Rpm and Feed Rates

[mrainey]

For a really nice, free program to make machining calculations, see my post in "Product Announcements" (ME Consultant 2.0).

[Jaws]

I've also got a TI 86. Would your program work in my calculator too?!

[Jaws]

Xeno,

Check this out, hope it helps a bit:

http://www.hougen.com/tech_tips/speeds/speed.html

[Mouldmaker]

Try out this one. It free-ware.

http://www.cimco-software.com/index-main.html

[mach3magee]

There are so many variables to this formula that your most reliable tool will be your personal experience. I believe that the best and least expensive way to gain a working knowledge of speeds and feeds is to understand tooling material types vs workpiece material types and how they translate into surface feet per minute (SFM) and chip load per tooth (CL) if that makes sense. My personal preferences are for instance:

Mild Steel:
Drill-(Dia-3/16-1in)
HSS Spot Drill: 90 SFM/.003 CL
HSS: 55 SFM/.0015-.004 CL
Cobalt: 90 SFM/.002-.005 CL
Solid Carbide: 240 SFM/.0025-.006 CL
Insert Carbide: 300 SFM/.0025-.007 CL

End Mill-(Dia:3/16-1in)
HSS: 90 SFM/.0005-.004 CL
Cobalt: 150 SFM/.0007-.005 CL
Solid Carbide: 220-280 SFM/.001-.006 CL
Insert Carbide: 300-400 SFM/NA-.007 CL

You want to reduce surface footage when working with harder/tougher workpiece material but dont monkey with the chipload too much. Too little chipload is just as bad for the tool as too much. You need to look at the chips you are producing too. Generally speaking, if they are ragged and dark blue or at all black then you most likely have too much RPM vs feed. If chips are clear or natural looking (like the metal you are machining) then you could do with more feed. Normally, a brownish or gold-colored chip is ideal. This usually means that most of the heat generated during the cutting process is being carried away by the chips. Exactly what you want for long tool tool life. Some tool materials that is. HSS would do better machining at a natural chip color state while a coated carbide almost demands to be pushed to achieve optimum cutting conditions.
Then there are some harder and more exotic metals in which chips will appear irradescent when cutting conditions are right. Variables are truly endless. Workpiece rigidity, tool length, depth of cut, radial cut, bla...bla...bla...
I'm tired and my grandma is invading my privacy so I gess its time to go. I hope I helped more than I confused, and I know my surface footage is conservative but thats just my style.
Good luck

[mach3magee]

I forgot to mention to get one of those machining calculators (slide rule) and learn how to use it. Tool salesman hand them out by the pile. It also helps you commit variables to memory better than a computer program that does it all for you.

[sirius]

Wow, what a source of info on here!!!

I was just looking for some info too!.

Im not an experienced CNC programmer, I was an apprentice on manual machines and now Im an office dweller needing to program!.

All I was looking for was a simpler rule of thumb like:

Mild Steel, general 6 tipped cutter roughing cycle + ??rpm & ??feedrate, ?? cutdepth.
As above, 25mm ballnose, ??rpm, ??feedrate.

you know, a kind of basic, everyday range you cant go wrong with!!! lol. No cut per flutes or whatever! lol, just 'common sense' values!! hahahah. nah, Im not that bad, but there is conflicts of 'its too fast it ruining tips and shaking the whole machine' and "its not cutting enough you will '****' the tips up!"

Well, thanks for the info so far, Ill have to try and figure it all out.


Cheers

Sirius.

[HuFlungDung]

Sirius,

It's not common sense until you've practised using the formulas for a while. If you apprenticed already, you should be halfway there

[sirius]

Yeah, Ill have to get my maths cap on!!!!

When I was apprentice, I used to look and listen and tweak till i was producing a good finish on the old manual machines.......it was oh so simple then! lol. Now, with a new 'machining centre' and being located off the shopfloor, the values can be hard to imagine in 'reality'. I should have payed more attention to what the readouts were giving me at the time!!.....but saying that, you could only take cut depths of about .75 of a millimeter on those old things whereas you can cream off 8mm per cut on this baby!! lol.

Im going to save the page on a floppy and take it into work. The chips usually coming off the machine are deep blue (due to feedrates recommended off the CNC operator)....I always thought it was going too fast, becuase tips wouldnt last, so at least if I start to get a balance back and the recommended swarf colour, I'll be laughing!

Thanks,

Sirius.

[HuFlungDung]

Unless you are machining with coolant, the chips are normally a deep blue color when machining steel. So I am afraid that is not too useful of an indicator any more.

One thing to watch for with carbide is sparking at the tool tip: if it sparks once in a while, you are going too fast. This would apply to machining dry, though.

Resist the temptation to overspeed and underfeed, which is a common fault of newbies. Some guys will run the snot out of the spindle and barely be moving. Tool life really amounts to so and so many chips, and if you use up that number on whispy little chips, you are not going to get much done before its time to change the insert.

[sirius]

Well, Ive yet to download some of the software mentioned earlier, but I had a look in the back of the Tooling Catalogs today armed with a calculator and a rusty remeberence of algebra........Jeezuz its compliacted!!!!!!.

What is the difference between surface speed and feedrate? They are given different 'labels' in both catalogs I looked in.

Why does the catalog give me the average chip thickness (ie .12mm) and why on earth would I need to know this? - it just falls in the scrap tray.

For example I tried to calculate a 20mm x r3.5 tip "Hognose" cutter for ToolSteel (BD2). The catalog reckoned a cut depth of 2mm (but the machine can only handle 1.25mm cuts) and an 8mm stepover. After gyrating through the mathematical hoops, I got a "Surface Speed" of approx 150 Meters per Minute.

Now, in the catalogue it says ( i think) to find the RPM.... Times the Surface speed x 1000, divide that by 3.142 x Dia of cutter.......which gave a RPM of about 2830!!! At which one guy said the tool would just disintegrate inbetween laughing, lol.

I worked out a feedrate per tooth which came at something like 0.51mm/minuite........how does this help me? I need a overall feedrate.

The job is a block of Toolsteel, roughing cycle with the above cutter on an old 'retrofit' style CNC that can usually only handle 1.25 cuts with that cutter without vibrating the head!!! lol.

There seems to be too much information required in the formula's than I know what to do with!! I just wanna know how fast I can go with tipped cutters on ToolSteel with 1.5mm cuts and what rpm that would be! hahah.

We have two old CNC's and a new 'Machining Centre' thats more robust, So I need to suss out what to do with each machine.

Im going to have look at the programs mentioned, but I think the ones I tried last time are only for endmills/slotdrills and I dont think this helps me with using the tipped tools, or working out for tipped ballnose or face mills.


Cheerio

Sirius.

[HuFlungDung]

Grasshopper

Surface speed = velocity of the largest cutting circumference of your milling cutter. This is a factor of the type of tool, whether it be high speed steel, plain carbide, coated carbides of various types.

Feed rate in terms of chip thickness is the most basic fact about the tool's capabilities, while assuming that you are already running it at the correct velocity (surface speed).

Take the rpm of the cutter * number of teeth * chip thickness = feedrate in inches or mm per minute.