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| madjackghengis:
Hi Craynerd, having come to model building from building full sized engines, I find looking at the model as if it were full scale, and taking the forces I would expect in full scale as a standard, I end up with solid mountings and good repeatability. Having rebuilt many engines and machine shop machines, I have developed standard procedures for differing levels of accuracy. The issue with steam engines is they tend to have many parts which must be either parallel to each other, or perpendicular, or either of the two to a round bore, and generally machined each separately. This means that more care must be taken, as error always creeps in, no matter how much care you take, and error usually "stacks", unless you take the opportunity to arrange it so such error as you know will arrive, can be put in the opposite direction as error expected in another part directly attached. With modern engines, most of the machining is in one casting, and it is much easier to get the full accuracy of the machine tool, as the limit of accuracy of the finished work. I learned about stacking error with the making of repair parts and having to match what I was making in replacement, to the accuracy already in place in what was being repaired. The good news is that people have been doing this as we do, for almost a couple hundred years, and with micrometers coming about only about a hundred years ago, so accuracy can be made to come to fruition by careful work, and most of all, an active imagination, as that is where you will see the error first, before you even measure it, once you are accustomed to how your machines behave. I commonly use cigarette papers to check fit, or to move a cutter to a piece without actually touching off, and getting a true "zero", because the it touched off on the paper, which is reliably .001 thickness, and tears away, with the cutter, while not touching the work. For making holes that must align, I always use a center drill in the center pop, always drill in steps, and use one piece to guide the final drill size in the other piece it fits to. My milling machine is twenty years old Taiwanese and when I first got it, I tried to "coordinate drill" six holes using the dials of the machine, and found no two holes within five thousandths of where they should have been relative to each other. A DRO is a tremendous boon, and improves accuracy enormously, don't know how I got along without it. The biggest mistake beginning machinists tend to make is related to the idea that the machines are solid cast iron and steel, and thus can't flex or bend, while this is actually exactly opposite what they really do. It was all brought home to me some twenty five years ago when I was in an old machine shop, having just moved to the area, and waiting my turn to speak to the owner. A machinist had the large lathe going, with about a ten foot face plate, with a complex piece on it being turned, with interupted cuts all the way to the end of the machine work. The tool was a piece of high speed steel a foot square, and four feet long, the tool post on this lathe was probably three feet square, and four feet tall, and the lathe was turning at about 2 rpm, for the proper cutting speed for cast iron. Each time the cut came around, I watched the tool flex about four inches, the tool post flex enough to be plainly visible, and the blue half inch thick chip fly off and travel a good thirty feet, smoking in the oil where it lay. Up to that day, I knew intellectually, that all things move and bend, flexing and such, but did not know in my "spirit", which required seeing it, absorbing it, and re-arranging my thought processes around to incorporate this old knowledge into the intuitive part of the brain. Having seen it on a work piece well over a ton, I now can visualize the minute flex in small parts clearly, and make plans to accomodate the flex, and work around it. I hope this is helpful and not just me running my mouth and flexing my "experience", but I am entirely self taught, and have only a year of "machine shop" in high school for formal school, started as a mechanic, and learning by watching, and even more, by reading how it was done in the very old books, those such as Lindsey Publications reprints, mostly from the turn of the last century and in the first few decades since. I highly recommend this publishing house, it's easily found on the internet, and has great old books show how to do things with far simpler machinery, and far less tooling for measuring. I also want to say I am as eager as any on this site, to see you successfully complete what will be a beautiful engine, and hopefully watch a video of it running, I'm a bit envious of your engine, to tell the truth, and wish it were my project. It is looking great so far, and your careful work shows. :jaw: mad jack |
| raynerd:
I appreciate your reply MadJack. I`m only just really getting to grips with machining procedure. It has been a steep learning curve. I`ve been on here for just over a year now but there is just so much to understand it takes a long time to get to grips with everything. I know for a fact I went through months of just machining aimlessly to get to grips with my machines and understand how they worked...then months of trying to machine accurately...then months of realising I can`t be accurate unless my machines are so and so I spent months setting them all up properly....and now I`ve hit the stage where I want to machine an engine and so not only to parts have to be accurate but they have to be true to one another and I guess this is where procedure comes in. I do need to read more but as I explained before I think I use to skim read procedure to just read and get to grips with the simple machining process that was going on...not necessary read and understand the order. Anyway...I`m getting there! I have to admit that I have always used the classic excuse of " a bad workman always blames his tools" and I did blame my tools or get stressed with them BUT since spending time and a little money on new tools and getting my machines sorted I feel so much more comfortable in my shop and getting on with things. I feel I don`t have to "make do" any more and normally have the tool to hand ready to use and it seems jobs are going more smoothly and better than then use to. Anyway...long may it continue!! Thanks again Madjack. I`ll keep pressing on! Chris |
| Powder Keg:
Your engine is coming along nice Chris! The months will turn into years. I've been paid to be a machinist for 15 years now. I'm Going to school to help figure things out. It will never end. Every job is different. Hang in there |
| raynerd:
Hello all, after drilling for the mounting bolts this eveing, I`m planning on moving onto the cylinder next. I have done a search "boring stuart cylinder" have have hit quite a few results but as with anything on the net, your never sure if it is done correctly (or like me, post for advice!!) I know I`m taking your time but would anyone care to outline the major things I need to consider and perhaps does anyone have any links to a method that seems accurate that I should follow? I have obviously identified that the bore must be at 90 deg to the cylinder bottom (and I am under impression that the top face is not essential). I also believe that the cylinder mount where the steam chest will fit needs to be parallel to the main bore. I`m clear that by boring and then facing one end I`ll have the bore at 90deg to the cylinder face but I`m not clear how to ensure or even machine the steam face parallel to the bore with both the lathe and mill at my use. Any help appreciated. |
| NickG:
Some great points there Jack. I found out that with my brand new chinese machine, the dials were accurate over short distances of travel, however, if I wound it more than a couple of inches there was quite some error, which obviously got worse the further you went. Must be pitch error in the screws. I think a DRO would transform my machine, I used them at university and it made things so much better. Since I got my 2nd hand Harrison lathe and this mill, my work has drastically improved, so I think it's sometimes right to blame your tools Chris, I was struggling on with the machines I had and it was those all along. Technique is obviously a bit part of it though. I also think that steam models generally are quite forgiving on tolerances though, it's better to have things with plenty of clearance rather than something with close tolerances and a boat load of friction. Limits and fits in model engineering are something, unfortunately, that you just have to get a feel for through experience, whereas if drawings were fully toleranced the builder would get results if he stuck to the tolerances. Even on my hot air engines and flame gulpers which are notoriously difficult to get working, I've probably given more clearance in working parts than most people would, with good effect. Friction is the enemy but clearance (in moderation) can be your friend! Chris, you could clamp the square end of the cylinder up against and angle plate and mill it that way (see attached diagram) or I guess if you can get a good grip on it, clamp it in the vice with that face vertical. Sure people will have better ideas though! Nick |
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