Do you need to accurately check bore size......and don't have a bore gage. Try this!

[Danzo187]

Yes, bore gages are expensive, if you need to measure a bore size to within .0001 use the three-pin gage method, all you need to do is use this.
3 PIN GAGE REVERSE CALCULATOR
https://www.holecalc.com/reverse
input (2) any diameter pin gages, enter the bore size and accuracy up to .0000 calculate, the calculator will give you the 3rd pin size you'll need to measure bore.
hope this is a help to new machinist, simple and helpful. Enjoy

 

[homebrewed]

For through-holes whose ID I really care about I make a tapered plug. I set my caliper to the desired ID and run it up the taper until it stops, then mark the location with a felt-tip pen. The taper angle can be as shallow as needed for the task at hand. It makes for a very quick check of my progress. It's not dead on since it's hard to get the calipers exactly perpendicular to the axis of the taper, but close enough to get me pretty close.

From there, I cut and test-fit. I have a plug I regularly use to make my Norman-style QCTP toolholders, just because I need to bore that size of hole on a fairly regular basis.

 

[Canus]

Danzo187 thanks for he link. Great information.

 

[Shiseiji]

It's not dead on since it's hard to get the calipers exactly perpendicular to the axis of the taper, but close enough to get me pretty close.

From there, I cut and test-fit. I have a plug I regularly use to make my Norman-style QCTP toolholders, just because I need to bore that size of hole on a fairly regular basis.

I'm on my 3rd "practice" piece because of not precise enough use of the caliper. Going to give this a try. Thanks!

 

[homebrewed]

If you're measuring the ID using plain calipers, you need to be aware that they usually have a built-in error due to the fact that the blades are not true knife edges, they are flat on the end. As a result you are actually measuring between two chords of the ID, which will indicate a slightly smaller ID than the actual. On a percentage basis this error becomes larger as the ID decreases. Since the "knife-edge" thickness can vary from manufacturer to manufacturer (and likely caliper-to-caliper) it's not possible to get more specific than that....although it should be possible to come up with a calibration factor for your particular caliper(s). This problem caused me all kinds of problems until I figured out what was happening. I was always missing my target ID's despite how much care I took.

The ends of telescoping bore gauges are radiused to avoid this problem. They're not all that expensive, but can be a little tricky to use until you get the hang of it.

 

[toprecyler]

If you're measuring the ID using plain calipers, you need to be aware that they usually have a built-in error due to the fact that the blades are not true knife edges, they are flat on the end. As a result you are actually measuring between two chords of the ID, which will indicate a slightly smaller ID than the actual. On a percentage basis this error becomes larger as the ID decreases. Since the "knife-edge" thickness can vary from manufacturer to manufacturer (and likely caliper-to-caliper) it's not possible to get more specific than that....although it should be possible to come up with a calibration factor for your particular caliper(s). This problem caused me all kinds of problems until I figured out what was happening. I was always missing my target ID's despite how much care I took.

The ends of telescoping bore gauges are radiused to avoid this problem. They're not all that expensive, but can be a little tricky to use until you get the hang of it.

You opened my eyes to this. I’ve noticed that my 6” Mititoyo calipers have a small gap at the inside jaws when looked thru held up to a light.
There is a slight gap at the tips of the OD jaws to. Knowing this, I have to account for a slight difference depending on where I am measuring parts.

I have realized that if I am shooting for a press fit for a bushing, if I make the ID part the same as the OD part, I have a pretty hard press fit.

It seem that the measured ID usually is about .001” smaller on my caliper.

I remember reading in an old book somewhere that a good Machinest will learn his tools, and will learn to compensate for their various shortcomings.

Trick here is to have several ways to calibrate or check your tools. I occasionally will check my calipers and mics against the standards, and 123 blocks, comparing all calipers ( at least 5 in my main tool box drawer) and mics to verify against each other.

I think it’s important to check the calipers at several different lengths because the variance may change at longer lengths, if the beam is slightly bent. I know I have dropped my calipers at least once or twice.


Sent from my iPhone using Tapatalk

 

[Danzo187]

If you're measuring the ID using plain calipers, you need to be aware that they usually have a built-in error due to the fact that the blades are not true knife edges, they are flat on the end. As a result you are actually measuring between two chords of the ID, which will indicate a slightly smaller ID than the actual. On a percentage basis this error becomes larger as the ID decreases. Since the "knife-edge" thickness can vary from manufacturer to manufacturer (and likely caliper-to-caliper) it's not possible to get more specific than that....although it should be possible to come up with a calibration factor for your particular caliper(s). This problem caused me all kinds of problems until I figured out what was happening. I was always missing my target ID's despite how much care I took.

The ends of telescoping bore gauges are radiused to avoid this problem. They're not all that expensive, but can be a little tricky to use until you get the hang of it.

yes sir, I only use calipers to get me close, then pins, bore gages or telescoping gages. Today I'm making a bull gear for a Standard Modern lathe.
the bore is 1.999 +.0003/-.0000. In this case without my bore gage, the only other way I'd attempt this is by making a plug gage. But then you have the problem of is the plug truly round, am I too fussy? Yup, a little slop in the gear will have negative effects on the rest of the headstock assembly.
on another note, I find a must have tools are B class gage blocks ($100.00), and a 12X12 cheap black granite surface plate ($75.00), there so versatile, you can check tools for accuracy, check ID bores using pins and blocks, heights, steps, OD's, use a kant-twist make go/no-go gage, and much much more.
here's some pics of my gear in progress, quite a ways to go. I'll post more pics when I finish. Also a video of the gear box running under glass.
Standard Modern 13" series 2000.

 

[RaisedByWolves]

You opened my eyes to this. I’ve noticed that my 6” Mititoyo calipers have a small gap at the inside jaws when looked thru held up to a light.
There is a slight gap at the tips of the OD jaws to. Knowing this, I have to account for a slight difference depending on where I am measuring parts.

I have realized that if I am shooting for a press fit for a bushing, if I make the ID part the same as the OD part, I have a pretty hard press fit.

It seem that the measured ID usually is about .001” smaller on my caliper.

I remember reading in an old book somewhere that a good Machinest will learn his tools, and will learn to compensate for their various shortcomings.

Trick here is to have several ways to calibrate or check your tools. I occasionally will check my calipers and mics against the standards, and 123 blocks, comparing all calipers ( at least 5 in my main tool box drawer) and mics to verify against each other.

I think it’s important to check the calipers at several different lengths because the variance may change at longer lengths, if the beam is slightly bent. I know I have dropped my calipers at least once or twice.


Sent from my iPhone using Tapatalk

And this will only grow over time.

I’ve mentioned before I usually have a “Good” set of calipers and a knock around set for doing rough work.

This rough pair bit me in the ass a few weeks ago as measuring with the outside jaws gave a good reading, but the inside jaws were work bad enough that my .003 clearance for turned out more like .010…….unacceptible.

It was actually a new heavy duty BP quill handle with three pins and a 5/8” handle used for reworking parts in our series 2 boat anchor. Gives it was work for “work” this didn’t hurt too much, a government job however…..

What led me to this folly is I hadn’t recently used both sets of jaws in conjunction to achieve a close tolerance fit. I was ok using the outside jaws to duplicate a part and vise versa, but using them in combination put me off much more than I previously thought possible.

They now sleep in the circular file.

 

[Danzo187]

And this will only grow over time.

I’ve mentioned before I usually have a “Good” set of calipers and a knock around set for doing rough work.

This rough pair bit me in the ass a few weeks ago as measuring with the outside jaws gave a good reading, but the inside jaws were work bad enough that my .003 clearance for turned out more like .010…….unacceptible.

It was actually a new heavy duty BP quill handle with three pins and a 5/8” handle used for reworking parts in our series 2 boat anchor. Gives it was work for “work” this didn’t hurt too much, a government job however…..

What led me to this folly is I hadn’t recently used both sets of jaws in conjunction to achieve a close tolerance fit. I was ok using the outside jaws to duplicate a part and vise versa, but using them in combination put me off much more than I previously thought possible.

They now sleep in the circular file.

here's my bull gear for my lathe ,finished except for dowel pins, without a proper way to measure this would not have been possible, everything is within .0003, angle is dead on 41 D 46 M gear was a off the shelf Boston Gear YD72 (ebay) and material 3.5 X 3 inch 4140 ,the whole set up cost less than $100.00, last year LeBlond quoted me 3 grand to make this bull gear, so if you have a busted bull gear (more than 1 tooth broken) your lathe is useless.
so, I'll input a good purchase are gage blocks, surface plate and gage pins, and to round it off to $300 throw in a sine bar.

 

[gard]

Danzo, Nice job on the bull gear and thanks for the 3 pin method of measuring bores, I had not seen that one before. I do not have a set of gauge pins but I do have an old starrette 196 set with accessories. I set it to measure bore and will use a a micrometer to get the actual dimension. The same micrometer will be used to measure the shaft. The back plunger indicator resolution is 0.0001". It is easy to rock it back and forth to find the center. I will see how it compares to using the telescoping gauges.