Another version.
Homemade measuring tool inner diameter
Homemade measuring tool inner diameter
Hi Minh-thanh,The problem with this measuring tool is that the measuring range is small .
Mine only has a 2.5mm measuring range .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.
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.
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 Ninefinger,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.
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.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.
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.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.
Hi Joe,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).
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.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.
Agree re touch and feel, I have had this very well since my apprenticeship.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.
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