taking one frame at a time since 1999

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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Most places/people are,the ones who aren't you'll know almost strait off the bat.As an introduction to "lathe" and "mill" terminology(in case you don't know),lathes work in an X and Z axis.The carriage travels in the -Z direction towards the chuck when turning, the cross slide goes in a -X direction towards the center of your work piece when turning.Mills have three axis of travel the X,the Y and the z. The X moves in both a + and - direction on the X and Y axis as does the z axis. Turning the handles to move the table in a longitudal direction is the X axis,the cross feed (the center handle/dial) is the Y axis and the quill controls the z axis.Down is -z and up is +z.Machine tools such as mills are laid out in a cartisan coordinates(four quadrants),it just depends how the machine is designed.Lathes are different than mills in design and performance,but operate off the same basics.I probably should put this bit of information in a different subject in this forum,but for now It'll stay here.When you mean by "a bit off" are you referring to excessive backlash in the dials/lead screws?I'm faced with a similar problem,so it might do you good to take apart the compound and clean everything out, real good,re-oil it,inspect the lead screws to see if there is wear that goes beyond just re-adjusting the gibs.You'll know it when you see it,then repeat the same thing for the cross slide.Just take pictures or really good notes as to what goes where,keep the parts organized so you'll know how to put everything back together.When/if you do disassemble the compound(when putting it back together),don';t over torch the bolts,you could strip them out or make it so things are funky and out of adjustment form being "too tight".I wouldn't crank on the nuts/bolts much more than 1 /8-1/4 a turn after they are snugged down good.Too bad machines like this didn't come with a Chilton's Maintenance and repair manual.

Yeah I got backlash on the top cross slide when I turn the handle. Yeah I definitely don't want to over tighten the nuts and bolts. That's what I'am hoping for that a nice cleaning will do the trick. Yeah your not the only person thats told me about Chilton's Maintenance and repair manual I really need to get that as well.

Even after a thorough cleaning,the gibs will need to be re-adjusted.Cleaning the gunk and chips out won't reduce the backlash.

Hmm interesting how do I go about doing that?

Good question because I am not even sure I even know how?I would look at/study the parts manual first,then go into the Practical machinist forum and "ask".look around first on "adjusting gibs or removing backlash in "lathes,compound rest,cross slide etc".If nothing is there/comes up then write a post and see "who bites".There may also be some tutorials online,but I have never looked myself.I'll be doing some of the same things you are soon,so I'll be new at doing this as well.

Yeah Ill see about looking that up then. Theres gotta be a youtube video, theres a youtube vid of everything nowadays lol.

I found one on a Logan Lathe,but I didn't watch the video all the way through.

Do you have a link? On a side note I found a guy by the name of Mrpete222 (Tubelcain) on youtube that has alot of basic vids on machining.  Heres a link:

No,I found it randomly searching on google trying to find something on stripping down the head stock to the spindle bearings and re-greasing the bearings.The videos will be good for you to watch if you are inexperienced using an engine lathe.I'll still post basic set up procedures in here at some point.

Awesome that would be fantastic, yeah looks like Ill have to see if I can stumble upon that video too.  Any info you got Id love to here, thanks again.

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