Being new to model engineering I was wondering what the concensus is on the need for all the various gauges of drill? Do I really need numbers and letters if I have metric and imperial? According to my Model Engineers Handbook (Tubal Cain) it seems I don't but Peter Wright (Model Engineering a Foundation Course) doesn't really answer the question. Roy
It depends to some extent upon the projects that you propose to undertake; number/letter drills may be helpful if the project is old.
(ISTR that the tapping drill recommended for one of the 5BA or 7BA threads favoured by Stuart (Turner?) kits has a number drill recommended)
Does not Tubal Cain's book contain a chart showing the nearest recommended replacements anyway?
Probably the best approach is to wait until you definitely need a number/letter drill before buying any. (Unless, that is, if you have loadsa money to spare to set yourself up with drill sets at £50 - £80 a time)
ISTR that the "Z" drill is most useful, being marginally larger than the largest 10mm drill normally supplied in metric sets. I know it's not as large as a 1/2 inch Imperial drill, but those sets of drills don't seem to be hardened with the drilling of steel in mind.
I have found that a set of imperial in 1/64th increments, plus a set of metric in 0.1mm increments, has met all of my needs so far. You will find that some writings, such as John Wilding's clock construction articles, mention number/letter drill sizes, but these can usually be substituted for metric/imperial equivalents, or the design modified slightly to suit the tools you have.
I bought two sets of metric drills 1.0-5.9 and 6.0-10.0 in 0.1mm increments because I figured that anything that needed to be more accurate than 0.1mm would probably need reaming anyway. Depending what size of model your building I'd recommend just buying the 1.0-5.9 set (as they are about 1/4 the price of the6.0-10.0 set) and odd larger drills as required. By and large I have found that this worked, and a taping drill never needs to be more accurate than 0.1mm so it covered most eventualities. I also bought a set of empty drill stands (letter, number and imperial sizes) for a few pounds and gradually filled them up with odd drills that I got from car boot sales, etc. If your willing to spend an odd evening sorting them out it is a cheap way to buy drills -and they are mostly good quality old English ones instead of cheap imports. Good practice at sharpening them as well
If you are just starting in the hobby, a good quality fractional set will do for quite a while. I have a full set of fractional, number, and letter drills, and believe me, they are a great convenience! So far, there has been no need for metric bits, but I live in Canada.
In my own recent experience, a set of 1mm to 6mm in 0.1mm steps will probably cover the majority of your needs. I bought such a set a couple of years ago and I can't remember when I had to delve into my older sets of number and imperial fractional sizes since then. If you have a bit of money to throw around, you could get the range in
For the really small sizes, there are a few sets of small number drills around - usually 60 - 80, or .035 - 1.00mm in 0.05mm steps. The only problem you have there can be the quality. I have bought one set which must have been poor quality carbon steel since the drills would not last any time when cutting metal. But at least the cost is not too high and you haven't lost a lot of money if you have to bin them.
For sizes larger than 6mm, buy individually when required since the prices of sets start going up exponentially :-)
Watch out for larger drills supplied in the DIY stores - some have their points ground in a form which probably assists the home handyman, but which doesn't work all that well in an engineering environment - ask me how I know :-) Also some of their so called imperial sizes tend to be the closest metric size - again, ask me how I know :-)
I work on a project basis, rather than tools basis, ie I start with the item I want to make and buy the tooling specific to the task. This way I spend only when I need to spend and I buy only what I need. It also means I can have more control over the quality of the tools I buy. (The only exception is if some shop or other is having a clearance sale when I can't resist buying stuff I'll probably never use!). If I need two taps, which looking ahead will get regular use I can buy "top notch" stuff. If one is an odd ball which is unlikely to get used again, I'll buy a cheaoer version. Specifically on your question, occasionally, a fractional drill is required for a metric tapping job, so you can't really say "I'll only work in metric".
I work like this for eveything. I wanted to turn some parts, so I got a lathe. Now I need to mill some parts to improve the capability of the lathe, so I've ordered a mill... It's a bit like the old lady who swallowed a fly I guess!
There are exceptions. One new part requires a 45deg Dovetail cutter with a
10mm cut. Huge money not worth it for two cuts in a lifetime. So a cheaper way (no pun intended) will be found.
Most outfits (J&L, screwfix etc) deliver next day, so on a hobby basis, not having a specific cutter to hand is hardly likely to be an issue.
Myford do a nice wall chart lsting number, metric and imperial sizes together, so its easy to find the nearest equivalent. I know you can look in the book - but the chart is easier.
Buy good quality drills.
Practice sharpening them - you can really feel the difference as you use a selection of badly and well-sharpened drills.
Nomally ground drills can snatch when drilling brass. You used to be able to get special straight flute drills, but conventional practice now is to stone a small flat on the cutting edge. But then the drill needs sharpening again for steel. - So buy an empty drill case and as you collection of drills grows you can house duplicates in there, ready prepared for brass.
I keep one set for brass and another for steel, ally ,etc. (The stoning of the drill's edge follows the practice of a negative rake angle for lathe tools for brass. The snatching is otherwise caused by the pointed cutter digging down into what is really a very soft metal)
I needed a 7mm hole which was fairly accurate in diameter. I drilled small and opened out to the maximum size of my 1mm - 6mm set then popped down to the local B&Q to get a 7mm drill. I didn't look too closely at the sharp end until I had chucked it up in the lathe. The end was like an end mill with a small point in the centre, so there appeared to be little centering action and the drill cut oversize.
As for the actual size, just put a vernier or a micrometer on a suspect drill and see what you get. I assume the DIY places reckon that the drills will be used in woodwork and for basic drilling jobs where accuracy is not really required and a few thou off the actual size won't make all that much of a difference. I've actually seen drills with their size shown as dual metric and Imperial - like
5mm/3/16" and it is up to you to guess whether it is accurate to the metric dimension or the Imperial dimension, or somewhere in between
- and you can't even take a mic or a vernier along since the drill is sealed in its package :-)
Trouble is it never ends. I'm now building up sets of cutting tools for plastics as well.
You don't really have a choice will serial production. We make a lot of repro stuff, most of which is spec'd in Imperial. If a part is spec'd as
3/32" then 3/32" is the dimension to work to. Conversion to metric either gives an unwieldy number (2.38124 mm) with implications for tolerances, or, if rounded, loses the centre of the original tolerance field. If the part has to fit with something else (like a plug and socket), loss of the centre dimension results in reduced yield in production.
The trouble with new standards is that the old ones don't magically vanish. You still have to work to them where they were specified.
It sound like the drill you bought were ground for cutting wood. Some of the HSS sets are supplied with this shape end looking at them quickly and you don't notice. Just like you say a flatter point angle and a small pip on the end. There is a company near to me that sells sets of drill at discount prices, Millhill supplies. I remember looking at these and noticing two different point styles.
Thanks for all your replies. I've sort of made up my mind now what t
get except that Wilfred brought up another problem! How do you tell what is good quality? I know what the DIY stuff is lik but do I assume that the likes of Chronos, GLR and Engineers Tool Roo will sell the quality required? Although the later do quote HSS BS328 DIN338, implying some sort of quality, none seems specific about th manufacturers. Ro
From your description, it sounds like the drill you bought was one designed to drill wood (not metal) - the small point in the cenntre is intended to give a centering action when drilling wood, but if you use it with a pilot hole, that centering action is lost. A conventional twist drill would have worked a lot better.