Drill bit accuracy

For a while I’ve been buying my drill bits from Warco, and I found them to produce a hole size that was pretty much spot on. My definition of being spot on being a nice sliding fit with ground silver steel of the same diameter. Drill bits I’ve bought from RDG tools, Amadeal, and Arc Euro trade were all just as good.

I then switched to one of the bigger ebay suppliers and bought sets of ‘precision ground TIN’ and results are all over the place. Price wise, and in packs of 10 they were maybe 2/3 the price of the suppliers mentioned above. Over a cross section of sizes, A 4mm ebay drill is producing a hole size of 4.32mm, a 6 mm drill 5.95mm, and 8mm drills were banana shaped.

Granted I can use a reamer for accuracy, but I’m curious to know if the usual suspects that we buy our stuff from are all particularly good, or is my ebay supplier particularly bad. Maybe it’s just a case of you get what you pay for.

I’m also curios to know what people deem acceptable hole size accuracy ?

The results are the same for drilling in either the mill or the lathe, with different chucks used for both, across a good sample of the packs of 10 drill bits, and results measured in aluminum, brass, and steel. Returning to the above ‘engineering’ suppliers with my old drill bits produces the same excellent results, so nothing has gone wrong in my machines.

David

Thinking about it, I should probably define what I mean by acceptable. For the sake of this discussion, let’s say it means an accuracy to keep 80% of the people on this forum happy when considered against a set of drills for general model engineering.

David

If the point of the drill is not on the centreline, it will cut oversize.

The longer cutting edge will rotate about the point and result in an oversize hole.

If you are sharpening drills, yourself, you need to be careful of this.

Not everything cheap is good value.

Howard

.

I usually buy Dormer drills and search for them.

I have however had an 11.5mm one supplied that was no more a Dormer brand drill than I am Neil Armstrong.! - It even had Dormer printed on it but at a low quality. I showed it to a pro machinist mate of mine who just laughed at the ground cutting edges.

I have never used that particular supplier again.

Like many things there are fakes out there. One of the biggest brands being faked is Scandisk memory cards to such an extent that many have stopped buying that brand. Even major retailers have been duped and unknowingly sold them on thinking they were genuine.

Nick

Hi David

​FWIW...Like you I buy my drills from the larger suppliers off eBay & in general they are not too bad although I have had some right bananas within the 'box of 10'. I find the better supplier for 'economically' priced, & reasonably accurate drills is usually 'UK drills'... plus others from Machine Mart, Toolstation, Screwfix... usual disclaimers... but as you know 'you pays your money'... I have a various makes bought,'obtained' from colleagues or collected in general, & if I can get hold of 'Presto / Dormer / Guhring, all the better, I much prefer the German Guhring, superb accuracy all the times I've used the ones I have... in my milling machine; also, still acceptably accurate, allowing for bearing play in the bench drill I have.

​I have drills from many moons ago, collected from wherever I've worked, usually 'Presto' or 'Dormer', & more the larger size which I still use on occasion, fortunately I can still freehand grind the larger drills to a reasonable accuracy albeit with the aid of a 'gauge' bi-focal glasses & a loupe for the smaller sizes. As mentioned if one lip is longer than 't other you will get an oversize hole hence the gauge, ( marked with lip length scale ).

I also tend to make allowances for any eccentricity by using one drill size smaller than the diameter I want & either ream, or more commonly, find that it has drilled 'to size'. I tend to use a 3mm spot drill to start off the smaller holes which I find is a great help in keeping holes on centre.

George.

My elderly "Dormer Twist Drill and Reamer Information Handbook" lists diametral tolerances according to BSS.

All tolerances are +.0000. Minus tolerances vary from - .014 mm for drills up to 3 mm. dia. to -.054 mm for drills between 80 to 120 mm. dia.

A 10mm drill for example would be +0, -.027.

Off-centre grinding would change this of course.

All reamers are plus tolerances

Edited By Clive Brown 1 on 03/06/2017 19:31:31

Edited By Clive Brown 1 on 03/06/2017 19:33:26

I buy as much of my tooling as possible from my local engineering tool suppliers in Sheffield, they supply industrial customers so even budget priced offerings have been tried out and found to be acceptable for production work.

I mainly use Dormer, Guhring and other major brands, I haven't seen the need to go cheap on drill bits yet as I suspect they don't work out cheaper in the long term.