Under & over reamers?

More a curiosity for me at this stage

I just watched a machinist video whereby the nomial diamter of a silver steel dowel didn't have a smooth sliding fit into a reamed hole. She then explains that's why 'under/above reamers sets' exist. Presumably, in Imperial, these would b +/- 1 thou of the nominal size? I've seen a couple of American sellers offer them.

After some searching, no one in this country seems to sell them? Is it instead common practise to buy buys the explicit reamer size e.g 0.501" for an oversize 1/2" hole.

Would appreciate some clarity on this. Thanks in advance.

H7 will provide clearance within certain limits, check online for details of engineering fits for shafts and bored holes.

Emgee

H7 is the standard tolerance for reamers. Other sizes are available but very expensive.

For hobby applications, bore the hole to size, make a D-bit to the required diameter or use an expandable reamer.

Paul.

Rotagrip do intermediate reamers in 0.01mm steps

Taylor Jones where the go to for non standard reamers when I was working.

Fits are an example of the sharp divide between amateur and professional machining. The relatively simple methods of our forefathers are well-suited to small workshops and prototype work, hence we can and should work with old-school fits. For this Tubal Cain's Model Engineer's Handbook is excellent. He gives generalised fit information for Shrink, Force, Drive, Wheel Keying, Push and Slide fits, plus dimensions for six other running fits. Shaft/Hole fits are on page 5.18 of my 3rd Edition.

The different fits aren't a fixed reduction or increment; rather the difference between hole and shaft depends on diameter, so a Shrink fit is undersized by 1.5 thou per inch of diameter, while a slide fit is oversized by 0.45 thou per inch diameter.

In my rough workshop I use Tubal Cain as a Guide rather than 'the law', and I suppose most of my efforts are either 'Wheel Keying' (requiring more-or-less light tapping) or something vaguely between a Push and Slide fit I guess many other Model Engineers would put more effort into fitting parts! Normal practice is to cut the hole accurately to size and then to adjust the shaft to fit: there's no need for special reamers.

The professionals are much more scientific due to the needs of interchangeable manufacturing. Their system covers adjusting holes to fit shafts as well as shafts to holes, and is much more complicated. Tubal Cain deals with fits in half a page; this PDF on ISO Dimensions and Tolerances is 39 pages long.

ISO is no problem to a production engineer: if he needs a few thousand unusual reamers, he rings up the supplier and calls them off in bulk. And in bulk, they aren't particularly expensive. No such luck when an amateur wants just one! Small quantities bought new retail are liable to give grandad a heart attack when the bill is presented. So my advice is to buy a copy of Tubal Cain and stick to that!

Dave

It's rather more complicated than that. My local tool emporium stocks metric reamers in 0.01mm increments. When I decided to buy a set of imperial reamers for my traction engine build I ended up with 4.76mm, 6.35mm, 7.94mm, 9.53mm and 1/2" reamers, by buying the cheapest in each (nominally imperial) size.

Andrew

If you want to produce your own fits, you can use adjustable reamers.

The disadvantages are :

1 That you would need to experiment a little until you obtain the fit that you require.

2 usually straight f cutting teeth, so may be more prone to chatter, if not used with care.;

In the old days when every vehicle had beam axles, they were very popular for reaming King Pin bushes, to produce the required fit of a King Pin in a front axle.

Howard