Holes for Reaming

I got fed up calculating hole sizes for reaming.

As all my rearers are imperial I now just drill a hole 1/64 (about 16 thou) smaller than the reamer, bear in mind most drills make a hole somewhat oversize so the allowance is probably a bit less in reality. This works from 2/16" up to 1/2" with hand and machine reamers. I do most of my reaming in a drill press or the lathe, even with hand reamers.

This seems to work fine on steel, cast iron, plastic, brass and phosphor bronze.

I would guess using a drill 0.5mm smaller could work fine for metric folks, although this is more like 20thou allowance which may be excessive for smaller reamers? ).3 or 0.4 might be ideal but most people don't have these sizes.

Neil

Hi Neil,

Not arguing about your sizing method, but you may get longer reamer life and better finish if you use the old rule of thumb " never ream more than .005" on diameter". I was taught this many years ago my an ex British Army armourer I trained under. I have a lot of respect for his experience and have practised many of his sayings advice and tidbits for more than 30 years.

( it is important when taking light reamer cuts to know your actual bore size and not just assume the drill cut to same size as marked on shank)

I also find a drop of cutting oil very useful during reaming of metals. Water works well when reaming many plastics.

Just my $0.02 your mileage may vary. JD

On metrics I go 0.2mm smaller, I have sets of 0.1mm increments and also stub drills in most of the reaming sizes. Also drill for imperial reamers using the same set of drills so usually quiet a bit less than 0.015" to come off

I will quite often drill 0.5mm smaller and then drill again with teh 0.2mm smaller drill to get a more accurately sized drilled hole.

As most of my drilling is done with the mill that also gets used for the reaming so you know the part is in line with the spindle and also generally slower rotating than a drill press.

Edited By JasonB on 12/09/2016 15:56:16

I do as you Jason, for the diameter range of holes Neil mentions, I use a 0.2mm under size drill. For larger holes may be 0.3mm under size.

Thor

Edited By Thor on 12/09/2016 16:48:10

Not wishing to be pedantic but, 2/16th, would that be 1/8th?

I knew that would start some debate!

All the advice I have read from reamer manufacturers says 'make it work' for longest life. I've just looked up the Dormer guidance:

To obtain the best results when using reamers it is essential to make them ‘work’. It is a
common fault to prepare holes for reaming with too little stock left in. If insufficient stock
is left in the hole before reaming, then the reamer will rub, quickly show wear and will
result in loss of diameter. It is equally important for performance not to leave too much
stock in the hole. (See Stock Removal on next page)

For 3/16" to 1/2" they suggest 8 thou for a drilled hole.

In addition the 0.1mm steps for metric sizes and 0.002 thou steps for imperial mean, for example that the metric recommendation for a 7.16" or 1/2" reamer is 12 thou so there's a fair bit of leeway in their sizes.

Dormer say their drills generally give a h12 tolerance with is size to plus 0.15mm or about 6 thou. I suspect my 1/64" below size is probably closer to 12 thou in practice.

When reaming a hole in the lathe I always drill out the bulk of it, but bring it to reaming size with a boring tool.

This ensures any hole wandering done with the drill gets eliminated.

Phil

So sounds like some of us have been doing it right 0.008" is 0.2mm.

Indeed, but only between 4 and 11mm.

But do you drills make holes dead to size? They could drill 0.15mm oversize and still be in 'Dormer' spec.

N.

Edited By Neil Wyatt on 12/09/2016 17:27:55

3mm & 1/8" are the smallest reamers I have so not much out of that range and anything 12mm or 1/2" and over then I'm more likely to bore to finished size than ream as they are the largest size reamers I have

As I said if its a critical hole I will use more than one drill so the final one is more likely to be on size. I actually write on the box what size tends to work best with a particular reamer so I know the next time what will give the right result

Marc lecuyer gave a simple formula for this on his youtube channel ( that lazy machinist) it was in the series on drilling and reaming .

Well, clearly the maximum allowance will always be your nominal size, so you definitely can't start too close to it, depending on how well your drill performed, so the minimum size will be the diameter tapered end of the reamer logically, any smaller and you're not going to get any purchase.

Michael W

My point kind of got lost in the quest for accuracy - i got fed up searching out the 'right' drill and now just use one from my 64ths set, and I haven't had any problems with hole size finish or apparent reamer wear/damage.

Neil

Good old British manufacturers used to give books away that had that kind of information in them. I bet you can not get one from the suppliers of the foreign stuff.

Bri

Can you read Chinese?

I was taught as an apprentice to drill 1/64" under for reaming, but that was on 3/8 diameter and upwards. I have more recently found that drilling 1/64" under and trying to ream with small cheap eBay reamers under 5/16 or so is a good way to break reamers (I think they are not HSS!). They seem to work best if drilled/bored to more like 5 to 10 thou under - close enough to an average 8 thou for bush work I suppose.

There is a "Reamers Guide" here, might give some guidance.

Thor