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measuring small bores

bell mouthing.

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RRMBK21/06/2016 16:54:20
154 forum posts
18 photos

Hi All . I am trying to bore some GM axle boxes to take a 12mm OD needle roller bearing. It requires an N6 interference fit but I cant seem to solve two issues .

Although I am using a tool with as large a stem diameter as possible I am still getting bell mouth/ taper down the length of the bore which is appx 15 mm.

Also any idea on how to accurately measure such a small bore please? vernier/ digital callipers don't seem to give repeatable accuracy and my expanding bore gauge doesn't go small enough.

would greatly appreciate any input having already scrapped two boxes!

Michael Gilligan21/06/2016 17:17:05
20185 forum posts
1053 photos

12mm is at the upper end of this set from Warco:


Haven't tried them, so I can't comment on quality, but the principle is good.


Bob Brown 121/06/2016 17:35:15
1021 forum posts
127 photos

I use this for small bores although a go/no go gauge may be an option image00003.jpg

Fatgadgi21/06/2016 18:24:15
178 forum posts
26 photos


Measuring small bores accurately is expensive.

If you don't want to do it too often, as Bob says a Go / No Go gauge is a good way to go and you can turn them up easily enough from mild steel. You can easily measure the diameters with a micrometer.

Cheers - Will

Boiler Bri21/06/2016 18:42:12
842 forum posts
199 photos

I make plug gauges to the size that I want and then keep them safe for use in the future. You can end up with quite a selection.


Tony Pratt 121/06/2016 19:56:53
1966 forum posts
12 photos

Telescopic bore gauges aren't too expensive, basically spring loaded plunger on a handle, tried an image but that didn't go well. Google or Ebay will assistwink

Draper are fairly cheap


Jens Eirik Skogstad21/06/2016 21:25:08
400 forum posts
22 photos

You can use steel ball in right size with handle soldered on as measure tool.

John Reese21/06/2016 22:04:07
1035 forum posts
Posted by Michael Gilligan on 21/06/2016 17:17:05:

12mm is at the upper end of this set from Warco:


Haven't tried them, so I can't comment on quality, but the principle is good.


MichaelG has recommended the best and most versatile tool for the job. The quality varies among the different brands. Here in the US I would favor Starrett or Mitutoyo. If buying an off brand pay particular attention to the finish on the ball end. A poor finish will impair the ability to feel when you have proper contact. There is also a flat bottom version of the small hole gauge that permits measurement close to the bottom of a blind hole. I also suggest a set of telescoping gauges if you are measuring larger bores.

Robert Dodds21/06/2016 22:55:07
320 forum posts
62 photos

Is nobody prepared to offer advice on the OP's primary problem? He scrapping parts because of bellmouthing!
My suggestion would be to acquire a good sharp 12 mm reamer to finish size the GM.
Bearing OD's are usually arranged to get the right fit with a nominal size bore but confirm this by checking your bearing OD.
The type of reamer, machine or hand, may depend on the through clearance you have beyond the 15mm bore for the reamer to run into.

Bob D

Michael Gilligan21/06/2016 23:14:02
20185 forum posts
1053 photos
Posted by Robert Dodds on 21/06/2016 22:55:07:

Is nobody prepared to offer advice on the OP's primary problem? He scrapping parts because of bellmouthing!


As Joseph Whitworth famously said:

'You can only make as well as you can measure.'


Fatgadgi21/06/2016 23:16:45
178 forum posts
26 photos

I have a nice set of M&W telescopic gauges which I use a lot and I would highly recommend them for general use.

But they are not super accurate, certainly I couldn't measure within an N6 tolerance with mine.

I did check how accurate they were once and using them very carefully in a ring gauge from a Micro Mag set I had readings of up to 0.02mm oversize. The error could well be my failing, but I did try hard to get good accuracy.

Never tried the Mitutoyo split sphere style, but I would guess they are of a similar accuracy because they rely on feel, probably more than the telescopic ones.

Cheers - Will

JohnF21/06/2016 23:50:49
1152 forum posts
190 photos

RRMBK, sound to me that it may be the tool causing the bell mouth [assuming your lathe is turning true?] what tooling are you using?

Personally I would suggest checking your clearance angles or if I was making one I would grind it from round HSS, grind to half the diameter for about 10mm then some clearance along the shank, only about another 10mm in this case. this will give you a very stiff tool, stone to a keen edge.

For measurement make a plug gauge to size then reduce the first 5/6mm by 0.001" or 0.025mm then bore until the first part enters you then know you have 0.001" to size. We used these extensively in jig boring when boring holes for liner bushes in jigs and fixtures. John

PS PM me if you want a photo of a boring bar made as described.

bricky22/06/2016 00:07:53
575 forum posts
68 photos

Bore close to size and make a D bit to finished diameter to finish with.


not done it yet22/06/2016 07:09:28
6812 forum posts
20 photos

KISS principle. A reamer for the purpose. Bob D got it in one.

Tony Pratt 122/06/2016 07:15:07
1966 forum posts
12 photos
Posted by not done it yet on 22/06/2016 07:09:28:

KISS principle. A reamer for the purpose. Bob D got it in one.

Sounds simple in principle & only takes a few words to say it but reamers/D bits are not without their problems. I would persevere with boring to size the OP has plenty of advice to go on.


JasonB22/06/2016 07:33:41
22751 forum posts
2653 photos
1 articles

Anyone know where to get N6 reamers, most are H7 or H8. Even a D bit will take quite a few trial runs to get it to form a hole to N6.

Tapered bore as said above could be tool rubbing, are you taking any spring cuts at the end? also need a very sharp tool your average indexable tool will get pushed off the surface trying to sneak up on a fine fit so Hss in a holder would be my prefered cutter. Plug gauge to measure

Not being a loco boy, how do needle rollers perform on axle boxes that move up and down and get coal dust on them?



Edited By JasonB on 22/06/2016 07:34:32

Hopper22/06/2016 08:46:19
6404 forum posts
334 photos

You could try using an adjustable hand reamer to sneak up on the final dimension. By using the tailstock centre to guide it through the hole, it should come out nice and parallel too. (Assuming it is not a blind hole)

+1 on ball gauges for small diameters (under half inch/13mm). Then measure the ball gauge with a good micrometer. They will give a better feel/reading than most digital calipers for most users.

Best final sizing is still checked by feel. Torrington-style Needle rollers need only the slightest of interference fit so you will feel it "almost start to go in" when the size is right.

Martin Connelly22/06/2016 09:09:33
2137 forum posts
222 photos

In an industrial environment the usual measuring tool would be a Bowers gauge. But these are too expensive a purchase for a home shop (I have a used 3/4"-1" that I got for free). How are you controlling the temperature in your work piece and workshop to achieve such a tight tolerance? Michael G got it right when he asked how well can you measure it. These sort of tolerances require some idea of workpiece and measuring tool temperature. The range of n6 at Ø15 (+0.012 to +0.023mm) is the same as a 6°C change in temperature in steel. If your workpiece is heating up as you bore it it could cause bellmouthing that is measurable. Flood coolant would control this.


Jon22/06/2016 09:51:46
1001 forum posts
49 photos

Put a cut on and watch the tool deflect.

Some of the bellmouthing can be reduced by angling flimsy tools inwards not at 90 degrees maybe 70 degrees ish. If must go for HSS buy decent once cobalt 5% English pretty good and last the cut.

I would be doing that 15mm bore with indexable 6mm shanked quality boring bar that don't visually flex where quality if it still exists HSS 1/2" will.

Andrew Johnston22/06/2016 10:06:38
6602 forum posts
701 photos
Posted by JasonB on 22/06/2016 07:33:41:

Anyone know where to get N6 reamers, most are H7 or H8. Even a D bit will take quite a few trial runs to get it to form a hole to N6.

Not a problem, my local tool emporium stocks reamers in increments of 0.01mm, so just choose an appropriate size. Of course one needs to allow for new reamers cutting slightly oversize, but slightly undersize in bronze, and presumably in GM.

As for measurement I'd make a go/nogo gauge.


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