Endmills – metric Vs imperial

[colinbParticipant @colinb]

Is there any functional reason to prefer one over the other?

Mine will be a metric shop, but of course imperial end/facemills are much more common on the secondhand market. I am trying to think of pros/cons, and so far, the only differentiators I’ve come up with are:

1. Plunging counterbores – or any other operation where the diameter of mill actually matters
2. Minimisation of error – unit conversions inevitably lead to errors, picking up a mill that’s a fraction over rather than a fraction under size

Everything else (roughing, facing, side milling, slot cutting) can all be done with metric or imperial, so long as the mill is smaller.than the final feature to allow for finishing cuts.

Do I have the right picture here?

 

[Thor 🇳🇴Participant @thor]

I mainly use metric end mills and slot drills because that is what is available where I live, I do have a few small imperial ones (bought in the UK).

Thor

[colinb]

On 18 May 2024 at 08:27 colinb Said:

Is there any functional reason to prefer one over the other?

Despite the endless, but ultimately fruitless, arguments on here about metric versus imperial it doesn’t matter a jot.

Metric cutters are more readily available commercially if buying new. But imperial cutters are still available from other sources such as Ebay.

My engine build is imperial, but for professional work I mainly use metric. My go to cutters for the vertical and CNC mills are 6mm and 10mm 3-flute centre cutting uncoated carbide cutters and the same sizes but polished, specifically for aluminium. For the engines I have a range of imperial slot drills (bought new) for keyways and counterboring. I buy others as needed; metric from commercial suppliers, imperial mostly secondhand. I have also bought a lot of larger cutters secondhand. They come in handy, especially on the horizontal mill, and I wouldn’t have been able to afford them new:

To summarise, buy a few metric cutters as standard; 3, 4, 6 and 10mm diameter covers most needs. Then buy other cutters, metric and imperial, as needed and new or secondhand based on need, availability and cost analysis.

Andrew

[John HaineParticipant @johnhaine32865]

I don’t know why unit conversions should lead to errors – 25.4mm has for decades been the exact metric equivalent of an inch (i.e. inch defined as).  https://en.wikipedia.org/wiki/Inch

Easy to compute metric diameters for all common imperial cutter sizes to more precision than any home milling machine.

Used cutters, whether imperial or metric, are not likely to be very sharp so if you are going to use a lot then equip yourself with a means to sharpen them.

[JasonBModerator @jasonb]

These days I’m only likely to take an imperial cutter out of the draw if I want to cut a narrow keyway or even less likely want to plunge an imperial diameter.

As Andrew says 4, 6 & 10 will do to start with for most jobs then buy anything you specifically need

On my CNC where I tend to use Carbide just about every cutter is 6mm or less and I have a reasonable selection.

[Clive FosterParticipant @clivefoster55965]

If, as I do, you have a DRO system, and set your reference to the centre of the material using a wiggler or edge finder on each side along with the 1/2 function its most convenient to use a cutter dimensioned in the same way as the job.

Metric cutter for metric jobs, imperial for imperial.

That way picking up from the centre means I can directly set where the edges of the first and subsequent cuts will lie. So the whole job can be done directly off the DRO scale with no measurement needed beyond final verification that all is within tolerance.

Using a cutter dimensioned in the other system means I either have to measure after the first cut to verify exactly where the edge lies or touch off the cutter on the workpiece before deciding on cut step-over. Either cigarette paper or feeler gauge methods work fine for me but its another step and potential for error.

The step-over doesn’t care whether the cutter is metric or imperial but you need to know here you are starting from.

Taking centre of job and centre of spindle as my references suits me just fine for all normal work but cutter and job need to be dimensioned in the same system.

Clive

[Peter Cook 6Participant @petercook6]

Drawbar thread?

I occasionally use a 1/2″ shank tool (sensitive drill) in my little mill with an 1/2″ MT2 collet – but that needs the drawbar changing to an imperial one. Metric collets and my ER16 adapter all use M10.

[JAParticipant @ja]

Like others, I do not understand the question. One uses both with very little concern.

Far more important is to keep the cutters (always slot cutters and end mills in my case) in their nice little plastic containers as in Jason’s photograph. The containers are for their protection, not your’s.

JA

[JA]

On JA Said:

Like others, I do not understand the question. One uses both with very little concern.

 

JA

Indeed so if you work in the traditional manner using dials and touching off the cutter. The step-over doesn’t care about cutter dimensions as its governed by dial calibrations.

However, as I described above, if you use and exploit a DRO set-up there are considerable benefits if both cutter and job are dimensioned in the same system. Many folk don’t really appreciate how different exploiting a DRO is to working with old style dials and how much of a difference seemingly trivial things make. Mostly in ease of doing things quickly and correctly.

I imagine running a CNC system almost manually setting travel limits on the machine supplemented by the MPG pendant has a similar crop of superficially trivial benefits that can add up to a lot. I really, really wish my DRO system could be linked in to control feed stops.

Clive

[JasonBModerator @jasonb]

It really only makes mental arithmetic easier, if you want to cut a 1.00 wide feature with a 1/2″ cutter you either offset 1/8 or 0.125 each side or move the spindle 3/4″ or 0.750 from an edge.

Error creeps in doing the same with a 12mm cutter that needs converting to imperial and then the sums doing for offset and you may well want to reach for a calculator rather than doing the simple maths in your head.

I tend to fully convert old imperial designs to metric eg metric nominal sizes so 3/8″ becomes 10mm not 9.53 so even simpler to do maths with whole millimeters rather than several decimal places. so 6mm cutter needs offsetting 2mm each side if a slot or 7mm in from an edge.

[DiogenesParticipant @diogenes]

Yes I use a mixture of metric and imperial cutters – since you mention counterbores, sometimes a cutter from the ‘other’ system will let you cut a counterbore, spotface, or what have you that is ‘neater’ than the ‘book’ for your native system – manufacturing does seem to like a (grossly) slack fit for such things.

If you have a DRO it’s not too hard to switch to ‘Imp’ to figure the offsets, and then back again to metric for the rest of the job.. ..I have mixed ‘systems’ within the same job a couple of times.. ..it’s good for one’s mental agility..

 

[Clive FosterParticipant @clivefoster55965]

Diogenes

At (almost) 70 I can do without any more things said to be good for mental agility!

Writing the DRO settings for each cut down on the whiteboard next to the mill before I start makes for a much more restful job as well as reducing the chances of a boo-boo. Now my machining is more fits and starts than really regular I’m much more conscious of the time it takes to get fully into gear and flowing in “the zone” where things just go right.

Given his prodigious output I sometimes feel that Jason doesn’t quite appreciate how different things are for folk whose workshop time is severely limited by family and other responsibilities. If a couple of evenings a week and the odd half day on some weekend is all that can be scrounged getting into the zone is hard, if its possible at all, hence a more measured approach with full awareness of the potential errors may be more productive.

Certainly in my days of motorcycle riding instructing and examining I was continually reminded how different folk are in the rate of learning / understanding and how much difference regular riding made to what, and how, you could usefully tell them. Post exam debriefs I found most stressful. Want them to progress but very conscious that telling it the wrong way could get them killed due to conceptually grabbing the wrong end of the stick.

A working life spent doing weapons systems guidance stuff does pre-dispose towards caution.

Clive

[JasonBModerator @jasonb]

Clive, that is about the amount of time I get in the workshop. I’m not retired and have other interests too.

[Howard LewisParticipant @howardlewis46836]

If the inly purpose is to remove metal from a dace, the size of the cutter will have little bearing on the choice. A 10mm end mill will remove 0.025″ just as effectively as a 3/8″, from a face.

BUT, if you need a counterbore or slot of a definite size, then Metric or Imperial will matter.

I use whatever seems most appropriate at the time.

Howard

[Nigel Graham 2Participant @nigelgraham2]

As with Howard: and using specific diameters only where really necessary.

Either way, and though helped by a DRO, I still measure the work as it approaches the finished size. Nevertheless it is better to use cutters that match the drawing dimensions, for anything more than simply squaring off the sides of the work.

If as I do you use a Clarkson ‘Autolock’ or equivalent you do need ensure the collets are correct metric or imperial as even the closest (probably 5/16″ / 8mm) are not interchangeable. The metric collets have a groove turned in their heads to help you differentiate them from Imperial – and I similarly marked the metric collets I made for a Hemingway Kits T&C Grinder.

Incidentally I sometimes use just the dials, if the milling operation is simple and not too critical.

[colinbParticipant @colinb]

Much input, and much appreciated – thank you.

Confirms what I thought – horses for courses, where the course comprises both the machine tooling and the operator!

In this case, as a someone who will be embracing DRO over dials, and a 100% metric child of the 80s, I’ll follow that path of least resistance. No fractions unless I absolutely have to! Any imperial cutters will be labelled with reference to their metric proximity e.g. 3/8″ will be “a bit less than 10mm”.

Time will tell how this pans out for me…

[John HaineParticipant @johnhaine32865]

Why not 9.53mm?

[John Haine]

Hi,

On John Haine Said:

Why not 9.53mm?

Because 3/8″ is 9.525mm

I’ll get my coat.

Regards Nick.

[colinb]

On colinb Said:

…

Confirms what I thought – horses for courses, where the course comprises both the machine tooling and the operator!

…

More I think the type of work that needs to be done.   My workshop is deliberately metric throughout because that very much suits the things I make.   I would only have gone Imperial if:

1. I was personally 100% imperial trained or an American
2. someone gave me a fully equipped Imperial workshop
3. I intended to specialise on repairing Imperial equipment – restoring old cars etc
4. My hobby consisted in building locomotives to any of the many excellent Imperial plans drawn in the last century

In practice, 99% of the limited Imperial machining I do can be achieved with metric tooling without much fuss.  Needing a short 3/8″ slot recently I cut it by widening a 8mm slot:  took a little longer.  But if I had to do a lot of 3/8″ slotting it would make much more sense to order a 3/8″ cutter.  Same applies to Imperial chaps needing the occasional metric dimension – not difficult to widen a 3/8″ slot out to 10mm, but might be worth investing in a metric cutter.

Though I can if I must I prefer not to prat about with inch fractions.  They just make life complicated, which is why Imperial drawings increasingly dropped fractions in favour of thou during the 20th century.

Dave

[colinbParticipant @colinb]

Simply reflecting my thinking – if I’m cutting a 10mm slot, I’ll be looking for a mill a bit less than 10mm for the main cut, then bringing the slot to size with. For reasons unknown, my brain probably won’t go “looking for 9.5mm, or 9.525mm” – so this labelling fits that.

Nothing stopping me doing both, of course!

[colinbParticipant @colinb]

I think this is more or less where I am at!

[colinb]

On colinb Said:

Simply reflecting my thinking – if I’m cutting a 10mm slot, I’ll be looking for a mill a bit less than 10mm for the main cut….

Use a 10mm slotdrill, one pass and job done. The slot width will be as accurate as the grinding of the cutter, usually a few tenths undersize.

Andrew

[Andrew Johnston]

On Andrew Johnston Said:

On colinb Said:

Simply reflecting my thinking – if I’m cutting a 10mm slot, I’ll be looking for a mill a bit less than 10mm for the main cut….

Use a 10mm slotdrill, one pass and job done. The slot width will be as accurate as the grinding of the cutter, usually a few tenths undersize.

Andrew

Oh for an ideal world!

If I did that the sides of the cut slot will be a very poor finish. When tidied up the slot would be wider than the cutter.

This is nothing to do with cutters and all to do with the quality of the milling machine used (in my case, well used and German). Some skill is just knowing the limitations of one’s tools.

JA

[Mark RandParticipant @markrand96270]

Having reached pensionable age (66 for me) less than a couple of months ago, I’m of that generation that is happy with both imperial and French measurement systems. I use whatever comes to hand. I did snaffle a number of cutters from the Blading Shop at work after it was closed and a small number of inserts. If I’d known that they were all going into a skip (several thousand pounds worth at scrap metal value, work paid to get rid of them!!!), I’d have overloaded the car with them 🙁 . Be that as it may, on the Beaver milling machine I use mostly metric cutters and Imperial cutters when needed or available. On the little BCA jig borer it’s either 1/4″ or 3/8″ threaded cutters (just got a £ton worth delivered from Tracy Tools) until I get around to making some FC3 holders to use cheaper cutters.

[colinb]

On colinb Said:

Simply reflecting my thinking – if I’m cutting a 10mm slot, I’ll be looking for a mill a bit less than 10mm for the main cut, then bringing the slot to size with. For reasons unknown, my brain probably won’t go “looking for 9.5mm, or 9.525mm” – so this labelling fits that.

Nothing stopping me doing both, of course!

If the first cut is “a bit less than 10mm” how will you know how much more to take off unless you know how much less the diameter is?

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