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how would you finish this bore?

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magu

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#1
I apologize in advance for being vague; I am building a small machine tool which I would really love to do a build journal for as I think others would find it fascinating, but in doing so I would inadvertently be spreading someone else's design around the internet. For anyone who is also into sheetmetal forming, I am building a mechammer from Ben's (bensmetalshaping) plans.

On to the question though: I am turning a piece of 304 ss which will be a cylinder for a sliding piston and needs a very good finish bore for the piston rings to seal against. The bore will be ~1", it is not critical that is 1.000, 1.001, or 0.999 as everything will be a one of matched set and the piston and other parts will be machined to match the bore. Once I have roughed out the bore, what would you recommend for finishing it? Options I have thought of are:

- a spiral fluted chucking reamer: this would probably do the job well, but they are not cheap, and I don't know if I would have any issues reaming a 1" hole from a power/machine standpoint

- an adjustable reamer: I am not concerned about being at an exact diameter, so the adjustability doesn't bother me, but I have never used one. This would be significantly cheaper though, and could be used for a somewhat larger range of bores going forward.

- a flexible hone: I don't know if by running one of these up and down the bore if I could improve the surface finish enough, or how consistent the bore would be, I have never used one.

-running a "dremel" type flap wheel up and down the bore while the lathe is spinning, I don't know how well this would work, how consistent the bore would be, or how much I would really improve a turned finish with this method

-feed the final pass of the boring bar extremely slowly and/pr multiple times and leave it as is

Anyone have thoughts or other ideas?
 

magu

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#4
Thank you both.

Bill, If I were to lap it, how would you go about it? The bore is ~5" long, I have lapped pistol slides and valves before as well as rotating components, but I have never lapped an extended bore, and have always had the male and female to lap into each other.

4SSSS, would you recommend a flex hone, or one with fixed blades? I haven't ever honed anything other than brake calipers and engine cylinders before.
 

4ssss

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#5
If you've honed brake calipers, it's the same process, unless you can get your hands on a Sunnen. If lapping, a speed lathe (or a fast lathe speed) with a wooden dowel cut to a pencil point and just going back and forth will do it, but being 5" long, you probably wouldn't get it straight.
 

whitmore

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#6
Iron oxide is softer than iron, but Cr2O3 (a chromium oxide) is 8.5 on the Mohs scale, while steel is
around 4.5, so rubbing at bits of stainless will be a lubrication problem. The iron and nickel oxides
will turn the oil (black? green?) but the chromium in the stainless turns into abrasive.
I think that means that the rings will lap that cylinder for you, in short time.
 

Bob La Londe

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#7
If its for a piston with "rubber" seals its doesn't need to be "perfect" diameter from end to end either. It just needs to be smooth. A brake hone should work fine. Once you have it very close if you use the hone with good technique I doubt you would wind up with more than a thousandth variance anyway. You might get a very slight belling a the ends as the stones come out or go in, but I doubt it would be huge or a huge issue.

I do have an micrometer adjustable fixed hone I used on a quill bore on a CNC Mill, but something like that would be WAY overkill for your application I think.
 

DAT510

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#8
You mentioned 304SS. Have you started machining it? It is a bear to machine. It work hardens just looking at it. If you already completed the machining, then as others have mentioned some combination of Grinding and Lapping should get the surface to where you want it, as my experience with taking light cuts on 304SS has never been good.

If you haven't started machining and other materials are an option, you might consider something like 303SS as it machines much better.
 

benmychree

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#9
Yes, 303 is a much better choice than 304; I made products in my shop before retirement and used 303 for everything we made for the wine industry, the only drawback is that it does not weld as nice as 304, it tends to undercut at the edge of the weld edge when using TIG.
The only other thing I'd question is the use of any 300 series of stainless, they tend to gall up with sliding fits especially against other stainless parts.
 

NortonDommi

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#10
What are the rings made of? What is being sealed? Iron rings need some ruggosity to allow them to bed in and to retain some lubrication,(honing). O-rings in a hydraulic cylinder need an almost polished surface,(lapping and polishing). Wet & Dry emery paper around a wooden dowl with a slot in it will clean it up as fine as you want, Kerosene for lube/wash. Whatever you end up doing scrub with hot soapy water and rinse when finished.
 

magu

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#11
The rings will be o-rings, more than likely buna-n, because I can get them easily, it will be "pumping" ambient temperature air.

I hadn't considered galling, but it is a good point. The only reason I was going to use it is because I have a piece on hand. hmm.... I could bore it to 1.125", insert a press-fit bronze sleeve and open that up to 1"... It would leave a very thin wall, but should work.
 

Bob La Londe

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#12
Buna works fine as a seal with stainless, steel, aluminum, etc. Buna is used as a seal at pressures upto 4500PSI in airguns. I tend towards Viton replacements, but viton doesn't perform as well at low temperatures. As a sliding element they may not last long without lubrication, but they will work. Fortunately since you are using an o-ring you can experiment with materials to see what works best, buna 75-90, viton, silicone, urethane... O-rings are readily available and relatively cheap in a number of materials. You can also consider double seals, x-rings, etc.

O-Rings are often uses in home made pneumatic cylinders. The best cylinders in my opinion are chrome lined, but often that puts a project outside the realm of the home experimenter unless they have a source for old hydraulic cylinders they can cut up for parts. To be fair their is many a small band saw with a home made air cylinder used as a feed regulator made out of aluminum with generic hardware store o-rings.

I have several low pressure plastic injectors with plain old Buna O-rings that are several years old. The cylinders and pistons are all aluminum and I haven't changed an o-ring in years. They get used with plastisol upto 350F, but pressures are admittedly relatively low. When I am leaning on one to fill a problem mold it might hit 50 PSI, but I doubt it.
 

magu

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#13
unless they have a source for old hydraulic cylinders they can cut up for parts.

That might be the golden ticket... I'll have to see if I can dig up a 1" cylinder to strip down. Even aside from the chrome bore, it would be less effort and waste to use that than to drill and bore out most of the material then fight to get a good finish...... Thank you for bequeathing you genius....
 

Bob Korves

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#14
Depending on the pressure expected in the cylinder, you might look into air cylinders, which are cheap and in the size range you are looking at. There are lots of choices of styles and end fittings. They will work as low pressure hydraulic cylinders (<200 psi) and are often used as hydraulic cylinders for adding a controlled down feed to a horizontal band saw.
 

NortonDommi

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#15
That damn cat! How many skins has it got left? What sort of pressure are you working with? HMWPE is cheap in cutting boards from your local $1,2&3 shop and makes great low friction bearings/packers for O-ring backup. Silicon greases are a big help where it is assemble and leave.
Aluminium Pnumatic tubing can be bought in Imperial and metrickery sizes with either a hard anodize or a hard chrome interior finish. A good place to enquire is at your local hydraulic service agent or Bearing Factor. Hydraulics, Pnumatics, Bearings, Seals & now electronic regulation & control are today so intertwined that any specialist shop or supplier wherever you live has access to information that as an individual you will never be able to access. Bonus is that it is a field that a lot of very smart people end up in and walking in with anything out of the ordinary or request for help usually results in a 'that made my day' moment for a very brainy person. Pays to do some basic research first ask a question or two and keep the taringa's,(ears), open.
I've often formed an idea of what I think will work and after a 5-10 minute talk with someone who knows their job have come away with something gaureenteed to work.
One thing I learnt very earley is that a good parts guy is worth whatever they ask for.
Sorry for the rant, little sleep
 
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