Hi guys - well we'll have another try - hopefully I won't hit the wrong 'key' tonight
Thanks for the tip Bez but where the problem lies is moving back and forth getting pics from the web album - I just hit the back button by mistake.
I'd like to say how surprised I am in the interest in this subject - it's very pleasing to see but I can't reiterate enough that there are several ways of going about this - it's very much down to what suits the individual. That said I'm quite happy to continue - as said I hope it lives up to your expectations
First off , my apologies Harry (GWR) - didn't mean to ignore you - I'm about 45 miles from Sudbury so if you're up around Beccles way and have some time to kill you'd be welcome to call. Theres also a very good stationary steam engine museum about 9 miles south of Norwich - it'll be in steam first Sunday of June just in case you need a fix
Okay back to the lapping
-
If you discount the wet and dry wrapped around a smooth mandrel I guess the next best thing would be a hardwood dowel turned to fit a bore, split down the end with a screw to expand it. This provides an expandable lap but one of 'single point' form though agreed, as the lap wears, the land will widen. Providing the amount to come off really is minimal this would suffice to apply a better finish but not neccessarily keep the dimensions uniform down the length of the bore.
As you can see most of the laps made so far have been of one type. I think I first saw it in an article by Len Mason - a well known engine maker - but whoever it was the merits of it's consistency were praised. Having made that first one for the Quorn there's been no reason to change since until I did the last engines - the Super Tigres - when a differing kind was tried. This was successful too but nowhere as easy to operate as this kind.
I don't know how small they could be made as there is this need to silver solder a small controlled area but I would think about 3/8 / 10mm would be the limit without it becoming a technical challenge.
George - I hope the following will be self explanatory without a sketch
So far they've been based around a 1/2" shaft - to fit a collet and if larger then a head is soldered on relative to the size required. The head is cut back like a D bit and a copper wrap is soldered to the leading edge. The cut off piece is inserted in the gap, A grubscrew locating it which is also used to expand it and the OD turned to about 1-2 thou / 0.025/0.05mm down on bore size.
The constituent parts
The head is split and half cut away
The wrap is annealed and formed around something to give the right diameter to fit the head - obviously the head has to be of such size as to leave sufficient for turning the od but not so big as once turned to leave little left for lapping. The wrap is drilled clearance for the expanding screw, the fixed half is tapped for it and the insert is dimpled to act as a locater to stop it falling out.
The inside leading edge is cleaned and the rest coated with a soft pencil - I believe Tippex makes a good resist for Silver Slder but haven't tried it and the same done to the head. Fluxed carefully on the cleaned areas the copper is held in place by a small screw through the copper and the edge soldered.
It's then set in the lathe and the OD turned, as said to a couple of thou max down on the bore - the abrasive has to have somewhere to go
Bear in mind if you solder it to the wrong edge this is the op where you find out as it unwraps itself
- no it hasn't, not yet
In use abrasive is mixed to a paste, applied to the lap and the part slid over - if it's too loose its witdrawn and the screw adjusted and tried until a good restriction is felt. Work the lap up and down to spread the abrasive then holding firmly set the lathe running at around 200 rpm slower is better to start with but not too slow. The part might grab at first but a good grip will soon have the two surfaces acting against each other as the abrasive begins to bite. I let the oil in the paste act as a lube for the first few seconds as the abrasive spreads out to an even film then I apply Paraffin (Kerosene) with some light lube oil mixed in (about 90-10) to keep things nice and wet. This is a messy process - I've tried using gloves but the loss of 'feel' and the problem of Nitrile gloves catching and winding up makes it much easier to get plenty of barrier cream on and volunteer to do all the washing up for a week
It should go without saying but make sure the lathe ways are covered too.
Here are a few more pics of the same ops - this time it's the lap used for the first Eta engines being 're-wrapped' - the first time this has been done. Not quite as neat as the first time but successful never the less
Despite the coating of graphite the solder has run a little further round than liked but none the less this worked well for all four cylinders
The lap turned and coated with abrasive for the first lapping. The grade is 320 grit silicon carbide - we'll cover abrasive later
The single point expansion gives a parallel lap at all times. I confess when I made that first one I could not see how it would remain round as it was gradually expanded but of course it is defined by the process - as mentioned before they gradually become slightly eccentric to the shaft but retain perfect (within the remit of what they are) circularity. As the op is one of a floating nature the slight eccentricity has no effect on performance. I think it would be fair to say that to lap successful bores for diesel engines goes some way to proving their viability.
I'd like to call it a day there - late night last night, early start this morning and a nice day sailing in the fresh air has finally caught up with me.
Regards for now - Ramon.
PS I see elsewhere Harry's preferrence not to have his post on his engine interupted. Without wishing to be seen as disagreeing with him, personally I would much prefer others to chip in/comment etc. To my way of thinking that way it's clear that the exercise is continually viable - I'm an inveterate worrier you see
