Milling Advice

Justin, There is a lot of stick out from the end of the mill spindle to the end of the cutter which will allow it to deflect. What type of tool holder are you using. It looks more like a drill chuck which are not rigid enough for milling.

Its a collet chuck, popular from China before ER became the main choice. Uses a similar style collet I use done when I first got my X3 without problem and do still use it on the odd occasion as the nut is smaller dia.

Justin, it looks as if the plate is secured to a wooden block by wood screws which I would have thought is nowhere near rigid enough. I can understand why you have done it that way. Others might be able to suggest how to mill a relative large (relative to its thickness) plate. I would clamp it in multiple places but expect it not to be completely flat. Perhaps soldering it to a heavier plate /block or using a suitable adhesive that would come apart with heat. Both techniques would have a parallelism problem wrt to its base.

John

Nothing wrong with stoning HSS lathe tools because they are easily reground, at least by experienced operators with a good eye. And stoning twist-drills for a special purpose is respectable too. But the idea of stoning a milling cutter upsets me! To my mind the sharp corner edges of a milling cutter are its most precious asset, to be cosseted as long as possible. When they're damaged the cutter loses much of it's utility, and needs to be replaced or sharpened, both a bit painful!

If finish on a large scale was important, I'd prefer Andrew's approach - buy a cutter designed for the job.

Of course being an amateur, I buy inexpensive cutters, use them for roughing out everything from squishy Aluminium to chilled cast-iron, then expect them to produce a mirror finish from fine cuts, and I'm certain they're rubbish unless they last at least 30 years. My mill's not very rigid either!

My first question is, how bad is the problem and does it need to be fixed. Andrew Skinner's photo is visually flawed, but could be acceptably flat (mostly).

Flatness can be established by shining a light on the back of a straight-edge and looking for light escaping between the work and the edge. Also possible to detect tiny ridges by feel - fingertips and nails are remarkably sensitive to surfaces.

Milling cutters almost always leave characteristic marks. They're obvious in Andrew J's excellent copper plate example, which is probably very flat and has classy finish with strong mirror effect. The example is as good as can be expected from a well-adjusted stiff milling machine, set up correctly with an appropriate cutter, and driven by a skilled operator.

The remaining milling marks are unavoidably tiny. If a better finish is needed, change technique. Usually polishing with a succession of ever finer emery paper, and then buffing.

When milling I try cut in parallel straight lines because changing course leaves deep marks. Also use the largest diameter cutter available. Fly-cutters generally produce a better finish than other types but are slow. And an HSS fly-cutter can be stoned without upsetting me!

Dave

I agree with Pete. I get a great finish with a fly cutter.

Actually Dave you can bring a blinted milling cutter back to life by roughly grinding a 45deg chamfer on each corner particularly if it has just been used for shallow facing cuts. This is not painful to the pocket and can simply be done on a basic bench grinder or with acoarse diamond slip.

I suspect what has also helped with Andrews finish is flood coolant to clear the swarf so none getting cought under the cutter and swirled around. A constant power feed also helps give a uniform finish.

The only downside to flycutting (and face mills) is that you want your tram reasonably good to avoid a concave surface

I found this thread interesting as I'm just doing a job that needs a flat surface with a good finish. It is similar to Andrew's application a heatsink for high power semiconductors. This is a aluminium heatsink and as usual with these it is near pure so soft and sticky. It's also used so I have to re-surface it and plug a couple of holes. The original finish was obviously made with a fly cutter. I did consider that approach but my mill is a small high speed model (SX1LP). so not Ideal. I decided to try a 90 degree (two insert) 25mm indexible end / side mill from ARC
https://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Indexable-Carbide-End-Mills/90-Indexable-Carbide-End-Mills
with APKT 1604 PDFR polished inserts. This has the added advantage that the semiconductor (MOSFET in SOT227B package) will just fit in the width of a single cut.
Had a go today. Using it dry with vacuum chip extraction, 2100 RPM and light cuts. Finsh is pretty good (not up to Andrews standard). It's clearly a milled surface but no roughness detectable by a fingernail .A check with some blue, a flat and one of the devices shows it is flatter than the MOSFET. The critical area of the MOSFET is in good contact with just finger pressure dispite not being perfectly flat. I'll sort out some photos. The holes that needed plugging were tapped M3 so I put plated brass screws in with Loctite 601 retainer. Left it for an hour and just machined the heads off.