Honing

[Ulma Doctor]

A slight bell mouth would not be the end of the world, as the majority of the piston is not likely to travel near or past the bell mouth

 

[C-Bag]

Just to be clear, the instructions to build the engine say it needs to be honed? What is the cylinder made out of? I’d not seen that kind of finish spec before and that fine of finish fell into what they called a “machined finish” which sounds more like a finish with reamer than a hone. Finishes with a hone were classified as rougher than that if I was reading the chart correctly.

is it a slug type piston with no rings like a model airplane engine?

I did a lot of engines in my past and honing by hand is an art. Most want to put it on a drill motor and crank away at full blast moving it slowly back and forth. Actually to get a proper crosshatch pattern it‘s the opposite with slow speed and vigorous movement back and forth. The speed of the drill and the back and forth movement(without poking the hone out too far on either end of the cylinder and fouling the hone) is the art I mentioned. I’d not honed anything in 30+yrs and pulled out all my assortment and every one of them had self destructed. I don’t have or ever use ball hones. I tried to get stones for my small hone like in the pic and talk about junk. They wore out before I could finish.

 

[RaisedByWolves]

Mike, what does this engine use to seal the piston?


Some kits use O-rings, some use a cast iron ring and others just a tight fitting piston..


Only on the latter would I be concerned about honing the bore for accuracy.

 

[whitmore]

Hello again,
I'm building a small steam engine and I'm going to be boring the cylinder to a specific size of 0.625"

I need to hone this cylinder and while looking online for a hone of that size (which I found much to my disbelief) they mentioned a finish of 35-50 Microinch(.9-1.4 Micrometer) (is this 0.9-1.4 thousands of an inch?) and this made me think that I might need to bore the cylinder undersize to allow for material removal.

I believe the finish is intended to be 'rough' enough to retain some lubrication, as a seal.
It's not the removed material that determines the effective diameter, if it's just a surface-finish
hone, rather than material-removal sizing operation.

 

[matthewsx]

I’d look to John York @benmychree for what a steam engine piston requires for cylinder to piston fit. I’ve done hundreds of small engine builds and honing can be used for both sizing and/or surface finish but have no idea how it works with steam vs. Internal combustion.

For the engines I built it was necessary for the first firing to get the rings to set properly and seal right. Does it work the same without fire?

John

 

[Pevehouse]

I hone engine cylinders on a regular basis but only to freshen up the engine with new rings and such. We usually just use a 320 hone from brush research. Does a great job at deglazing and leaves a nice crosshatch. Now my machine shop when boring my engines will cut the cylinder to size then hone to specifications based on measurements of the piston skirt

 

[Jake M]

I need to hone this cylinder and while looking online for a hone of that size (which I found much to my disbelief)

I wish that would happen more often when I need something that "shouldn't" exist....

they mentioned a finish of 35-50 Microinch(.9-1.4 Micrometer)

Nothing to do with steam engines, but that's in the range you'd find called out for older gasoline engines with high tension rings. (Normal tension rings, no descriptor needed prior to low tension rings...) Back before they started calling out not only the profile of the ridges and valleys contained within the surface finish.

(is this 0.9-1.4 thousands of an inch?)

No. That is 35 to 50 microinch. A microinch is one millionth of an inch. So what you're looking at there is one third to one half of a "tenth". A micrometer is one thousandth of a millimeter. That surface finish inside the boor will "look" like you just absolutely ruined it with the hone, but (if the bore were bigger) you could drag your fingernail on it and it'd feel dead smooth and with moderate pressure and a long attention span, you would not be able to file your fingernail with it.

and this made me think that I might need to bore the cylinder undersize to allow for material removal.

I doubt you're going to find performance profiles for any stone in a proper hone that size, and the ball hones don't take much out of the middle diameter at all. (They're sure good at opening up the ends though).

Ok, enough picking on those little heaps of crap, as you might actually be able to pull it off with a ball hone in a bore this small, simply due to the practicality of having a reamer to put it on size and true, and then you would be literally just scratching up the surface, instead of actually sizing with the ball hone. (Don't size with a ball hone...) You'd want to keep it inside the bore though, pulling those ball stones out and back in is brutal on the precision of your bore. You'll get picture clearly when you first insert it.

Honing a cylinder is new to me and I have only one shot to get it right so please let me know if I'm understanding this correctly.

Thank you,
Michael

That means you've got no practical option but to do some testing. And if you're ball honing (I'd give it a go, as you're gonna be a LONG time finding a hard hone and figuring out the pressure and removal rate that generates a finish like you're looking for with any given grit of stone. They're usually rated at "full cut", which means max material removal rate, and you will NOT be doing that in this size range. So the ball hone might, in your case, have an advantage there. (Dam that was hard to say. I hate those things....) But you need test holes for two reasons. First, you need to know how fast you're removing material, how fast to spin the thing and oscillate it (They're looking for cross hatches, even if they didn't specify), and VERY, VERY importantly. Crazy stupidly important- You NEVER, EVER make a final finish with a brand new stone. On a hard hone, the stones would have in theory removed material before they ever came close to final dimension. With the ball hone, you want it used some before you can expect any consistancy, as right out of the box, the grit will be very uneven, and there will be some deep scratches until the high spots wear down to the low spots to "equalize" how much pressure is being applied at each contact point within each little ball on the ball hone.

So yeah... That's how I'd go about it. Practice. Make three or six holes in the same material. Or as close as you can come to it. See what it does, and react. Then make your "good" hole accordingly, and go from there.

 

[JFL4066]

My 2 cents...
I would make a brass or hard maple lap under size by 5-10 thou. Slot the lap almost full length. Put a machine screw at the end of it to adjust the diameter. Get some fine valve grinding compound or similar. Since the bore(s) were bored with a single point boring tool or an undersize reamer you want to remove the fine "rings" on the bore surface left from the boring process. Ideally you want to lap the final 2-3 thousands of an inch max because as you remove more material you will not maintain cylindricity or ovality. A surface finish of .9 to 1.4 microns (which is indicated by an Ra of .9 or Ra 1.4. is typical of a ground finish. Lapping by a drill press at 100 - 200 rpm is good. To achieve cross hatch you slow down to 80 - 100 rpm with a rapid up down stroke a few cycles. Done. I would strongly suggest you practice on some test bores to get a feel for the process. It is highly accurate and has been done for 100 years.

 

[Makintrax73]

Hello again,
I'm building a small steam engine and I'm going to be boring the cylinder to a specific size of 0.625"

I need to hone this cylinder and while looking online for a hone of that size (which I found much to my disbelief) they mentioned a finish of 35-50 Microinch(.9-1.4 Micrometer) (is this 0.9-1.4 thousands of an inch?) and this made me think that I might need to bore the cylinder undersize to allow for material removal.

Honing a cylinder is new to me and I have only one shot to get it right so please let me know if I'm understanding this correctly.

Thank you,
Michael

First let me just say that I am FAR from an expert. I did however just recently complete a running compression ignition engine from bar stock. These engines use no piston rings, a cast iron piston, and steel cylinder liner. To say that extreme precision in fitting up the piston is critical is an understatement. It simply must be near perfect or the engine will not have the compression to light off the fuel.

A far more experienced engine builder than myself recommended this article as explanatory for the steps to correctly get a fitted piston in the type of engine I built: https://www.adriansmodelaeroengines.com/catalog/main.php?cat_id=178

Assuming a similar type piston and cylinder arrangement: Based on what I know from building my engine I would NOT use a ball hone. A ball hone might be great for an oil lubricated internal combustion engine with piston rings, but I don't think that relatively rough surface finish is going to do you any favors in an engine in which the cylinder walls are not constantly splashed with oil and sealed with piston rings.

Based on my understanding that steam engines are far more forgiving of lack of exact fit of the piston in the bore I doubt very much you could go wrong following these instructions. I don't know whether the tapered bore which is beneficial in a compression ignition model engine is appropriate for a steam engine, but I doubt it would hurt anything since. If you want a parallel bore you can certainly create a different type of lap which will do that. A parallel bore lap simple is rather more complex to build. Blondihacks did a parallel bore lap on her steam engine build and you could certainly follow her example.

I used the 1,200 grit diamond lapping compound here: https://www.amazon.com/gp/product/B071CH37Z6/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

As you work at it the above described lap and lapping compound will work to create a VERY close fitting piston if you take your time and start about .001" or .002" over sized and lap it to nominal.

I would worry far less about hitting a dimension and far more about making sure the piston and bore work together.

Here is the Blondihacks cylinder lapping video, well worth a watch........

It may very well be the case that simply using a brake cylinder hone will be perfect based on the design of your engine too, and probably far easier than making your own lap if you don't want to go that route.