Newbie- Aluminum milling advice needed...

[damusernames]

to make a long story short i have been donated a home made cnc and i desperatly need some advice as far as milling aluminum, speeds, rpms, how to calculate them on my own basically everything...im am brand new to the milling world and am trying to learn as much as possible...

my current delemma is how fast and at what rpm should i mill aluminum, keep in mind that the options for cutting are a 26000 rpm trim router or a hand held variable speed drill mounted on the cnc...this is not a precision machine i just need it to cut away material no matter how fast or slow it goes...and i would like my end mill bits to last as long as possible given the 2 different power options...i will be cutting aluminum that has been sand cast as far as how hard or soft that is...i have no idea...it comes from everything from a transmission to a lawn chair...

i realize that these are not the ideal conditions for milling but at this point i just need it to mostly work...i plan on building a stronger more accurate rig later...tolerance is not a word this machine has ever heard of...

any advice is welcome... the bits im using are
http://toolinghouse.com/qnea14.aspx
if someone could give me advice as far as how good these are for that purpose and what other type i could get that would work better im all ears...

i havent used them yet for fear of ruining them...currently im using carbide tipped strait fluted wood bits(try not to cringe they were cheap and unwanted) and actually do the job somewhat...

[Geof]

Given that you have limited options you are going to have to compromise. The variable speed drill is probably the wrong choice, it does not have bearings suitable for side loading. The router can work but it is necessary to keep the cutter size down around 3/8" or smaller. Aluminum can be cut at a surface speed of several thousand feet per minute and if you do the math you will find at 26,000 rpm the sfm of a 3/8" cutter is around 2500.

Lubrication is essential, absolutely essential if you have a hodge podge alloy cast from all manner of stuff. These castings are going to have a very low temper and will be very soft and gummy for machining. The alloy will tend to stick to the cutting edge of the tool without an form of lubrication. If you do not have access to proper cutting fluids a perfectly good alternative is automatic transmission fluid just painted on the material along the line of the cut, or sprayed on from squeezy bottle.

Rigidity is likely to be a big problem. It is best to cut in climb milling mode which tends to pull the cutter up onto the work if the machine is not rigid and backlash free.

To improve the machinability of the castings you could try a crude tempering procedure, if they are small enough. Heat them to about 470F in a regular oven then quench them in water; this more or less anneals them. Actually you need to go higher than 470 but most ovens peak out around there. However, if you have access to one of the self cleaning type put the castings in and run the cleaning cycle which is quite a bit hotter than 470 and then quench them. After this annealing procedure you want to age harden, also called precipitation harden, them and this is done by holding them for several hours at about 350F, several being something like 12 hours. As soon as you try machining any castings that you have tried to temper using this procedure you will know if it has worked because the difference in machining is like night and day.

By the way, nothing wrong with carbide wood bits for machining aluminum they can work quite well. I use router bits for machining radii on aluminum parts and they last forever.

[damusernames]

good to hear about the router cause thats whats already mounted on it...ive done some milling of aluminum with it and its worked fairly decently considering the setup...when my brother made it he used wood...which has way to much play in it...but it was origionally designed as a EDM machine so there were no horizonal or vertical loads on it...

1) surface feet per minute...is this the distance you cut the material in a minute or is this the distance the outside of the cutter travels in a minute? ive been trying to figure this out for a while

2) ive heard that you can use wd-40 instead of transmission fluid...i have access to both but i think the wd-40 would be cleaner and easier

3)climb cutting...what exactly is this i hear everyone saying it but i dont know what it means? preferably a definition an idiot can understand...i may be going into my third year of engineering...but its structural engineering...not mechanical...

4) for hardening of the amuminum if i put in a bbq grill then quenched in water for the first step would this be suffecient? then using an oven for the long term low heat?

[Geof]

Surface feet per minute (sfm) is the peripheral speed of the cutter.

Inches per minute (ipm) is the rate of advance of the cutter through the work.

The sfm for a particular material and cutter diameter is used to determine the correct rpm which is then used with the chipload, i.e. the depth of cut per tooth multiplied by the number of teeth to get the ipm.

I don't think WD40 works as well as transmission fluid. The best of course is a proper cutting lubricant.

Climb cutting is when the rotating action of the cuts tends to pull it into the work. A cutter rotating clockwise and passing along the lefthand side of the material viewed in the direction of travel is climb cutting.

Almost any method of heating will work for the initial annealing of the aluminum provided it is not overheated. Idealy you should get to somewhere around 80% of the melting point which is actually well above oven heat but not bbq heat. Just don't get it so hot it starts to soften.

[Shev]

download G-Wizard from Bob over at CNC Cookbook...

http://www.cnccookbook.com/CCGWizard.html

[damusernames]

ok thanks for the help...ill probably rig up some form of a halfass pump system for transmission fluid ive got a few quarts of merc 5 floating around...

im a little curious why do people recomend transmission fluid instead of a standard motor oil? ive used motor oil for years while using a drill press on metel would transmission fluid be a better alternative for this also?

the hardening sounds like a pretty practical method would quenching a casting right after pouring work for the first step rather than reheating it? as generally by the time i pull it out of the sand its still easily in the 500 degree range or is the reheating a necessary process? is there anything i could do to molten aluminum to increase its hardness...ive read before that silicon is used to increase its machinablilty...i dont know if thats accurate but it wouldnt be that hard to throw a few old computer chip boards in the aluminum to introduce silicon

[Geof]

In my experience transmission fluid works much better than motor oil for machining aluminum or steel, why I don't really know.

Silicon is included in aluminum alloys for casting because it improves the fludity of the molten aluminum. It does not improve the machinability, actually it makes it worse because high silicone alloys are very abrasive. Cast aluminum can be difficult to machine with HSS tools simply because they do not survive very well and even carbide tools will have a much shorter life than on extruded alloys.

[damusernames]

well the transmission fluid was a significant improvement im noticing a much cleaner cut on the material and it seems to be a much smoother operation alot of the vibrations and unpleasant noises have gone away...i had tried water and motor oil in the past and neither had worked as well...

is there anything i can put into aluminum that will make it machine easier? within reason and easy to obtain obviously...

[Geof]

Originally Posted by damusernames ....is there anything i can put into aluminum that will make it machine easier? within reason and easy to obtain obviously...

Not really. Pure aluminum, which is difficult to get, does not machine very well but it is also not much use because it is very soft and weak and cannot be tempered. It is the alloying elements in aluminum alloys that make it possible to temper the alloy and improve its strength, which also vastly improves the machinability. You should be able to find something on Wikipedia about different alloying elements used with aluminum.

[aucran]

Quick and easy info on Aluminum:
http://www.mcmaster.com/#alloy-aluminum/=7l93f5 (Scroll down to materials)

http://www.mcmaster.com/#about-aluminum/=7l93se


I use mostly AL6061 and it machines well.

[Pete_Read]

Paraffin (kerosine in the US) works well as a lube. Then again, I used to work on alluminium airframes, jet fuel is basically kerosine so we weren't exactly short of the stuff!

[Geof]

Yes kerosene works and I used to use it years ago. Unfortunately I discovered that a mixture of fine steel turnings, fine aluminum turnings and kerosene can burn very well. Brilliant whites flames coming up the back of the machine and licking across the basement ceiling can be nerve wracking; fortunately I had a dry powder extinguisher so I did not burn my house down.