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lwalker
10-23-2007, 12:36 PM
A few days ago I needed to make a small pin. I don't have a lathe, so I cutoff a 1.5" length of 12L14, put it in a 1/2" collet and placed that up in the spindle of the mini-mill with a lathe tool held in a vise.

Final diameter was about 0.185 or something like that. I carefully measured the hole's ID with dial caliper and got to work turning down the bar. Stopped a few times to measure, double checked measurements against hole dia, and when the pin was about 0.002 oversize I took it out to double check fit since I figured I could use sandpaper to get the remaining bit down and have a nice snug fit.

The pin slid easily into the hole! Remember it should have been .002 over at this point. When I was done cursing myself, I measured again. Everything checked out. Hole was 0.185, pin was 0.187 ??? WTF!!!

OK, so figured out later that the caliper was not measuring the real diameter of the hole because it was touching on the edges of the (inside) jaws, not the flats. So my question is what tool should I have been using? I have seen inside calipers for large holes, should I get a small pair for tiny holes, or is there some other tool that's better for this?
Couldn't use the outside jaws of the caliper because the hole is chamfered on both sides.

DR
10-23-2007, 12:42 PM
For small holes like that a set of gage pins are the only practical way.

BTW, calipers are notoriously inaccurate for precision measuring.

jimmstruk
10-23-2007, 12:52 PM
Check out E-BAY #280165035379 for an inexpensive set of pin guages. JIM

tattoomike68
10-23-2007, 12:57 PM
I have had good luck with small hole gauges.

http://cgi.ebay.com/4-PC-SMALL-PRECISION-HOLE-GAUGE-BORE-GAGE-TOOL-SET_W0QQitemZ320171345648QQihZ011QQcategoryZ58231Q QssPageNameZWDVWQQrdZ1QQcmdZViewItem

mechanicalmagic
10-23-2007, 01:02 PM
I have an inside micrometer, MUCH better than a caliper.

http://www.kbctools.com/usa/Navigation/NavPDF.cfm?PDFPage=629

But, they only go down to .200"

Pin guage is about the only way to go at your diameter.

Or, you can make a stepped guage.

heavysteamer
10-23-2007, 01:32 PM
I too use small hole gages. That way I can use the same mike on the gage and the pin.

Carld
10-23-2007, 02:32 PM
I too use Starrett small hole adjustable gauges. They are a split ball with an expander in the middle. Use them like telescoping bore gauges.

SGW
10-23-2007, 03:17 PM
I also use the Starrett expanding ball gages, although it takes a while to develop the proper feel for them -- just like the telescoping gages. It helps to drill/ream a few test holes you know the diameter of, and measure them for practice. The feel you get in a particular hole depends in part on the surface finish, too, which adds another variable into the equation.

DR's right -- the most accurate way is with a set of pin gages. Even with those though, if the hole surface is rough...what "diameter" are you measuring?

Bmyers
10-23-2007, 03:52 PM
what is the difference between plus tolerance and minus on pin gages?

rkepler
10-23-2007, 06:16 PM
what is the difference between plus tolerance and minus on pin gages?

The pins gage isn't exactly on size, it's going to be a little over or a little under based on the class of gage. For a ZZ class gage the tolerance is .0002", so a 1/2" plus gage in ZZ will be between .5000 and .5002", a minus would be between .4998 and .5000"

There is a type of gage for holes smaller than the normal ball end gage will measure:

http://www.kepler-eng.com/images/small_hole_gage.jpg

This is one from a set by Moore & Wright (I've seen them in the SPI catalog as well) and is intended to allow measuring holes from .0625 to .080, the rest of the set extends the range to the bottom end of the ball end gages. These are similar in that there are balls and a tapered pin that drives them apart to engage the sides of the hole. It takes a pretty delicate feel with these little guys.

lwalker
10-23-2007, 07:40 PM
So is using one just a matter of going through a range of sizes until one doesn't fit and so the hole diameter is between the current one and the last one that fit?

I looked at a few sets online, but they didn't say what the progression was from one pin size to the next. .001? .0005? .0001?

oldtiffie
10-23-2007, 07:59 PM
Try these.
http://i200.photobucket.com/albums/aa294/oldtiffie/idmeasdevice1.jpg

The "ball" type - for smaller ID's.

They are "tops" and require only a little practice (on known hole sizes) and you will get and keep to within 0.001" and with more practice (not much) down to +/- 0.0005" - and better, repeatedly.

But a caution: try to check your "feel" on a "known" bore/hole size every time before you use them on the "real thing".

I use my spring calipers next (with a micrometer) next - +/- 0.0005" (and better) easily.

Telescopic type are next but can be hard to get "set" so I leave them a little bit "slack" and then use an excellent utility on Marv Klotz's site to "correct/allow" for the "error/slack" - and it is "spot on" (needs "feel" for the job - but so do most things on most jobs).

I hope this helps.

rkepler
10-23-2007, 09:37 PM
So is using one just a matter of going through a range of sizes until one doesn't fit and so the hole diameter is between the current one and the last one that fit?

I looked at a few sets online, but they didn't say what the progression was from one pin size to the next. .001? .0005? .0001?

That's pretty much the way it works. Standard pin sets are by .001", for specialty hole gaging at the .0001" you turn to Deltronic pin gages. They normally come in sets of 20 or so covering the hole size and all tenths around it. Typically class XX, meaning .00004 or better on diameter.

darryl
10-24-2007, 01:19 AM
I'm not surprised that so many different gauges exist for so many different tasks. I suppose that is the way to go- but I am surprised that nobody mentioned the fact that calipers can measure inside diameters reasonably well if you take into account the overlap of the jaws. They have to overlap or they could jam when the caliper is closed all the way. All my verniers and dial calipers have some overlap, typically about 2 to 3 thou. I haven't checked that with the digitals, but I would expect the same.

You must be careful to hold the calipers properly and 'feel' the fit of the jaws on an inside measurement in order to get repeatable results, but you can do so.

Iwalker, I would bet that if you bored a hole about 2 thou undersize according to your caliper, you'd find that the pin you're boring the hole for would fit. You'd also find that other sized holes would measure out the same, in other words they would all be about 2 thou larger than the measurement shows. Easy enough to add 2 thou to your reading to get the actual hole diameter.

I'm not claiming this to be the better way to measure hole diameters, as obviously other tools exist to obtain precise readings, but it works. In many cases, you'll be using some method other than boring to bring the hole to some exact size, if that is required, so if you can get the hole to within a thou or so small of final size using a caliper (and it's correction factor, 2 thou I believe in your case) then a reamer or a d-bit would be the final tool you'd want to use anyway. If you're simply boring to final size, you'd be checking fit once you're down to the nitty gritty anyway, so being able to measure a hole diameter to a fine degree of accuracy isn't going to assure that you get the exact hole diameter you want.

Many factors are involved- one is bellmouthing, which is easy to do, another is the often unknown depth of cut you'll be getting from the boring cutter in the final stages. It's very easy to dial in say, one thou, and then find that the hole has increased by 3 thou, when you expected 2 thou.

Find yourself a few examples of pins and bushings that are a very close fit, and measure ods and ids with your caliper. You'll probably find the difference in readings to be fairly consistent. Then you'll know the correction factor for that caliper.

oldtiffie
10-24-2007, 01:56 AM
If your final size is 0.185" (pretty close to 0.1875 = 3/16"???) then try using your lathe to make a pin with a taper from 0.180>0.185" over say 1" and parallel to 0.185" for a short length.

You will be able to "eye-in" your sizes as it will "rise" 0.001" per 1/5" ("zero" = 0.180, 1/5 = 0.181; 2/5 = 0.182; 3/5 = 0.183; 4/5 = 0.184; 5/5 (end) = 0.185".

If you really want to get "fussy" then you can make it into a "Go/No-go" guage by making the end/parallel bit to the limits of what you require - eg part nearest the taper 0.1845/0.1847" and the larger (next) to say 0.1849/0.1850. Adjust these sizes or limits to suit the job.

Use an an accurate and correctly "set/zero-ed" 0.1" micrometer for preference as getting accuracy to 0.0001" or 0.0002" (1 or 2 "tenths" is relatively easy).

Its a good turning job that will work and you will get a lot of satisfaction from it.

Oh, and get the surface finish as good as you can - just a light final "rub" with USED ie not "sharp" - "wet and dry" (used "dry") will do fine.

You might be surprised at just how accurate this can be - and it will beat a caliper of any type "hands-down" - every time.

It won't cost you any money and just a little time.

Don't get concerned as there is a whole lots of "making things for making things" going on - as just about anyone on this forum will tell you.

I hope this helps.

lwalker
10-24-2007, 02:11 PM
Thanks for all the help.

I like the idea of making a stepped pin for gaging and I'll probably do that if only just for the practice.
Since I don't have a lathe yet (this was done using the mill), the only practical way I can think of to turn a taper would be to rotate the mill head, and since it's an X2 mini-mill, that means tilting the entire column! Easier to just make steps.

oldtiffie
10-24-2007, 09:20 PM
Thanks for all the help.

I like the idea of making a stepped pin for gaging and I'll probably do that if only just for the practice.
Since I don't have a lathe yet (this was done using the mill), the only practical way I can think of to turn a taper would be to rotate the mill head, and since it's an X2 mini-mill, that means tilting the entire column! Easier to just make steps.

Thanks lwalker.

Using the mill as a lathe is quite practical.

I only suggested the taper as I didn't realise that you didn't have a lathe.

Why not make 2? One "stepped" as you say ans the other a taper (use a fine file - surprising how accurate and easy it is with a bit of patience and perseverance - "broken-in" fine "wet and dry" paper works well too).

Let's know how you go.

I hope it helps.

BadDog
10-24-2007, 10:33 PM
I've used the "turn a stepped plug" a numerous occasions. Works quite well.

Shaidorsai
10-25-2007, 01:00 AM
"Cerrosafe" or other low melting temp alloy can work well in some cases. Melt it in a double boiler arrangement over boiling water. Please don't use food utensils. Build a modeling clay dam around the hole. Plug it with a bit of match wood if it is a through-hole. If possible, warm the substrate a bit to keep the alloy from freezing to soon. Pour it in and add a handle of some kind to the sprue before it sets. I usually use a small key ring. Let it cool to room temp, then pull the casting and measure it with a mic. Depending on the alloy you use, the alloy may shrink slightly, or it may even expand a bit, but it should be close. It probably would be best to limit this technique to things that you could readily heat to a high enough temp to melt out the alloy if the plug locks in a hole (rough surface, tapered - the wrong way of course, etc.) Also there is probably a size limit based on the surface tension of the liquid alloy. A hole could be too small to allow the alloy to run in to it. The smallest hole I have used this on is about .085. Do two holes this way. Partially extract the plug from each hole, and you can get a very accurate center-to-center measurement. Won't work in all cases, as the hole needs to be smooth, clean, and tapered the right way if at all. If the accuracy you need would be adversly impacted by CTE errors at these temperatures (160-180 F), this method may not be useful to you.

oldtiffie
10-25-2007, 01:18 AM
"Cerrosafe" or other low melting temp alloy can work well in some cases. Melt it in a double boiler arrangement over boiling water. Please don't use food utensils. Build a modeling clay dam around the hole. Plug it with a bit of match wood if it is a through-hole. If possible, warm the substrate a bit to keep the alloy from freezing to soon. Pour it in and add a handle of some kind to the sprue before it sets. I usually use a small key ring. Let it cool to room temp, then pull the casting and measure it with a mic. Depending on the alloy you use, the alloy may shrink slightly, or it may even expand a bit, but it should be close. It probably would be best to limit this technique to things that you could readily heat to a high enough temp to melt out the alloy if the plug locks in a hole (rough surface, tapered - the wrong way of course, etc.) Also there is probably a size limit based on the surface tension of the liquid alloy. A hole could be too small to allow the alloy to run in to it. The smallest hole I have used this on is about .085. Do two holes this way. Partially extract the plug from each hole, and you can get a very accurate center-to-center measurement. Won't work in all cases, as the hole needs to be smooth, clean, and tapered the right way if at all. If the accuracy you need would be adversly impacted by CTE errors at these temperatures (160-180 F), this method may not be useful to you.


Thanks Shaidorsai.

I wish I'd have known or thought of that recently - great hint/help.

I had trouble with awkward "centre distance problem". I solved it after waste time and "heart-burn" that I could have done without.

"Cerrosafe" would have done the job nicely as you describe it.

Note to self: check for supply in OZ.

speedy
10-25-2007, 02:11 AM
I was about to suggest using a standard taper pin as a rough and ready gauge. I have a small supply handy and have used them in the past when I was in a squeeze. It gives a measurement that is nearabouts (with the eyeometer or mic)and quite often that is enough to suffice.

Paul Alciatore
10-25-2007, 02:33 AM
.....
BTW, calipers are notoriously inaccurate for precision measuring.

I have found that all calipers that I have used have had a few thousanths difference between the outside jaws and the inside jaws and the depth rod. One set I have had a 0.003" difference on the depth measurement. I closed them tight and ground the back end for a better zero there.

As for the inside jaws, they can easily be a thousanth or two off, but even if they are dead nuts, they will have a small flat face. That flat will contact a round hole at the two edges and span the arc of the hole as a chord. As the hole gets smaller and the curvature increases, the error from this chordial effect will increase. So even the best calipers will not make an accurate measurement of a small hole.

In larger holes, you must take care to wiggle the caliper in every direction while gradually increasing the pressure on the jaws and watching for the largest reading. It is often difficult to remove the jaws from the hole with that "true" reading still showing. Most often the act of removing the jaws from the hole will "kick" them down a few thousanths.

I would say that calipers are notouiously difficult to use for internal measurements and the difficulty increases as the size goes down. I would use the gauge pins when more accurate measurements are called for.

Pete H
10-25-2007, 12:51 PM
The other name for it is "Wood's Metal". Scientific supply houses - the kind that serve schools, mainly - used to list it. McMaster-Carr lists several different kinds, for U$ 25-40 per pound - a lump about the size of a granola bar. They call it "Casting Alloy". It contains lead and bismuth - not good for you. Don't smoke while handling it.

There's a non-toxic substitute called "Field's Metal". I didn't see that listed.

I recall being told years back that you could use melted sulfur the same way, except that sulfur is brittle when it hardens. (Get it at the pharmacist's - the "yard and garden" variety has other things mixed in.) The melting point is higher, and of course there's the chance of it catching fire. But if you melt it carefully, it probably won't even stink very much.

Pete in NJ

Shaidorsai
10-25-2007, 10:53 PM
I got "Cerrosafe" from Brownell's Gunsmith Supply several years ago. They still have it, I believe. Gunsmiths use it for making castings of rifle and pistol chambers to get accurate measurements for making chamber reamers or diagnosing chamber problems. Unlike Wood's Metal, which expands upon cooling and locks itself into a hole, Cerrosafe will shrink slightly during the first 30 min of cooling, so it will release easily. After an hour, says Brownell's, it will expand to exactly hole size, and after 200 hours it will be about .0025 oversize. In my experience that is an accurate description.

Other handy uses for it have turned up. Use a round, square, or conveniently shaped container and cast an awkwardly shaped part into it with Cerrosafe so you can chuck it in a lathe. Do the deed that needs doing then drop the whole assembly into a pot of near boiling water to remove the finished part. A "liquid chuck" as it were.

I got out of a tight spot once when I needed to know the volume of a small odd shaped cavity very accurately. Warmed it, filled it with Cerrosafe and used a mill to trim it even with the top of the cavity. Dropped it in boiling water and measured the resulting liquid metal in a graduated lab beaker. Instant answer. Could have gotten it closer by casting the removed metal in a mold and using the displacement method by measuring how much water it displaced. Actually the problem was a little more complex than that as I had to machine a specificly sized and contoured hole in the frozen metal to account for a plug that was supposed to partially fill the cavity.

Steve Steven
10-26-2007, 12:39 AM
Good note about using sulfur to get hole size, I use it for rifle chambers. You have to be VERY careful not to heat it up too much, it has two states when liquid. The first (lowest temperature) state it is a light yellow liquid, this is the state you want to have it in when you pour it into the hole. If it goes up a few more degrees in temp, it will turn black and get very thick and cannot be used. Throw it out if it does that, can't be reused.
Steve