom ports were symmetric. I have usually spaced the top (or bottom) ports so the distance between the two holes is the same as the diameter of the cylinder port hole.
 Here's a link that might explain things better than I can: http://modelsteam.myfreeforum.org/archive/designing-and-building-a-simple-model-steam-engine__o_t__t_49141.html
 I hope this helps.

Thor

Offline Gas_mantle

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Re: Large Wobbler
« Reply #8 on: August 01, 2017, 03:20:15 PM »
Ben,

Here's a photo of a simple double acting one I made last year, as you can see the ports are equally spaced from the pivot point. I agree with Thor that its usual to have the space in between the ports as the same diameter as the port holes but in this case it seemed to work well with a larger gap so I left it that way.



I've seem Mr Pete use the transfer method like you say but to me it seems like it's easier just to make a drilled template out of some scrap strip or do it mathematically.

Peter
« Last Edit: August 01, 2017, 06:23:52 PM by Gas_mantle »

Offline bwal

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Re: Large Wobbler
« Reply #9 on: August 02, 2017, 11:29:41 AM »
Ah Hah, I see what your saying about the pivot hole location and port locations.  I just randomly placed the pivot hole and port holes in the cylinder (with some guestimation involved) and then transferred port holes onto the pedestal.

My next engine I will work from plans!

I had a quick read of Tony Birds write up, I knew there was more to model engineering than just having a punt.

Thanks Ben.

Offline bwal

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Re: Large Wobbler
« Reply #10 on: August 19, 2017, 09:07:03 AM »
Hi,

I've managed to get the wobbler running.  I changed the design some, after reading all the helpful advice I've received.

I made up another cylinder block, this time the pivot was centrally placed.  I re-did the ports (I had to fill the top ports and the original pivot hole). the ports are internal, something I haven't done before. Somehow I stuffed the cylinder bore so I had to remake the piston which I'm not really happy with but it works.

I bought most of the model home last weekend to see if it would run - it didn't.  The cylinder wasn't perpendicular to the pedestal.  It really wanted to run but was loosing too much air.  So I took it back to work and replaced the pivot rod with a larger (6mm Diam) rod.  Which solved the problem.  I also cleaned up the mating surfaces and shaped the pedestal.

I haven't finished just yet, I have an old pulley wheel from somewhere which I'm going to rig up. So I'll have to make a bearing block and pulley.  I'm not sure how long that will take as I've resigned from my job of 15 plus years so no more nice lathe and mill etc.  Just my old machines at home (no DRO's) but at least I'll be able to spend some time on them.

Ben.


Offline Gas_mantle

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Re: Large Wobbler
« Reply #11 on: August 19, 2017, 01:25:08 PM »
Hi Ben,

Glad you got it running :-)

If you think the cylinder is not sitting right against the pedestal and you are still getting leaks it may be worth cutting a small amount of relief where the faces slide against each other like I did in the photo above.

A very simple piston ring might help too if you haven't yet fitted one. It doesn't need to be anything fancy, I find even a bit of oily PTFE tape rolled into a string will improve things dramatically :-)

Peter.
« Last Edit: August 19, 2017, 01:30:34 PM by Gas_mantle »

Offline Thor

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Re: Large Wobbler
« Reply #12 on: August 19, 2017, 01:29:55 PM »
Congratulations Ben,

 Your wobbler looks nice. On wobblers it is important to get the cylinder and pedestal portfaces to mate closely, and the pivot must be perpendicular to the cylinder portface. And the cylinder bore must be at 90 deg. to the cylinder portface, on  the first wobblers I made I used a square (or rectangular) block of cast iron and mounted it on a small anglepalte on the faceplate. I described the procedure in the link I provided earlier.
 I am looking forward to seeing some photos when you have made the pulley.

Thor

Offline b.lindsey

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Re: Large Wobbler
« Reply #13 on: August 19, 2017, 02:26:36 PM »
Looks like you are making some great progress Ben. You will have it running well in no time. Just one comment... Please resize your pictures in the future to something like 800x600 pixels. These megabyte sized ones for many of our members take forever to download and really provide no more detail than the resized pictures.

Bill

Offline bwal

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Re: Large Wobbler
« Reply #14 on: August 20, 2017, 12:35:16 AM »
Hi,

Bill, on re-sizing the photo's is a that phone/camera thing? Or on the computer before posting?

In regards to the problems with the cylinder, there's a few.  The end caps both have minor leaks - nothing a gasket wouldn't fix.  I like the idea of cutting some relief on the mating face - I think though I'd struggle to cut it on my little Hercus mill. I won't worry about a piston ring on this one but its definitely something I'll have to do eventually.

Thanks for the advice.

Ben

 

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