What tool to measure small bores?

Hi everyone.

I’m sorry this post is probably going to be long winded and possibly not make much sense. I’m looking for some recommendations on measuring small diameter bores.

I’m currently building a rc Scania truck chassis. I’m using pretty small bearings. From 4 mm to 10 mm outside diameter. I’m struggling to decide what’s the best way to measure the bores to fit the bearings properly. At the minute I’m using a cheap inside jaw micrometer I got of eBay and calibrating it with my Mitutoyo Digimatic QuantuMike outside micrometer. Still sometimes I end up with the bore slightly too big and the bearing fit is loose.

I’ve been debating with myself for weeks about the best way to measure the bores. Do I stick with the inside jaw micrometer but get a decent make or do I go with a 3 point inside micrometer, again of a decent make.

Next, the question is what make do I go for. Obviously I would like the Mitutoyo inside micrometer, but the prices are scary. Arceurotrade and Chronos Tools both sell 3 point micrometer sets but only down to 6mm. The only other website I can find is Digital Micrometers Ltd who sell a set of 3 point micrometers from 3mm to 6mm. Roughly half the price of the Mitutoyo set but they’re still over £700.

Has anyone used DML micrometers or calipers and could give an opinion on their quality? I’m guessing they’re probably made in China, which doesn’t necessarily mean they would be poor quality, but would I really notice the quality difference between the Mitutoyo and the DML sets?

Looking forward to the different opinions or perhaps other options to measure small diameter bores.

Darren

You could make some plug-gauges: an o.d. is a lot easier to measure accurately..

Turn spigots on each end of short lengths of mild-steel bar; one end a known fraction under-size; the other to size appropriate for the bearing. Mark the rod with which end is which.

Aim for the under-diameter plug just entering, first.

You may well find the spring-cut on a boring-tool will bring the hole to size, even a touch over, quicker than you think.

Darren

I use these on small bores, fit the gauge in the bore and then measure over the gauge with a micrometer.
You may have to turn a stub piece to the finish size needed for checking as you make the shaft.
The top set are split and the bottom set have 2 balls to measure over.
As with all gauges of this type you will develop a feel for them the more you use them.

Emgee

Hi

Chronos have a dial bore gauge in their Dasqua range that measures between 4 to 6 mm ,not cheap at about £165 and also 6 to 10 mm at around £70 ,sometimes MSC J&L have sets of three point micrometers in the Advantage cat that range between 3.5 to 6.5 in 4 micrometers with setting rings even more expensive but measure down to .001 um .Not in this months catalogue.

John

0.001 um is a nanometer! Maybe just a micron?

As suggested use plug gauges or small bore gauges.

Tony

The bearings will only come in a limited range of sizes and will be at full- of half millimetre dimensions. Perhaps consider some second-hand go-nogo gauges. Alternatively, some under- and over- gauge pins.

The point is that you do not need measuring capacity spanning the full 4-10mm range with a resolution of 0.02mm. You only need a very, very limited subset of this range.

With such small bearings, Loctite is your friend.

Cutting a few appropriate ‘go-no go’ plugs would be my solution. Press-fit and/or loctite-fit should easily be possible to achieve. Less precise measurement,until close to the desired size, can be carried out with current tools.

No need, IMO, to go to great expense for such a job. A reamer for each bearing size might be a better investment?

Reamers may not work for the kind of stepped holes that bearings fit in? A machine reamer has a small tapered lead which will leave a chamfer on the bottom of the hole and may stop the bearing seating, hand reamer has a long tapered lead so wouldn't cut to nominal size on a blind bore. And if the reamer didn't cut to nominal size, what do you do next?

If you can make a plug gauge, maybe make a D bit to cut the hole anyway?

Hi
I've woken up now and taken a photo of these micrometers ,of course they
only read to 1 um ,made by Spi not as good quality as Mitutoyo but they
are plenty good enough for use in the workshop ,as far as i can remember
they worked out at about £105 each they do sell them individually but
work out more expensive this way as the setting rings are not supplied.
The smallest here will measure into a depth of about 16 mm.

John

Edited By John Pace on 01/03/2021 09:48:19

Hi

Thank you to everyone who commented. Lots of food for thought. Possibly the plug gauge method might be the most cost effective option, but I really like knowing what size the bore is as I go along so I can sneak up on the dimension I would like.

Plus its an excuse for new shiny tools.

Like I really need an excuse.

John Pace - Would you mind telling me where you purchased the SPI micrometers if your allowed to post the name the supplier on the forum?

Thanks

Darren

Hi Darren

The Micrometers came from MSC J&L They usually appear in the monthly Advantage catalogue ,they are not in this months catalogue March.

They are useful as you say to be able to measure where you are and eventually get to the size you need ,i doubt you will find a similar instrument within this price range .

John

Thank you John for the information. I just looked MSC for the SPI micrometer set. Still fairly expensive, but not as scary as the Mitutoyo. What’s the quality like, if you don’t mind me asking? I’ve got a 0 - 25 mm mitutoyo digimatic quantumike which is really nice to use. I have some unbranded larger micrometers which work ok but aren’t as nice to use as the Mitutoyo. You know the old saying “You buy cheap, buy twice” Don’t want to make the same mistake again, by buying cheaper metrology kit.

Thanks Darren

Hi Darren

I've found the page on the December 2020 Advantage for these micrometers.They are listed in the main book on page 871 as part no SPU 14346 K , as you may see there is difference in price than the listing on the Advantage page as part no UESPU 14346 K , they re-appear regularly in the advantage leaflet from time to time.

As far as the quality  aspect   i can't  fault them   as i have nothing else  to compare them with  ,as they follow  on  in size range  from one to the other  they are within   .002  mm  using the ring gauge   to check  ,that is within the specification  as  advised .  

John

Edited By John Pace on 01/03/2021 12:05:09

The cheapest way is to make the plug gauges, as already mentioned. For short term use, they don't have to be hardened, and are easy to check. If the ends show wear, they can be faced back easily. For a 6mm hole, the go would be 5.99mm and the no go 6.01mm.

Diatest

This set goes from 1.5mm to about 20mm. allows easy measurement to 0.002mm.

Around £200+ on ebay. (this set was a bargain at £75!).

Jim

Darren

Have you considered Starrett gauges as Emgee suggests - RS Pro have them at £88 ish including VAT:

Thank you everyone for the lard amount of responses. I'm still tempted to splash out on a set of 3 point micrometers. Just really cant justify the amount it would cost for the Mitutoyo sets, although I know they would be really nice to use and last a lifetime.

I have a cheap set of the expanding gauges, eBay again, but find them extremely difficult to get the diameter just right. I think its because of the hand surgery I got 3 years ago. Severe Carpel Tunnel Syndrome. The surgeon did warn me that the damage was already done just because of the length of time I was on the NHS waiting list before I got my operation, but that's another story. Since then I find my dexterity isn't what it use to be and I had to give up my miniature model figures painting, as I just don't have the fine control I once did. I always find I either over tighten the expanding gauge or not tight enough. This makes repeatability difficult. That's why I've been looking at micrometers with their ratchet thimble ensuring consistent tightening force.

The dial bore gauge looks interesting. I didn't know they could be got for use in such small diameters. I think I'll look at either turning gauge plugs or getting a few pin gauges plus and minus the bore diameter I'm making at the time and see how I like using that method. I'll also keep an eye out on MSC to see if they have the bore micrometers on offer again.

Many thanks for everyone's thoughts, time and input.

Darren

Just out of interest are you hoping to press fit these bearings if so be very wary as even a micron underbored on a 4 mm bearing will shorten its life by a large amount. Much better to make them a very light fit or sliding with bearing fit, this way the outer race is not over stressed. This info came from a clock man who realised pressed in small bearings started to bind and increase friction. Hope this helps.

Hi Bernard, No I'm not making them a tight press fit. Normally if I can get the bore correct, then they bearing press in by hand. I just don't want them to fall out easily and be loose enough that the outer race can spin in the bore. They don't really spin at a high rpm, not like my rc cars. I'll put a few photos of my rc hydraulic front loader I built last year and the progress of my Scania truck in my album when I get a chance.

Many thanks

Darren