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Very small holes

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Steve Withnell30/07/2022 17:17:07
848 forum posts
215 photos

I have a little motor with a prop shaft which mic's up at 1.48mm. The coupler that came with it is a sloppy fit, not a good idea at 10,000 rpm.

So I turned a new one then drilled that 1.5mm. Sloppy fit.

If I was talking 14.8mm it wouldn't be a problem, but how to I get a sliding fit at the small diameter I have? I did wonder about freezing the thing and drilling it 1.4mm whilst it was very cold, but I doubt that will get me close.

Any thoughts on how I get to a true sliding fit?

JA30/07/2022 17:36:11
1401 forum posts
81 photos

Have a look at Drill Services, Horley, web site ( They can supply hand reamers starting at 1,0mm diameter with 0,1mm steps after that.

I have always found them a good company to deal with.


Dave S30/07/2022 17:54:32
373 forum posts
90 photos

Drill 1mm and then make yourself a single flute drill from silver steel, also known as a d-bit.

Or make a 1.48mm spade drill. Watch makers use them, so a bit of Google in that direction will show you how to make them.


speelwerk30/07/2022 18:16:18
447 forum posts
2 photos

Try drilling 1.3mm, open it up with a 1.4mm drill and again with a 1.5mm drill. Niko.

SillyOldDuffer30/07/2022 18:22:17
8862 forum posts
1995 photos
Posted by Dave S on 30/07/2022 17:54:32:

Drill 1mm and then make yourself a single flute drill from silver steel, also known as a d-bit.


Dave got in first, but I have a picture!


The silver steel is turned to the wanted hole diameter. Then harden the rod and grind half the diameter off to make the 'D' outline. Finally, the top is ground at a slight angle. Important the 'D' profile be exactly half the rod's diameter, or no more than a smidge under.

D drills cut slowly and need a pilot hole and plenty of lube.


Clive Brown 130/07/2022 18:26:14
862 forum posts
47 photos

I.45mm dia. is a commercial drill size eg; here. Might be worth a punt to see if it cuts slightly oversize.

duncan webster30/07/2022 19:24:40
4105 forum posts
66 photos

making a 1.5mm D bit will be a challenge. If a bit of 1.5mm silver steel is a good fit chop a bit say 50mm long off and harden and temper the end. Then grind away at a shallow angle. For something that small I'd use an oilstone. Voila a toolmakers reamer. Drill the hole 1.4mm and ream it out.

If this isn't clear I'll take a picture of one I made to very slightly open out 4mm holes

Speedy Builder530/07/2022 20:26:10
2642 forum posts
217 photos

Does it need to slide once in position ? If not, sloppy fit and Loctite bearing fit (other makes are available)

blowlamp30/07/2022 21:10:01
1632 forum posts
105 photos

Take a stone to a 1.5mm drill to adjust its size.

Neil Lickfold30/07/2022 21:42:41
890 forum posts
195 photos

On the outer edge of the 1.5mm drill, radius the corners with a stone. Then relive on the grinder the back outer edge to create clearance for the radius to work. I do this after using a 1.4mm drill. It is like using the drill as a reamer. Depending on the materials, but using a water based coolant can make the hole very close to the size, but I find that castor oil will make the tool cut to the closest size of the tool . It is messy and you need to clean everything after, otherwise it will go gummy. Then support the part between two centres to make the hole concentric to the outside for the rest of the part being made. The thread etc can be then turned off the trued outer.

lfoggy31/07/2022 21:07:09
196 forum posts
18 photos

A clockmaker would drill to 1.4mm and then use a tapered 5-sided broach to open out the hole to 1.48mm. This can be done with a surprising degree of accuracy by hand. The resulting hole will of course be slightly tapered which may or may not be relevant depending on your application. Works very well for clock pivots. I use this technique all the time in my workshop. One advantage of the slight taper is that it will trap a bit of oil.

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