Author Topic: Fixturing question  (Read 7345 times)

Offline Allen Smithee

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Fixturing question
« on: January 06, 2016, 11:22:57 AM »
OK chaps, be gentle with me!

Having spent a year re-equipping the shop and making tooling I'm finally looking at making something! My chosen project is a coffee-cup stirling engine based heavily on the Jan Ridders design with a few minor variations to suit what I have lying around.



In my case the con rods will be made from 1.5mm carbon fibre rod rather than 1mm brass sheet, the displacer piston will be from depron sheet and the con rods will be ballraced at both ends (because I have loads of spare small ball-races from RC helicopter linkages). I was thinking of making the displacer cylinder from a 4" pyrex caffeteire jug (available for well under £5 from local shops), but after considering the various methods of cutting a 20mm slice out of it I chickened out and opted for the conventional 100mm pespex tube.

I'll also be making the upper and lower plates of the displacer cylinder from 4mm aluminium plate rather than 5mm slices of bar stock for the simple reason that I have quite a lot of 4mm plate in the stockpile whereas I'd have to buy-in the 5mm slices at some ridiculous cost (unless anyone can tell me where in the UK I can buy slices of 120mm bar for less than an arm and a lower leg). I'm aware that low-temp stirlings are very sensitive to the fit/friction balance in the power piston and displacer rod seal (a small bronze bush in this case), but I figure that (a) both parts are small and cheap so I can have multiple attempts at making them until I get it right, and (b) what I learn about lapping and honing in the process will be good experience for later projects.

My plan is to roughly bandsaw the disks from the plate, and then turn the OD and rebate groove. For the upper plate this will be fairly easy as it has a number of holes in it (including a bore in the centre) so I can easily mount it on an arbor to true the OD and then grip the OD in a chuck to cut the groove that seats the perspex cylinder. But the lower plate is essentially featureless - it just has the groove and six holes near the outside edge for the pillars between the plates. After racking my limited brain the only way I can think of to make this plate is to put the rough-sawn blank on a rotary table in the mill to accurately drill these six holes, and then make some short stand-offs so I can use screws (M2!) in these holes to clamp the plate to a faceplate with a gap between them so I can finish-turn the OD and bore the cylinder seating groove. Clamping with 2mm screws feels a bit weedy for turning what would initially be an interrupted cut, but can anyone suggest a better way of doing it?

All thoughts welcome!

AS
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Offline Bluechip

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Re: Fixturing question
« Reply #1 on: January 06, 2016, 11:39:22 AM »
Aluminium Warehouse will do 5mm 6082 sheet cut to size 100mm min dimension , but shipping is £12 / order IIRC ...  :insane:

Hope this link works:

http://www.aluminiumwarehouse.co.uk/cutting_calc.php

Dave

No, it didn't. Does not keep the options I did ... just goes to input screen. 130 x 130 x 5 was £1.33 ex vat.
« Last Edit: January 06, 2016, 12:01:45 PM by Bluechip »

Offline Ian S C

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Re: Fixturing question
« Reply #2 on: January 06, 2016, 12:19:20 PM »
To turn the aluminium plates, get some MDF, or wood, and PVA wood working glue, and some paper (news print is ok). Put some glue on the MDF, then the paper, some more glue, then the aluminium.  You can hold it in the 4 jaw chuck, or attach it to a face plate.  When you are finished put a wood chisel, or a knife at the joint, and give it a light tap, the paper should split, wash in warm water to remove the glue and remaining paper.  If you don't like that, use MDF, or wood to go in the chuck as above.  Make a pressure plate with a centre hole from metal a bit smaller in diameter than the aluminium disc, sandwitch the aluminium between the pressure plate and the MDF with a rotary centre in the tailstock, and screw it up fairly tight, take fairly light cuts, job done.  You probably knew all that, but I felt like rambling on at any rate.
Ian S C

Online kvom

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Re: Fixturing question
« Reply #3 on: January 06, 2016, 01:01:16 PM »
If you can attach the plate to the rotab with backing, why not mill the OD and the center bore rather than using the lathe at all?

Offline Allen Smithee

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Re: Fixturing question
« Reply #4 on: January 06, 2016, 01:12:08 PM »
Aluminium Warehouse will do 5mm 6082 sheet cut to size 100mm min dimension , but shipping is £12 / order IIRC ...  :insane:

Hope this link works:

http://www.aluminiumwarehouse.co.uk/cutting_calc.php

Dave

No, it didn't. Does not keep the options I did ... just goes to input screen. 130 x 130 x 5 was £1.33 ex vat.

Thanks, but that's just more plate - as I said, I have plenty of plate! I got briefly excited by their "CNC-sawn bar stock" page (even if it does have diameters in inferial and lengths in metric) but then saw [sic] the bit about 100mm minimum length. I just don't fancy parting-off a 5mm length of 5.5" bar!

AS
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Offline Allen Smithee

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Re: Fixturing question
« Reply #5 on: January 06, 2016, 01:17:56 PM »
If you can attach the plate to the rotab with backing, why not mill the OD and the center bore rather than using the lathe at all?

Mainly because my envisaged method of attaching it to the rotab would be clamping at the edges(!), and while cleaning up the OD in the lathe would take seconds in the rotab it would take ages.

But it's an interesting thought which I shall cogitate upon.

I do have several faceplates, and I'm wondering about whether I can bring myself to simply put one in a rotab and drill/tap six holes in it so that I can crew the blanlk to it with 3mm alloy stand-offs (or even a sheet of MDF).

AS
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Offline Allen Smithee

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Re: Fixturing question
« Reply #6 on: January 06, 2016, 01:43:47 PM »
One other thought - Jan's drawing shows the bottom plate left at full thickness (5mm). I was wondering if the thermal capacity of all that aluminium soaked up a lot of heat before passing it into the cylinder. I'm tempted to thin the middle region of the lower plate down to 1.5 or 2mm to help it pass the heat from the coffee cup. I'm thinking that it's not as if it handles any vast pressure, so does it really need to be that thick?

Any thoughts?

AS
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Offline Bluechip

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Re: Fixturing question
« Reply #7 on: January 06, 2016, 02:02:14 PM »
You said quite a bit of 4mm plate ...  :headscratch: err .. I think ..

Dave

Offline Allen Smithee

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Re: Fixturing question
« Reply #8 on: January 06, 2016, 03:55:04 PM »
Ah, ok - I see what you were saying now (apols!). I think I'll stick with the 4mm plate because I can't see any real reason for it needing to be thicker. Certainly not sufficient reason to justify the shipping cost, anyway.

Having said that, their material prices look reasonable and I will also need a 100mm dia piece of >7mm plate to make the flywheel from (if I decide to use his flywheel design) so the combination may reduce the sting of the shipping costs. I was originally thinking of making a smaller flywheel from steel (which I have in the scrap bin) but I need to do some moment of inertia calcs to see whether it's practicable to make a steel flywheel from the stuff I have available.

More thought needed.

AS
Quidquid latine dictum sit altum sonatur

fcheslop

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Re: Fixturing question
« Reply #9 on: January 06, 2016, 04:02:55 PM »
I tend to friction turn them or use a wax chuck/super glue
<a href="https://www.youtube.com/watch?v=K6Ai5F8O3iI" target="_blank">http://www.youtube.com/watch?v=K6Ai5F8O3iI</a>
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Offline sshire

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Re: Fixturing question
« Reply #10 on: January 06, 2016, 04:13:16 PM »
Given the surface area, this should hold up to major fllogging

http://www.modelenginemaker.com/index.php/topic,4711.msg88782.html#msg88782

Best,
Stan

Offline Allen Smithee

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Re: Fixturing question
« Reply #11 on: January 06, 2016, 04:33:59 PM »
True, but it's almost as expensive as buying aluminium bar stock and parting it!

AS
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Offline sshire

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Re: Fixturing question
« Reply #12 on: January 06, 2016, 05:34:23 PM »
Everyone says that. Cheaper than a decent end mill (and much less than a decent bottle of fine Scottish produce) and infinitely reusable.
It's interesting how we spend a fair amount of $ on machines, tooling, coolant, stock, inserts, fasteners,  Loctite @ $1.50 per ml, but something like Crystalbond seems excessive.   Go figure.
Best,
Stan

Offline Allen Smithee

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Re: Fixturing question
« Reply #13 on: January 06, 2016, 05:47:11 PM »
Cheaper than a decent end mill

Depends on who your supplier is...

 :-X

AS
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Offline Jo

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Re: Fixturing question
« Reply #14 on: January 06, 2016, 05:57:12 PM »
Depends on who your supplier is...

:lolb: Another convert.

JasonB just has to get brave  :naughty: ...but then he might pinch the not yet mine sets of model engine castings  :hellno: Pete don't tell him where we get them from  ;)

Jo
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