|bernard towers||06/07/2020 13:48:28|
|20 forum posts|
Unfortunately the spade tip of even a small center drill is larger than the intended drill which can allow the drill to wander. If you have a piece of 1/8 carbide or hss grind 3 or 4 facets giving a approx angle of 60deg and a sharp point, make a small divot which will be the lead in for the drill. The idea came from a cnc website and they were charging over £40 for just one of these!!
|410 forum posts|
Bernard, centre drills are available down to .3 diameter, I would think that is small enough for most work.
|bernard towers||06/07/2020 15:20:32|
|20 forum posts|
The only problem with a .3 centre drill is that the .3 tip is very short and fragile why have the problem when there is a solution that’s cheap to make and almost impossible to break,
|Rod Renshaw||06/07/2020 16:05:52|
|117 forum posts|
Spotting drills are available in small sizes and have much more robust points than centre drills. They are made for the job of "starting" drills and have better geometry for the purpose than centre drills; which are really made for drilling centres for turning work "between centres". Use of centre drills for starting twist drills is traditional amongst model engineers but they are not ideal for this purpose.
|Michael Gilligan||06/07/2020 16:58:01|
15891 forum posts
I suppose that could be seen as a ‘machine version’ of the watchmaker’s graver
This is promoting Eternal Tools' carbide gravers ... but it illustrates some standard processes rather well:
[skip to about 1min 30sec if your’e in a hurry]
Edited By Michael Gilligan on 06/07/2020 17:04:07
|410 forum posts|
Rod, using centre drills for starting drills has been traditional in industry until perhaps twenty years ago when the new kid on the block, the spotting drill turned up.
|Rod Renshaw||06/07/2020 19:05:20|
|117 forum posts|
Baz, I had not realised that spotting drills were so recently developed. I have not worked in industry and imagined spotting drills being used for years without MEs getting the message.
|Martin Whittle||08/07/2020 20:19:42|
|94 forum posts|
Thanks for the advice, many interesting comments much appreciated.
I thought I would give an update on this. I was looking around on eBay, and found many PCB drill sets on 3.175mm shanks. There are suppliers in China doing sets very cheaply, below £4 for a set of 10 drills typically in the 0.3 to 1.2mm range. Some sets go down to 0.1mm!
I bought a set from a UK seller, ‘Flux Workshop’, for a set for £5.09, and it was delivered on the second day after ordering, the postage label indicating a 24 hour service. The 0.3 to 1.2mm set was one of the available options under item 113799603202. They are of Japanese manufacture rather than Chinese.
Looking at them, they look well made, with the point ground reasonably symmetrically. There is some variation of the grinding angle on the tip between the various bits, but this of minor importance. The helix angle is quite fast as is usual for a PCB drill.
I briefly tried a 0.6mm bit on the lathe, going into a brass rod which had already been ‘pecked’ with a centre drill, it worked very satisfactorily.
I am not clear on the bit material, the description on this item does not give this, but many similar adverts mention ‘tungsten steel carbide’ which I assume relates not to tungsten carbide but a form of HSS? The bits are strongly magnetic (tungsten carbide is usually just weakly magnetic), and shiny like HSS. I would personally expect a fairly high breakage rate if they were tungsten carbide!
I am very pleased with them, especially at this price.
NB Pero asked about sharpening small drills. I have never tried this, but there is a simple jig on https://modelengineeringwebsite.com/Small_drills.html by Graham Howe, which may be worth looking at.
|113 forum posts|
Bit late getting back on this but thanks for the comment on sharpening. I have seen the photos and plans for one of these (or very similar). OK for the larger small drills, if that makes sense, but the problems that I have are being able to see the tip that I am working on and supporting the tip being sharpened. I can check the tips using a dissecting microscope but don't have a way of seeing what is happening as I am trying to sharpen.
I have come to the conclusion that the answer is to treat them as disposable items and it they are not working move onto the next one. One possibly bright note is that some of the manufacturers on the Alibaba site sell in small quantities so the quality may be better than the reject stock on Aliexpress - or possibly not!
I'll give one a try next time round.
|Stewart Hart||13/07/2020 20:27:32|
643 forum posts
I’ve had a few comments about the video that the unit is out of line bending the drill I’ve taken this criticism on board and checked the alignment and yes it was slightly out and I’ve corrected it.
I’ve used it today to make an injector using drill number 63 56 and 68 from the cheep kits under the eye glass they looked okay. I started them off with a spotting drill and they went through at high speed like a hot knife through butter. To give small drills a fighting chance you really need high speed my unit was doing 3000rpm
I hope this helps if any one wants details of the unit just drop me a private message
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