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View Full Version : why can't I mill at recommended SFM?



Kart29
04-22-2009, 08:46 PM
So, I just got little benchtop mill a few weeks ago and am finally starting to cut some metal. I was trying to cut some 1018 with a 5/16 carbide EM - side milling. I looked up the recommended SFM in a book and it told me something like 125-175 sfm. I made my calculations, RPM=CS X 3.82 / .313 and come up with something like 1600 RPM. So I sets my belts to produce an RPM near that. But the EM was howling like a banshee at that speed. I slowed it way down to something like 500 RPM and suddenly it started cutting with a nice low hum and producing short curly chips.

It wasn't just this case, either. I almost always find that things cut better for me when I run speeds producing a fraction of the recommended sfm. Why is this? Any ideas?

sansbury
04-22-2009, 08:53 PM
"Little benchtop mill" is probably part of the reason. Most of the books assume you're running a full-size knee mill or similar which has far more rigidity and vibration-damping.

500 still sounds a little slow to me as I use around double that on my mini-mill with good results. How deep/wide were you trying to cut?

spope14
04-22-2009, 08:54 PM
Rigidity of set-up and the machine are a major factor. Speeds and feeds are set for optimal conditions, such as a Bridgeport or larger machines, full size vises for example. Even then for bridgeports, with the quill right up in the machine head. You may want to cut your speed x 1/3. Your motor is probably underpowered as well, less than 3 to 5 HP changes all of this

Then again, maybe it is also feed. Too little feed will cause chatter. Cut the speed and maybe give it a bit of an extra push.

However, all said and done, rigidity. I have machines that are extremely rigid and I can really push out the speeds and feeds probably 20% higher. My little sherlines I tend to run by sound and my mid size benchtops I run probably 1/2 for steel and aluminum.

You may want to take a few notes regarding what you find successful on your machine.

BobWarfield
04-22-2009, 09:10 PM
Kart29, that was a bit much. Try the HSS numbers instead of the carbide for a little margin on a small mill. My speeds and feeds calculator suggests:

1262 rpm
103 SFM
0.100 max depth of cut. less for a slot, but you said you're side milling.
5.4 IPM feedrate

I always found it useful to convert the feedrate to handwheel turns per minute, and then seconds per handwheel turn. Count of a couple until you see what's needed and then hold that pace.

Beyond that, all the other things talked about. Rigidity of machine. Rigidity of setup. Runout on the cutter. Coolant. Yada, yada.

Cheers,

BW

Kart29
04-22-2009, 09:24 PM
I was cutting .030 deep on a 1/2" thick piece of stock.

Bob Warfield, thanks for the data. That's a good idea on estimating the feed also. I never thought of that - I always just sort of went by feel.

I have started a little notebook keeping track of material, type & size of cutter, type of cut, and RPM. Hopefully I can start to build up my own little knowledge base.

Rigidity could be a real issue in this case. So far all I have is a CHEEP, super-crappy little vise that came with the mill. "Rigid" is not a word that comes to mind when viewing this vise. Rather, "floppy" or "loosey-goosey" would be far more technically accurate. I didn't realize rigidity of setup or machine could have such an effect. But it only makes sense when you stop to think about it.

A real vise is the next thing on my acquisition list.

Carl_In_NH
04-22-2009, 09:45 PM
Not just flex in the vice - but in the entire machine as well. Test by eliminating the vise and bolting a workpiece right to the table; that should tell you if there's any real effect introduced by the vise.

-Carl

airsmith282
04-22-2009, 09:51 PM
i find my self carbie for HSS i use about 400 sometime a bit slower depending on the material ,, brass i usualy give hell on and face cutting on aluim full speed ahead steel much slower just all depends on the debth of the cut as well.. many factors involved but for the most part 500 or 450 ish,, i have also leaned i can do alot more dry milling as well as turnning onthe lath if i take the extra time and slow it down a bit to alot less mess to that way..

anyhow i keep on my junk in my brain i hate paper
...

bobw53
04-23-2009, 12:42 PM
Kart, as everybody else said, 99% probability that you have a rigidity issue. My guess is your 5/16 was a standard 4 flute finisher, not the best choice for moving material.

I would suggest giving some of the high zoot variable flute endmills a shot. They are great on a big ass machine, but can really liven up a smaller less rigid machine. Simple concept, breaks up the harmonics, that are giving you fits right now. Maritool has double enders that are incredibly cheap for what they are.

Paul Alciatore
04-23-2009, 01:03 PM
In line with what everyone has said about rigidity, check the gibs. Adjust them as tight as possible while maintaining the ability to move the ways. Lubricate the ways and the screws first. And while you are cutting, lock down any axis that you are not actually using for the cut: this can also improve rigidity and accuracy. I always do this even on a full sized machine.

pcarpenter
04-23-2009, 01:11 PM
I know this is digressing, but I checked out the variable flute end mills referenced from Maritool. This video is worth watching. They are milling a .45 inch deep pocket in one pass in 4140 at speeds that make it look like it was wood or aluminum. Not that the rest of us will ever need to do that, but it makes the case that it can be done with reasonable tool rigidity and enough machine rigidity and HP.

http://www.maritool.com/Cutting-Tools-End-Mills-Variable-Flute-End-Mills/c78_79_91/index.html

lazlo
04-23-2009, 01:27 PM
I checked out the variable flute end mills referenced from Maritool. This video is worth watching. They are milling a .45 inch deep pocket in one pass in 4140 at speeds that make it look like it was wood or aluminum. Not that the rest of us will ever need to do that, but it makes the case that it can be done with reasonable tool rigidity and enough machine rigidity and HP.

http://www.maritool.com/Cutting-Tools-End-Mills-Variable-Flute-End-Mills/c78_79_91/index.html

Here's a Mazak running a 5/8" variable flute endmill at 1900 IPM in Aluminum, then another run at 400 IPM in 4140 Pre Hard. Don't try this at home! :)

http://www.youtube.com/watch?v=YG_A6gvOS24