A word of Caution

Masters Mate

Masters Mate

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Nov 6, 2019
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Location
Sunshine Coast Queensland Australia
Perhaps some of our more experienced members have already seen this problem but I never expected to have to check commercially produced drill bits. Until now.
I have bought a quantity of these .7mm bits over time but it was only when I tried using .7mm nails through drilled brass that I discovered the discrepancy.
All these bits at .6 mm but marked as .7mm
IMG20240302075926.jpg
IMG20240302075948.jpgIMG20240302080047.jpg
The nails and brass rod are .7mm as marked.
 
I think that a drill bit size as measured is always "a bit" smaller than the hole they make. I attribute this to the fact that every drilling machine has some degree of axial runout which in turn produces a slightly larger hole than the drill bit diameter. This may allow vendors selling a slightly smaller size bits marked with larger diameter.
 
Last edited:
Masters Mate said:
Perhaps some of our more experienced members have already seen this problem but I never expected to have to check commercially produced drill bits. Until now.
I have bought a quantity of these .7mm bits over time but it was only when I tried using .7mm nails through drilled brass that I discovered the discrepancy.
All these bits at .6 mm but marked as .7mm
View attachment 431800
View attachment 431801View attachment 431802
The nails and brass rod are .7mm as marked.
Click to expand...
The thing is that we are usually taken for a ride with these drill bits. If you buy a boxful - a collection say from 0.5 mm to 2.3 mm - you will find that at least half of these are not the size they supposed to be. These are mostly imperial sizes and they are just close to the nominal size they are marked with. However the same story applies if we buy bigger drill bits at Bunnings or Mitro10 - they are mostly imperial sizes, smaller, close but not identical to the size what we expect them to be.
János
 
Y.T. said:
I think that a drill bit size as measured is always "a bit" smaller than the hole they make. I attribute this to the fact that every drilling machine has some degree of axial runout which in turn produces a slightly larger hole than the drill bit diameter. This may allow vendors selling a slightly smaller size bits marked with larger diameter.
Click to expand...
Probably true as far as physics goes, but it doesn't explain why the finished holes with these bits are still not .7mm diameter. Maybe the finished holes are .65 but they still would not accept a measured .7mm rod or nail.
If I were to purchase .8mm bits next time would they drill a .7mm hole or something else? For me there's too much guess work involved and working at these tiny sizes is hard enough already.
 
janos said:
The thing is that we are usually taken for a ride with these drill bits. If you buy a boxful - a collection say from 0.5 mm to 2.3 mm - you will find that at least half of these are not the size they supposed to be. These are mostly imperial sizes and they are just close to the nominal size they are marked with. However the same story applies if we buy bigger drill bits at Bunnings or Mitro10 - they are mostly imperial sizes, smaller, close but not identical to the size what we expect them to be.
János
Click to expand...
I believe this might be the answer. It doesn't solve the problem but might explain it.
 
I think you have to measure the hole that the drill makes, not the drill itself to get the correct result. Also, if you want to be careful, drill a lot of holes, say 100, and then calculate the standard deviation. Then it all also depends on the type of material you are drilling. All this is taken into account in professional manufacturing. Therefore, factories have machining technicians who only work on this.
 
I would use a lot of these little bits, but only in my Proxxon MF70 as the tend to break far too easily hand held. As for the size, nearly always they are too small, I don't know if this anything to do with the fact they are designed for PCB's, could be.. I use a lot of 0.4mm brass wire and it will never fit into a hole drilled only with a PCB drill bit and now as a matter of course I run a high quality titanium bit from the likes of Proops, (jewellery suppliers), through the hole using a pin vice. But the PCB ones are cheep.

Cheers JJ..
 
I have bought over the years many packs of cheap Chinese tiny drills of .6mm, .7mm, .8mm, .9mm The main problem I find is that they are not sharpened correctly when new and probably 75% of them won't drill at all. At 1mm and above they do start to be sharpened OK and usually drill OK.
The more expensive shouldered drills such as shown in the above picture I find to be correctly sized and sharp but are very brittle and break easily making them an expensive proposition. I use small drill bits mainly for drilling brass and white metal, wood should be an easy drilling job.

Doug Hey NZ
 
Reminds of my early career as a buyer for and machine tool manufacturer.
In 1969/70 the UK was going through the pains of metrification and as we were making German machines on license from US drawings it was thought that we get ahead of the game and go completely metric.
Try buying metric drill bits in the UK in 1969. No chance. The UK used imperial sizes of course but also had "Letter Drills" and "Number Drills". Perhaps you guys in the USA still use them as I see there are conversion charts available these days.
Many a happy time spent trying to find the nearest imperial, letter, or number size to the metric that we needed!
 
If I need a good fit I always "sneak up" on the hole. If you drill with a 1/4" bit, quite often it will be a bit large but if you sneak up on it you end up using a 1/4" drill and it fits.
 
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