Difficult to actually size reamers, I did a lot of reaming to tight tolerances on aircraft parts. Different sizes are easily achieved with one reamer in holes of the same diameter and materials.

Ream dry, the largest size hole.

Wipe the reamer with an oily cloth and remove as much as possible, smaller hole.

Oil the hole and reamer well, smaller still.

There are other methods I would not want to talk about.

I suspect that I would struggle to afford any of these : **LINK**

http://www.re-al.ch/en/technical-features/standard-tolerances

… but it’s an interesting glimpse at ‘how the other half lives’

MichaelG.

If they are old imperial reamers I doubt yours will have ISO tolerances. Most of my newer ones have it on them.

If you look up the tolerence you want on the link posted then you buy a suitably spec'd reamer though as Old mart says the actual hole you will get will vary depending on several factors, I suspect there is some criteria for testing reamers so they can be sold as H7, H8, etc

This is why I prefer to bore anything 8mm and over where possible as you have more control over the fit, For example on the crankshaft of an engine just finished I had a press fit for the crank disc, push fit for the flywheel and running fit for the bearings. Had I done it with reamers I would have needed 3 different sizes or tolerances and possibly mucked up parts testing to see how they actually cut.

You can buy a standard tolerance reamer, and carefully lap the OD to make the closer tolerance hole. The better reamers I find are the ones that have un even flute distribution. For accurate holes, I pre bore with a undersize reground end mill or I make a single point cutter that effectively will bore the hole in the correct position. Then ream.

For important hole sizing, I make a test hole and measure it. Then if needed, lap the od some more and re test again. I am finding that the hss reamers that are readily available out here from the tool merchants, are on the upper limit of the H7, rather than lower limit.

Different cutting fluids etc will vary slightly the result , and so will the cutting allowance etc. Castor oil can make a hole smaller than the reamer, but only by 2 to 4 um. But is not absolute guarantee though. Castor oil will allow oversized parts to assemble that would other wise be a press fit. Down side, castor oil makes an ugly mess over time.

Some places that make carbide reamers, will often ask for a size limit or the size you want with the maximum and minimum size. Like a Ø4.494mm reamer with an upper limit of Ø4.496mm , these will cost like 60 to 70 pounds each like the example I gave .

The coding on the reamer, such as H7, defines the expected tolerance that it would give. While this is of little interest to model engineers it is of great importance in manufacturing where interchangeability is a must.

I will give a simple description on how it works.

For example, we want a 10,00mm shaft which slides in a bush (a sliding fit is between a push fit and a nice rotating fit).

The established standard kindly gives a chart

which suggests a hole and shaft tolerance – H7 & g6 (upper case = hole, lower case = shaft). The number gives the machining process that will achieve the required tolerance with ease.

It shows that we should be able to achieve the tolerances by grinding, broaching, reaming, boring or tuning. Finally we need the tolerances and required dimensions.

The hole requires a tolerance of 0 to +18 μm and the shaft -6 to -17 μm. It follows that the dimensions are 10,000 to 10,018 mm for the hole and 9,983 to 9,994 mm for the shaft. A 10mm reamer marked H7 should give us the correct hole dimension.

Fits and limits can get horribly complex and, unless one’s interest was manufacturing engineering, there were usually better things to do.

JA