Here's an important category for "machining" purposes regardless if you are machining armatures or not.If you are *NEW* to running a drill press, mill or lathe,these are the essentials you need to know.This formula is used for virtually every machining procedure.The calculation you will use most often is the calculation for determining the correct rpm's whether you are milling,turning or drilling.A similar calculation(with a twist) is also used for determining the correct rpm for reaming unless you are intending on "hand reaming" much the same way you would tapping a thread manually.

Why is this important? Optimizing the best rpm  not only insures maximizing "tool life" on expensive tooling such as end mills,twist drills and reamers, but your parts will have the best surface finishes you can get out of your machine as well as avoiding catastrophes and potential accidents that can injure you and worse,possibly cause damage to your drill press,mill or lathe.

So,what is the secret formula for calculating rpm's? Before I offer it up, much depends on the material your are machining.Low carbon steel is machined at different SFPM (surface feet per minute) than aluminum,tool steels and stainless steel as well as brass and copper.Most often in armature fabrication,the armature parts are turned and milled of stainless,tool steels and aluminum and occasionally "other" ferrous and non ferrous metals.

Knowing what number to "plug into" the formula is important.I've seen the wrong RPM formula used and watched aluminum melt into  droplets and puddle as an end mill plunged into and milled the periphery of a part as well as seeing low carbon steel come off smoking as purple and "bright red" chips or carbon steel tools literally "weld" themselves into parts because the rpm was way,way off.

Of course you will know when you have the rpm's turned up to high,your machine will "let you know".Often the tool/machine will make horrible,loud noises.The machine can *shake*,cutting oils will smoke terribly and your "chips" will come off in irregular shapes(a continuous chip from a lathe often a result of "too fast an rpm&feed rate)very hot in bright blue,deep purple or "red hot" colors and sometimes you will see "sparks" when there shouldn't be.

Even if you have done the formula correctly,always look and listen to your machine and what it is doing.There is no harden set rule in this,and "when in doubt" turn your rpm's down.Knowing your machine and it's capabilities and limitations is important.It is never good practice to push your lathe or mill beyond its design limits.That's how things break and you might get away with that a few times and maybe only break a turning tool or a drill,but that one time is when something expensive breaks in your mill or lathe and then you are "in trouble".

So,the following applies to SFPM of some basic materials used in machining.Choose the correct calculated number and below I will show/demonstrate how it works.These are rounded up/down figures from far more "precise" calculations found in "The Machinery's handbook". I'm going off of memory too,so if I am incorrect I will make subtle changes/corrections,but the following I am almost 100% certain of.

low carbon steel: 100sfpm

aluminum: 200sfpm

tool steel:50-80sfpm (look&listen,when in doubt,reduce rpm's)

stainless:50-80sfpm (look&listen,when in doubt,reduce rpm's)

How is the formula applied?It depends on whether you are turning or milling.Its a mind set you have to think about until after you do it so much,you won't have to think about it anymore.The formula is as follows:

4Xcutting speed(SFPM) = RPM's


example 1: you are milling some low carbon steel with a .500HSS endmill

4 x100sfpm divided by .500 = 800rpm's

example 2: you are turning 1.00 in. diameter aluminum stock in your lathe using a HSS tool bit

4 x 200sfpm divided by 1.00 = 800rpm's

example 3: you are drilling "water hardening" tool steel using a 1/2 inch HSS twist drill

4 x 50sfpm divided by .500 = 400rpm's

A word on spot drilling,reaming and power tapping: There are always exceptions,and one exception to the rule in this formula is "spot drilling".We need to "spot frill" to assist in guiding the twist drill in and giving it a good starting point to drill "straiter"  and prevent the drill from "walking,bending or breaking on the part we are drilling into.While the formulas about hold true to milling and drilling,the spot drill is a slightly different animal.

Proper spot drilling procedure is you only plunge the tip into the part surface approximately1/2 to 2/3's the way in.You DO NOT "peck" with the spot drill.A slow,steady pressure is exerted to final depth.Basically,take a rough measurement for the diameter and when you get your rpm,roughly "double it".You more or less play this "by ear",so if your rpm is 400rpm's you can spot drill somewhere between 600 and 700rpm's.A word to the wise,if you spot drill at too low an rpm,you run the risk of breaking the tip off inside your part and unless you have access to an EDM machine,you'll never remove it. does the rpm formula differ when reaming? For the most part the rpm formula is unchanged with one exception,the number or cutting teeth(or in a reamer's case) the number of flutes.Reamers are not drills and they have more flutes than  your average two fluted twist drill.So,if you are reaming a hole out to be tapped be it in on your mill or chucked ij your lathe,the following formula applies:

RPM= SFPM x 4 divided by diameter(this being the reamer) then "basically" you divide your calculated RPM "in half".

example: you are reaming out a drilled hole  in aluminum to a finished size of 1/4 inch using a HSS .250 reamer.

RPM= 4 x 200 divided by .250 = 3200rpm's

3200 divided by 2= 1600rpm's

Another thing to consider when reaming. Reaming out a hold to finished size you are not removing a large amount of material. This is where the useful charts in the Machinery's Handbook is useful.There in,you can look up what drill size to end with in order to "ream out" to it's finished hole size.Often it is not more than a few thousandths of an inch and as the reamer becomes smaller, the amount in turn is less and less.

Power Tapping.What is power tapping?Well it is exactly what it says it is,"tapping under power".Can/should you power tap?Yes you can power tap,some occasions call for it,however for our purposes,many of the taps we use for screws holding our armatures together are very small.Power taping wouldn't really be necessary in my opinion and I wouldn't tap under power unless the tape is at least a 1/4 tap due to risk of breaking the tap in a finished part.

Keep in mind if you are power tapping you run the risk of breaking the tap in your finished part,just something to think about.If you are going to power tap you really have to be "dead nuts on center" over the hole to be tapped.This is where a DRO comes in handy (digital readout).A DRO allows you to position your tools exactly within .001-.0005 accuracy over the center of holes and parts to be milled to stay within your dimensions/tolerances.The DRO is "repeatable".You can find the same location over and over again as long as your set up has been done correctly.Most of us won't have this on our milling machine,so I think "hand taping" would serve most of us best.Also,(in any tapping procedure) you want to countersink your holes to be tapped.This is a necessity.The 45 deg countersink gives the tap a "good start" and things go smoothly.

If you are going to power tap (aside from being "dead center"), you have to shift your mill (or lathe) into LOW GEAR and dial your rpm's down to the lowest it can go.You want the slowest rpm your machine will turn.So,if you can run your mill or lathe at/below 50rpms that's what you want.Make sure you mill table is locked in place and the quill "is not locked.While the tap turns in your spindle bring it to the surface of the hole/part.Often I would "hand tap" the first  two to three threads just to get it started before power taping(it was just something I did).As you do this,the tap will turn under power and slowly draw the tap into the hole.As it goes through and the point of the tap pushes through the back side stop your mill,your threading is done.Just "reverse" your mill and the tap will back out.

Why would you want to tap under power?There are a few reasons and I can't remember all of them.Mainly it saves time.If you are tapping allot of holes,you definitely don't want to be hand tapping all of them.Also(if memory serves),you get a more uniform thread,but I'm not really sure how much better it is compared to hand taping?If by chance you are power tapping in your lathe,you basically follow the same procedure.Move the saddle of the lathe as close to the chuck as you can without wrecking into it,have your tap mounted in the tail stock locked down (quill unlocked) and as the threads start the quill moves forward until the threading is complete,then reverse out(or back out manually with the machine turned off).

Anyway,I have covered allot in here,and there are more calculations for machining procedures that I have not mentioned. One of which is calculating feed rates.Since most of us in here don't have a CNC mill or lathe and not all conventional mills (except for lathes) have power feeds.Power feeds are expensive and you will often "lose" travel in the X axis of your mill(a few inches) and most all of us will be turning handles to mill parts done by "feel".Other things to consider are "depths of cut" which I may cover another time,but often it is done by "feel" and how the machines is reacting to the cut as well as "what material" you are machining.For now what I have provided will be useful and serve our purposes in here and I hope it helps some of you out there.

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I'm sure this is all good info.

But what if you don't have power feed?

I've researched this off and on since I got a mill, and here's where I think all these numbers break down.

I'm not calibrated. All my axes are fed by me, a mere human. I don't have a speedometer on my axes. What the hell good is all this if it's not a power feed? Can you tell I've been frustrated by this for a while? Every time I look into this it's always about power feed.

And people don't even agree on the effing formulas. I could easily find a couple of other sets of calcs on the intertubes at sites that have enough of a pedigree that they could be considered reliable. What's up with that? This is physics! They should match.

There don't seem to be any guides for manual feed. Not sure it's even possible to produce such a thing because of the aforementioned speedometerlessness.

And I don't have any gizmo spraying water or oil all the time. I spread some oil in the path and add a little now and then. I have to back drills out occasionally depending on the depth because the flutes get clogged.

So I go slow. I use cobalt bits, always. I work on 6061, 2024, 1018, and O1. Checking chip color doesn't work because regardless of how slow I go I can get blue.

If it sounds unhappy then I back off.

I limit cuts to between 10 and 15 mils depending on the material. And when reaching a critical depth I'll come in to within 3 or 4 mils and make the final cut.

Tapping. Manual all the way using the chuck as a guide to keep it straight. I think LIO talks about it on his web site somewhere ( It took me a while to figure out what was being described, but then a light came on and I got it. On a recent armature I tapped 48 0-80 holes, some large number of 2-56, some 4-40s, and a handful of 6-32s. Without some really nifty pro equipment, I don't see ever having completed those without breaking every tap in SE Michigan. And no, I'm not just picking on SE trolls* - it's where I live.


(*trolls - it's a MI thing.)

Hey Dave,I did mention "power  feeds" and the fact that "most" all of us if we are running a mill won't have it.Power feeds are pricey.Really all a power feed does is allow you to optimize your surface finishes(plus give you a rest at turning handles).I don't think having  a "pretty" armature is really important considering it is about to be cast in silicone,urethane or foam latex unless you want to just "show of" the armature and how shiney looking your machined parts look.

Flood coolent isn't terribly necessary for our purposes,so swabbing/applying some cutting oil (as needed) as you turn the handles is fine(nothing wrong with that).Flood coolent is really a production thing.It is comparing apples and oranges.It is necessary in most production applications because these are large CNC machining centers and much of it has to do with a multitude of factors.The material machined,speeds,feeds,HSS,carbide,dimond or ceramic tooling along with "tool geometry".Non of which applies too much towards conventional machining aside from rpm calculations or machining armatures.

The SFPM figures I put up there are "rounded up".SFPM again(if you are going to be a real stickler on it)applies to advanced CNC machining centers and production work.Just like I mentioned and what you are doing,look,listen and "get a feel" for the cut.If the cut is too deep,your mill/lathe will let you know.It will shake and you will feel some resistance,the endmill won't want to cut.

Typically,your "max" depth of cut should not exceed  1 1/2 - 2 times the diameter of you endmill,but again this greatly depends on what material you are milling.Machining aluminum with a solid set up and "larger mill"  will machine fine than a say... "a Sheerline type model".This might not hold true,but theoretically should.

Know your machine and its limits (in which case I think you do).You can always reduce your rpm's.The surface finish may not be perfect,but the result is the same.Blue chips mean that allot of heat is being generated and because aluminum is cut at higher speed and feed rates,coolent is often used(but not always,you can mill it dry,even with HSS or cobolt).

Carbide is a different animal.In many applications flood coolent is used,but in some instances you cut "dry".There is great debate on this as well and you will get many different opinions between text and seasoned machinists.Flood coolent(as well as cuting oil) is used to cool the tool and part,provide lubrication between the cutting edges and flush chips out so they don't become compacted.

Advanced tool geometries are designed specifically for removing large amounts of material (FAST) as well as machining some specific materials and providing excellent surface finishes.I'd say,if you are getting "dark blue" colored chips to not only crank your rpm's down but to reduce your depth of cut and see if that helps.A little blue is "Okay" as long as bad things aren't happening.If your rpm calculation is 800 rpms,bring it down to 650 or 700.Maybe you have already tryied that?Anyway,I hope this helps and by all means ask questions.I will try to give the best information and advice I can,and when I can't...I turn to a book like the Machinery's Handbook or from a experienced machinist like my instructor.

Lots of good info here, and somthing Ive been dreading to mess with since I havent used my mill yet. My mill has no way to show me RPMS soo Iam assuming theres some after market product that I can get that will tell me no?

I used a tachometer to calibrate my mill. Well, *one* of my mills. My SEIG SX2 has a variable speed control, so I borrowed an optical tach to sample some speeds and mark them on the dial. The tach I used needed to see light vs dark, so I put some gaff tape for the dark background and a strip of white tape on the spindle.

My other mill is a belt-drive ZX30 that has specific speeds it can reach based on the belt/pulley arrangement. I haven't had a chance to do anything serious with it. My first task is to add DROs. Then I can use it for armatures.

Interesting looks like Ill be getting a tachometer, because I have no way of telling the rpms on mine. Thanks for the info.

Zaid,is your mill "belt drive", have to stop the mill and manually change belts to a different step pulley for the nearest rpm?There should be something on your mill (at the head somewhere) showing the various positions for low to high gear and what the rpm's are about for rpm ranges.I'm not sure what kind of mill you have?If variable speed,you just "dial it up or down" depending what gear you are in.You might also want to think about contacting the manufacturer if they have a web site,they may be able to provide you the information you need,and if that fails,possibly contact someone else who may have the same model mill and get the information.There are various mill/hobby mill groups out there.I'm sure they would be happy to assist you.A tachomenter is also another choice,but I don't know how expensive they may/may not be?I know my shops uses one because the most of the mills are so beat up that you can't get an accurate rpm even with the variable speed.That's what happens when you get someone who is inexperienced and they literally *break* the mill.

It has a knob that lets me dial up or down the speed. Its this guy right here.

My Mill

Yeah not the greatest mill in the world but my neighbor sold it to me dirt cheap cause he never used it. I just need to do some more research, because it doesn't, or at least I haven't found it, a way to find out the rpms. Yeah Iam in the inexperienced department lol. Thats my next step is buying the tools I need now, but alas dont have any money atm lol. Thanks for the helpful reply.

I see why you don't know the speeds exactly, and why you'd want to calibrate it.

This is a SIEG SX1. It doesn't have the kind of "musical belt" arrangement that John was talking about that my ZX30 has. The box on the top/back of the column is a speed controller. In the SX1 it's simple and has 2 preset speeds. I have an SX2, and the control box generates a continuously variable speed up to about 2500 RPM.

I think if you do happen to buy a tachometer that you can then dial in your speeds,when you do,maybe you can "mark it" so that you'll know where 400rpm's is,600rpm's,800 rpm's and so fourth.Aside from not being able(at the moment) to know where to set your speeds,the other big challenge with that small mill/drill(which is basically a drill press with "heavy duty spindle bearings") is the stability of the machine.It lacks the mass and heavy castings that larger machines have to reduce vibration and a "solid" foot on the floor.You can mainly do lite passes,which is fine for "armature purposes" I think.However,if you really want to hog off allot of metal in one pass,I think your machine will *hop,skip and jump* if your depths of cut are too much for the machine.

I think.However,if you really want to hog off allot of metal in one pass,I think your machine will *hop,skip and jump* if your depths of cut are too much for the machine.

That's why I cut only .015-.020 deep per pass.

I've attached shots of my machine and how I marked the speeds based on reading a tach.

Well,there ya go.Hopefully Zaid can do the same and move on to bigger and better things.

Yup Dave thats my mills brother lol, except with alot more options on it. Yeah I knew my mill going into it that it was gonna cut much material per pass, and Iam ok with that. As for stability I currently have it sitting on a big iron table that Iam planning on anchoring down. Those Digital read outs on your mill Dave? Unfortunately my work space and time are still a bit aways till I can actually use them . Plus I still need to but alot of tooling. Sigh my road to where I want to be is still far away. Either way thanks for all the advice so far guys .

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