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Very basic milling questions

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Goran Hosinsky13/03/2010 12:29:59
41 forum posts
I am just beginning my career with the mill drill and have two questions:
 
In the litterateur I have found several thumb rules, like 500 rpm @ 10 mm to start and max 1/3 diameter deep cuts, but I have not seen anything about feed speed. My mill is mounted at the back of the lathe so I can use the lathe feed screw also for milling. Is there any rule of the thumb for feed speed? 
 
A second question is that I understand that very fine cuts might be difficult. So, how much material should I leave for the final cleaning up cut?
 
Thanks
Goran
Canary Islands


LADmachining13/03/2010 12:39:23
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124 forum posts
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Rules of thumb are not often much use when considering an activity with as many variables as milling.  A lot depends on the type of material, type and condition of the cutting tool, rigidity of the machine and workpiece etc, etc.
 
A much better way, I have found, is to start with light cuts of 0.25 to 0.5mm, with a new cutter, and see how the machine reacts.  You will soon find out if deeper cuts, or a faster/slower spindle speed is/are required.
 
The same can be said for feed speed - depends on material, spindle speed, depth of cut etc.  Again, start at a slow speed (but not too slow, as you don't want the milling cutter to rub on the material and become dull).  The machine will soon tell you by sound and feel if you are asking too much of it.
 
For finishing cuts, I generally leave 0.1 - 0.25mm remaining, depending again on the type of material and its rigidity.  A new cutter helps achieve a nice surface finish...
 
Best advice is to practice on some scrap material first, to get to know your machine.
 
Anthony
Peter Gain13/03/2010 13:25:58
103 forum posts
As Anthony states, it is a very good idea to practice on scrap. But try to use metal with a known spec as a poor result may be due in whole or part to a difficult to machine material. Some metals are a difficult even for an experienced worker. Good luck!
Peter Gain.
 
Steve Garnett13/03/2010 14:55:00
837 forum posts
27 photos
Goran, you haven't indicated which machine you have. If it's a machine with coolant, then you may well get away with slightly greater speeds in general anyway, and also probably a better finish (tools not withstanding). And since, with the best will in the world, any mill attached to a lathe isn't going to be as robust/substantial as a standalone one would be, having a coolant system available sounds like a good idea anyway, since everything you can do to ease its lot will help.
 
Even if you don't want to go down the coolant route, it's still worth making sure that you can apply some sort of cooling/lubrication to your cuts whilst they are happening, even if it's just by the simple 'paintbrush' method.
 
As for the 'practice on scrap' idea, it's excellent advice as ever. The only thing I'd add is that as a beginner, try practising on a scrap of the actual material you want to mill, if you can. That way, you get the clearest idea you can about how fast you can proceed.
Versaboss13/03/2010 19:23:00
493 forum posts
71 photos

It is usual practice to calculate the 'tooth load' or 'cut per tooth' when milling. Say you have a 4-tooth cutter rotating at 500 rpm, that's 2000 cuts per minute. A fine cut is maybe 0.05 mm, so you get a maximal feedrate of  2000.x 0.05 = 100 mm/min.

But I confess when milling aluminium I usually just turn the handle as fast as I can !

Greetings, Hansrudolf

Ian S C14/03/2010 00:00:10
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7468 forum posts
230 photos
When I bought my mill I had no idea how to use it, I asked the "expert" in the shop where I got the machine (the tool was a 3 point TC cutter 50mm dia) how fast etc etc, he said give it about a mm, 900rpm, and crank like hell. So I halved that, and found that was OK. Ian S C
KWIL14/03/2010 08:36:15
3562 forum posts
70 photos
There are charts which enable you to "look up" material, diameter of material or cutter and hence derive rotation speed and feed rate.  You can choose max metal removal, industrial style, or max tool life with a slower removal rate, or somewhere in between.
John Haine14/03/2010 09:49:54
4714 forum posts
273 photos
Lathe is probably less rigid than a mill, so reduce feed rate/cut depth a bit.  Best advice though is to start slow and go by what sounds right.
Steve Garnett14/03/2010 11:01:30
837 forum posts
27 photos
Yeah, that's the other thing - how well any of this works at all rather depends on the amount of backlash present on the carriage and top slide, doesn't it?
 
Definitely suck it and see...
Goran Hosinsky16/03/2010 16:48:06
41 forum posts
Thanks for the comments!  I will be testing and see what happens.
Goran
Nigel McBurney 116/03/2010 20:36:45
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1004 forum posts
3 photos
hi    Assuming the use of high speed steel tools,and economy ,run 10 to 12mm cutters at around 500 rpm on mild steel,cast iron,and bronze bronze, always use soluble oil on mild steel and bronze, cast iron cut dry.aluminium at twice the speed,brass anywhere between 500 and 1000rpm.  Hss will cut at higher speeds but cutter life is reduced.Running hss cutters at slower speeds increases their life where as carbide tools suffer if run too slow.For economy when machining flat surfaces try making and using fly cutters with a single point hss tool bit,they are easily ground by hand,cost virtually nothing and give an excellent surface finish,because they can easily be reground,a flycutter with  a tool at 25mm radius can be run at 1000 rpm on brass or aluminium and will give a beautiful finish just keep out of the way of the flying swarf.
Goran Hosinsky16/03/2010 21:10:39
41 forum posts
Do I understand this, that flycutters can be run at about the double speed of normal cutters of the same diameter?
Goran

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