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View Full Version : How to destroy a milling cutter?



The Artful Bodger
10-27-2013, 12:09 AM
Twice now (yes, I know I am a slow learner) I have brazed a few bits of mild steel together then destroyed the cutting edges of a an end mill when trying to clean afterwards.

So what destroys the cutter? Surely not the steel at it cannot be hardened, the brazing material or the thin scale?

Whatever, in future do I start with a grinder before milling? Surely some abrasive would be left to damage the mill.

PixMan
10-27-2013, 12:12 AM
What was the designation for the steel that has you believing it couldn't be hardened? What material are you brazing it with? Cutting speed?

darryl
10-27-2013, 12:18 AM
I imagine that the process of welding has left hard spots. Aren't such 'weldments' often annealed before being machined on?

The various oxides are hard on cutters also.

The Artful Bodger
10-27-2013, 12:24 AM
I only know the steel as "mild steel", 6mm (maybe 0.25") plate and has a dark (scale) surface when delivered. The cutter is a 20mm HSS end mill running at 250rpm and very slow hand feed, lubricant/coolant is hand applied ROCOL RTD metal cutting liquid 10% in kerosene.

The Artful Bodger
10-27-2013, 12:26 AM
I imagine that the process of welding has left hard spots. Aren't such 'weldments' often annealed before being machined on?

The various oxides are hard on cutters also.


Yes, I assume it must be an oxide product of the brazing. Does annealing do anything to mild steel?

lakeside53
10-27-2013, 12:51 AM
Annealing takes it back to its "non-hardened" state (yes, welding/cutting temps can make it "hard" in the Heat Affected Zone (HAZ) ), but brass (?) brazing temps and normal cooling won't make it make it "hard" enough to mess with your cutters.. If you have the original black mill scale - that can be hard on cutters and is most likely your problem. You are best to "cut beneath" mill scale, or strip it off with acid before milling. The worst you can do is "skim" the scale surface with an end mill.

However... are you sure you don't have "glass" inclusions from the brazing flux? What is your braze material and what if any flux is is used?

I've cleaned up plenty of mild steel HR mill scale, braze, HAZ from torch/plasma cutting, and mig/stick welds, but I reach for carbide when doing so. Even mild steel has a harder HAZ area when cut with heat, and even though I grind of the slag etc first, the cutters take note.

The Artful Bodger
10-27-2013, 12:56 AM
The braze is brass by all appearances, it is the cheapest brazing rods available and is known here as 'Steel Braze". The flux is, I think, borax paste, sold to go with these rods.

Glass, maybe, the temperatures are only orange/red not steel melting temperatures. The torch is oxy/lpg.

But whatever, I know now not to try milling a brazed edge so what is the alternative? Is grinding before milling acceptable practice?

lakeside53
10-27-2013, 01:04 AM
Strip the mill scale off before brazing and milling, or "get beneath it" with a tough cutter. For such work, I also use with great success insert end mills

There nothing wrong with grinding before milling, but I'm not sure that will solve your problem.

Ian B
10-27-2013, 04:35 AM
+1 on grinding before milling (or other machining). A light cleanup with an angle grinder's perfectly acceptable, and is a great way to remove any surface nasties. Grinding of oxy acetylene cut edges is also good idea.

Ian

The Artful Bodger
10-27-2013, 04:37 AM
Thanks, the first end mill is likely beyond salvation but I thing today's casualty is saveable.

DATo
10-27-2013, 05:10 AM
I agree with those who say the flux itself can act as an abrasive to your cutter when left on the part after brazing but I wouldn't think it would produce a dramatic dulling of the cutter. In the old days used to soak brazed parts in a very diluted mixture of water and nitric acid till all the flux softened and washed off and the material was etched clean by the acid. Now I just bead blast it off.

Just taking a shot in the dark here but could it be possible that the flux you are using is acting as a medium to case harden the surface of the material?

vpt
10-27-2013, 08:38 AM
I was wondering if the braze being softer galled up in the cutter and then the steel made the cutter blow apart.

Doozer
10-27-2013, 08:49 AM
I have some braze (bronze alloy I think) that when I got done
with the repair, the braze was so hard, it trashed a HSS end
mill easily. I did not want to chance ruining my carbide end mill,
but an insert cutter, maybe. I ended up grinding the part on my
surface grinder. Bronze can be 80,000 psi. More than mild steel.
And very tough material. People do not realize how strong some
bronzes are.
--Doozer

bob ward
10-27-2013, 10:02 AM
For that sort of miscellaneous cleanup work I really like this indexable end mill. I run it at 1600RPM and it just ploughs through rust, mill scale, weld, oxyslag etc. The inserts are cheap so you don't worry about trying to go easy on them, but even so they last surprisingly well.

http://www.ctctools.biz/servlet/the-630/45-DEGREE-INDEXABLE-CHAMFER/Detail
http://www.ctctools.biz/servlet/the-631/TUNGALOY-CARBIDE-INSERT-SDMB26152/Detail

boslab
10-27-2013, 10:06 AM
Take a heavier cut, they dont like rubbing
Mark

CarlByrns
10-27-2013, 10:59 AM
I only know the steel as "mild steel", 6mm (maybe 0.25") plate and has a dark (scale) surface when delivered. The cutter is a 20mm HSS end mill running at 250rpm and very slow hand feed, lubricant/coolant is hand applied ROCOL RTD metal cutting liquid 10% in kerosene.

I clean up brazed joints without using coolant. I've found that when using coolant the brazing material chips tend to ball up and wedge between the cutter and the work, which will kill the cutter but quick.
Use a small brush and constantly sweep the chips out of the tool path.

radkins
10-27-2013, 11:21 AM
Brazing rods are BRONZE not brass!

Jaakko Fagerlund
10-27-2013, 11:23 AM
Brazing rods are BRONZE not brass!
Yes, bronze rods are bronze and brass rods are brass. They are both available, so brass != bronze.

JoeLee
10-27-2013, 11:45 AM
I've ran into the same problem a few times in the past. I always thought that the flux may be surface hardening the steel in the heat zone area.

JL...................

Toolguy
10-27-2013, 11:48 AM
When I do brazing my flux leaves a clear glass like film that will definitely eat an end mill. The easy fix is to soak the part in hot water for a while which dissolves the flux and any remaining can be brushed off with a wire brush. I use a small toothbrush size with stainless steel bristles.

boslab
10-27-2013, 12:43 PM
Brazing rods are BRONZE not brass!

Usually silicon bronze
Mark

beanbag
10-28-2013, 03:00 AM
Although I have not done this myself, a "real" machinist told me to use a conventional cut instead of a climb cut so that the teeth don't slam into the hardened outer layer.

jlevie
10-28-2013, 11:50 AM
My suspicion is that your "mild steel" isn't exactly what you think it is. Take a piece of it and heat it to brazing temperatures and see if it is harder afterwards (check with a file).

Rustybolt
10-28-2013, 12:19 PM
Art. For a minute I thought you were talking to the guys in my shop.
They could give lessons.

Stern
10-28-2013, 12:25 PM
Well if you want to destroy a mill cutter, send it to me LOL. Seriously, I have learned a lot and now know why I have killed so many. Speed is probably a factor, but I think most died a horrible death by taking light surface cuts on the steel I have (which is mostly layered in a thick rust coat). Never would have figured rust was harder than steel, but thats why Im here, to learn why I screw up all the time LMAO

The Artful Bodger
10-28-2013, 03:15 PM
Art. For a minute I thought you were talking to the guys in my shop.
They could give lessons.

;) Some of us are slow learners!

The Artful Bodger
10-28-2013, 03:23 PM
Well if you want to destroy a mill cutter, send it to me LOL. Seriously, I have learned a lot and now know why I have killed so many. Speed is probably a factor, but I think most died a horrible death by taking light surface cuts on the steel I have (which is mostly layered in a thick rust coat). Never would have figured rust was harder than steel, but thats why Im here, to learn why I screw up all the time LMAO

I think I must be learning something here as the tool destroying incidents in the shop do appear to be happening at (slightly) increasing intervals!