Reamer reaming oversized

What do you mean by "a bit"? The tolerance on all reamers is slightly over nominal size. For a 5mm one it's +0.015mm to +0.006mm. I wrote an article on drilling and reaming tolerances some months ago.

Yep, new reamers always cut a touch oversize. I've recently been reaming holes with a 2.5mm reamer that always come out a shade under

2.52mm from a 2.3mm drilled hole. It just needs to wear a bit.

Here's my previous post from Dec 2008

I'm sorry but that's really nonsense. Drills have a cutting tolerance in correct usage which is always +0.00 to -something. They are intended to make holes that are never bigger than the nominal size. The Dormer tolerance on a

4mm drill is -0.018mm or about 7/10ths of a thou which is a light tap fit on a nominal sized dowel. Running too fast or not double drilling will use this up, and perhaps more, leading to a nominal sized hole or an oversized hole.

Reamers by contrast have a tighter tolerance but which is slightly oversize. They are intended to make holes which are not smaller than the nominal size so that the mating part, say a gudgeon pin or shaft, is a running fit. The tolerance on a 4mm reamer is +0.004mm to +0.012mm or about bugger all to

+5/10ths. Again running too fast will also lead to an oversized hole.

If you want a press fit on a job then you're better off using a drill at a slow speed than a reamer of the same nominal size. Tim's results were exactly what you'd predict from correct usage of a decent quality 4mm drill. No reaon at all to suppose that triangularity played any part in it.

It's commonly thought that reamers are intended to cut to exact size and that drills cut oversize but this is invariably caused by incorrect use rather than design. In fact the whole raison d'etre behind how things worked out this way is you have a choice between a press fit hole or a sliding fit hole using readily available nominal sized tooling by either just drilling or drilling then reaming. If both drills and reamers were designed to cut oversize you'd have to have special tooling made to get press fit holes.

That also explains why it's drills that cut undersize and reamers oversize. If you want a press fit you're not so worried about the quality of the finish. If you want a running fit, say for a bearing, you are. If reamers were designed to cut undersize and drills oversize this would end up the wrong way round.

In article , Dave Baker writes

Dave,

Thanks for (re-) posting the excellent summary. Makes up for the other b******s which gets posted on usenet.

David

Dave,

Is this available online? If so, please post a link, I'd like a read of this.

Basically, I'm making couplings to fit model electric motors which have precision ground shafts. I made some for 4mm shafts and they were a good sliding fit, ie., no wobble. When I made some for 5mm shafts, these came out with a tiniest, and I mean tiniest, bit of wobble - enough to be unacceptable at 40,000 rpm.

I'm using chipping-grade aluminium and white spirit as the lube/cutting fluid, drilling at 1000rpm and reaming at 500rpm. Perhaps 500rpm is still too fast? The reamer is a "Titex Plus" (not a brand I've heard of before, I was given the reamer) and I'm drilling 4.8mm.

I will play around with reaming at a lower speed.

Regards, Paul

Has the reamer got H7 engraved on it, quite a few reamers are sold as a nominal size but actually produce a H7 tolerance hole which for a 5mm reamer would cut approx 0.01mm oversize.

Jason

Yes, it has H7 marked on it. If I want reamers that ream at nominal size, ie., a 5mm that reams a 5mm hole, what do I ask for?

Cheers, Paul

and from a post in 2003

My copy (unknown date) of The Dormer Twist Drill and Reamer Information handbook says the following:

"It is good practice to first use 50% of the drilling speed used on the pre-forming of the holes, though in fact present ideas tend to reduce this even further."

The first time I ever used a reamer that's what I did - used 50% of the drill speed. It was to make bronze valve guides and it ended up as a complete disaster. I spent ages turning, parting and drilling a batch and then ran an

8mm reamer through them all at several hundred rpm on the lathe. On the naive assumption that a reamer could only ever generate the correct sized hole I didn't measure any until I'd finished the batch. To my horror they were all several thou oversize and consequently useless. 50% of the drill speed might be achievable on a totally rigid and perfectly centered setup but for HSM you can chuck it straight out of the window. A reamer running too fast on an old machine chatters its way through the job and cuts on the sides of the teeth as well as on the lead in. Ever since I've used about 80 to 150 rpm depending on the reamer size and never had the same problem recur. I've even made pistons in house from castings which involved reaming the 24mm gudgeon pin bore to very fine tolerances and managed that on a shagged old Bridgeport copy without too much difficulty at its slowest speed of 80 rpm. For an 8mm reamer on the lathe I'd maybe double that speed. 20 rpm is far too slow though. You need a decent feed rate to stop the reamer burnishing and blunting itself and that's hard to do at very slow speeds. However, when in doubt start slow and build the speed up until a problem manifests.

So whatever Dormer's "present ideas" were when they wrote that guide I wish they'd printed them before I made my valve guides instead of just saying "50% of the drilling speed".

On or around Wed, 1 Apr 2009 10:43:47 -0500, paulwilliams enlightened us thusly:

a 4.9mm one?

:-)

Speed pay a large part, faster cuts larger. Lubrication also plays a part, Oils tend to cut tighter than solvents.

It's possible to dial in a reamer to a set size and has been used for may years by the old manual toolmakers. Take a hardened dowel pin or piece of round HSS stock and slide it once down each flute, end to shank, following to top land to reduce size.

A reamer that's cutting too tight can be nudged up by doing the same but going shank to tip and run the dowel pin on the front edge of the flute.

John S.

Sorry Dave, you're still wrong on this one.

A two flute drill enlarging a hole will produce a three lobed profile. It's an inevitable side effect of the geometry and stiffness of the drill.

You might not be able to see the out of roundness in most cases, but it can be measured and plenty of learned papers and articles have been written on the subject.

regards Mark Rand RTFM

Hi Paul

1 use a floating reamer holder, Hemingway do a kit. 2 use a d bit to bring nearly to size 3 use a lubricated steel ball to finish, pushed through with smooth positive action under a press. This will give you a work hardenend mirror bore at low cost. Peter

Peter,

I'll try a D-bit, I can't use the steel ball idea as the holes are blind.

Cheers, Paul

I seem to remember special reamers for steel dowels used in press tool fabrication :rolleyes:

If there's enough meat below the hole you could drill the hole 1 diameter deeper and leave the ball in there.

Regards, Tony

There's a very easy way to do this even if the holes are blind. Accurately turn and polish a length of steel rod to the exact diameter you want the finished hole. Hardened is nice but in alloy a bit of decent unhardened steel will do. Chamfer the bottom end and leave the top 15mm or so longer than the hole depth. Drill and tap a thread into the protruding part of the rod. Now you can press or tap the rod into the hole to swage it to size and withdraw it with a bolt, washer and a bit of hollow tube to pull against.

With a small bushing like 5mm you could make the swaging rod top hat shaped with a larger diameter top you can get a decent sized thread into. Say a

10mm diameter top bit with a 6mm thread in it and a bottom section a bit longer than the hole depth and 5mm diameter.

If you want to be clever you can polish the rod down a bit everywhere apart from the very bottom part so it just swages on that short length and doesn't lock into position on any part above that. Probably not necessary on something as small as 5mm though.

You want to drill to within about 1 thou of target size for the swaging to work and a nominal 5mm drill used slowly in a good concentric chuck and with double drilling after say a 4.9mm drill should do that. Drills are always sized to drill from +0.00 to - a tad. About -0.02mm in the case of a 5mm drill which should be ideal.

If there is any springback in the swaging process you might need to play with the rod size a bit to obtain the exact target size. i.e. a couple of tenths over 5mm to get a hole that's exactly to 5mm.

Cheers Dave, I'll try that. I need these couplings to be absolutely spot-knacker as I'm going to be turning them at 40,000 - 50,000 rpm.

I've just gone through my collection of brushless motors with a micrometer, and not a single one has a shaft of exactly 5mm - they're all 4.95, 4.97, 4.96 etc. - surely a precision ground shaft should be the exact size, ie., 5mm?

Regards, Paul

Drill 'em deeper, leave the ball in.

sur> ely a precision ground shaft should be the exact size, ie., 5mm?

Depends on what tollerance they shaft has been ground to, this is signified by a lower case "h" followed by a number, Silver steel for example is usually ground to h6 this would give a location/clearance fit with your H7 reamer. For the fit you want then the shaft needs to be k6 or n6 which are light interfearance fits.

Jason