# Why Am I Unable To Plunge Cut A Bronze Casting



## tomw (Feb 18, 2016)

Dear All,

I am working on a bronze cylinder casting from PM Research. I am trying to get the steam chest milled, which requires that I make plunge cuts. I have tried 2 and 4 flute center cutting mills, but they do NOT plunge! They just sort of buff the surface.  I have done lots of plunge cutting in aluminum, CI, and brass. What the hell am I doing wrong.

I have tried 4 different end mills in two different sizes.  All center cutting. 

Any help will be much appreciated. This seems like an easy part to work on!

Cheers,

Tom


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## Tony Wells (Feb 18, 2016)

If needed, punch a small hole with a drill right in the center of where you need to plunge.


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## TommyD (Feb 18, 2016)

Are you sure they are center cutting? I have seen some cheap 2 flute endmills that LOOK to be center cutting but don't. As Tony said, maybe a pilot hole first.

Could be improperly ground also, too much 'land' on the cutting face.


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## tomw (Feb 18, 2016)

TommyD,
I tried 4 different center cutting mills, both 2 and 4 flute. One was an unused mill from McMaster. I also just did a test on a chunk of aluminum. No problem plunge cutting that. 

Tony,

I will try that. Thanks


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## mmcmdl (Feb 18, 2016)

If that bronze is hard , the center of your end mill has a very low surface speed compared to the OD of it . Wouldn't make that much difference on aluminum . The pilot hole would remedy that if it's the case or a carbide CC end mill would work .


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## tomw (Feb 18, 2016)

I just ordered a carbide 1/8" end mill. Damn, their expensive!

Tried the pilot hole trick. Unfortunately, I still can't plunge below the drill tip angle, and I need a flat surface. Hopefully the carbide will help.

Thanks for the help.

Tom


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## Tony Wells (Feb 18, 2016)

The face of an end mill is not truly square. You will have a bit of a inverted cone if you just straight plunge. If there is any pocket milling to do, this will pretty much eliminate that, and it may not be severe enough to worry about anyway, but you should be aware.


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## tomw (Feb 18, 2016)

Thanks. It is a pocket that I am milling (a steam chest). 


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## TOOLMASTER (Feb 18, 2016)

brass is kinda self lubricating....once is grabs watch out


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## Metal (Feb 18, 2016)

Another thought might be to "slightly" tilt your milling head if that is allowed, letting the endmill have a slight leading edge and working into the material from its tip slower, this works really well with a cnc but would be a pain to do manually.

since its a casting, I'm thinking the outer surface might be denser then the inside of it depending how it was cast, once you break into the material your going may be much easier.


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## Billh50 (Feb 18, 2016)

There are many types of bronze. Some much harder to machine than others. Don't use too slow of feed as the cutter has to bite in to cut with bronze. Most types of bronze will cut fine with HSS tools with proper feeds. If the feed is too slow the tool will just rub and dull quickly. I have only turned bronze but do remember a friend bringing me a 1" square bar he wanted me to cut in half for him. Took the band saw with a new blade a little over an hour to cut it. That piece was tough. Good Luck!


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## TommyD (Feb 18, 2016)

Just be careful drilling brass or bronze with a sharp drill, we used bronze castings for hinges on our compression molds and have seen more than one vice turn into a helicopter. You have to brace the vice and dub the cutting edges, soft metals like this like to grab the cutting edges when it starts to break through. Dubbing is squaring the cutting edges on the drill flutes.


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## derf (Feb 18, 2016)

Bronze cuts better with zero or negative rake on the cutter.


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## Wreck™Wreck (Feb 18, 2016)

tomw said:


> Dear All,
> 
> I am working on a bronze cylinder casting from PM Research. I am trying to get the steam chest milled, which requires that I make plunge cuts. I have tried 2 and 4 flute center cutting mills, but they do NOT plunge! They just sort of buff the surface.  I have done lots of plunge cutting in aluminum, CI, and brass. What the hell am I doing wrong.
> 
> Tom


My best guess is that the feed is way to slow and the spindle speed to high letting the tool rub rather then cut, many bronze alloys are rather hard and abrasive.

Also be aware that hard bronze chips will come off the tool like  shards of very unfriendly glass,


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## Flammable_Solid (Feb 19, 2016)

You might have a casting with a martensitic microstructure.  A quick hardness check might allow you to heat treat it, otherwise you probably won't be abke to machine it.


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## EmilioG (Feb 19, 2016)

Guhring makes an end mill called the diver that is supposed to make all types of cuts including plunge cuts.
The video looks amazing. Msc has them.


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## Billh50 (Feb 19, 2016)

Some bronze materials can be tough to machine. 9D aluminum bronze machines like 316 stainless. Not an easy to machine material sometimes. It is hard to give good advice without knowing what bronze alloy you are working with. So there may be a little trial and error before you find the right answer. I know you have had some good advice for most bronze alloys so far.


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## TomS (Feb 19, 2016)

Billh50 said:


> Some bronze materials can be tough to machine. 9D aluminum bronze machines like 316 stainless. Not an easy to machine material sometimes. It is hard to give good advice without knowing what bronze alloy you are working with. So there may be a little trial and error before you find the right answer. I know you have had some good advice for most bronze alloys so far.



Bill - my thoughts exactly.  Many years ago I was given a job that required a specific type of bronze.  For the life of me I can't remember the alloy designation but it ate up end mills quickly.  Just because it's bronze doesn't mean it machines easily.

Tom S.


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## tomw (Feb 19, 2016)

Thank you all for the advice on feeds, speeds and bronze in general. This stuff is hard, but also in places gummy! And it loves to eat 1/8" end mills. I have now dulled all of mine, and am awaiting some new ones. 

The weird thing is that I had no problem on the lathe doing the bore and head surfaces. The chips were like sharp sand, but I got a nice surface. 

But, I was using carbide cutters for the lathe work...

For the carbide end mills I have, what plunge (Z AXIS) feeds and what spindle speeds do you think I should try? They will 1/8" dia center-cutting 2 flute mills for roughing, 1/8" 4 flute mills for finishing. The machine is a 5400 Sherline mill, so no power Z AXIS feed, but I can try to regulate my manual cranking speed. Normally for plunging I go pretty slow, but this would appear to be bad. However, too fast and I risk breaking the end mill or crashing the mill. 

Thank you all so much for your help on this problem. As a newbie, I bow at y'alls feet, tip my hat, and genuflect to your experience and wisdom.

Cheers,

Tom

p.s. Never thought I would use "genuflect" on a machinist forum.


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## mmcmdl (Feb 19, 2016)

Any way you could post a picture of what you are machining ? Perhaps a sketch ? RPMs you are running ? Coolant . spray mist etc ? This may help with finding a solution . If you are milling a pocket , do you have to use a 1/8" for corner radius ? Just curious ?


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## chips&more (Feb 19, 2016)

Sorry, a Sherline? You should have a rigid machine when using carbide. Any unwanted movement and carbide is not forgiving, it will break, especially a 1/8” end mill. Maybe try Cobalt instead…Dave


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## Ernienoatrainz (Feb 19, 2016)

I have had some PM research cast iron that was impossible to drill. Try moving the table at the same time you are starting the plug cut. It may allow the tool to start to cut. Once at one end move back the other way as you do the plunge. This may also break through the skin of the casting which is harder than the rest of the metal.


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## T Bredehoft (Feb 19, 2016)

I remember milling a steam chest that had a pocket about .25 square and .100 deep. Or was that the valve itself.  Maybe that was it.


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## francist (Feb 19, 2016)

I wonder if it's possible to find out from PM research what type of bronze they would be using for said casting? If they know and would tell you, there might be some peculiarities about machining it. As others have said and I agree, there are a number of different types that go under the bronze heading. The only one I know for sure to have worked with is 660, and it was a pleasure. I would be interested to know what this troublesome one is.

-frank


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## lvflats (Feb 19, 2016)

You said you did drill a small pilot hole so the material is machinable. I would drill the pilot hole ( if possible ) the depth of the flat bottom hole
you want and then plunge. Maybe drill 7/64" and leave a little side clean up.
Art


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## tomw (Feb 21, 2016)

I will post a picture of the part and the mess I have made, once I have taken the photo. 

Although Sherline's aren't super rigid, they are fast (2700 rpm) so I am hoping that speed plus carbide will help. Of course, I might just melt the bronze and break the tool!

I have sent an email to PM to ask about the type of bronze they use and what they recommend. I am sure they will answer. 

Overall, I am really frustrated with this model. It's not PM research's fault, but my own ignorance. I am going to spend the next couple of workshop times filing the rough castings and pondering if this is just a bridge too far.

Cheers,

Tom


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## kvt (Feb 21, 2016)

tom,   Doe it seem like the same as the stuff on #7.  The other thing is to watch once it starts it may and take off and want to go quick.  Another can you use a bigger end mill to cut the center of the box and then the smaller one to finish it off.  
Ken


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## tomw (Feb 21, 2016)

Ken,

It seems like the same stuff, but I never tried to plunge mill on the other engine cylinders (just regular milling, boring and drilling). And when I did regular side/surface milling on this part it seemed to go okay. I agree that it can be grabby (but much less than brass). Definitely don't want to down-mill it, particularly with mini machines like Sherline. The part just doesn't seem that hard, but man is it giving me fits.

I am loathe to dull my other end mills, so I am going to wait for the new tooling.

I hope your progress on #7 is going better.

Cheers,

Tom


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## kvt (Feb 21, 2016)

Right now I am waiting for some hard 5-40 screws that I can put in the lathe and make me 6 of the things to locate where to drill the holes in the new casting so that they exactly match the cylinders.   So I started making me an additional tool holder for my QCTP,  But had to do the chores first, Mow yard etc, now down with the back spasms and shooting pain.   will try to get back at it tomorrow.  Oh,   are you still wanting socket head cap screws for your #7,   Found a place to order them,  Stainless and not real expensive.  Let me know and I will send you the link.

Ken


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## derf (Feb 21, 2016)

Take one of your "dull" endmills, and stone or use a diamond file some negative rake on the cutting edge. At this point you have everything to gain.


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## lvflats (Feb 21, 2016)

Tom, You can call tomorrow. Best after 7:30 PDT. For some reason my reply to your PM bounced.
 Art


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## tomw (Feb 22, 2016)

I heard from Jim at PM research. This is what he says:



> It is the same Bronze used for the 3 and 7 cylinders.  It is bronze ingot 305 part # 937 Bronze.  Paul recommends that you mill it out with a ¼ end mill first at speeds of 1500-3000 rpm and when you get to the smaller end mills, the speeds should be 3000 – 6000 rpms.



Interesting, and impossible for me without buying yet another attachment for the Sherline (the high-speed spindle pulley). I can get close to 3K spindle speed now, but not twice that fast.

Here is some information on the bronze 937 alloy: http://www.anchorbronze.com/c93700.htm, and the Wikipedia page on copper alloys: https://en.wikipedia.org/wiki/List_of_copper_alloys.

Hmmm.

Tom


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## francist (Feb 22, 2016)

tomw said:


> I heard from Jim at PM research. This is what he says:
> 
> 
> 
> ...



Well that's just weird. With the high lead content in there I would have thought it'd cut like a dream.

-frank


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## chips&more (Feb 22, 2016)

Ingot 305 has a machinability rating of 70%, even with the lead in it? But, still not a problem to machine, even with HSS. And I would not be overly concerned about not being able to get the mentioned high spindle speed. I think your achillis heel is your Sherline mill, sorry. I think you will need to learn its idiosyncrasies for achieving proper machining results. This is where I think a good background in making chips on a conventional machine can be very helpful. Then, when you step in front of a Sherline, Unimat or the like you will have the background knowledge to overcome the pitfalls of said machines…Dave


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## tomw (Feb 24, 2016)

Update:

I received my 1/8" carbide end mills. They are 3/8" shanks, 1/8" diameter cutter, 3/8" depth. 

I didn't notice the last bit.

Anyway, milling with the carbide end mills at 2800 rpm cut this stuff like butter! The cuts were smooth in feel and produced a nice smooth surface. Unfortunately, the cutter depth (length of the 1/8" dia tip) was not enough to mill all the way to the edges of the pocket. Since side milling with my HSS bits didn't seem completely terrible (terrible, but not completely), I finished off the pocket with a hopefully sharp 1/8 2 flute HSS end mill that had a long enough cutting tip.

I then milled the steam ports with a 1/16" carbide mill I already had.

Below is a photo showing the result of my milling. The crappy surface along the margins is from the HSS bit. The smoother center is from the carbide bit. The screwed up edges are from my previous efforts.




It looks really ugly in the photo, but after a bit of sanding with 400 on a flat tipped sanding bar (a bit of dimensionally sound brass about the right width) things looked not bad. The margins are still all effed up, but the critical central section that was milled with the carbide bit looks great. Unfortunately, no photo. I will try to remember to post one later.

Also, the valve ports look great! Kind of...

BTW, anyone know of a source for 1/8" carbide bits with a cutting depth greater than 3/8"? If I had that, it all would have looked great!

Thank you so much for your help. 


Cheers,

Tom


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## lvflats (Feb 25, 2016)

Hope this link works

http://www.travers.com/2-4-flute-lo...grain-solid-carbide-single-end-mills/p/38546/

Art


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## tomw (Feb 25, 2016)

Art, thanks!


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## Richard White (richardsrelics) (Feb 26, 2016)

My approach would be to drill holes of the appropriate size in each corner, to the proper depth.  Then using choice of proper sized end mills, go to any of the holes starting location. touch off the top of the part, lower cutter .05 in depth and traverse left to right or front to back depending on your preference, using a helical pattern, following the outside contour, remove all material within the pocket. Repeat as necessary until you reach your proper depth.
The holes in the corners simply allow a spot for you to take a break.  For best results, leave .015 on the outside wall for a finish pass that you cut using the climb milling direction and at a higher RPM but same feed rate. This will give you a very nice finish.

This is the pattern I was referring to, I did not put the holes in the corner so it would be more clear.


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## Quai_Oui (Feb 29, 2016)

tomw said:


> Dear All,
> 
> I am working on a bronze cylinder casting from PM Research. I am trying to get the steam chest milled, which requires that I make plunge cuts. I have tried 2 and 4 flute center cutting mills, but they do NOT plunge! They just sort of buff the surface.  I have done lots of plunge cutting in aluminum, CI, and brass. What the hell am I doing wrong.
> 
> ...



Tom,

An End mill is for milling at the end but not for plunge cutting.  A slot drill is for plunge cutting, but be aware that as the cutting faces have one overlapping the centre of the tool it will tend to cut slightly larger than its own diameter.  To counter this tendency use a smaller slot drill and then finish with a slot drill or an end mill of the chosen size.  The hole will still be slightly over but less so.  If the hole needs to be very accurate to then finish with a reamer.

Generally most Bronze is not had to cut.  It is said that some grades of Aluminum Bronze are harder, but in most cases you will not have a problem, unless the casting has been poured to hot and you have "burn on", that is when the mould sand tends to burn into the surface of the metal.  The sand of course is very abrasive.


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## 5.56 (Feb 29, 2016)

OK,

LOL Against all odds, here is my opinion.
If it is gummy and grabbing here is my experience. I machine C110 Copper on a daily basis. It is EXTREMLY gummy and grabs tooling.

Drilling with a .125 bit into the face, BTW I make X-Fire bullets. At 5,000 RPM with a feed rate .0025 I was shearing bits every 5-25 drills. I bumped the RPM to 6,000 and dropped the FR to .0020. I have gotten over 1,000 drills to this current bit which is a high speed steel. When running gummy material, if you wish to damage your tooling by all means go slow. I turn the C110 at 5.000 RPM with a 55 degree tooling. Larger drill bits I turn at 5,000 RPM, Smaller are 6,000 with slower feed rates.

Nick


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## bfd (Nov 9, 2016)

what you might try is to feed in the right direction as you feed down to let the sides of the ill start to cut. feed down and back and forth at the same time. bronze can be difficult to machine. that size endmill needs higher speed. bill


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