Milling Small Circles

Many thanks Rory, I have a number of questions, so I'll keep them to this thread if I may, rather than start multiple ones...

The project I have in mind will involve a great deal of drilling small holes accurately, and cutting of shapes, mostly in flat alloy sheet of thicknesses ranging between .05mm and 3mm. With that in mind, would you recommend the HF Spindle in 500w over the Proxxon?

Drilling holes accurately. As per topic opened in other thread.

Firstly - when it comes to CNC accurately...? Can you be more specific? STEPCRAFT approx 0.08 under optimal setup. Larger routers with ISEL ballscrews - can achieve 0.02mm. Precision milling machines can be 0.003 and lower.... some machines are setup so that they have to be at a specific temperature in order to maintain tolerance. On a Monday morning they will not produce parts to spec untill "warmed up" to operational temperature and thermal expansion of all aspects are stable.

Its very important to be relative to a specific reference. Of course - we won't achieve 0.003 on our little steppers! But equally asking a 3mm cutter to plunge aggressively into aluminium will cause tolerance issues - the machine will bend and twist and will loose tolernace. So the "accuracy" is related also to the machine stiffness and performance.

For example - under certain operational parameters - the tolerance will be held.

then there is the tool itself and the mounting - if we are machining even on a heavy milling machine with lots of cutter stick out - the cutter can starts to be influenced by slight changes in the materials. How many of us have manually drilled a hole in JUST the wrong place. And then the drill can't drill a hole JUST beside it as it wanders into the hole 🙂 Large cutter stick out can cause tools to wander and break. the machine might be stiff enough to machine heavily but the cutter cannot cope - wanders and breaks.

for drilling this is a little different than milling as we are plunging only. If you look at a drill bit - its designed to remove material efficiently - as a results its weaker than other slot drill type cutters. So - its easiy for the drill to "wander" off the centre point. the way to avoid this is to use a "spot drill". This is a drill bit that is often larger than the drill bit - 6-8mm diam - and you "spot" a small hole with the same cutting angle as the drill bit you will then use to do the drilling. this way - the much stiffer spot drill has made a very precise hole that "guides" the drill bit on its way.

More specific to the STEPCRAFT is the vibration levels. you will need to make sure the machine is very well setup with as little vibration as possible - both from the cutter side and from the workpipece side. Vibration - especially with small small cutters is a big issue.

what sized holes are you looking to make? remember the cutting speed V (the speed of the tip of the cutter that interacts with the work) is related to the radius of the cutter x angular velocity (RPM). So at same RPM - half the cutter radius halves the cutting speed at the cutter tip.

I wold go for the Hf spindle. Although I don't have hard vibration / run out figures for the two systems. I would expect the HF to flex less (have higher stiffness in the Z axis mount) and have less runout as the ER11 is used.

There is no one clear answer to this...

my answer may not also be the correct one - just my opinion based on experience. More practical machinists might have different views.