Author Topic: Agnes.... 1/16 Scale Pollit & Wigzell Tandem Compound Condensing Engine.  (Read 59175 times)

Offline MuellerNick

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This would be my machining sequence:
Face side 1
Flip over and face 1' (the ' means second clamping)
Face 1', 2' and 3' (all side cutting). Either in a rotary table or (easier) by CNC.
Might be worth roughing (with allowance for lathe op) the outer circumference, so you have a nice reference for the next steps.


To bore the faces 2' and 3', you need two fixtures.


Sorry if I come up with a different solution. :) But I always have to take the challenge to find a machining sequence and clamping strategy  for interesting parts like these. For me, that's just interesting and is never meant as an offense to the other.




Nick

Online Jasonb

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Given the mill that you have with the ability to tilt the table I would asume the ali block goes in the vice against a vice stop you machine one face, flip the block around and machine the other.

If the mill angle is set right they will all fit together if its a fraction of the cumalative error will either mean they have to be machined again or the last segment can be made to fill more or less than 1/8th of the circle

Offline steamer

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Given the mill that you have with the ability to tilt the table I would asume the ali block goes in the vice against a vice stop you machine one face, flip the block around and machine the other.

If the mill angle is set right they will all fit together if its a fraction of the cumalative error will either mean they have to be machined again or the last segment can be made to fill more or less than 1/8th of the circle


YUP....hope you have a GOOD sine bar!... 8)
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline MuellerNick

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Quote
if its a fraction of the cumalative error will either mean they have to be machined again or the last segment can be made to fill more or less than 1/8th of the circle


If the angle is off, it will only change the diameter.


Nick

Offline pgp001

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Well both Jason And Dave have the right idea but its not quite how I did it.

The next step was to attack the castings with an angle grinder to remove any lumps and bumps, the final finish will be corrected with filler and paint towards the end.



I thought it might be an idea to get under the skin in the critical areas with a roughing cut, the last thing I needed on the finishing cuts was to hit a hard spot and blunt the cutter before all the segments were finished.
Here is the first cut holding them in a normal vice.



Agian the vice was used to hold them for the second face to be cut, I uded my digital angle gauge to get them positioned somewhere near flat.



This was where I nearly spoilt my one spare casting, as soon as the cutter bit into the metal it just dragged the casting over in the vice, apart from a couple of unwanted cutter marks on a face that was going to be re-machined anyway that was all that happened.
It just shows how easy it is for something to move in a vice though.
So after a bit of thought I jury rigged a back stop and approached the part from the opposite side to ensure the cut was pushing the part against the stop.



Dave mentioned a "sine bar", well I dont have a sine bar big enough, but I do have a sine table.
This is a lovely bit of kit that I normally use with a pair of centres on it for measuring tapered shafts etc.
In this case I need an angle of exactly 22.5 so I can tip the milling machine table to that angle as Jason suggested earlier.



Once I had the sine table set to the correct angle, I clamped it to the table making sure it was square to the tee slots. The mill table is then tipped up until until the top of the sine table is dead level again.
This is done by using a verdict indicator on the "Y" axis slide, I took a great deal of time over this, as any error here would be multiplied by eight when all the segments are bolted together.



Jason had the right idea about flipping the fixture round but I did not use the vice, I chose to use the rotary table to ensure repeatability.



I have drawn out the profile on Solidworks 3D, so I know what some of the key dimensions need to be. But in reality it does not matter too much if the finished size is slightly out. The most important thing is that each segment is identical.
So having machined the casting that had the least amount of spare metal first, I settled on the final cut depths and zeroed the DRO for both "Y" & "Z" axis.

This is one cut.



And this is the other.



After spinning the job 180 the same two cuts were done at the opposite end with same DRO readings, thus ensuring that relationship between the angled faces were all the same.
Jo mentioned the grub screws in the base of the fixture, these were individually adjusted to make sure there was no "rock" and that one of the machined side faces was square to the top of the rotary table.

I did use the same fixture for one other job, and that was bringing the flange thicknesses down to the correct size.





My next challenge was how to get all the holes in the correct place, there are four holes for the bolts that pull the segments together, and two in each end for the bolts that holds the spoke flanged end so that it spans the joint.
I wondered about using the same fixture and the DRO on the mill, but eventually chose a different approach.

More later.
Phil

Offline pgp001

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Quote
if its a fraction of the cumalative error will either mean they have to be machined again or the last segment can be made to fill more or less than 1/8th of the circle


If the angle is off, it will only change the diameter.


Nick

Nick

You are correct in one respect that the diameter will indeed change, but when you have seven segments all bolted together at the wrong angle, you will find number eight does not fit. the gap will be either too large or too small.
So you would have to make a "special" segment of a completely different size with completely different angles to try and fill the gap.
For me personally that would be a really bad thing to have to do, but in reality that is exactly how they did it on full size engines all those years ago.

Phil

Offline pgp001

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You know what it's like when you just can't wait to see if it has all worked out. Time to get the clamps out.



Oh dear I have run out of clamps............I wonder if I could just balance the rest in place.



So far so good no unwanted gaps anywhere.  :whoohoo:

Phil

Offline peatoluser

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BRAVO!!!  :praise2: :praise2: :praise2: :praise2:

superb set ups and a superb outcome!

following with much interest

peter

Offline steamer

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Nice Work!

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline kellswaterri

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Hi Phil,
           lovely work on the flywheel, the grinder bit brings back come memories of my corliss build,
                                                       All the best and good luck with your build,
                                                                                                              John.

Online Jo

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Very nicely done Phil  :ThumbsUp:.

I am beginning to like your mill even more  :embarassed: I also like the design of those tool maker's clamps  8)

Jo
Enjoyment is more important than achievement.

Offline pgp001

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Jo

I made that set of little clamps nearly 40 years ago, I use them all the time.

The clever thing is they can be used "one handed" because of the knurled wheels used to tighten them up.
They are much easier and quicker to use than conventional ones.

Maybe I should patent them  ;D

Phil

Offline garym

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Hi Phil,

Fascinating engine this one. Best of luck with the project. Here is a video of the full size engine I took at the beginning of June on a visit to Markham Grange. First time I'd seen an engine with two piston rods connecting the low pressure cylinder.

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

Gary

Offline Alan Haisley

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This is going to be a lovely engine.
I'm looking forward to the rest of the flywheel build since I have always wanted to see one of the segmented ones go together.
Alan

Offline pgp001

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You might be interested in this, a rare opportunity to see a mill engine in build.

http://www.markhamgrangesteammuseum.co.uk/agnesrebuild.htm

Phil