Bore Gauge

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Interesting concept !

The reason that I went with three balls is the self centering. Two certainly makes construction simpler.

Also using a 40 tpi micrometer thread gives 25 graduations on a dial. I didn't get 1/1000" per division, but it did make it more comparative.

The ball size verses slope makes calculation difficult because you have to take the slope distance into account. I'm crap at math...
 
The problem with this measuring tool is that the measuring range is small .
 
Here's a tool I made years ago. I thought I had posted it but couldn't find it. It consists of a shaft with a key and a slotted end to accept whatever DTI you have. Most of the small ones have a dovetail on the side and top. The main arm with the ball on the the end is reversible for large and small holes. It has a secondary block with a vernier screw for fine adjustments. It's nothing more than a DTI snap gauge. You put the indicator tip into the bore and slide the arm up until it touches then lock the vernier block. Now you adjust it until you get -0-. Withdraw it from the bore and measure across the ball tips until you get -0-. If you don't like using telescoping gauges (snap gauges) then this one is extremely accurate.
 

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Hi Minh-thanh,
Which one do you mean ?
Mine, the one in the picture, only has a range of 3 mm. Primarily governed by the size of the balls used.
Mine only has a 2.5mm measuring range .

@gbritnell .
it's a great tool
Maybe I'll make a similar tool .
 
Hi Guys,

I said that my maths was rubbish. :rolleyes:

I've been chatting to a math teacher, now retired. I was chatting about the bore gauge that I had made and he pointed out that a slope of 45 degrees is a rise of 1:1, and that I had made it hard for myself by trying to work in fractions of an inch.

Now thinking about it, I now understand that if I had used millimetres instead of inches, 1 mm of movement of the taper would have resulted in 1 mm expansion of the balls regardless of the ball diameter. However this would not accurately measure the bore diameter without taking the expanded diameter of the three balls into account.
 
Here's a tool I made years ago. I thought I had posted it but couldn't find it. It consists of a shaft with a key and a slotted end to accept whatever DTI you have. Most of the small ones have a dovetail on the side and top. The main arm with the ball on the the end is reversible for large and small holes. It has a secondary block with a vernier screw for fine adjustments. It's nothing more than a DTI snap gauge. You put the indicator tip into the bore and slide the arm up until it touches then lock the vernier block. Now you adjust it until you get -0-. Withdraw it from the bore and measure across the ball tips until you get -0-. If you don't like using telescoping gauges (snap gauges) then this one is extremely accurate.

Whilst all these gauges are essentially just comparative instrument, I was attempting to make a direct reading device.
 
Hi Guys,

I've not been able to find the original drawing so I've re-drawn it.
Bore_min-max.png

On the left looking into the measurement end, the grey circles represent the balls. I've used 6 mm diameter for them as an example. The minimum measurement diameter is 13 mm.

On the right the small blue circles represent the maximum expanded position of the balls. This gives a 19 mm maximum measurement diameter.

The green circles represent the minimum and maximum diameter of the taper.

Hope this helps !
 
Gordon, wrote in the thread, Homemade measuring tool inner diameter

So how do you measure the diameter with three contact points? You said with a caliper but a caliper will not measure over three points.

Hi Gordon, Guys,

The idea of three contact points is that the device is self centering.
generally you would use a calibrated reference ring and the amount of movement of a taper pushing the three balls outwards.

In my device the idea is to use a micrometer movement to make a direct measurement of the unknown bore. The device has a calibrated taper that is pushing the three balls outwards. Knowing the taper angle and the ball diameter will/should give you a direct measurement.

The drawings posted earlier should make it more clear and explain things.
 
I would argue that you'd have a near impossible time trying to build it and set it (calibrate) initially without a ring gauge.

After that you can take direct readings based on the taper angle and the thread pitch chosen, within the tolerances of your construction.

45 degrees gets you 1:1 as you say for thread pitch movement to ball movement, on the radius, not the diameter. So this would be a pretty coarse measurement with say 40 divisions per rotation on a micrometer barrel as one turn of the screw results in 2mm change in diameter. 2/40=0.05mm (~0.002")

A better taper angle would be 4:1 or ~14.036 degrees. so every unit down results in 1/4 unit radius or 1/2 unit diameter change.
keeping your 1mm pitch and using 50 divisions (barrel starts to get bigger or markings smaller) would yield 0.5/50=0.01mm (~0.0004") measurement or 5 times more resolution than the 45 degrees and 1mm pitch.

Taking it one step further, change the screw pitch to 0.5 and again you increase resolution to 0.005mm per division which for the imperial crowd is close to 0.0002" resolution.

Like anything, there are trade offs. More resolution makes it slower to use over a large range, and the shallow taper will make it a longer tool with more projection past the balls if you want a large range, reducing the ability to use in blind or stepped diameter holes.
Also, the balls don't really matter except that they end up contributing to determining the minimum bore that can be measured (along with the body of the tool and the taper shaft).

Then there is the possibility of adding an vernier scale to improve the resolution, again at the added complication of making it.

Fun stuff for a thought exercise, but I'll stick to buying one, or making a plug gauge when needed for my ultra low volume work, or making Georges DTI snap gauge - that looks like a great tool for all sorts of measurements.
 
I would argue that you'd have a near impossible time trying to build it and set it (calibrate) initially without a ring gauge.

After that you can take direct readings based on the taper angle and the thread pitch chosen, within the tolerances of your construction.
Hi Ninefinger,
Thanks for your constructive post.
You can see in the pictures that I used a 13 mm ring gauge for the initial testing.
45 degrees gets you 1:1 as you say for thread pitch movement to ball movement, on the radius, not the diameter. So this would be a pretty coarse measurement with say 40 divisions per rotation on a micrometer barrel as one turn of the screw results in 2mm change in diameter. 2/40=0.05mm (~0.002")

A better taper angle would be 4:1 or ~14.036 degrees. so every unit down results in 1/4 unit radius or 1/2 unit diameter change.
keeping your 1mm pitch and using 50 divisions (barrel starts to get bigger or markings smaller) would yield 0.5/50=0.01mm (~0.0004") measurement or 5 times more resolution than the 45 degrees and 1mm pitch.
An initial taper angle of 14 degrees was used, but as you pointed out, it took a lot of turns with a micrometer head. I've considered re-designing it using an M6 x 1 or an M10 x 1 thread with a large calibrated dial.
Taking it one step further, change the screw pitch to 0.5 and again you increase resolution to 0.005mm per division which for the imperial crowd is close to 0.0002" resolution.

Like anything, there are trade offs. More resolution makes it slower to use over a large range, and the shallow taper will make it a longer tool with more projection past the balls if you want a large range, reducing the ability to use in blind or stepped diameter holes.
Also, the balls don't really matter except that they end up contributing to determining the minimum bore that can be measured (along with the body of the tool and the taper shaft).

Then there is the possibility of adding an vernier scale to improve the resolution, again at the added complication of making it.

Fun stuff for a thought exercise, but I'll stick to buying one, or making a plug gauge when needed for my ultra low volume work, or making Georges DTI snap gauge - that looks like a great tool for all sorts of measurements.
Yes the steel ball size does make the minimum bore diameter around 13 mm. A flaw with using balls is that you cannot push them out more than half their diameter without increasing the diameter of the measuring head, or you loose the support for them. Also you have to take into account the space in between the the three balls. Which was why the end of the taper started at 1 mm diameter.

Thank you again for your comments, they certainly add food for thought.
 
I would argue that you'd have a near impossible time trying to build it and set it (calibrate) initially without a ring gauge.


Me being a right proper cheap sob found that using bearings instead of a ring gauge - - - pretty much worked.

A ring gauge is likely a better idea except it was so bloody easy to get bearings in so many different sizes and ring gauges - - - not so much (and their cost - - -ouch).
 
Me being a right proper cheap sob found that using bearings instead of a ring gauge - - - pretty much worked.

A ring gauge is likely a better idea except it was so bloody easy to get bearings in so many different sizes and ring gauges - - - not so much (and their cost - - -ouch).
Hi Joe,
I agree, ball races are manufactured to extremely close tolerances ! Since the manufacturer states the bore diameter and the tolerance, there is no reason not to take advantage of that.

In fact I've used ball races a number of times for precision measurements. Also very useful when using a caliper, as a second check.
 
Hi Joe,
I agree, ball races are manufactured to extremely close tolerances ! Since the manufacturer states the bore diameter and the tolerance, there is no reason not to take advantage of that.

In fact I've used ball races a number of times for precision measurements. Also very useful when using a caliper, as a second check.
The trouble is they are very flexible, and go out of shape as soon as look at them. There is a reason that ring gauges are so substantial.
 
The trouble is they are very flexible, and go out of shape as soon as look at them. There is a reason that ring gauges are so substantial.


If you're finding that bearing races are 'very flexible' when you're trying to use them as a pseudo setting ring gauge I would suggest that you've not got a very 'nice' technique for setting your bore gauge or other form of measurement.

In my experience - - - by the time I got the feel of finding the diameter points on a cylinder is when I also found that I was getting far more accurate readings using my calipers. A more careful touch that made using those pesky 'snap' gauges more accurate settled in at about the same time. (It didn't hurt getting an opportunity to by a set of Tesa micrometers where I could actually feel more of the differences - - - there action was/is just so smooth!!) There are just some things in life that experience trumps theory on hugely - - - and, imo anyway, precision measurement is one of them.
 
If you're finding that bearing races are 'very flexible' when you're trying to use them as a pseudo setting ring gauge I would suggest that you've not got a very 'nice' technique for setting your bore gauge or other form of measurement.

In my experience - - - by the time I got the feel of finding the diameter points on a cylinder is when I also found that I was getting far more accurate readings using my calipers. A more careful touch that made using those pesky 'snap' gauges more accurate settled in at about the same time. (It didn't hurt getting an opportunity to by a set of Tesa micrometers where I could actually feel more of the differences - - - there action was/is just so smooth!!) There are just some things in life that experience trumps theory on hugely - - - and, imo anyway, precision measurement is one of them.
Agree re touch and feel, I have had this very well since my apprenticeship.

I was just warning the uninitiated that bearings are more flexible than they may first seem.
 
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