If you know a site, or if you have a list of the speed requirement for turning metals on a lathe, I would appreciate it. On my quick change gearbox on my lathe it shows the speeds, but I like to know how fast I should have the spindle turn if I'm turning let's say aluminum and soft steel maybe...The same maybe, if I could impose for a milling machine as well....I used to have all this stuff, but whenever my grandson visits me, and he needs small things, he borrows it, and then forgets to give it back to me. Then if I need it, I can't put my hands on it....Peter
Peter, I don't know of a site with that info, must be somewhere.
But it's rather easy; Metals have an optimum surface speed, based on tool wear, and productivity. Surface speed is how fast the tool travels over the work, in feet per minute.
FPM for common metals using High Speed Steel tools: Hard Steel/Stainless 50 Soft Steel/Iron 100 Brass 200 Aluminum 500
(Many other speeds are quoted, this is generic, and nudged for memory sake.)
So, if you have a 6" diameter steel blank in the lathe. The circumference is 6" x 3 (Pi) = 18 divided by 12 to get feet = 1.5' circfunference. Since steel is 100 fpm, 100 / 1.5 = 66 RPM for the best productivity.
All these numbers are approximate, too slow increases tool life, fine for a home shop, that likes to watch the machine. Coolants and lubes will make a big difference. Faster wears the tool tip quicker.
Is there not some negative impact on the cutting tool if it is running too slowly? I have recently gone through the same deliberations and have to say that good tables were hard to find on the net.
I read a book which included correct speeds in a chapter. Their caveat was not to let the tool "rub" - whatever that means. My take on it was that this happens at slower speeds than recommended but could not understand it completely.
They also suggested that the speeds you quoted should be adjusted according to the operation: For instance for countersinking they suggested using speeds 1/3rd of those normally recommended.
There were also notes about the feed speeds and the material of the cutting tool affecting the correct cutting speeds, so in the end I was rather confused. Nowadays I rather tend to err on the slow side...
Make a spreadsheet with the speeds for your lathe or mill as rows, common work diameters as columns, and the FPM formula in the cells. If you don't have MS Excel you can download Calc from Openoffice.org.
Sounds like me and the kid working in the shop a few years ago. I was the tortoise, he the hare. He went fast and broke shit, I finished first by going slow and easy. But now, he's been a pro for six years. he just plain works circles around me. He still asks me to do the manual lathe work. The old guy has a better touch for that.
There are great tables for carbide inserts. Every manufacturer has a web site for this, giving optimum speed and feed by material for each insert.
I will probably be shot down in flames for this, being a mere tyro, but...
At school, we were told where to look in the textbook for the optimum cutting speeds - they do vary with the material. BUT - these were the max speeds in a production environment and were designed to get maximum ECONOMIC life out of the tooling - faster meant unacceptable tool wear and so increased downtime and cost, slower meant...you didn't get as much work done. (The temptation is, of course, to run it fast and take big hogging cuts - fine if you know what your doing...)
Run it at a speed you feel comfortable with, slow is not a BAD THING if you aint doing it for a living and being paid per job. Dont let it rub on one spot, heats up the tool, stuffs it.
Feed rates are probably more important, IMHO. But agin, slow is better than too fast.
So saying that, one of the instructors faced off a topslide mounting plate I was making - insanely and dangerously fast to me, but he got a BEAUTIFUL finish, in one pass - but then, as a toolmaker now a teacher, he knew what he was doing....
From memory, (again, probably wrong - its in my locker at school..) the Southbend 'How to run a Lathe" book, widely available as a free download has the formulas - a great quick reference guide. Pity its in imperial, I am being taught exclusively in Metric. Can think in metric now for machining, still in feet/inches for everything else. Makes life confusing, to say the least. Got a imperial micrometer here I got at a junk sale - no idea how to read it...
Oh, yes - was told to run reamers at the slowest possible speed/feed - lessens chance of damage to expensive tools...
(photocopying charts and nailing them to the wall behind the lathe/ mill is a GOOD IDEA -)- nomographs? - is that the right term?
As a sort of rule of thumb erring on the side of slower cutting speeds with higher feed rates will generally remove more material in a day then high cutting speeds with lower feed rates.
The lower cutting speeds usually let you get more cuts between sharpening too :-))
Bruce-in-Bangkok (correct email address for reply)
Nope, you can run it as slow as you want. It is the high speeds that do you in.
"rubbing" would be insufficient clearance on the tool bit.
That is usually correct but it has less to do with cutting speed then being a multi edge cutting tool. Reamers are also prone to this problem which has to do with being a multi edge cutting tool. Reamers are also prone to this problem which has to do with multiple cutting edges not cutting evenly or "chattering" leaving a poor finish or out of round hole.
Look at it this way. The cutting speed is the highest continuous speed that the tool will sustain. Now, stock removal is a somewhat different story and you may find that because of old age, a lack of machine rigidity, more frequent tool dulling, or other factors a slower cutting speed coupled with a higher feed rate MAY remove more material per hour/day the a higher cutting speed with a lower feed rate.
Bruce-in-Bangkok (correct email address for reply)
no math needed. make a facing cut across the end of the billet. set the speed of the job such that the smoothest finish area is out where you are cutting.
usually you will find one area that cuts just right for a lovely finish. if you dont see one sharpen the tool again and try another facing cut.
eons ago there was a guy creating a table for 'model engineer' magazine giving the cutting speed vs rate of removal. the table was a rectangular grid of figures. typical sweet spot being in the middle of the left hand end. for the simple reason that he wanted to fill in a value in every square in the table he ran the lathe and made the cuts at all the speed combinations.
down in the right hand lower corner was discovered a range of speeds where the cutting rate increased substantially. the tools ran red hot but the removal rate was far higher.
that bottom right corner is now where most of industry works. we work at home in the left slower end where it feels right.
Cutting speed is only half the equation-- you also need to consider the feed rate (thou per tooth or whatever).
The cutting edges have to be moved into the work fast enough that they bite in and don't just rub against the work (feeding too fast causes different problems).
The feed rate in thou per tooth is independent of the cutting rate, so if you are turning very slowly the feed rate will be slower in terms of inches per minute, but the same in terms of thou per tooth, if that makes any sense.
The optimum feeds and speeds are dependent on the type of tooling, coolant used, the material being cut, whether it's a roughing or finish cut, and other factors. Google for some numbers. I've generally used 80 sfm for HSS tooling into mild steel.
I made a simple spreadsheet and pdf file several years ago using conservative figures for smaller equipment. You can find it here:
formatting link
The spread sheet should work in MS Excel, but I don't have anyway to test it. The pdf is just a spread sheet printout with suggested values. That values should work with either drill bits or lathe stock.
It lists several materials and suggested rpm's for material from 1/32 to 8 inches in diameter. If you use the spread sheet you should be able to just change the value for the "SFM to calculate" field and come up with your own custom values.
It was built using an old version of Quattro Pro, so I have a Quattro version around too if some needs it. I know that version works :)
SFM. The spindle speed is dependent on diameter, cutting tool material and material you are machining. Surface Feet per Minute is the speed of the material in the cut in feet per minute.
-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
Hmm ... what lathe is this? Most quick-change gearboxes on lathes are for setting the thread pitch being cut, or the feed rate when turning, not the spindle speed.
There may also be a set of levers in a gearhead lathe to select various speeds.
Well ... there are multiple factors which are involved in determining the optimum speed for a lathe. (And bear in mind that the people defining "optimum" are those doing lots of work for money, so a speed that wears out the tool a bit faster, but gets more work done in a day is "better" for them.
And if you are threading up to a shoulder, you will want to run a
*lot* slower than the optimum speed, to keep from crashing your lathe.
But here are some factors which apply here.
1) The material being cut.
2) The material of the cutting tool -- usually HSS in a hand-ground tool or some carbide in interchangeable insert tooling.
3) The *diameter* of the workpiece. The larger the diameter, the faster the material is passing by the tool for a constant spindle speed.
So -- the figures given are SFM (Surface Feet per Minute), calculated from the diameter and the spindle speed.
You look up the proper SFS for the combination of the tool material and the material which you are cutting, then calculate the spindle speed using the maximum diameter of the workpiece (you can speed up as you reduce the diameter, if you want to re-calculate. To be precise, multiply the diameter in inches by Pi (3.14159...), which gives you the circumference in inches. Then divide by 12 to get the circumference in feet. Divide this into the SFM for the tool and material combination, and this will tell you how many RPM you want --
*maximum*. And this assumes a powerful enough machine to produce this speed. Yours may or may not be that powerful. Don't be afraid to divide the speed suggested by four if you are not in a *production* hurry.
To determine the SFM -- look it up on the web or with a program (you've got some pointers for this), or buy a copy of _Machinery's Handbook_, which has all of this you need. Pick up an old copy, it will be as good as a new one for a hobby machinist.
Now -- there are speeds which will give a better finish, but this is the sort of thing which you learn for your machine, tools, and material.
Remember -- not all steels are the same. Figure that a free cutting steel can probably be run four times as fast as an unannealed steel like 4140.
With carbide and aluminum, you can probably run as fast as your machine will turn -- though this might produce a poor finish as the aluminum melts and welds to the tool.
The same principles, except that you care about the diameter of the milling tool instead of the workpiece -- and you also have another factor -- chip load per tooth. Count the number of teeth on the cutter, and multiply by the chip load per tooth to get how fast you can feed, again if you are in a *production* hurry.
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