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| kwackers:
Don't bother. The slide valve is adjustable and there's enough room in the chest to adjust out the error without it hitting the end of the chest. Even if it did, you could easily take of the 15 thou of the inside of the chest to give the valve space. |
| Bogstandard:
You are hitting and hilighting every known problem imagineable when working from castings. People imagine it is easier to work from castings than making from barstock, nothing could be further from the truth. The first thing you must do when working with them is find your most critical datum point on each and every casting, and work everything out from that. In this case, it is the centre of your steam ports. Find that position first by using your height gauge on your surface plate, blue up and scribe the centre line. Without touching anything, with a bit of maths, you can easily work out the height gauge settings for the top and bottom of the cylinder. Scribe those two lines on there. Now you should be able to split those lines by machining and you shouldn't be more than 0.002" out from centre and the actual length requirement. If you don't go too mad with the cutting, and take it very steady, measuring all the time, you should be able to get the cylinder length spot on. As already mentioned, in this case, you are lucky in that the positions of the slide valve can be adjusted out. I have shown above how you should have gone about it if starting from scratch. Your piston will be the next critical bit, and for that, your working datum should be the centre of the gudgeon pin hole. By working from that, your should be able to get the top and bottom in just the right places. So really before doing anything else, go over your plans, find the datum points for each and every casting and mark them up on the plans with a pencil (not ink), and also put your new dimensions working out from it. Some parts might have more than one critical datum point at the start, so be careful when you are starting out. As you machine each datum point, more datum points will be made that you can work to. If you just dive in and machine willy nilly, you will end up buying a lot of replacement castings. In a few months, I will be starting to work from castings again, after many years layoff from them. I have already started on my plans, finding out just where I am going to make my first cut. Bogs |
| raynerd:
Thanks for the replies...it seems I have been lucky here in the fact that I can adjust the slide valve - thanks Kwacks, that did reassure me when I read it :med: John, nice to hear from you again and thanks for your replies. I haven`t had to buy a new casting yet thankfully although I am in two minds as to whether I should buy the badly machined con rod that was done before I recieved the kit or if I should make it again from bar stock - however that is of now fault of my own. I have however, been waiting and putting off ordering this to save on postage should I need to order anything else while I build this! Anyway...I`ll keep plugging away and be thankful that I got lucky this time. If I remove it from the vice and bolt it to the table, does my method for facing off the steam surface look OK? - I just need to figure out how to set it up as parallel with the table as I can prior to cutting. Cheers Chris |
| Bogstandard:
The main critical bit is getting the valve face parallel to the bore. So your valve operating linkage runs parallel to the con rod centre. I have attached a C-o-C to show how that can be achieved. The diameter of the bar isn't critical as long as it doesn't bend when you push down on it and it has parallel sides, but the larger the better. If you can mount it up like this in your vice, push down and tighten up, the port face will be parallel in the X axis to your bore. The only thing you have to watch out for is that it is square in the Y axis as well before taking your cut. But if you do it right, that should almost automatically align itself. Bogs |
| NickG:
Chris, either of the ways shown will work for milling the valve face. In the first one which I and a couple of others suggested, you've got to assume your angle plate is square and your end face is perpendicular to the bore - it should be as you did that in one setting. .... this is the reason you can easily just put it back in the lathe with the square face against the chuck - skim one end to length - they're now both square to bore, flip round skim other end to ensure ports are dead centre. Bogs method is technically better as you're only only assuming your milling table is good there - it is probably also a more rigid setup. Take the cut in the x axis though as otherwise you'll need your mill trammed spot on too. As somebody else said though, it's not going to make a great deal of difference to the running, you can adjust the valve on its rod and having 30 thou dead space in the top of the cylinder won't matter a jot - what is the clearance when at top and bottom dead centre? Good points about castings by Bogs, I tried to make some stuff from castings when I was really young, the first things I tried to machine - you won't be surprised I just wasted them all. Haven't touched a casting since but am beginning to think I want to try a small engine like yours. Cheers, Nick |
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