Final Bore is under spec

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EOsteam
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Final Bore is under spec

Post by EOsteam » Mon Nov 13, 2017 2:13 am

Howdy all,

I'm boring the trailing truck journal boxes for my 4-8-4 Northern and the final bore is 1.8504" with a 0.0000 to +0.0008 tolerance for the bearing pocket bore. I'm using a Criterion type 2" boring head with a Criterion insert boring bar. My final boring pass came out at 1.8502" measured with a Dial Bore gauge. I'm 0.0002" under the minimum allowed spec. Nothing has been moved on the bore setup. I'm pretty sure that if I don't move anything and take another boring pass at the current settings the spring in the tool will probably take out more than my tolerance allows. The journal boxes are cast brass. What is the accepted next step?

HJ

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wlw-19958
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Re: Final Bore is under spec

Post by wlw-19958 » Mon Nov 13, 2017 2:18 am

Hi There,

Maybe a wheel cylinder hone?

Good Luck!
-Blue Chips-
Webb

Harold_V
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Re: Final Bore is under spec

Post by Harold_V » Mon Nov 13, 2017 2:46 am

EOsteam wrote: What is the accepted next step?
It's a matter of procedure. When you work to tight tolerances, you don't take irregular cuts, nor do you take a light cut after a heavy cut. Not ever. When you bore properly, you should be able to go back in the hole and remove a tenth or two, no more. You do that by taking three light cuts when you approach size, each being 1/3 of the amount of remaining stock. A good target size, before finish cuts, is generally about .030" under. By taking a third each time, the tool works under identical tool pressure and removes a predictable amount of material. If it's properly sharpened, and is NOT dragging anywhere, very little will be removed with a spring pass, as it should be.

That said, if you took a large cut for finish, or if the tool is dull (insert tooling leaves a huge question mark), you most likely are correct, as going back in the hole may lead to an oversize bore.

If you have any doubts, blue the bore, and back off the bar a few thou. Keep backlash in mind when you do.
Begin by lowering the not-rotating bar in the hole, to see if it leaves a witness mark. If it doesn't, advance the tool a half thou, and try again. When you see a hint of contact, mark the location, and back the tool off a thou. Now, with the spindle running, try the tool. If it takes a hint of blue off, take the cut, stop and measure. Once you have achieved tool contact, you should be able to advance the tool a few tenths and achieve the final size.

This is slow business if you hope to achieve success without scrapping the parts.

An insert tool may or may not have the degree of sharpness that is desirable. For that reason, I suggest you learn to hand grind boring tools, so you can present the exact tool geometry, as well as the desired sharpness, to the work at hand. Relying on insert tooling, especially for the novice, is asking for problems that can be difficult to overcome, and it cheats the operator from learning the fundamentals of tool geometry and sharpening.

H

Edit: You made mention of using a dial bore gauge. How was it set? If you used a micrometer, you may or may not have an accurate setting. It's VERY easy to miss dead center (don't ask me how I know). A ring gauge of known (certified) size is the most reliable way to set a dial bore. Even gauge blocks leave a lot to be desired, due to the ability to miss center.

A wheel cylinder hone may or may not yield success. They are notorious for yielding bell mouthed holes, and it's easy to do. If the bores are through, with no shoulders to interfere with a mandrel, a Sunnen type hone would remove a couple tenths in a matter of a few seconds. I, personally, would NOT trust a wheel cylinder hone, but then I own a Sunnen, and have the proper mandrels.
Wise people talk because they have something to say. Fools talk because they have to say something.

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wlw-19958
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Re: Final Bore is under spec

Post by wlw-19958 » Mon Nov 13, 2017 10:01 am

Hi There,
Harold_V wrote:A wheel cylinder hone may or may not yield success. They are notorious for yielding bell mouthed holes, and it's easy to do. If the bores are through, with no shoulders to interfere with a mandrel, a Sunnen type hone would remove a couple tenths in a matter of a few seconds. I, personally, would NOT trust a wheel cylinder hone, but then I own a Sunnen, and have the proper mandrels.
Perhaps, but seeing that he is only removing 2 tenths of a thou,
I thought it would work if he is careful. It would also depend
on the grit of the stones (seeing that the material being honed
is "cast brass").

Another thought occurs to me. Depending on the tool geometry
and feed rate of the boring tool, there could be fine shallow ridges
in the bore and his bore gauge maybe reading the crest of those
ridges.

But, what do I know? My advice is worth every cent one pays for it.

Good Luck!
-Blue Chips-
Webb

John Hasler
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Re: Final Bore is under spec

Post by John Hasler » Mon Nov 13, 2017 10:55 am

wlw-19958 wrote:Hi There,
Harold_V wrote:A wheel cylinder hone may or may not yield success. They are notorious for yielding bell mouthed holes, and it's easy to do. If the bores are through, with no shoulders to interfere with a mandrel, a Sunnen type hone would remove a couple tenths in a matter of a few seconds. I, personally, would NOT trust a wheel cylinder hone, but then I own a Sunnen, and have the proper mandrels.
Perhaps, but seeing that he is only removing 2 tenths of a thou,
I thought it would work if he is careful. It would also depend
on the grit of the stones (seeing that the material being honed
is "cast brass").

Another thought occurs to me. Depending on the tool geometry
and feed rate of the boring tool, there could be fine shallow ridges
in the bore and his bore gauge maybe reading the crest of those
ridges.

Good Luck!
-Blue Chips-
Webb
In that case a few swipes with sandpaper might suffice.

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SteveM
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Re: Final Bore is under spec

Post by SteveM » Mon Nov 13, 2017 10:13 pm

Make a lap and lap it out to final size.

Laps can remove tenths.

Steve

Harold_V
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Re: Final Bore is under spec

Post by Harold_V » Tue Nov 14, 2017 4:10 am

I'm not trying to make anyone look bad in this situation, but if you have any experience in honing, you'll understand what I'm talking about. The only purpose in my posting my original comments was in the hopes of avoiding a scrapped piece.

If one is not familiar with working to tenths, it can be a daunting task. You have to pretty much abandon everything you think you know about general machining. That's assuming one hopes to make good parts. If you're not interested in upholding stringent requirements, then, yeah, cast it all to the wind and take your chances.

It should be noted that the first thou, often more, goes away almost instantly in a bored hole when honed. That's one of the reasons why it's a good idea to avoid a spring loaded cylinder hone, as it isn't selective, and has no ability to correct an erroneous bore, very unlike a rigid hone.

It should be understood that a machined finish behaves totally differently from a continuous surface, as the peaks of the ridges are removed almost instantly. The end result will generally be less than acceptable, unless enough material is allowed for a honing operation, and then only with a rigid hone.

One thing to consider. If the bore has visible ridges, and it's only a couple tenths under, it may perform perfectly well with a pressed bearing, as the ridges present almost no resistance to being swaged, especially in brass. I'd chance that long before I'd put a hone in the bore, pretty much guaranteeing it will end up a disaster. If the bearing, once pressed, isn't lumpy, it most likely will perform exactly as desired.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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wlw-19958
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Re: Final Bore is under spec

Post by wlw-19958 » Tue Nov 14, 2017 10:59 am

Hi There,
Harold_V wrote:I'm not trying to make anyone look bad in this situation...
Oh come now. When it comes to making one
look bad, you are like a pickle.

Good Luck!
-Blue Chips-
Webb

Harold_V
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Re: Final Bore is under spec

Post by Harold_V » Tue Nov 14, 2017 5:01 pm

wlw-19958 wrote:Hi There,
Harold_V wrote:I'm not trying to make anyone look bad in this situation...
Oh come now. When it comes to making one
look bad, you are like a pickle.

Good Luck!
-Blue Chips-
Webb
Chuckle!
Well, if that's what it takes to get others to pay attention to guidance that will keep them from making scrap, so be it. I'm not new to this type of work, and have more than my share of battle scars that have provided me with the wisdom to avoid bad choices.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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SteveM
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Re: Final Bore is under spec

Post by SteveM » Wed Nov 15, 2017 9:55 pm

Harold_V wrote:It should be noted that the first thou, often more, goes away almost instantly in a bored hole when honed. That's one of the reasons why it's a good idea to avoid a spring loaded cylinder hone, as it isn't selective, and has no ability to correct an erroneous bore, very unlike a rigid hone.
That's why I suggested a lap. You would be better able to control the final size.

I've been surprised at how much just abrasive paper will remove on a bore, as all it is doing is removing the high spots.

Steve

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mcostello
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Re: Final Bore is under spec

Post by mcostello » Wed Nov 15, 2017 10:04 pm

What ever You do try about 3 revolutions first and check. 2 tenths can disappear quickly.

Harold_V
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Re: Final Bore is under spec

Post by Harold_V » Thu Nov 16, 2017 1:55 am

SteveM wrote:That's why I suggested a lap. You would be better able to control the final size.
I agree, but most folks don't have a lap, and the fix provided will happen much faster than making one. It also goes a long ways towards helping one understand the procedure required to successfully work to close tolerance with reliability.

What's really important in this issue is gaining that understanding. If one hopes to achieve reliable results, a different approach is required. Precision takes more time, and is often a difficult process for those who don't understand the approach to success. The smallest variable can be the cause of failure, and it often isn't obvious to the operator. Uniform cuts, modest feeds and temperature control are all very much a part of the degree of success one can hope to achieve.

Precise measurements are also mandatory. One hits size by chance if the exact measurement isn't known. That's why I asked how the dial bore was set. It may provide repeatable readings, but they may or may not be the actual size if not set by a ring. I've been embarrassed by that issue more than once in my life.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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