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Drilled Hole Tolerances

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ChrisH24/04/2021 10:16:28
1009 forum posts
30 photos

I have a need to accurately drill a 10mm diameter hole transversely in a 16mm diameter bar. Actually, a hole a few thou under 10mm would be ideal. 25/64th inch is 0.3906", 0.003" under, so possibly ideal.

However. I was concerned that it would not drill accurately - my 10mm diameter drill drilled a hole nigh on 0.014" (0.35mm) bigger than 10mm - so I did a trial.

I have a stub of 20mm bar so set that up on the drill and drilled progressively holes of diameters 1/8", 3/16", 1/4", 5/16", 3/8" and finally 25/64", starting with a centre drill and using a pecking technique rather than just blasting through. From 1/4" and upwards I measured each hole diameter with an inside mic; the 1/4" hole was 0.0025" oversize, the 5/16" hole 0.003" oversize, the 3/8" hole a whopping 011" oversize and the 25/64 hole 0.006" oversize.

So the 25/64, far from being 0.003" in theory under 10mm ends up 0/003" over 10mm.

Question is, are these tolerances the norm (not really bothered to check before) or would a better quality drill be more accurate?

And would getting a 9.8mm - 0.3858" - drill on the basis that if it drilled few thou oversize as expected it would end up roughly where I want it to be, be a good option, or would a high quality drill drill more accurately, and if so, what make is recommended?

All my drills are bog standard HSS drills of no particular parentage BTW.


Edited By ChrisH on 24/04/2021 10:18:20

Edited By ChrisH on 24/04/2021 10:19:11

JasonB24/04/2021 10:19:40
21967 forum posts
2531 photos
1 articles

A drill is not really an ideal tool for an accurate hole, bore or ream for better results

ChrisH24/04/2021 10:20:45
1009 forum posts
30 photos

Putting a bracket ) after a " and I get a winking smilie when the post is posted, not what I wanted! Why is that?

ChrisH24/04/2021 10:29:03
1009 forum posts
30 photos

Agreed Jason, but the bar is about an inch down in a block and will be mounted on the mill table. Just concerned that I would be able to accurately achieve that using the mill's boring bar, hence the easier option of a drill so the question remains.

I could try a setup on the lathe if I could hold the assembly in the four jaw and set it up to run true, easier to set up true on the mill, I'm certainly more confident of accuracy in boring on the lathe.

I have also tried a 10mm end mill to fo the final (skim) cut, but with the same result as the drills, 10+thou over.


PS      Plus I don't have a reamer and reluctant to buy one just now!

Edited By ChrisH on 24/04/2021 10:30:31

Emgee24/04/2021 10:34:29
2310 forum posts
277 photos

As Jason says for better accuracy it's drill, bore and ream but you may get a good enough result using a 10mm milling cutter at low rpm after drilling 5/16", a lot depends on the stiffness of the set-up, material being drilled and the quality of tooling used.


Tony Pratt 124/04/2021 10:38:43
1820 forum posts
12 photos

As already stated normal twist drills are not an ideal tool for accurate holes but say drill 9.5 mm to start with and then finish with a 10 mm drill that has the 2 cutting lip corners radiused off with a stone should give good results. Done it many times to get out of trouble when no reamer is available. BTW an endmill is usually made oversize to start with but not 10 thou, you must have cutter run out also.


Andrew Johnston24/04/2021 11:03:30
6390 forum posts
682 photos
Posted by ChrisH on 24/04/2021 10:16:28:

Question is, are these tolerances the norm (not really bothered to check before) or would a better quality drill be more accurate?

I'd say they were pretty poor. Out of interest I've just measured a 1/2" hole I drilled in hot rolled steel last week. Using a Mitutoyo hole gauge:


it measures 0.502". Not sure I believe that; one bush that measures 0.500" just slides in, the other, at 0.501", doesn't. My drills are Dormer, the 1/2" one is old school 2 facet.

For 16mm depth I wouldn't be pecking, just drill straight through. Neither would I start with a centre drill, I use spotting drills instead. At 10mm I'd be tempted to drill through 10mm to start with. If I did use stepped sizes I'd drill one smaller hole around 5mm.

A 10mm hole through 16mm bar doesn't leave much metal at the sides. For a good proportion of the depth the drill is cutting intermittently which may well cause it to go walkies. After spotting and pilot drilling I'd probably use a 10mm centre cutting carbide endmill as it'll be much stiffer and less likely to wander. It's my standard size, so I've got lots of them. I'd also be drilling on the vertical mill with the work securely held in a machine vice bolted to the table.


SillyOldDuffer24/04/2021 11:06:59
7883 forum posts
1724 photos

Other potential problems with twist-drills:

  • If the drill is blunt or pushed too hard, heavy pressure tends to open the helix, enlarging the hole.
  • If the cutting edges aren't a close mirror, one side will cut better than the other. Unbalanced drills wander, overboring, and maybe veering too.
  • Make sure swarf doesn't accumulate

Twist drills are so inaccurate, a special tool is available to correct them. Reamers! Never tried Tony's trick, but that sounds good too - unlikely to be quite as accurate as a reamer, but cheaper and 'good enough'. I always bore larger holes because big reamers are expensive and I don't do many.

I have an unproven notion that the more expensive twist drills are less likely to overbore because they're accurately sharpened and made of a tougher steel that resists the helix opening out.  



Edited By SillyOldDuffer on 24/04/2021 11:09:58

J Hancock24/04/2021 11:23:42
798 forum posts

Is it worth trying a 10mm centre drill ?

Nigel Graham 224/04/2021 11:24:13
1896 forum posts
26 photos

Picking up on Dave's point about the lack of side support, what may help (with careful setting-up) is to lock the work-piece closely-fitting inside a cross-drilled piece of similar material, so the drill is effectively cutting a solid block.

Nevertheless, simple twist-drilling is not intended for precision holes, but as a starting-point for finishing to precise size.

Paul Lousick24/04/2021 11:41:06
1896 forum posts
669 photos


How did you hold the 10mm end mill to do the final (skim) cut. In the drill chuck or in a collet ? A drill chuck is not very rigid and will make an oversized hole. Standard size reamers are not expensive. Making a D-bit out of tool steel is not difficult if you have a mill and lathe.


Trevor Drabble24/04/2021 11:43:26
271 forum posts
5 photos

Chris , When l worked for Intal in Sheffield ( then part of Clarkson International ) the points on our drills were produced to within a concentricity tolerance to the lands of 0.003" , thus , under certain cirmstances the drill could drill up 0.006" over size if the point was at its limit of eccentricity , with the body of the drill being ground to tolerances of +0.000" and nominally undersize by a small amount dependant upon diameter . The drills were also subject to strickley controlled heat treatment regimes . All these factors contributed to the finished product's accuracy , hence the reason why ours , and our other direct Sheffield competitors such as Presto ( Easterbrooke and Allcard ) and Dormer were so expensive

Ronald Morrison24/04/2021 11:58:05
75 forum posts
4 photos

From what I have been reading, a drill is to make a hole. Boring that hole makes it round because a drill may not. A reamer is used to make that round hole accurate. Now you have to decide what is important enough to go through the 3 steps. My farming equipment is usually loose enough in fitment that a drill is sufficient. It doesn't move fast enough that the hole being oversize a little or being slightly out of round will matter in the end. Some exceptions are bearing mounts. That hole needs to be accurately fitted so the bearing will go into the hole with the proper clearance so it doesn't rotate in the hole nor be so tight fit that the bearing breaks during installation.

Clive Foster24/04/2021 12:05:49
2978 forum posts
105 photos

Sharpness and point concentricity make a huge difference to drilled hole accuracy.

My drilled holes have become much more accurate since I got my Clarkson drill sharpening attachment set up.

So much so that I see no real need for reamers as sizing tools, nowadays just used as a surface finisher for bearings.

The Clarkson is fundamentally far more accurate than the usual Model Engineer / Home Workshop type sharpeners. Within its operating range its on a par with the more sophisticated drill grinders like Brierly, Dormer et al although not quite as good as Christen and the like. Being easy to use and always set up I automatically sharpen any drill that looks anything less than really good and always before needing a precise hole.

A big, rigid drilling machine, mine is a Pollard 15AY, doesn't hurt either.


Andrew Johnston24/04/2021 12:09:39
6390 forum posts
682 photos
Posted by Trevor Drabble on 24/04/2021 11:43:26:

......the body of the drill being ground to tolerances of +0.000" and nominally undersize by a small amount dependant upon diameter......

I knew my Dormer metric 4-facet drills had an undersize shank, but didn't know about the imperial 2-facet drills. Time for measurement. A 10mm drill measures 9.95mm on the shank and a 1/2" drill measures 0.497" on the shank. So both sets are slightly tapered. Paying a premium price doesn't ensure a premium product, but paying a bargain basement price definitely ensures a bargain basement product.


Andrew Johnston24/04/2021 12:14:57
6390 forum posts
682 photos
Posted by Clive Foster on 24/04/2021 12:05:49: real need for reamers as sizing tools, nowadays just used as a surface finisher....


While I still use reamers for sizing, the primary goal is better surface finish. On my traction engines I reckon 95% or more of holes are plain drilled.


Tony Pratt 124/04/2021 12:25:33
1820 forum posts
12 photos

I think you will find that twist drills are meant to be very slightly tapered with the small end being the plain shank.


Martin Kyte24/04/2021 13:26:19
2634 forum posts
46 photos

Stoning the corners of twist drills so they are rounded off closes the flutes slightly and produces a slightly undersized hole.

regards Martin

larry phelan 124/04/2021 16:25:57
1137 forum posts
14 photos

Takes me all my time to try and sharpen them, never mind about the "finer" points.

Anyone else in this club ? Dont be shy "laugh

Dave Wootton24/04/2021 16:51:57
227 forum posts
56 photos

I'm in your club Larry, had no end of lessons by exasperated co workers when I was an apprentice, even had some from the toolroom manager ( who I believe could walk on water) but have never been able to sharpen a decent drill without using a jig . Lathe tools no problem but a mental block with drills I even had a little help from a tutor on my end of college test.

Can't really explain it although a few years ago after a small eye operation followed by some vision tests which showed that I have little or no 3D vision, which might explain why I've never been able to catch anything or play ball games. Had a lazy eye when a kid ( heard all the jokes about spread to the rest of me thank you!) and evidently the brain misses the 3D bit, more common than you might think.

Well thats my excuse anyway!!!!


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