Thread: Imperial vs Metric bits

As I am in Australia my view will be metric so it is bias.

All my router bits are metric and I have been buying as and when I need a certain bit. There is a downfall to this in that I am delayed between starting and completing a task. As this is a hobby for me it does not effect me too much but if it was a business I would require spare bits at hand.

It also helps to deal in one format for a build from start to finish, in my case metric.

Personally, I'd call it a bit of a storm in a teacup. I know I would not really be able to see the difference between a 6mm and 1/4" radius, 10mm and 3/8", 13mm and 1/2" and so on.

Then there's the fact that your 19mm thick timber might actually be closer to 3/4" in thickness because 19mm is a nominal size and they might actually be aiming for 19.05mm, or that 25mm thick timber gaining 0.4mm (2%) after a few a humid days to make it 1" (and that's before you worry about how tightly sizing tolerances are held in a product like wood anyway).

I can't actually think of a time when I'd worry about that level of accuracy in wood...a simple change in temperature/humidity will create more change in size than the variation between commonly used metric/imperial sizes.

The only thing I'd worry about is the shank size - imperial collets need an imperial shank, metric ones metric (6.35 just does not fit in 6mm!).

Mastersplinter - you make a good point re infinite size measurements not being an issue.

The issue of collett is not one I considered. My colletts are metric so I guess that answers the question for me. As a newbie I seem to have fixed on the idea, which I know to be incorrect, that colletts for routers are like those in drills where it adpats to the size of the bit being used...doh!

Metric it will have to be. At this stage its not an issue as I probably won't be buying dozens of bits.

It doesnt matter if u use imperial or metric, once the cutters have been sharpened the size will always end up some odd size.

Originally Posted by Gerryattrick

Mastersplinter - you make a good point re infinite size measurements not being an issue.

The issue of collett is not one I considered. My colletts are metric so I guess that answers the question for me. As a newbie I seem to have fixed on the idea, which I know to be incorrect, that colletts for routers are like those in drills where it adpats to the size of the bit being used...doh!

Metric it will have to be. At this stage its not an issue as I probably won't be buying dozens of bits.

Are you sure? I can't stress highly enough the importance of correct shank/collet matching.

To mix them is to court disaster. As far as I know all routers marketed in Oceania have imperial collets: as do the corresponding bit shanks. The principal market for metric colleted tools (and bits) is for domestic continental European consumption. The UK and most of the rest of the world is imperial, at least as far as routers are concerned.

Even the Euro brands sold in imperial markets are "imperialised" for obvious reasons (availability of suitably sized bits).

Unless you've purchased your tool directly from a continental European retailer it's much more likely to be imperialised.

It makes very little difference in my opinion whether you use metric or imperial but as already stated by other members it is imperative that the cutter shanks match your collets and don't try to fit metric into imperial or vise versa they simply don't fit.

All of my collets are either 1/4 or 1/2" and my cutters were either purchased in the U.K or USA some of mine are are 25 years old! Purchased when I first started serious woodworking.

Originally Posted by Gerryattrick

Mastersplinter - you make a good point re infinite size measurements not being an issue.

The issue of collett is not one I considered. My colletts are metric so I guess that answers the question for me. As a newbie I seem to have fixed on the idea, which I know to be incorrect, that colletts for routers are like those in drills where it adpats to the size of the bit being used...doh!

Metric it will have to be. At this stage its not an issue as I probably won't be buying dozens of bits.

Hi Gerryattrick

please check your collett sizes very very carefully

Most routers sold in Aus and NZ will have an imperial collett.
Standard collett sizes in both markets are 1/4in and 1/2in -- but 8mm ones are also available, and sometimes in the box.

However, if you bought your router from an ebay seller, you may have been sold one with a 6mm or 12mm collett. But if you had, my guess is you would have by now had problems gripping your bits.


as to router bits, many bits sold in Australia as metric are really imperial bits in disguise. e.g. a bit sold as 25mm will often measure 25.4mm

To confuse things even more, I have seen routers sold in the big green shed that say 12mm collet when in fact is is a 12.7mm (1/2" collet). Given it is the cheaper brands.
Les

The only metric collet I have come across in Oz was in a router sold by Aldi, and even then there was enough slop in the engineering for a 1/4 inch bit to fit with some persuasion. Even Festo supply 1/2 and 1/2 collets.

Oh, and most of the 'metric' sized cutters are just the nearest equivalent Imperial sized bit with a new label.

When I talked about imperial vs metric I meant the cutter size as opposed to the shank size. With a router with a 1/4 & 1/2 collets I assumed that blades themselves (as opposed to the shanks) were either metric or imperial. Is that not the case?

I guess that the difference between an imperial measurement and the closest metric bit is indestinguishable unless one is working in fine tolerances - not something I am likely to be doing. My router currently has imperial (1/4 & 1/2) colletts and I know I will need bits with the same shanks. I have no idea why I said the collets were metric...more brain fade

So...at the end of the day I need imperial shanks and don't worry about the actual cutter unless I am working to very fine tolerances

Originally Posted by Gerryattrick

, like Australia, we deal in metrics now. Its not helped by the fact that most plans available are also imperial.
My wallet says buy a basic set of cheaper Imperial bits from USA and once I find out the ones I use most replace them with metric over time.

My head tells me not to be a tight and buy decent metric bits (maybe from Australia??)

I would appreciate the views of others that may have faced this decision.

Ok; Australia is not as metric as many believe which is why sheet goods often still come in 2440mm X 1220mm (8' X 4') and all timber sizes are priced by the meter but sold in feet and yards (900, 1.5m, 1.8m, 2.4m, 2.7m, 3m, 3.6m, 4.5m, etc). If you take a set of calipers to the hardware stores you'll find a mixture of Imperial and Metric thicknesses of ply but sold under "nominal" metric sizes; eg 1/2" ply will be sold as either 12mm, 12.7mm or 13mm.

To be honest; the only real need you'll have for specific Metric or Imperial sized cutters are in the basic straight cutters where you need to make an accurate slot. Template following and roundover bits etc; does it really matter? I have matching roundover and cove cutters in Imperial sizes; I have never needed a Metric one ever. All my beading, profile, thumbnail and moulding cutters are whatever size and shape I wanted at that time.

As for brands; CMT (best in world IMHO), Freud and Trend are the top of the food chain; think Mercedes & BMW. Carbitool are Australian made and easily available; to me they're the Ford/Holdens of the router world. Linbide (NZ) very close to carbitool.

Everything else is a Great Wall at best!

I have a mix of imperial and metric and I see no issue whatsoever in mixing them provided any jigs etc you have you clearly label with what bit they suit if it matters.

As mentioned by other unless you are doing a dimensioned slot then it really does not matter if its either, example is I have 4 flush trim diameters (1/4, 10mm, 12mm 1/2 inch) and the actual function between them is effectively the same (yes smaller radius corners etc)

There are many times where it is important to use metric cutters as an example of Inserting an Inlay where the material is routered out and the insert has to be a perfect fit. Example 2 when routing small circular or elliptical boxes and a 'Plug' is required when routing the external edge. I personally stick to the metric cutters
Tom

Like many others I quite happily use both metric and imperial. If using a template I've made for a particular task I always make a note on the template of the guide bush and bit that should be used with it. My router came with 3/8", 1/4" and 1/8" (a plastic/nylon one that I'm a bit doubtful about ) collets and I also have 1/2" to 8mm and 1/2" to 6mm collets. When it comes to buying solid carbide bits these last 2 collets are very handy - think much cheaper bits available for CNC.