Post By ADFToolmaker
Post By Limy Sami
Post By CWB
Post By Bruce Griffing
Best way to finish inside of cylinder?
I've got a bit of a unique challenge here.
I am building a steam/air engine I designed. It has 8 aluminum cylinders that are 4 inches long and the double ended piston is about 2 inches long. It piston has a built in slide valve and the piston/slide valve is made of delrin. All of this means both ends and the center of the cylinder (the whole thing) needs close tolerances as does the slide valve/piston. In other words I want the cylinder bore as perfect as I can make it for it's full length. The bore size doesn't matter too much as I'll machine the pistons to match but it would be nice to have interchangeable parts. I tried a reamer but didn't like the finish as I got a bit of chatter (adjustable reamer at 1.015"). Then looked for hones. I heard Sunnen hones where the best but they only go down to 1.25" and that's actually the cylinder OD so they are too big. I resorted to a brake cylinder hone. It only had 1.15" long stones so hopefully it didn't mess up my dimensions too much. Unfortunately these hones come with 220 grit stones. Way too course, the cylinder is going to 'eat' my delrin pistons so I need to finish them in a manner that removes the 220 grit scratches.
So, what is the best method to get the bore of the cylinder as dimensionally consistent as possible for its full length and smooth enough for a delrin pistion....I'm thinking 1000-1500 grit sandpaper like finish. The cylinders are currently open at both ends....as in a open ended tube so the bore is very accessible.
Honing is the way to go. Sunnen make hones way smaller than 1.00". The info you have may be for a specific type, of which there are many.
Find a hydraulic or automotive engine rebuild shop, they'll have honing equipment that size for wrist pin bushes etc etc, IME charges are very reasonable.
For finer grit for the brake cylinder hone buy some self sticking sanding pads in the grit you want and stick them to the hone.
I think I'd stop here and take Sami's advice before continuing with the brake hone. The brake hone will give you a smooth surface, but it is probably going to cause taper and, if your cylinders have any porting in the sides, it will go out of round as there is less material to cut there than on the rest of the inside surface. A Sunnen hone holds the stones parallel to each other and at a fixed radius from the center, so they can't cause taper or go out of round.
And all that work will be for naught when you install them in the engine. As soon as you put torque or load on them they will distort. They need to be honed fixtured in a manner as close to the "As Installed" condition as possible.
With the clearances required for a delrin piston with it's higher coeficient of expansion? I wouldn't worry on that score.
Originally Posted by Tonytn36
I'd be afraid that life of uncoated Al bores will be short. I'd use SS or anodize. I've honed cylinders with a homemade split wooden mandrel with sandpaper glued on You need to hone wet, or your stones or paper will load right away, stop cutting, and likely score the bores. I use Diesel fuel. Have a buddy or a kid squirt it out of a detergent bottle. I'm assuming the job won't pay for the "right" equipment.
Sunnen makes multi stone mandrels a long as you need in very fine grits. You also need to have the correct shoes for the material. First get the bores straight and finished, then have them hard coat anodized. You can run the hone through them after anodizing and get a slick finish. With Delrin pistons, you are not going to be able to run very high steam pressures because high pressure steam is also hot. Besides, Delrin absorbs water and swells. Some investigation of materials and methodology now can save a lot of expense and failure later. The way you are going now doesn't sound very promising.
I had a similar problem with racing kart brakes. I had the bores hard anodised to eliminate wear and scouring. This process penetrates the base metal by a thou and raises the surface by a thou.
There is only one process that will produce your stated goal, "I want the cylinder bore as perfect as I can make it for it's full length." It's lapping. Lapping can be done with shop made tools to very high standards with some knowledge and practice. If you're not familiar with the process have a look around this site for an introduction.
Internal Laps, Arbors and Expanders
The process can be done on a cheap, crap machine by a good operator and be on the money in onesey, twosey numbers such as your job. Hones make sorta round straight holes with decent geometry and surface finishes. Laps take over after they're done with what they can do.
If you don't need that level of precision adjust your stated goals with finish and tolerance specifications. Perfect is impossible but with enough elbow grease you can get real close, within millionths of an inch for the geometry of a through hole: circular and straight.
The previous comments about the material choices described for air/steam service are correct, bare aluminum cylinder bores and Delrin pistons don't perform well long term. If it's some sort of model for brief demonstration with air the pairing might meet your requirements. A hint of lubricaton will do wonders for its longevity. Put anything hot (steam) there and the Delrin will expand greatly more than the aluminum and stick it.
There are other ways to do things, search "hydraulic wear rings" to learn how materials such as Delrin are used inside cylinders for their good bearing properties while avoiding the problems their high thermal expansion rates can cause.
I design recip engines for a living, and would tell you to try to design and build the best you can, but not go overboard too much. Being that youre designing it yourself, I dont imagine this being a critical application of any kind (please correct me if Im wrong), and would guess you are building this mainly for entertainment. In that case, I wouldnt worry about overbuilding for longevity, simply build it within reason and if you run into issues rebuild it. Reciprocating engines tend to have very tight bore and piston related manufacturing tolerances for longevity relating to thermal growth, but if I read your original post correctly it sounds like you will be running on air. In this case your delrin piston would likely work and tolerances inside the cylinder arent super critical. Even on fuel powered recip engines many of these tolerances and surface finishes arent super critical to actual operation, simply performance. We all know multiple Bubbas who have "rebuilt" junkyard engines in their garages, and getting something like this to run a few hours minimum isnt rocket science either.
For a good quality hobby project, I would simply bore then run a piece of 600 grit down the bore and call it a day, but I also would use an iron or aluminum piston instead of Delrin.
Exactly. I doubt you will really be putting hours on this engine anyway. Probably could use a mudball hone.
There are two types of "honing"...the brake cylinder hone or "dingleberry" hone is an uncontrolled process for the most part, it could level off any high spots I suppose, but it can never really make the bore more round or straight then it is now.
The "other" type of honing is a controlled process that WILL make a bore straighter and more round than it started out, the sunnen engine cylinder hones will do this, they were originally intended to allow use on and engine that was still in the vehicle however, so they will not work as well probably as the machine tool type sunnen hone.
Lapping shares some of the same attributes as the sunnen controlled honing process.
For a lap to work it needs to be softer than the material you are lapping, aluminum makes an excellent lap for OTHER materials, but it will not help you here, I'd guess off hand lead may be your best bet to lap aluminum ? I'd single point bore them, then give them a final lap to make them really nice, it is good to learn how even if THIS job really does not require it :-).
You MIGHT be able to make up an expanding lap that is knurled on the OD, and wrap some lead roofing around it, or warp it in 50-50 solder :-). Or buy a few lead anchors for concrete nearly the right size and modify them into a lap
6 in. x 3/4 in. Lead Anchors (24-Pack)-5075-R-001B at The Home Depot
If your finished cylinder has some clearance between the piston and the bore, the piston will more or less float on an air (or steam) bearing and never touch the bore.
Okay.. Go buy yourself a 1/4 or 3/8 diameter half round drill. Put a chunk of your aluminum in your lathe. Using a standard drill & center drill (for accuracy) drill the hole to a test depth you want, and then follow it with the half round drill which was selected for a finish pass. In effect it becomes a one fluted reamer which can't go over size and can't chatter. Use some real cutting oil.
The Ol' Man told me the aluminum will expand a bit during the cutting process, and then on retraction it will cool and shrink a bit and the inside surface will become burnished. NO GRIT Necessary. If that works for you then obtain a 1" half round drill. You don't even have to measure anything.
Some folks call that a "D reamer" :-). He is talking a 1" bore though...suppose he could make his own D reamer, there are a few threads here if he does a search.
Originally Posted by Metalcutter
Wow! Tons of good info here. Thanks all.
For clarity it is an air engine....I say steam because almost anyone who builds model 'steam' engines actually runs them on air as it's easier and safer to run a compressor than a boiler. I don't typically like to say 'air engine' as that makes people say why would you use an engine (air compressor) to make air to then turn another engine.....they don't always get that the concept of most air engines are based on steam engines.
It is just a concept model so won't run long periods (if it runs) so I'm shying away from the metallurgy and challenges of machining different types of metal. Hell, the machining itself is enough challenge for me. I'm not a machinist by trade, this is a hobby. Every day my respect for true machinists increases. Had I known the challenges and skills necessary to become a competent machinist I probably would have taken up latch hooking or paint by numbers.
I suspect I'm being a bit overzealous about these tolerances but one of my goals is to learn to machine to a high degree of accuracy....might not actually be required for this job but is still a valuable bit of information or a valuable skill for when it is needed.
Another way to get a smooth interior surface is to force a very slightly oversized ball bearing ball through the cylinder. Lube it up and force it through with a press. It will leave a burnished surface that can be borderline shiny.
Finally finished the engine . It can be seen here;
It ran right from the get go with no adjustments/modifications required (don't ask about it's predecessor ).