Hi Colin,

You are quite correct the figures can be rounded up or down on the M/C in question, and I am not a novice, but I was many years ago

Tony

Michael – I'm not sure I know what you you were trying to point out. My question was regarding the necessary precision for cutting speeds. The revolution values in the table provided by Neil were precise calculations from the table's cutting or revolving distances (e.g. carbon steel = 15m/min) – which in themselves were probably gained from empirical experience rather than any theoretical or experimental work. From a practical point of view, I questioned whether the table entries could have been rounded up to the nearest 10, 50 or 100 revs/min without serious consequences to the overall finished product. I said to round up because many mini lathes/milling machines cutting speeds are based on reducing the electric motor power supply rather than on gear changes. Such motor power reduction also reduces the torque of the motor and so rounding up will help to preserve the torque.

The topic appears to be, in general, somewhat personal in that many books on lathes etc have cutting revolutions that differ to the table provided by Neil. Some books lump together cast iron, steel and stainless steel in one column and brass, bronze, mild steel in another column. So the science is definitely not that precise – so the cutting revolutions should not express a precision that is not justified. I would not tell a cook to use a kitchen measuring jug marked in 50ml divisions to add 37.25ml of vegetable stock to a recipe when there was no appreciable difference to the meal if he just added 50ml.

If there are any engineering or scientific papers regarding the basis for the different cutting speeds for different materials, I would be interested to have a link to these or pdf downloads.

Colin,

Re your initial question the tolerance is 50% or more. Ie it is very very vague. The speeds sort of work out ok and you can mess around according to how it seems to go. If you go too fast the tool may overheat but HSS is so much better than old carbon steel which many of these tables were originally planned for. If you go too slow it mostly doesn't matter but with a small machine you can degrade the surface finish as the tool cuts in microscopic jumps because it is a bit like pushing a big rock – once you get going it is easier if you can keep it moving.

Theoretically if you have the power and heat tolerance (ie carbide) the optimum speed advances the cutting edge at the speed of sound in the material which is at least twice these figures.

For milling cutters don't forget to allow for the multiple teeth. This is significant when you come to run a slitting saw which are often run too fast. (it is nothing like a saw cutting wood which needs to run as fast as possible for differnet reasons.)

Hi pkg pkg,
I do not believe the number of teeth on a milling cutter or saw have any bearing on the cutting speed. It will effect the tooth loading. If you double the number of teeth each tooth will remove half as much on each cut providing the feed rate is the same.

Les.

Ta for the explanation,

I was going to post this yesterday, then I thought I'd got the answer. Now, I've become confused again! So, I've bought some module 1 involute gear cutters from CTC and I'll be using them shortly to cut a number of gears for my toolpost mounted spindle. My question is: what sort of speed should I run them at? They are 50mm outside diameter, with 14 teeth. I thought I would start at around 2-300pm in my mini mill. What do others think?

Sorry if this is a thread hijack!

John

Years ago I was given a tip on cutting speed- just fast enough for the cutting edges to blur into one. Sounds daft but does seem to work for lots of things, and a good starting point to try. May be something to do with the eye /brain seeing 24 shots per second ?

The trouble with using ones senses to set feeds and speeds is that instead of leading to common(sense) it sometimes leads to non(sense).

The most common cause of machine/tool vibration is too low a feedrate or depth of cut. Instinct is to reduce it further, which just makes things worse.

Andrew

Thanks for the guidance. I'll be cutting gears in both EN1 steel and brass, too, so I guess there's going to be a bit of experimentation going on. I'll publish the results in my photo album of the build, as and when it happens. I suspect something called Christmas will get in the way of progress.

After converting a couple of slitting saws into large diameter washers (by using FAR to high a speed), I'll start off slow and steady and see how I get on.

John

Thanks, Neil, that gives me a good starting point.

John.