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Which size drill bit

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Joe McKean16/08/2019 20:34:59
21 forum posts

I need to ream a hole to 9/32nds, could anyone advise me what size drill bit I will need to use prior to reaming.

Thanks in advance


Emgee16/08/2019 20:39:11
1355 forum posts
212 photos

Joe, I would use a 1/4" and reduce the rpm to halve drilling speed, using coolant (spray or brush) for both ops.


Neil Wyatt16/08/2019 21:04:18
17088 forum posts
690 photos
76 articles

I'd be tempted to run a 17/64 through after drilling 1/4" IF I could be certain it wouldn't cut oversize, perhaps do a test hole first. I've found 1/64" just about right for small reamers.


SillyOldDuffer16/08/2019 21:10:02
5138 forum posts
1074 photos

I like to minimise the work done by my reamers and would go metric. 9/32" is 7.14mm making 7.0mm a convenient off-the-shelf match.


Mike Poole16/08/2019 22:46:55
2329 forum posts
52 photos

Well we have a choice here of 30 thou ish from emgee or 15 thou from Neil or 3.5 though from Dave (SOD). Given that most drills tend to drill slightly oversize then you cold be living dangerously with a max of 3.5 thou to play with which may not be there if your drill cuts a bit oversize. I aim for something around Neil’s suggestion as you should have something for the reamer to cut but not too much, if your drills are cutting 15 thou oversize the I think it’s time to look at your drill sharpening process. It is useful to know what the size of a drilled hole is using your normal sharpening process and drilling technique.


MadMike16/08/2019 23:45:16
198 forum posts

Reaming allowance is dependent upon several factors, hole diameter, material and method of reaming (hand, in a drill with hand feed, or auto feed in drill or lathe or similar). As a principle you should try to remove as little material as possible to avoid chatter and reaming an oversize hole.

A rule of thumb for decades has been to remove approximately .005 inch after drilling. Remember to keep the reamer well lubricated with coolant, and to slow the cutting speed tp between 50 and 60% of the drilling speed in Feet per minute. Feed rate should be higher than when drilling by around 1.5x or 2x faster in fact. Of course the condition of the reamer is also a very important factor in all this. HTH.

not done it yet17/08/2019 07:14:29
3944 forum posts
15 photos

Do give us some idea of the depth of this hole. It may have some bearing on the decision of which under-size hole to go for. Personally, I would likely use an end mill, after drilling a smaller pilot hole, rather than a twist drill when accuracy and straightness is required.

JasonB17/08/2019 07:34:14
17079 forum posts
1839 photos
1 articles

For that sort of diameter these days I would drill 0.3mm smaller so something like a 6.8mm, under 1/4" I tend to go with 0.2mm smaller and have stub drills in these sizes eg 3.8, 4.8, 5.8 for metric which I find myself using mostly now , if imperial then I have 0.1mm increment jobber drills upto 10mm. Over that size I would drop 1/64th if I have the drill or find the nearest to it though if in the lathe I'm more inclinded to bore to finished size once things get to over 10mm.

Mick B117/08/2019 11:20:53
1361 forum posts
75 photos

I've thought for a while that a drill's tendency to cut oversize is mediated primarily by imbalance between cutting lip angle and/or length, so that the heavier pressure encountered by one side deflects the drill elastically. The closer any pilot hole is to finished size, the lower the pressure on the leading lip and the lower the tendency to flex.

There's no way I can afford the money to buy, or the mental concentration to maintain stock control of, reamers for every small precision hole I might gotta make. So I make extensive use of the 'Dagenham reamer' procedure where you drill a pilot hole undersized by a few percent of diameter, then finish with a drill to size, run at about quarter or third speed and whatever feed 'feels' right, usually several times normal drill feed.

If I needed a precise 9/32" hole I think I might do it 17/64", then 7,0mm (thanks SOD ! smiley) and finish with a 9/32" drill or reamer, whichever I happened to have - because if the predrills came out right I don't think it'd much matter.

old mart18/08/2019 14:14:29
1099 forum posts
113 photos

A lot depends whether the hole is blind, how deep, or a through hole. All reamers have a lead in angle, or they wouldn't work. A hand reamer has quite a long shallow angle lead in and would be unsuitable for use in a short blind hole as about one diameters length is tapered. A machine reamer has somewhere in the region of 1/4 diameters length of taper. If you are not very experienced in using a reamer by hand, the hole will end up bellmouthed, so using a machine, or a pre made guide will help to keep the reamer straight, turning the machine by hand, unless a very slow speed is available. A reamer can be made to cut different size holes, without resorting to butchery of the reamer. The smallest size is achieved by copious amounts of cutting oil. The intermediate size is by oiling the reamer and wiping most of the oil off with cloth or tissue. Reaming dry will give the largest size, although usually a poor finish at the same time.

Solid carbide drills used correctly will give a hole to size and finish without any need for subsequent reaming.

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