Newbie question on milling speed

[just old al]

I dont get is the SFM, surface speed per minute. How can this be applied on my lathe

SFM is one of those weird and wonderful things that makes endless sense with a bit of explanation.

Imagine a lathe tool bit cutting a round bar 4 inches in diameter. That lathe tool bit is going to be happiest cutting that kind of metal at a certain rate of speed - call it 100 SFM just for argument's sake.

The piece is 4" in diameter - so a bit of maths tells us its circumference is 12.5" or a teeny bit over 1 foot (Pi x diameter). So for 100 SFM you need to spin the piece at a little under 100 RPM so that the speed the part is moving presents the material to the tool at the rate the tool is happy cutting it.

All of this can be done in the metric system - it just happens I'm most comfortable in Imperial and explained it that way - apologies.

With your milling example we're spinning the cutter instead - so it's the cutter circumference we're working with here. That kind of metal is going to be happiest cut at a speed that presents one cutter tooth at the proper SFM.

Let's say you're using a 1" diameter cutter made of high-speed steel on brass. Brass likes 300-350 SFM. So, cutter circumference (3.1 inches, or about 1/4 foot) to cut brass should be spun at about 1200 RPM or so (300 SFM X 1/4 foot) to get the cutter speed about right.

None of this is cast in stone, and minor variations on speed are not an issue - slower is better than too fast, though.

Carbide allows for much higher SFM, but most hobby sized machines aren't suited to take full advantage of it.

Al

 

[Reeltor]

Aris,

The on-line calculator that I posted has a list of different materials and what the SFM is recommended for each material. That calculator is setup for HSS endmills, but will get you close to a feed/speed that will work.
I am not in any way an expert and am just passing on what has worked for me; it may or may not work for you.

Here is a link to a calculator that must be downloaded. It has many more advanced features. In this calculator you can choose if you are using a HSS or Carbide twist drill, HSS or Carbide end mills, Face Mills. It is more versatile but can be a little confusing. Many times I reduce the recommended RPM and feed setting. It also has a setting for Turning.

http://download.cnet.com/ME-Consultant-Professional/3000-2064_4-10522439.html

good luck

Mike

 

[Aris]

OK guys, I spent many happy hours working on the mill the last three days, and following your advice I can proudly say I'm just a tiny little bit wiser about milling
First I checked for loose parts and discovered that some could be tightened so I now have a more rigid setup, which helped. Next, I went down to 100rpm (the lowest I have) and removed three of the four inserts. Initially the results were poor, with many 'jamms' and two broken carbide inserts, but I kept trying this and that and at the end of the day I found out that I needed much 'deeper' cuts, very low-low speeds and a continuous supply of liquid. I learned along the way that my lathe experience is not applied to milling !
Today I tried with two inserts and achieved fewer jamms and a much better finish. I also realized that jamming is also due to my somehow 'weak' belt-driven power transfer, which seems like a drawback, but also a blessing for beginners like me that would otherwise damage the equipment!
Like you wrote, it took ages to mill five tool holders, but so be it, I'm sure it will be different with softer metals.

Here is the job in progress


I realize expertise comes with time and experience so I don't pretend to become an expert overnight, just to learn slowly while enjoying this hobby when I have time, so would appreciate more advice, such as a link to a good youtube tutorial. Also some brief answers to my following questions will help me at this point!

1. SFM, I learned it means Feet Per Minute, but how do I implement this value on my machine? Here is a pick of what I have, are any of those numbers SFM's?

2 On the carbide inserts, here is what came on the 50mm face plate, they look quite 'oval' to me, like if they might be specific to finishing work on soft metals. I wonder, would it help with 'jamming' if the inserts were more 'pointed' like the regular triangular carbide ones? If so, could I find 'pointed' inserts to go on the face mill I bought?

3. Should I mill always with the 'moving table' going always 'one way', or go back then forth? Or should I use a last backwards swipe for finishing?
4. Should I try to mount the work so that the carbide insert 'comes on' to the metal at an angle, to avoid bouncing or this makes no difference?

I'm done milling my AXA tool holders and now plan for my next projects whenever I find the time. I'm contemplating a Steady Rest and have already the material for it, it's steel and I trust it will prove easier to work with ! Here is what I'm aiming for, a bit complicated, but a challenge! http://www.toolsandmods.com/lathe/lathe-steady-rest

THANKS to all for your support and advise and HAVE a NICE EASTER !!!!!
Aris

 

[Reeltor]

3. Should I mill always with the 'moving table' going always 'one way', or go back then forth? Or should I use a last backwards swipe for finishing?
4. Should I try to mount the work so that the carbide insert 'comes on' to the metal at an angle, to avoid bouncing or this makes no difference?



THANKS to all for your support and advise and HAVE a NICE EASTER !!!!!
Aris[/QUOTE]

The quick answer is No, always go "one way" and that way should have the cutter entering with work for conventional milling not climb milling. I have some diagrams but they are on another computer that I cannot access. I'm sure some of the real experienced experts will chime in and explain it better, in the mean time; please look up Climb Milling, you do NOT want to climb mill on your combination machine, or any mill that is not 100% rigid and setup for it.

 

[dalvorius]

1. SFM, I learned it means Feet Per Minute, but how do I implement this value on my machine? Here is a pick of what I have, are any of those numbers SFM's?
View attachment 231033
Aris

Hello Aris,

I am also very new to machining, but I have found some good information here and also on the web.

Surface Feet per Minute (SFM) is dependant on RPM (revolutions per minute) and the diameter (size) of your work.

It is the speed of the surface of your work, as it revolves around the diameter at the RPM your machine runs at.

I use this table: http://www.tapdie.com/html/sfm_to_rpm-_surface_feet_per_minute_to_revolutions_per_minute.html

You probably know how to convert inches to mm but just in case - for reference with this table in relation to the diameter of your work : 1" (1 inch) = 25.4mm, and 1/4" = 6.35mm (approx.)

I hope this helps.

Cheers...
Dalvorius

 

[TORQUIN]

I very neat trick I learned from an old machinist is to take note of the RPM speed for various SFM's for 1 inch diameter cutter/work piece (in case of a lathe). Then when you need to calculate your RPM you just look at your note and divide the RPM speed for the 1 inch piece by the diameter you are actually cutting/cutter you are using.
For example: 70 SFM is 267 RPM for a 1" work piece (lathe) or 1" cutter (mill). If you are actually using a .5" end mill, you would divide 267 by .5 and get 534 as your RPM for that cutter. Works the same for a lathe. You just use the work piece diameter instead. If your SFM was 100 then you'd divide 382 by your cutter diameter.
SFM Spindle Speed
40 153
50 191
60 229
70 267
80 306
90 344
100 382
110 420
120 458
130 497
140 535

Sorry, I don't know of a way to do this trick in Metric.

And when you do not have the speed it requires on your machine, go slower. It will keep you from burning up your cutter.

Chris

 

[jlsmithseven]

Here is what I was taught:

To Find RPM - Take cutting speed (1018 steel) which is 70 X 4 (inserts of carbide) / by size of cutter. So if you're cutting steel it would be 280 / 50mm which is 280 / 1.9685. Your RPM for this would end up being 142.2 RPM. Pretty slow because you're using a 2 inch cutter almost. But then you times this number by 3.5 because you're using carbide which likes a faster speed. Your RPM would be about 500, but you can go 600 if you'd like.

To Find IPM- Inches per minute feed rate. Take RPM (500) X .0015 (chip rate) X number of cutters. End up with about 3 inches per minute feed rate.

This example is cutting plain steel with a 2 inch carbide cutter with 4 blades on it. Just an example.

 

[aliva]

Check this site, Tom has some very useful charts here's a link to his reference page and you can download and print the charts
http://tomstechniques.com/reference/

 

[scwhite]

I very neat trick I learned from an old machinist is to take note of the RPM speed for various SFM's for 1 inch diameter cutter/work piece (in case of a lathe). Then when you need to calculate your RPM you just look at your note and divide the RPM speed for the 1 inch piece by the diameter you are actually cutting/cutter you are using.
For example: 70 SFM is 267 RPM for a 1" work piece (lathe) or 1" cutter (mill). If you are actually using a .5" end mill, you would divide 267 by .5 and get 534 as your RPM for that cutter. Works the same for a lathe. You just use the work piece diameter instead. If your SFM was 100 then you'd divide 382 by your cutter diameter.
SFM Spindle Speed
40 153
50 191
60 229
70 267
80 306
90 344
100 382
110 420
120 458
130 497
140 535

Sorry, I don't know of a way to do this trick in Metric.

And when you do not have the speed it requires on your machine, go slower. It will keep you from burning up your cutter.

Chris

Here is a good rule of thumb
Run your end mills or cutters at the speed that you can see the flutes - just before they start to blend together.
To fast you can't see them . To slow you see them to good .
Regardless of the size end mill
This rule of thumb will put you in the working
Envelope Of the SFM
You can go up or down from there
No math needed just watch your flutes
Six flutes run slower than Four flutes two flutes
Run faster than Four flutes .
Feed rate is based on the depth of cut .
Rule of thumb
Steel or cast iron

 

[benmychree]

Just to qualify things, I tested my Aloris holders for hardness with my Rockwell hardness tester, and came away with a Rockwell "C" hardness of between 37 and 40. At this hardness they should be machinable with carbide, with some difficulty at cutting speeds below 100 sfm, or about 200 RPM or less.
My cutting speed calculator, taught in my machine shop class in high school and JC takes the allowable cutting speed (say 100 for steel) times four, divided by the diameter of the moving part. Going to the internet aside, the easiest way is to get a slide rule type cutting speed and feed calculator; these were given away by all the carbide companies in the past, and it should be easy to find one on E Bay.