Vernier caliper accuracy

I chose the vernier calipers because I wanted both imperial and metric on the same tool, and I wanted them to last. I hate replacing tools.

I've not seen a three-point bore gauge intended for measuring engine cylinders. All the gauges I found on the market were two-point with a centring device. I picked the one I mentioned because it's complete with all the anvils, locking nuts and the spanner, and is in good condition. There were more accurate gauges available (measured down to 0.0001" instead of 0.0005"), but they were much more worn and had missing parts. Also, I figured that for investigating wear as opposed to reboring, having a 40 thousandths range on the dial might be more useful than a 10 thousandths range. If people are interested, I can take a picture of the bore gauge and calipers.

Best wishes,

Chris

Reply to
Christopher Tidy
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Thanks. That's interesting. I wasn't sure what an air gauge was.

That picture isn't actually my set of calipers, but is a very similar set. Same brand, but shorter and perhaps made a little earlier.

Chris

Reply to
Christopher Tidy

Looks like I might regret buying verniers as I get older :-).

Chris

Reply to
Christopher Tidy

Because the thicker sections, which are the boss areas for the wrist pins (piston pins) expand with much more force than the thin sections. So the pistons have a smaller diameter across the boss area.

Almost all production automobile pistons made today are elliptical. In fact, they're often elliptical with the major axis in one direction at the top of the piston, and in the other direction at the bottom. The bottom ellipse is for better sealing, to meet emission requirements. It has to do with differential friction and heating between the neutral axis, which is parallel to the crankshaft, versus the other axis.

When I was at Wasino we had some drawings from Ford that actually had three different ellipses along their length, from top to bottom, and they had to blend into each other.

If you don't get an expert to chime in here, I'll see what I can dig up for you. There is one guy who stops in here from time to time who is an engineer for one of the world's top piston manufacturers; you won't get any better info than that from him.

If you're eager to search on it yourself, try both "elliptical piston" and "oval piston." They're often, incorrectly, called "oval pistons" in the trade.

Reply to
Ed Huntress

You'll get in the habit of wearing a swing-away loupe on your glasses when you're reading gages, after you pass a certain age.

You original reasons for buying a vernier caliper were good ones. Dial calipers are no more accurate; they're just easier to read. The same is true for the digitals.

I use my TESA vernier caliper when I'm not in a hurry and I want to keep up the skill of reading them. But if I'm doing a lot of measuring, I'll pull out one of my digitals. I've never owned a dial caliper.

Reply to
Ed Huntress

On Tue, 23 Feb 2010 05:04:18 +0000, the infamous Christopher Tidy scrawled the following:

We all do.

-- "Politics is the art of looking for trouble, finding it whether it exists or not, diagnosing it incorrectly, and applying the wrong remedy." -- Ernest Benn

Reply to
Larry Jaques

O.K. There *are* dial calipers with dual scales, and two separately geared pointers -- but you still have the problem of the pinion skipping a tooth shifting the zero point either from a shock or from chips getting into the rack gear in the bar.

Digital are nice for switching between metric and imperial, but have the problem of keeping fresh cells around to power them, as some will kill their cells in six months, and I think a year and a half is pretty long for one to last.

And -- they are more easily damaged -- things like the liquid crystal display are rather fragile -- behind the clear plastic window.

For checking *wear* in the cylinders (as you want) what you got is better -- quicker to use and all. And it lets you measure parallel to and at right angles to the crankshaft to check for oval wear.

For checking a reboring job, I would prefer the three-point, since the boring had *better* be cylindrical. :-) But since this is going to be in an automobile, and you have dissimilar metals between the cylinder and the piston, you have differing thermal coeifficents of expansion, and the temperature range between sitting outside on a really cold day (maybe -40 in some areas, where special lubricants and coolants are also needed, and a bit over 212 F (100 C) would really require a good starting clearance, or it will seize at one extreme or the other.

Any idea what the required starting clearance is between the pistons and the bores? *That* would settle how accurate you need the measurement to be for checking a rebore.

If so -- post it to the dropbox, or on a private web site and post the URL to find it.

Enjoy, DoN.

Reply to
DoN. Nichols

As someone else mentioned, the joint between a properly wrung pair of blocks is on the order of two microinches (0.000002"), so only with the highest grade blocks do you need to take account of the interface thickness.

It is a plate (or a bar -- the more common ones are sine bars) which has a pair of cylindrical surfaces at each end, separated by a precise center distance. The most common is 5.000", though I have one at 2.500", and have seen some offered at 10.000").

For a sine bar, there is no captive base plate, so you set it on a surface plate, and with both cylinders in contact, its top is parallel to the surface plate.

Now -- let's say you want a precise fifteen degrees. O.K. Look up the sine of 15 degrees (0.258819045) and multiply by the length of the bar (5.000"), so you get 1.294095226". Wring a stack of gauge blocks to get 1.2941" and you will get very close to 15 degrees. Calculating back from that, I get 15.000056 degrees +/- a bit given the accuracy of the gauge block set. I stopped the blocks at 1.2941" assuming a cheap Chinese set with +/- 0.000050 accuracy. You can get greater precision with the more expensive and accurate sets.

But -- to get that 1.2941" -- we need to build a stack. Let's see -- start with a 0.1001" block, add a 0.1040" block, then a 0.1900" block, so we are up to 0.3941" and need only 0.9000" to make up our total size. When calculating/building a stack, always start with the last decimal place and work backwards. Here, for example, if you had started at the big end, you would have picked up a 1.0000" block, and when you got the lesser digits you would have discovered that the total was too long.

Now -- a sine plate is like a sine bar, except that it is wider (the sine bar may be 1" wide) and is captive to a base, so you can build the angle and lock it in and then carry it to the magnetic chuck of a surface grinder to grind the desired angle on something mounted to the top (angled) plate.

To see a sine bar -- here is one on MSC's site:

Or MSC part number 85005502 in case the url above turns out to be a temporary one built by my search.

The toe on one end is to keep the workpiece from sliding off the end while measuring.

An example sine plate (much larger and *much* more expensive than the one which I have) is MSC item 08020216

You can see the near roll under the top plate in the image, and a raised block for the zero point, which is a precise height above the base plate. So you can either build blocks above the raised block, or above the base plate - whichever lets you reach your desired height more easily.

Note that there are double sine plates -- a second one hinged at right angles to the first to allow compound angles.

I've used mine to make Acme threading tools to fit a boring bar. I first used the smallest sine bar that I have (2.5" long between centers) to machine a 14 degree angle plate (half of an Acme) guide to hold a HSS tool bit at that angle in a small vise, then mounted the vise on the sine plate, and set the gauge blocks under it for the desired side relief on one side of the bit. This went onto the surface grinder, and was used to make the desired angle on that side. Then I reversed the bit and angle plate, and built a different stack of gauge blocks to provide the different relief angle on the other side of the bit. (This was calculated based on the helix angle of the thread. After this was complete, I put the bit upright in the vise and set a nose relief angle to grind while I ground the nose back for the proper nose width for that particular Acme thread pitch.

Sorry -- yes, 0.100" is a bit large for automotive pistons at least. :-)

O.K.

Good Luck, DoN.

Reply to
DoN. Nichols

Nice description.

A sine bar can also be used to measure an unknown angle, such as the taper on a shaft. Lay the taper on a flat surface and clamp the sine bar on top of it, rolls up. A ground toolmakers vise works well for this. Measure the difference in the heights of the rolls.

If you have only a short-range 0.0001" test indicator and can't measure that much distance, tweak an adjustable parallel to make up the height difference, so the indicator reads the same on both, then measure the parallel with a micrometer.

jsw

Reply to
Jim Wilkins

Thanks!

Aha! A nice trick which I had not thought of.

I've got one much smaller sine bar -- 1.000" between rolls -- which is mounted on a micrometer, with a bar of the same length as the anvil, so it can be used to measure the sine of existing angles within a limited range of diameters.

Of course -- you lose the multiplier effect of a 5" or 10" sine bar, so you don't get the accuracy that you can with one of those, but it is still quite good and a lot more convenient for quick readings.

Enjoy, DoN.

Reply to
DoN. Nichols

Hi Don,

Thanks for the description of the sine plate. Sorry I've taken a few days to reply. Bad week!

Not what I was expecting! I had imagined a steel plate with a surface shaped like a sine wave, sitting on a table (though what you'd use that for, I don't know). Thanks for the explanation. Some day I'll probably need one!

Best wishes,

Chris

Reply to
Christopher Tidy

Actually, it's a big motor lawnmower (80 mm bore).

I've found that figure in the manual. I'll post it along with my measurements in a new thread shortly.

Here's a picture of my actual calipers:

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Best wishes,

Chris

Reply to
Christopher Tidy

Ah, I think I misunderstoood you. I thought you meant a piston shaped like a beer barrel. Instead you mean a piston which is slightly elliptical when viewed from the top or bottom?

The thought I had about vibration only applies to a piston shaped like a beer barrel. And if the piston expands to become a near-perfect cylinder when it's heated, I can see why wear wouldn't be an issue either. Thanks!

Best wishes,

Chris

Reply to
Christopher Tidy

Yes. I wish I still had the 3D versions of those Ford programs that I produced in Rhino, for our promotional material on cutting elliptical pistons. I applied multipliers in Excel to the values in the CAD drawing until you could actually see the shapes with the naked eye.

I don't want to confuse things, but they were shaped a lot lie beer barrels. As I mentioned, there were three different ellipses from top to bottom, with major axes arranged differently, and the appearance from some angles was that they were widest in the middle.

Yes, that's more or less what happens.

Reply to
Ed Huntress

You're welcome.

A lot of that going around -- just from the weather. I hope that is all that yours was.

[ ... description of appearance and use snipped ... ]

Actually -- there is something like that -- but a pair of them go in a milling vise to support the workpiece. They are less prone to topple over when the vise is loosened than some standard parallels, and they can also be positioned to support workpieces with gaps near the edges in some places and nearer the center in others.

I think that they are called "wave parallels". I've never owned any, or had a chance to handle some belonging to someone else, but they strike me as useful for production runs.

If you see one used cheap -- go for it even before you need it. I've gotten a lot of my tools that way against future needs.

Enjoy, DoN.

Reply to
DoN. Nichols

Got it. I can see the shape now. That's complicated!

At least the concept sounds simpler :-)

Chris

Reply to
Christopher Tidy

By the way, Ed, how are elliptical pistons machined? Are they made using a CNC mill, or a special lathe on which the tool can move in phase with the rotation of the workpiece? Just curious...

Chris

Reply to
Christopher Tidy

A darned good question. I wish the piston man would step in here (Anthony?). He's a world-class expert on this subject.

In high-volume production, they've traditionally been turned on special lathes with cam-operated cross slides. That requires a cam for each piston profile. Takisawa makes, or made, a programmable machine that, IIRC, uses solenoid-actuated slides. It's slow. There also have been some piezoelectric actuators. And that was all ten years ago. There may be something new.

I was involved with these things due to a joint project we had going at Wasino lathes, where I worked in those days. We were using a magnetostrictive actuator, made of the material used in submarine sonar. It worked great but programming the hysteresis out of it was a nightmare. The project was dropped about the time I left.

Anyway, the challenge has been to come up with a programmable system that provides adequate thrust at high turning speeds. They're doing it, I think; I just haven't kept up.

Reply to
Ed Huntress

Thanks for the information. Sorry I've been slow to reply; I've been away. I worked myself as part of a team developing electrostatic actuators a few years back. The actuators consisted of a sheet of a rubbery material with conductive coatings on each side. When subjected to a high DC voltage, the conductive coatings could be made to attract or repel, squeezing or stretching the material (the actuation direction was perpendicular to the electric field, as this gave a greater movement). One idea was to use these actuators to move a car wing mirror, although I'm unsure if a prototype was made as I'd left the laboratory by then. But the actuators we had at that time were definitely too flexible to be used for moving a lathe tool.

Best wishes,

Chris

Reply to
Christopher Tidy

It requires a lot of force. The material we used, Terfenol-D, is one of the few that can deliver the force with good speed (several thousand cycles per second with high force; up to 20 kHz at lower force levels, IIRC). Piezoelectrics, which are the other option, are used in stacks to get the required travel, but they're a little fragile for the application:

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Terfenol is the material used in advanced sonar detectors. We were first looking at it as a counter-vibratory attachment, but my boss got the idea that it would work for turning elliptical pistons. We had a demo machine at IMTS 2000, I think, and it attracted a lot of attention. We could get it to work extremely well and with high accuracy and repeatability. But if you make a small change in the program, magnetic hysteresis would complicate the programming, making the whole thing problematic.

Reply to
Ed Huntress

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