Repeatability of Mill Vise & Stop?

I am milling a part I want to fit reasonably closely to something (close slip fit, ideally). Something like -0.000"/+0.001". I have it clamped in
my mill vise, and I have a stop set to locate one end. I'm milling out a small pocket, and the best I can do for measuring the width & length is a digital caliper. The thing it fits on is too big to try in place.
Thus far, I haven't taken it out of the mill. I few more passes, and it should be done. My question is whether I have a prayer of taking it out & returning it with adequate precision if it ends up a bit tight? I was figuring that if it doesn't quite fit, I could count on it not locating exactly the same. That way another pass with the same settings should open it up a hair.
My experience says that I should (barring chips getting in the way) be able to relocate the part within 0.001", but I'm not sure how much better than that I can count on. Depth isn't as critical, so a little "vice lift" won't kill me.
Comments? Suggestions?
Thanks!
Doug White
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If you are only making one part and it turns out too tight, then get out your files and make it fit. If you are making many of the item, you might invest in DRO for your mill.
Paul
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wrote:

I ended up cutting the most critical dimension a mil oversized & filing it to fit. A DRO doesn't really help here. I can read the dials on my mill to less than half a mil. Knowing where the table is is only part of the equation. As it turns out, I ended up cutting the slip fit about 1 mil larger than I'd wanted. This was without moving the piece. I suspect the problem was flex in the mill. It's an old Clausing 8520, and although I was taking light climb mill cuts, I'd been taking 5 mils off per pass quite reliably until I got close. Then I tried to take off 2 mils, and it took off ~ half a mil more than I'd expected per side.
Live and learn. The thing works fine, and I know more than when I started. All in all, a successful project.
Doug White
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My 8525 misbehaves that way sometimes. I have better luck when I surface-grind the reference faces of the block after milling, and pad the moving jaw with wood. It's more precise if I tighten the knee and quill locks.
jsw
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With power off, back off the cutter a little and advance it until it barely pinches a strip of paper. After you check the fit you can see if it still pinches at the same dial setting.
If you blacken the cut surface with a marker you can detect very light contact with the spinning cutter.
jsw
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    [ ... ]

    What is good for that is rolling papers. They are quite thin (I think 0.001" but double check with a micrometer first) and you can wet one before sticking it in place, then advance carefully until it moves from where it was stuck.

    Indeed so.
    Enjoy,         DoN.
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wrote:

You should be able to locate an edge to within half a thou with one of these: http://www.use-enco.com/CGI/INSRIT?PMAKA$0-0127&PMPXNOH22122&PARTPG=INLMK3
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    [ ... ]

    [ ... ]

    Yes -- and the final question is -- what is the actual size of the end mill, once you have set things with the edge finder. You know that the edge finder is 3/8" or 0.200" diameter (these) or I have an older one which is 0.500" -- but if the mills have been resharpened, you need to take that size into account.
    The prices on those edge finders look pretty good.
    Enjoy,         DoN.
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Doug White writes:

Then you must make a gage. Shop-made gages, jigs, and fixtures are a critical part of competent metalworking. You are very limited if you only work with what you can buy from a catalog.
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Doug White wrote:

Usually the back edge of your mill vise is not going to move on you.
You will get a bit of lift when you tighten the vice but a little tap with a mallet will unually bring your part snug against the paralells.
The position against the stop is usually consistant so yo should not have too much trouble there.
Also since it is a slot, can you use a pin to measure the width of the slot? That might be easier than your calipers. Also a pin that is a few thou under size can be used a feeler gauge to measure.
Hope this helps.
Roger Shoaf
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    [ ... ]

    I would suggest that you take a set of fine adjust inside and outside calipers (spring hinge and a knurled nut on a fine screw) and use that to transfer the fit from the workpiece pocket to the part which it fits. It takes a bit of "feel" but that should be better than the digital calipers overall. That is how machinists made things to fit in the old days. If you have two pair of each style, you can set the outside calipers to fit both dimensions on the thing you are fitting, and then transfer to the inside calipers for both dimensions and use those to check the fit.
    Good Luck,         DoN.
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wrote:

Don, is there any place for using telescopic gauges in this instance? Personally, I have had limited luck in transferring dimension from outside to inside calipers and vice versa. I have taken to setting the dimension on the micrometer (or use the micrometer to measure the male part of the assembly), set my telescopic gauge to that dimension and then us it to check the part being machined.
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Michael Koblic,
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wrote:

I like the caliper idea. I'm not sure I have one of each, and my recollection is the one(s?) I have are fairly large. The pocket was narrow & shallow, so it might have been a bit tricky to fit an inside one down in the pocket. I've used telescoping gages in the past, but the pocket was definitely too small in this case.
I'll have to add a matching pair of small calipers to my Xmas list. My wife is always beating up on me because she doesn't know what to get me. If I decide I really need/want something, I usually just go & buy the damn thing. The less expensive it is, the more likely I'll snag one within a day or two of identifying a need. That makes it tricky when folks are trying to buy gifts with any sort of lead time.
When I was a kid growing up in New Mexico, there was a two week delay built into almost every project, and it used to drive me nuts. I'd send out an order via the mail, and wait for the postman to bring the order back, hopefully correct & complete. Now, I can fire off an order to MSC and expect it to be delivered in one day. Amazon Prime gets stuff here in 2 days with no shipping charges. If I'm impatient (who, me?) I can track every step of it's progress with on-line tracking. I am SO spoiled.
"The problem with instant gratification is that it takes too long..."
Doug White
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[...]

Would even these be too small?
http://www.use-enco.com/CGI/INSRIT?PMAKAa5-6260&PMPXNO0942&PARTPG=INLMK3

Best thing ever is to teach people not to get hung up on this Christmas thing. Here it is routine to get Christmas presents in September or March. The September ones are often accompanied by some suitable statement such as "we thought you looked a bit ill and were not sure you would last..."

What planet is this?
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Michael Koblic,
Campbell River, BC
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The smallest of these might just work, and I'd forgotten that I had a similar set. Thanks for the reminder!
Doug White
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    [ ... ]

    I prefer the ones which are flat bottomed starting at the equator. Easier to measure shallow grooves. The best ones I have are an old Lufkin set. Everyone else has more meat on the underside of the equator.
    And you never actually said how small the pocket needed to be. That might have changed quite a few of the answers. If it was too small for the telescoping gauges, but would likely work with the smallest of the small hole gauges, it is pretty small, and I would likely not have suggested the calipers.
    Enjoy,         DoN.
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<snip>

&PARTPG>>> INLMK3

small
The pocket is ~ 0.10" deep, 0.240" wide, and ~ 1.25" long. Too shallow & too narrow for a typical telescoping gage, but the truncated small hole gages would probably have worked. The width was the place I wanted a close fit. The pocket is actually open on one end, and the critical dimension there was the thickness of the wall on the far end. That I cut slightly oversized & filed to fit.
Doug White
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    [ ... ]

    If you have them -- and can get them to work well for you -- *and* if they fit where you need to measure -- sure.

    There are people who have trouble getting repeatable measurements with telescoping gauge, too.
    With both -- it is a matter of developing the right feel to get repeatable measurements or dimension transfers.
    Granted -- something which gives you numbers makes it easier to figure out how much to advance the cutter for the next pass. (But with the standard calipers, you can get similar information (in a non-numeric form) by the angle of the clipers when you start feeling contact with both sides. (Of course, this will change with the size of the dimension you are measuring.)

    Hmm ... another way to do this is to use some adjustable parallels -- find one which fits reasonably in the pocket, adjust it out to make firm contact with the walls of the pocket, and measure the part which is projecting from the pocket using your micrometer. (Do I remember you asking what adjustable parallels are for in an earlier exchange?)
    Here is a web site showing the use of adjustable parallels to measure an internal dimension:
<http://www.wttool.com/product-exec/product_id/16863?utm_medium=cpc&utm_source=froog>
    Anyway -- there are always multiple ways to accomplish something in machining -- and in many other things as well. The trick is to find one which works for you.
    Enjoy,         DoN.
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