Holding +-.0001” on 7/8 bore with cnc lathe

Ianagos · Aug 10, 2021

Hey so the title sums it up pretty well. I have a quote where I need to hold +0 -.0002” on a roughly .875” bore 1.5” deep. Material is 316 stainless

I’ve been able to hold these tolerances on external features but internal features may be a little harder. Qty on the parts is low about 25parts quarterly.

should also mention the machine positions to .00001”

I was thinking 5/8 solid carbide bar for semi finish Would let me hold a few tenths but to inure I hit that +-.0001” I was consider other options.

Internal grinding is one option but the part has almost no way to be refixtured to go on to my cylindrical grinder and I don’t have a high rpm internal head either way.
I did consider grinding on the machine. It has a 12k spindle only though and for the 1/2 or maybe 5/8 stone required I don’t see that really being optimal.

Next thought was finishing to within a few tenths on the lathe then setting up a hone of some sort. But trouble with this comes in the form of no space to setup a sunnen hone although I do see them for sale every few months and also absolutely no experience at all with honing and whether I can even hold those tolerances on a hone. I have drill press I could stick a hone in?

Parts will be checked with a 3point digital bore mic checked against a master ring gauge. I may also pick up a .0001” bore gauge for some additional gauging.

So that’s my thoughts. I know there are some folks that hit these tolerances all day on the forum.

What do you guys recommend?

Mechanola · Aug 10, 2021

Typical hone job in my understanding

Proper hone oil and air gauge. Of course will a Sunnen find room, if the job runs well on it.

FredC · Aug 10, 2021

You might try these guys for a single pass diamond hone. Di-Coat Corporation
800-637-9194

I have 2 of their units and they work quite well for me.

Dan from Oakland · Aug 10, 2021

Hone it, but bigger question is how ya going to measure it? I would imagine that just holding it in your hand might well use up your tolerance. Sunnen mechanical gages to 50 millionths if your part is not too large to get it on the gage.

BobM3 · Aug 10, 2021

I've wondered about using roller burnishing:

What is Roller Burnishing? | Cogsdill

Seems like the pre-burnishing would have to be held to a tight tolerance so I'm not sure you gain anything geometry-wise.

modelmakerblue · Aug 10, 2021

Dan from Oakland said:
Hone it, but bigger question is how ya going to measure it? I would imagine that just holding it in your hand might well use up your tolerance. Sunnen mechanical gages to 50 millionths if your part is not too large to get it on the gage.

This has got to be a honing job, obviously in a temperature controlled environment & checked with decent measuring equipment.
Tony

michiganbuck · Aug 10, 2021

I think a decent reamer for -.0003, a Sunnen hone, an air gauge, and so a master need be bought. Likely hard to re-coup on a 25 piece order.

I once met a guy who made a small fortune bidding on building a fixture, perhaps another grinder, buying gauges and everything on a thousands pieces repair order on a government small part. He said he didn't event want to take the job because he was full-up so bid way high. He found the parts were all near dead on at -.0003 or so and ran them on a Sunnen machine for like a 1234 (or what) swipes to make good the part.
I forget now but I think he said he was making a thousand or thousands an hour on the job.

Ianagos · Aug 10, 2021

Well seems like honing is the recommended way.

This is a repeat job but seems as though it may not be worth it. 125pcs/year

I figured the digital 3 point mic would be accurate enough to measure it as it reads in .00005” but maybe not.

Seems like an air gauge is the appropriate tool. Anybody got one for sale?

Can the honing be done on the cnc machine possibly? Or setup in a drill press. I really do not want a sunnen hone for this one job…

Ianagos · Aug 10, 2021

barbter said:
:blink:
:blink:

And a 3 point digi mic isn't accurate enough for this (IMHO).
A Diatest bore gauge would probably be (and you can also "feel" the bore with these, but an Air gauge is what i'd be using for a repeat job.
Also, a temperature controlled environment is obviously a biggie, as you're using the ring as a master etc.

Hey it positions to that not necessarily accurate to .00001”.

Took me several days of settling and a .0001” per 10” level to get this thing setup… probably time to check it again

I’ve held +-.0001” on od of parts before but it’s really a pain. Thermals in the machine means I can’t trust it to do better than .0003” without verification.

Also shop is reasonably temperature controlled. Stays 68-75 degrees. On real hot day if door is open a lot ac can struggle.

gappmast · Aug 10, 2021

You will need to hone it twice to hold size because of heat, even if it doesn't feel warm it will shrink. I would make sure you are on the same page with your customer when it comes to size and measuring. It doesn't natter what you measure their measurement it what will sign the pay check.

Sunnen has the PG800 gage that will measure down to 50 mill it should do the job and they lease it won't break the bank, give them a call and check the lease price.

otrlt · Aug 10, 2021

Ianagos said:
should also mention the machine positions to .00001”

Nothing in existence positions to .00001, you also have another problem; assuming that you can position to .00001, can you repeat within .000005?

HuFlungDung · Aug 10, 2021

I find it easier sometimes to do a precision bore on the cnc mill, using this tooling:

Micron Precision Boring Head Set MariTool

The boring head has .002mm adjustment capability, and it seems to actually be capable of such very fine adjustments. I was in fact boring 35mm bearing bores in 316 SS today, and after getting it dialed in to what I wanted, I never noticed any diameter variation in a dozen parts. But, that is my particular combination of tool and mill, so it might not work out for a different machine.

Gordon Heaton · Aug 10, 2021

If you get them close, maybe these guys can hit the spec at reasonable cost: Colonial Bushings, Inc. – Colonial Bushings, Inc.

Under 'Other Services' they say they can hold .0002 'with no issues' which leads me to think they might be able to hit your spec too.

Ianagos · Aug 10, 2021

FredC said:
You might try these guys for a single pass diamond hone. Di-Coat Corporation
800-637-9194

I have 2 of their units and they work quite well for me.

Gonna give them a call here but what does pricing look like on these units? And will yours hold these kinds of tolerances?

Kallam · Aug 10, 2021

Try ball sizing Balltec sells crome steel and carbide balls
in .0001 increments.I had a job to hold a bore .0002 .750 dia
with a 32 micro inch finish. I would bore to about .0005 undersize and press the balls thru the hole each one a little bigger until it was in spec. It also produced a mirror finish.

Bal-tec - Report On Ball Sizing

Conrad Hoffman · Aug 10, 2021

If the part lends itself to honing, it will be almost trivial with a decent Sunnen hone and an air gage and some setting rings. Not cheap for 25 parts unless you job it out to somebody who does that sort of thing, but you'll still pay for the gage head and rings. It's the measuring that's the killer. No experience, but I suspect it could be done on the lathe if you found the right insert, but lots of ways to go wrong. With the Sunnen you just sneak up on it.
Honing

Milland · Aug 10, 2021

Kallam said:
Try ball sizing Balltec sells crome steel and carbide balls
in .0001 increments.I had a job to hold a bore .0002 .750 dia
with a 32 micro inch finish. I would bore to about .0005 undersize and press the balls thru the hole each one a little bigger until it was in spec. It also produced a mirror finish.

Bal-tec - Report On Ball Sizing

The issue with ball sizing is the ball can drift a bit side-to-side depending on local hardness. So even if you measure to spec, you could have unseen "wander" in the cylinder, which is likely a no-no here.

I'm with the honing crowd, ditto those that say check with the customer as to how they plan on measuring the parts. You'll need to copy their method.

Mechanola · Aug 11, 2021

Roller burnishing is another method that might be appropriate. You can reach down to almost full depth of the bore which is not the case with balls and honing stones which actually need to overrun somewhat.

Dan from Oakland · Aug 11, 2021

Once you get a hone in your shop, you will never look back.

Conrad Hoffman · Aug 11, 2021

Well, actually you will. You'll look back to not having a stinky oily mess and a process that's more of an art form than something you can stick a button pusher on. You'll look at a machine that sits unused way too much of the time. You'll wonder why you never ever have the right (expensive) mandrels or stones for the job. I've been honing to stupidly tight tolerances for about 40 years and enjoy the process greatly, but I've never found management or "normal" machinists to be very fond of it. Honing and grinding should be the easiest jobs in the shop, but they're both becoming relics of the past, given what you can do with a CNC machine.

Holding +-.0001” on 7/8 bore with cnc lathe

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