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Getting near the limits?

DPittman

DPittman

Ultra Member
I'm still learning with my mini knee mill (6×26) and have basically been afraid of pushing it past its limits in fear of damage to machine, work and self. I have been really conservative I think on my cutting depth and feeds but I'm wondering if I am getting close. I know every "depends" but I figure the more experienced operators could look at this info/video and suggest if I'm anywhere near using my machine to capacity.

I'm using a 5/8" 4 flute hss end mill running about 930 rpm.

I am running the x power feed at unknown rate.
I'm taking 2mm (approx. 078") depth of cut.

Material is steel and the only description I can add is that it is "salvaged farm machinery flat bar".

The milled finish seems good in either climb or conventional travel. I tried with and without wd40 and didn't seem to make much difference.

The machine did not seem to grumble or shake whatsoever.

I just figured I'd ask before doubling the cut to see if anyone figured that I'd be getting close already to a reasonable maximum cut with my machine.

 
140mower said:
..... I am interested in what others think on the subject as well, 'cause I don't know that my method of "push it until it breaks and back off a little" is necessarily the best approach. But, my jar of future round shank tool blanks is growing nicely.....
Click to expand...
That's kind of my method with lots of things too but in this instance I want to learn the easier way instead. I remember running combine (long before any digital/analog sensors ) and being told that I wouldn't really know what the limits were until I plugged the combine.
 
DPittman said:
Oh wow, so I'm a long way yet.
I'd think a rpm or inches per minute meter on the power feed would be a good/ helpful thing???
Click to expand...
Does your power feed not have values, or is it just numbers?

If just numbers, do a test. Don't have to turn your spindle, just measure say 12" and time how long it takes to get from one end to the other. Do it for each number on your feed mechanism, then convert those numbers into inches per minute. Make a chart.

Also, if you are using a VFD, it can probably display the amperage while cutting. My 2hp on my mill is rated at 6amp. So I'll increase feed until its around 4 amps. If I'm cutting at 5amps, I'll back the feed or speed down.
 
Feed looks very slow. Speed for HSS seems quite fast - I usually would run 1/2 slower than you run 5/8. I would run 1/2 at 700 and 5/8 at say 550 or 500.

With slow feed you are rubbing a lot.

I would aim for a cut of 0.001+ per tooth. So at 500 rpm that would be at least 2in per min.

The shaking of the machine has to do a lot with how sharp your cutter is. When the sucker is dull even reasonable cuts will get your machine angry and produce strange results (EM pulling out, EM breaking, EM pushed in, parts dislodged from vice etc.)
 
thestelster said:
Does your power feed not have values, or is it just numbers?

If just numbers, do a test. Don't have to turn your spindle, just measure say 12" and time how long it takes to get from one end to the other. Do it for each number on your feed mechanism, then convert those numbers into inches per minute. Make a chart.

Also, if you are using a VFD, it can probably display the amperage while cutting. My 2hp on my mill is rated at 6amp. So I'll increase feed until its around 4 amps. If I'm cutting at 5amps, I'll back the feed or speed down.
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No my power feed doesn't even have numbers or hash marks, I've thought about measuring it and making a basic chart but I don't think it is consistent over the range and maybe not even in the same direction. I guess I need to confirm that with some measuring.

I do have a vfd and I forgot about that feature. Thanks.

What would likely be more taxing to the machine doubling the depth of cut, or doubling the rate of feed (in/min).?



20230217_134540.jpg

Tom Kitta said:
Feed looks very slow. Speed for HSS seems quite fast - I usually would run 1/2 slower than you run 5/8. I would run 1/2 at 700 and 5/8 at say 550 or 500.
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Good to know thanks.
 
I use my Machinist Toolbox calculator to give me the absolute maximum I might want to run since it appears to be commercially oriented for maximum throughput.
This is free machining steel. Whatever that is.
1676669614110.webp

And this is low carbon steels.
1676669718419.webp

Surface speed drops and as a result RPM and IPM. Max radial depth of cut and axial are about the same. I suspect flood coolant would be required for these rates like you see on videos using HAAS machines for example.
 
How long do you want your tool to last? In machining nearly everything you do has a trade off. I’m not in a hurry, and I like to make my tools last - so like others have said, reduce the spindle speed. Often the #1 problem is recutting your own swarf, second is tool life goes down as the temperature increases, so avoiding the rubbing (like Tom pointed out) will give a longer life and lower temperature (and is a benefit of climb milling - less rubbing coming into the cut). Flood coolant provides these benefits (and obviously removes heat, but that is #3 on the benefit list). Get an air blast in that will help. Your cut is very shallow, you’ll wear the corners, with much of the flute still good, but won’t cut well.
 
One indicator is listening. You are re-cutting chips there, that is what the odd click or chirp is telling you. This tells you 2 things: Your RPM is already fast enough (sometimes this tells you it is too fast.) But re-cutting chips is shortening your tool life, due to extra pressure and wear in re-cutting.

You can use air or coolant to help with chip evacuation. If you do, it looks like you can increase your feed a 'smidge'... There is also squeaking, but I don't know if that's your mill or the cutter. If it is not your mill, then you need to back off you feed a little.

... complex much??? :rolleyes:
 
DPittman said:
Oh wow, so I'm a long way yet.
I'd think a rpm or inches per minute meter on the power feed would be a good/ helpful thing???
Click to expand...
Too fast. Beyond the calculated max rpm, tool wear vs removal goes non linear; you're wearing out cutters more quickly than you need to , to remvoe the same material.

The calculation is very simple, and is the same for milling, drilling and turning, just do it in your head as you're setting up.

Memorize RPM = 4 x CS / diam. CS for HSS in steel is 100, AL 400, Brass 200, chrome moly or tool steel 50-80, stainless, slow right down and control the chip so you don't get work hardening. do the calc and round down to the nearest machine speed.

Thats it, all you need to know to get the right rpm on all your machines.

RPM = 4 x 100 / .625 = 640. 500 or 550 is perfect (there is no penalty for going slower with HSS.

I also often go even slower, less feed and a really big depth of cut. Uses more of the cutter. Otherwise you're wearing out that bottom 2mm and the rest of the cutter never sees any action.
 
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In aluminum I'm running closer to 20ipm full width 1/2" carbide endmill with a 0.050 depth at 3000rpm.

Steel definitely less.

Couple of things to consider for how hard you push it.

Carbide or HS cutters, want to push carbide is a must as there is less flex, and flex can cause unintentional grap.

Conventional cut or climb cut. If you have any play in your system climb cut while a better finish (and trend CNC) is going will pull work into the cutter causing grab.

Finally cooling/lubrication as you increase cuts heat is generated quickly which is both bad for the work and cutter, causing all sorts of problems. Dap lube, spritz lube are ok to a point, mist lube maybe a bit better, ultimately you are going to need flood cooling.

HP/Torque does you machine have enough?

RPM? inceased RPM allows increased feed rates as chip load remains similar though load on motor increases see above.

Machine rigidity, as you increase the loads on the machine increase dramatically, adding vibrations, flex all which can cause issues.

Vibration, sometimes a slight increase or decrease in any of the variables can remove this issue. It is a case of playing around to find the best that works for you.

So a good starting point is the charts, start cutting and see where things sound, look bad or break in terms of cutters.

As the saying goes, try and try again till you get where you want.
 
140mower said:
..... I am interested in what others think on the subject as well, 'cause I don't know that my method of "push it until it breaks and back off a little" is necessarily the best approach. But, my jar of future round shank tool blanks is growing nicely.....
Click to expand...

The chip color and formation will tell you what the cutter is asking for, prior to wear and breakage. ( This is better than any calculated speed and feed chart, well, unless you find an old machinist handbook 25th addition or older )
A new HSS endmill will want less RPM with a little more feed, until it wears in a bit. That will help keep them from squawking and breaking when using smaller diameter endmills.
 
After misunderstanding some of the advice here and the online calculators I am a bit gun-shy of repeating mistakes and having a wreck.

I have this little railroad anvil I made and have never worried about resurfacing the top surface of it as it really didn't matter much to what I typically hammer on about. But now I see an opportunity to try out a face mill I got with my machine but have hardly used.

I've tried a few online calculators and I know that they are a good starting point IF one understands them correctly. I did not understand things correctly last attempt so I am going to run my plan by the folks here first this time.

The face mill is 50mm diameter with 4 apkt1604 inserts. The railroad anvil is likey 1084 medium carbon steel.

My calculations say about 200 rpm and 3.25 ft/min feed rate. I'm not 100% sure I have all info entered right?

The depth of cut says .375" and width of cut says 1.867". Does that mean I attempt to remove .375" of material across nearly the entire width of the tool???

I will squirt wd40 as a lubricant as needed?
Screenshot_20230223-150354.jpg
20230223_150249.jpg
 
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