HOBBY TYPE BRASS TUBING AS BORE LINERS AND BEARING SLEEVES
The drilling, reaming, boring and honing of perfectly sized and parallel holes or bores are fairly easy things to accomplish if you happen to own a good set of reamers, hefty flex free boring bars and a lathe with a perfectly aligned bed and head stock. If your equipment falls a bit short of the just descried requirements you can still achieve great results building small bored (1/4 -1/2") steam and air powered engines by following this very simple application.
So called hobby brass, as it is sometimes referred to in the industry, can be obtained in rod form up to 3/16" diameters plus many sizes of round, square, rectangular and angular shaped telescoping tubing as well as flat stock. It is the round tubing that I have adapted for our use. This tubing comes dimensioned so the next larger size slips almost perfectly over the previous one. It does so with a nice wobble free running fit which tells me that the clearance has to be in the range of .001" or less between the two tubings. I have tested this by inserting a 1"long piece of ground .500" diameter drill rod while blocking one end of the tubing with my thumb. If I drop the rod from the top while I plug the bottom, it takes about 5 seconds for the heavy drill rod to fall all the way down a 12" long length of tubing while if I do not plug it, it will fall freely through the opening. If I apply some oil to the inside walls of the tubing to further seal it, it takes about three times as long for it to fall through. If I flip the tube over while still plugging the end and allow it to fall, it produces a pretty good vacuum. This material is rolled to shape and pushed through a die so the metal is effectively work hardened as you get it from the store and will provide a long lasting and wear resistant surface. I specifically like to use tubing with inside diameters of 1/8"- 3/16" - 1/4" - 7/16" and 1/2". One of the main applications of this material is as a liner for cylinder bores on steam engine blocks. Say you just finished boring the bore so it just accepts your perfectly turned piston. As you insert it into the front to check the fit, you discover that the fit gets tighter and tighter as you try to push the piston further into the bore. It could be the other way around, with it getting looser the further you push it into the bore. Any way you look at it, you have a tapered bore situation which is as useless a condition to have in a cylinder bore except in some internal combustion situations and a few very advanced type of steam engines. The only way to prevent this condition during boring is to bore between centers or finish the bore by reaming. A quick and much easier cure is to simply enlarge the bore just enough to accept a length of the proper diameter tubing as a light press fit. It is important to create a level of fit so the tubing has to be pressed in with minimal pressure of a vice. That way it can be replaced in the future if and when it wears out. After pressing the tubing all the way in, you can face it smooth and de-burr the bore openings to prevent scoring the piston's walls. For designs that call for a piston of 1/2" diameter I use the tubing with a 1/2" inner diameter and merely use a brand new length of 1/2 aluminum rod for the piston. If you begin with a brand new piece of rod, you will find that it may only need to be reduced by .0005" or so for a perfect running fit. This can be accomplished with a quick 400-600 grit paper polish along the sides of the piston blank as it spins on the lathe. You should polish the piston to final diameter this way anyway even if you have turned it down from larger stock. Do all your machining steps while the piston blank is still mounted on the lathe and then part it off to length. Re-chuck with soft jaws and slightly chamfer the top edges of the piston and cut the oil grooves or piston ring slots if called for in the original design. I have gotten some the smoothest running engines that will run very slow with a minimum air or steam pressures by using this method of cylinder bore lining. The same technique can be used to fabricate bearing blocks for all of your steel crank and flywheel shafts using stock sizes of ground drill rod. Drill and ream the holes to accept the tubing as a press fit and trim flush with the bearing block. Oil holes can and should be cross drilled through the top side of the bearing block. Run a reamer of the same diameter as the inner I.D. through the bearing to clean up any drilling burrs caused when the drill breaks through the brass liner.
Not only does this simple lining method work to produce close fitting and long wearing bearing and cylinder bores but it allows you to substitute the more cheaper aluminum, considering the high cost of solid brass, to fabricate the bulk of the part with the brass tubing providing the bearing or sleeve material. These bearings can of course be easily replaced when needed by pushing them out with a flat faced piece of drill rod of the same outer diameter as the tubing used for the bearing or sleeve.
Even if you do have an abundance of reamers in all sizes, you might still want to give this system a try just for the savings in brass alone. You can still put your reamers to work by drilling out and reaming to accept the out side diameter of the sleeve. The tubing that provides a 1/2" bore is actually 17/32" in diameter as the wall of all of the tubing sizes are a consistent 1/64" thick. A 17/32 reamer is all that would be needed to finish the bore to accept the tubing as a perfect fit. By reaming the primary bore, you will automatically take care of any existing slightly out of parallel condition possibly introduced during the boring out process. Always use drill rod for all your shaft needs. Steel, especially in drill rod form is perfect for shafting requirement as it is ground to exacting diameters and works well in contact with brass. Aluminum is not compatible when in contact with aluminum but will do well against brass or steel and vice-versa. Aluminum would not be rigid or hard enough for shaft use but because of its lightness and relative softness tends to be the perfect material for pistons as it will wear itself to a perfect leak proof running fit during the breaking in procedure. A heavy piston will require a counter weighted crank disk to minimize vibration at the higher running speeds due to the reciprocating action. A steel piston will also tend to wear out the cylinder liner prematurely if it is not perfectly glass smooth when it is first installed. A steel shaft on the other hand, because is usually attached to a pretty hefty flywheel tends to run pretty much suspended in the oil used to lubricate the bearings so there is substantially less wear involved. An aluminum piston can be made even lighter by hollowing it out by drilling or boring like that of a automobile engine without detracting from its strength and dimensional stability in the least. In fact the lighter the piston is made, the smoother the engine will run at the higher speed ranges. Steel drill rod is made by grinding to diameter and it normally runs very true to the stated size so it can be used in reamed or sleeved holes without any extra adjustments or treatments. Because of its high carbon content, drill rod can also be easily heat treated to make shafts even stronger when used in extreme high stress situations. One neat little trick that we can do on a shaft to be used in conditions which do not require hardening is to heat the shaft as slowly and evenly as possible until the polished surface has gone through the yellow and brown heat color range and has turned a deep blue color. As soon as the blue color has been reached, plunge it into an oil bath to quench it.. The oil can be simple common 30 weight motor oil, used or new, it doesn't matter. The resulting blue colored surface is very resistant to wear, will probably never rust and will tend to retain a thin film of oil which will impart a slippery touch to the surface. Treating shaft material in this manner will increase its life several fold and will give a pretty nice color contrast against well polished aluminum or brass engine components.