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Endmill smear of metal

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Nick Welburn21/03/2021 21:11:57
81 forum posts

Sorry beginner question again. I’ve been milling some brass for the bearings on my 10v.

Good news! It’s getting thinner. Bad news it’s kinda smearing metal over the edge of the piece and the finish is decent but looks ‘wavey’.

I guess this is a speed question. How fast should a 3/8 end Mill run on brass?

Chris Evans 621/03/2021 21:23:22
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1960 forum posts

Can you fill in a bit of information ?

What machine ? HSS or carbide end mill ?

Thanks.

Nick Welburn21/03/2021 21:25:01
81 forum posts

Sieg sx2p mini mill. Think the end mill is HSS

Andrew Johnston21/03/2021 21:29:15
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Posted by Nick Welburn on 21/03/2021 21:11:57:

I guess this is a speed question.

Probably not. If by smearing you mean a burr, almost as if the metal has flowed, that's a characteristic of gunmetal and bronze. It can happen on brass but is less common. A wavy finish implies that something is moving. Could be the work not held properly, or the tool not rigid, or too slow a feed causing the tool to cut and rub alternately.

I'd be running a 3/8" cutter at 2500rpm on brass - could easily be higher but my manual mill starts to get really noisy over that speed.

Andrew

Jon Lawes21/03/2021 21:33:36
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652 forum posts

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.

Chris Gunn21/03/2021 22:10:10
392 forum posts
27 photos

Nick, my guess would be that the end mill is not sharp.

Chris Gunn

Ramon Wilson21/03/2021 22:31:49
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Posted by Chris Gunn on 21/03/2021 22:10:10:

Nick, my guess would be that the end mill is not sharp.

Chris Gunn

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

Paul Lousick22/03/2021 06:06:06
1855 forum posts
661 photos

The direction that you are cutting will effect the edge of the machined part.

If the endmill is cutting from the inside and finishing the cut at the edge of the part, you will get a burr but if the cut is started at the outer surface and the endmill cuts towards the inside, you will get a sharp edge.

Paul.

As demonstrated by Joe Pieczynski in this Youtube video. **LINK**

Tony Pratt 122/03/2021 06:34:34
1692 forum posts
8 photos

Blunt cutter.

Tony

Paul Lousick22/03/2021 07:11:11
1855 forum posts
661 photos

Depends on the direction of the cut (climb milling or conventional milling) but a sharp cutter can also leave a burr on the trailing edge, just smaller. (see video above)

Edited By Paul Lousick on 22/03/2021 07:19:17

JasonB22/03/2021 07:19:58
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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.

Martin Connelly22/03/2021 07:55:23
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Often beginners overspeed and under feed. Set the speed (RPM) correct for the material and tool (diameter and tool material are taken into account as well as the number of cutting edges) and feed fast enough to make chips not filings.

There are plenty of feed and speed calculators on line but using Andrew's speed of 2500 rpm and a 3 flute cutter you need to be feeding about 50-60mm/minute. If it is a 2 flute cutter then feed at 2/3 of this and a 4 flute cutter feed at 4/3 of this. If you are turning handwheels find how fast you have to turn a handwheel to achieve something like 50mm/minute (2"/minute) feed and it may surprise you.

A tool that is rubbing not cutting will spend a lot of time rubbing its cutting edge off compared to a tool that is fed fast enough to cut. If for example you were being overly timid and were feeding at 5mm/minute the cut would take 10 times as long and so the tool will suffer 10 times as much wear doing the same job.

Martin C

JasonB22/03/2021 08:23:10
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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

Nick Welburn22/03/2021 08:45:19
81 forum posts

Ok - I’m running to slow and feeding too slow. Sounds like I almost ‘buzz’ through it than pass gently over it.

SillyOldDuffer22/03/2021 09:41:22
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Posted by Nick Welburn on 22/03/2021 08:45:19:

Ok - I’m running to slow and feeding too slow. Sounds like I almost ‘buzz’ through it than pass gently over it.

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

Andrew Johnston22/03/2021 10:29:42
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Posted by SillyOldDuffer on 22/03/2021 09:41:22:

Brass particularly likes sharp tools, which is why I keep a set of new drills and files for use on brass only.

People say that, bit I've never understood the logic? The recommended side rake for a lathe tool on brass is zero, or even negative. And the recommendation for drills is to stone a flat edge on the cutting face. Neither of those seem sharp, at least in the conventional sense. The cutting action is purely shear?

Andrew

roy entwistle22/03/2021 10:36:55
1408 forum posts

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

Brian H22/03/2021 10:53:52
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At the company where I served my apprenticeship, files were always used on brass first, if possible before being used on steel or C.I.

Brian

Ramon Wilson22/03/2021 11:28:53
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1194 forum posts
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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

Dave Halford22/03/2021 11:56:18
1729 forum posts
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I have a HSS cutter purely for brass no top rake, just both sides. It cuts fast with a nice fizz and a shower of small chips till it goes blunt. I also have a file that cuts fine on steel and skates over brass or gunmetal like it's chilled cast iron.

Watching the vid on the best way to mill a cube you avoid the outside burr by always cutting into the metal even if it means climb milling a finishing cut. If you still get a smeared burr inside check the corners of the cutter, they may have gone missing.

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