I'm quite new to milling too. I get this more with the shell mill and less with smaller tipped tools. To be honest I just thought it was one of those things and just run the file over it (one pass is usually enough) and its gone.

Agreed and possibly been used before on steel? As Andrew says brass is unlikely to exhibit this 'pushing' characteristic to any great extent but bronze and gunmetal certainly will if the tool has been used on anything but brass before.

I mark all new cutters used on brass as such using them on steel only as the edge wears off.

Tug

Blunt cutter.

Tony

Most likely blunt or poor quality caausing the burr. Would need to see a picture of the surface to see if its down to cutter or something else leaving the pattern.

I'd be feeding more rapidly than that, 4-flute @2500rpm 250-300mm/min in brass/bronze. Subject to how you were using the cutter – side or end.

Edited By JasonB on 22/03/2021 08:24:47

Could be, and being generally 'too slow' adds another gotcha in that it tends to blunt the cutting edge. So, even when the first mistake is corrected, you get the same symptoms, this time because the cutter is blunt.

That treating cutters gently damages them is counter-intuitive. Surely delicately applied cutters will last longer? No, the problem is the edge rubs rather than cuts, and wear is concentrated on the delicate tip rather than shared by the whole tool. Rubbing also causes a lot of heat, which tends to reduce HSS hardness just at the edge where it's needed most.

Deeper cuts reduce wear and heat, because after the tip has penetrated, much of the heat is carried away by the swarf, and because much of the cut is done by the flanks wedging metal off rather than edge contact. Carbide can be driven so hard that almost all the cutting is done by wedging and the sharp edge isn't in contact at all because the metal peels off just in front of it, but this requires more power, rpm, and rigidity than most hobbyists can manage.

Brass particularly likes sharp tools, which is why I keep a set of new drills and files for use on brass only. When they become blunt on brass, they're still plenty good enough for other metals, but using a twist drill on steel is enough to spoil it for brass.

Few ways of proceeding:

• Tolerate the burr and tidy up after. (This is what I do)
• New cutters for brass. (Expensive)
• Reduce the effect by keeping the cutter sharper longer by being more aggressive.

Finding the right balance can be tricky because it depends on the machine, material, and type of cut. Without being too scientific, the important thing is to avoid extremes. Pussyfooting causes rapid wear whilst Urgent Gorillas strip gears and burn out brushes, motors and controllers. After selecting a suitable rpm and cutter, I like to adjust depth of cut and feed rate by ear so the machine can be heard to be working, but isn't labouring. At first I adjusted rpm, doc, and feed experimentally on almost every job, but it didn't take long to learn the ropes, and now I only have to experiment on new materials. I'm sure I'm not cutting optimally, but it's good enough for me. The work gets done, finish is reasonable, nothing breaks, and cutters last much longer.

Final point, keep an eye on the material. Brass can be hard or soft, and some types work harden and have to be annealed. Likewise, steel can be anything between soft and malleable and extremely hard and brittle. Many alloys are absolute pigs to machine, so beware unknown scrap.

Dave

Any cutter, file, saw or drill if used on steel will be next to useless on brass. Keep them separate. Will be OK on steel after brass.

Roy

Andrew – the recommendation to stone a flat on a drill when drilling brass is only if it's opening a previously drilled hole. A drill if not treated in that way will quickly pull itself into the brass up a pre existing hole. The flat may be there but it still needs to be sharp to perform at its best.

With regard to the original post any feshly sharpened tool either a new cutter or one freshly ground will cut bronze effectively and with a good finish. Use anything other than that then the finish is as described, wavy, looks 'pushed' rather than cut and will have a large burr in the direction of cut. All metals if the cut goes over the edge in the direction of cut will create a burr but this is usually on the very edge of the part. Using a used cutter on bronze the edge will literally form a considerable burr that is far more difficult to remove cleanly than that on any other material.

Ron is right in his advice though as far as drills go I do not bother to keep some for brass only – just regrind them to suit as and when – but milling cutters and files definitely get used on brass first before moving over to steel

Tug

My 'logic' is Brass is 'slippery' and with any kind of rounding on the cutting edge it skates off the surface. With lathe tools this can generate a series of cut skate cut events without achieving a stable continuous cut. Sharp tools as in comes to a fine edge at the cutting edge rather than has acute angle geometry.

With your massive cut perhaps you don't notice. You really do feel it when filing.

regards Martin

Exactly. I go as far as honing my brass tools.

regards Martin