Model Engine Maker
Supporting => Engine Ancillaries => Topic started by: Steve Crow on January 02, 2022, 04:53:45 PM
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I've just made the cylinder liners for my model V8.
They are bored out to 7.2mm and the OD is just over 8mm for a nice friction fit in the block. I made go-no go gauges to keep the bores consistent to 0.01mm.
(https://www.model-engineer.co.uk/sites/7/images/member_albums/163849/902614.jpg)
I am not happy with the finish of the bores. I am looking for someway of smoothing/polishing them while maintaining the "roundness" and "parallelness" of the bores.
I am considering turning down a bar to a whisker under bore diameter, charging with an abrasive paste and polishing it that way.
Is that a good idea? And if so, what material for the polisher? A softer metal like ali. or harder like silver steel?
Also, would an axial or radial motion be the way to go?
I was thinking of Eucryl mixed with light oil (a toothpaste powder, I've used it before) followed by Autosol.
Any advice, suggestions or guidance would be much appreciated.
Cheers,
Steve
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Steve-
Your best bet is to make a lap and use lapping compound. The lap needs to be softer than the part that you wish to lap. Ramon did a rather extensive write-up on lap making.
-Bob
Hopefully this works. It is a link to my “Tiny” I.C. build where I made a lap.
https://www.modelenginemaker.com/index.php/topic,6968.msg155006.html?PHPSESSID=j74mh9ftmo4bm4nfmvhhocfma6#msg155006
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Should all be in Ramon's long thread
https://www.modelenginemaker.com/index.php/topic,1908.0.html
I usually go down to 1000grit silicon carbide on a copper or aluminium lap,
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Hi guys
I have replied to this question earlier on the Model Engineer forum.
The liners appear from the image to be brass and as such I would not recommend lapping because of the effort that has gone in to producing the bores to a consistent dimension of fine limit. Brass is easily lapped and of course makes ideal piston laps but wears very quickly in use
To 'polish' these bores as Steve asks would to my mind be simply achieved with judicious application of fine wet and dry paper wrapped carefully around an appropriate (accurate) mandrel.
Lapping is designed to remove metal - polishing will do the same but to a much less degree and particularly so on brass.
I'm afraid I don't see this specific task as a conventional lapping process.
Tug
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I wouldn't use lapping compound on brass liners, as abrasive being embedded in a somewhat definitive way into brass IMHO...This ability to incorporate grits and micro swarfs is maybe the reason why brass appears to wear so quickly !
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Hi guys
SNIP
To 'polish' these bores as Steve asks would to my mind be simply achieved with judicious application of fine wet and dry paper wrapped carefully around an appropriate (accurate) mandrel.
Tug
Agreed, but the cylinders could be manganese or aluminium bronze for all we know?
Lovely job though...
John
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As its a soft bore the old technique of pressing a hardened ball bearing through each comes to mind. Not sure how effective it is :noidea:
You can't use a ali or copper lap as the brass is about as soft.
Jo
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If I was going to lap the bores I would use timesaver lapping compound as it will not get embedded into the material .
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Using balls did cross my mind but was worried that with such thin walls the liners may expand and muck up Steve's fit into the block.
But there are many ways to do it, As I said I have lapped a couple of 12mm ID brass engine cylinders and for piston valves going down to 5mm dia just using very little of the 1000g and none of the coarser ones all with aluminium as the lap - solid not expanding as there is so little to take off you don't need to expand it as the lap wears.
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I lapped the 16mm bore brass cylinder for my vertical petrol engine using an Acrolap and 40 micron diamond paste. This was then cleaned in an ultrasonic bath. It runs with an aluminium piston and single cast iron piston ring.
http://acrolaps.com/index.htm
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When boring out the cylinders, the geometry (as well as lathe speed and feed rates) of the boring cutter also make a huge difference. How sharp a point vs how large a radiused end on the cutter tip makes a huge difference (talking profile when viewed from above, not sharpness of the edge). I've had great luck with one boring bar with a fine tip, leaves a shiny smooth surface on brass bores. When boring out larger diameters, there are plenty of passes to experiment with settings to find the sweet spot and get a better surface. On small diameter ones like you had, not as many to experiment with. On those, I've used very fine diamond paste to polish them up.