Why do some claim that RPM is irrelevant on a manual machine?

Hello!

I have posted a few other questions in the General and Bridgeport sections and have heard a few times now where some feel that RPM of the spindle is unnecessary information, yet it seems like there are those who want that information displayed. Why the two camps?

Thank you!

Andy

Andy,

After years of operating both machines, watching chip color and chip formation and listening to the sounds and feels of turning and milling, the speeds and feeds become automatic without the need of a RPM display...at least that's what I think I think!!!

Maybe the display would be great as you learn to get the feel of the machine, then it would become more automatic. There are so many variables that RPM only is only part of the big picture. The material being machined, type of insert, coolant, finish required, etc. RPM would be the starting point but lots of other things need to be factored in.

Stuart

Guncraft said:

Hello!

I have posted a few other questions in the General and Bridgeport sections and have heard a few times now where some feel that RPM of the spindle is unnecessary information, yet it seems like there are those who want that information displayed. Why the two camps?

Thank you!

Andy

Click to expand...

Please reference to those comments that RPM doesn't matter. There may be some context conflict.

RPM (Surface speed) does matter for "most" applications regardless of machine type.

eta

atomarc has brought up a good point. A "display" of spindle speed is not needed for ANY machine, close enough usually is!

But with a CNC, you really can not program without it.

Perhaps if you were developing an SOP for certain parts to have operations performed on them by semi-skilled workmen, than a visual display of the RPM might be useful. If you were selling rebuilt machines, it might also be something cheap and sexy you could install to enhance the perceived value. For me, my ears and my eyes are pretty good at determining the optimal RPM on a manual knee mill.

Context conflict looks the best explanation. Remember that manual machine tool design concepts and execution were pretty much frozen before the relatively inexpensive electronics and VFD era. Industrial class manual machines have never really embraced modern electronics as the gain ahsn't really been seen as worth the pain. Might as well go CNC than make the design shifts needed.

i dotting t crossing mode on

Historically manual machine spindle drives come in two varieties.

1) Fixed settings whether by gears, step pulley or a combination so the speeds are what it says on the levers or in the book unless some fairly major changes are made to the input drive side shuch as different pulleys or motor. Even then the speeds are fixed so RPM indication is no great help. Just choose the best match from the available range between tooling and workpiece making due allowance for its size and material.

2) Mechanical or electrical varispeed devices having essentially infinite adjustability over a decent range, from 3 to 1 up to 5 to 1 is common. Classical mechanical include the expanding contracting pulley pair on a Reeves drives and variators. Classical electrical include Ward Leonard and similar motor generator units along with various DC drives with old line controllers. RPM display is essential as otherwise you have no idea what speed you actually have, mostly setting indicators rather than true reading speedometer types.

Modern VFD drives confuse the issue as they allow motor speed to be adjusted at will filling in the gaps, or even overlapping ranges, on fixed setting machines. If used with mechanical varispeed devices they bypass the calibration. VFD boes can completely replace classical electrical variable speed drives and motors. With a VFD rpm display is pretty much essential. True spindle speed being best but fixed setting machines can get by with the VFD frequency display and a bit of mental maths.

Tweaking the speed by observation of the chip when you have seemless control is fine. But you'd better be pretty close to start with or your tool pit may be damaged.

Clive

The point made in one thread you might be thinking of was not that RPM didn't matter -- only that you didn't need to see it to three or four digit accuracy on some display to dial in correct feeds and speeds.

Without variable speed (e.g. most manual machines) your RPM increments are likely to be in 50% to 100% steps anyhow. And the nominal rpm on the machine plate is likely to be off a few percent due to such things as where v-belts ride on pulleys. No one's choosing between 999 and 1001 rpm.

Most folks running manual machines will be happy to set feeds and speeds best they can to start. You hope to begin within a couple significant digits of recommended speed -- and then judge such things as chip form, heat, noise, chatter etc. in deciding whether to dial speed up or down. Variable speed is great for that (a small change, for example, might avoid chatter) -- but you don't have to actually see three or four digits on a display to set things up right.

In production -- where you might want to return to some optimum speed and feed -- that might be another matter. In that case you might also want to see such things as HP in the cut, to get the most economical production possible.

Specifically, the material and cutter material set best SFM, which will then relate to RPM. But, as mentioned, those are good if close, there are no hard limits where 1 rpm more and the cutter suddenly burns up.

All you need to remember is 4" diameter is about 1 foot around. 6" half again, 1" a quarter of that, etc. RPM then can be quickly estimated to see if you will burn up the cutter, which is more important with ground HSS than insert carbide.

After that, it's feed, and checking chips to see what's going on.

I've got twelve speeds on my manual lathe, and probably none of the speed labels are closer than two significant digits accurate to the true speed. Does that matter? No. Do I need to know the difference between 450RPM and 1000RPM? Yes, I certainly do. Do I need a digital read out? No.

I have three speeds on my manual Nichols mill, with no labels whatsoever. Does that matter? No. Do I need to know the difference between (roughly) 175, 350 and 700RPM? Yes, I certainly do. Do I need a digital read out? No.

I still stand by unneeded on a manual machine.

SFM is important.

Machines that only had large gaps between available speed selections, meant using closest available speed.

Many times I work on things that should be running at xxx rpm/xxx feed... But clamping/fixturing a delicate part, is tough sometimes.

Stick to book, watch part go flying, or get bent..

Or use common sense, learn your machine, break a few endmills, learn when and how much you can climb mill a finish pass..

Exact spindle rpm will not solve any problem on a manual mill..

Spend money on a DRO, There... is a use for a 5 digit display..

I agree with abarnsley, the speed chart is useless. It looks good though. On machines I fix up (for myself) I spend extra time getting the chart to look good but never look at it! First after years of running the machine I know which combination of levers on my lathe give about what speed on my variable speed toolmaster I turn the handle until it sound and looks right then on either machine adjust the speed/feed to how it's cutting. I also have Moore Jig Borers. They have an analog meter tach. along with a motorized variable speed change. I never look at the tach. Haven't burnt-up a cutter yet

I wouldn't spend much time on a rpm display given sfm is where the cutting happens. OTOH I like having the placard on the lathe gearbox so I can remember which position gives me a rpm close to the sfm where I want to be cutting. For kicks I put my rpm counter on the spindle sometimes just to see whats going on. I've been misled by chip color with shell endmills, burning the cutter even though the chips looked fine.

abarnsley said:

I still stand by unneeded on a manual machine.

SFM is important.

Click to expand...

Well, if you have no idea what the RPM is, how do you expect to estimate the SFM? A chart of speed vs pulley plus an idea of work or cutter diameter (lathe or mill) should be good enough to go from.

And you would really only need that if you are trying to optimize something more closely. Otehrwise, you know what works and just keep doing it that way.

I have no idea why someone would need a fancy tachometer unless they are trying to tune a particular manufacturing process for some specific factor. And then, like as not, the material is different in next batch and it's all out the window.

Personally, I don't find not knowing what my RPMs/Feeds are on manual lathes and mills to be too much of a bother. Its more about feel,sight and sound for me. They're definitely important to manual machining, just dialing them in precisely and knowing what the exact numbers are, are much more important in CNC/ large production runs where your trying to optimize tool performance/cutting time. The more time you spend on a particular machine, the more you know what dials and levers should be where for particular circumstances. For example, I run my tool room lathe "A -E" rpm with an "E-F" feed most of the time when turning 1018 around 1"OD , what those combinations of letters translate to actual numbers, ask and i'll tell you I don't know lol. All I know is it sounds and feels good, minimum vibrations and chatter, I can take some nice heavy cuts, and the chips look good to me. Change the EF to AB for the feed and the finish is great.

Corey

Coming from a world of the shittiest import lathes management thinks they can get away with, and turning mostly aluminum and plastics, RPM is pretty much defined by how fast do I think I can go without losing the part. The Bridgeport clones, however, have a relatively useful speed dial right on them for large drilling.

I never calculate speed and feed, takes too much time! Possibly for CNC operation but in a manual machine I never do it. Experianced machinist can look at the spindle and know if it's too fast for the cutter or turning to fast as in a lathe for the diameter to be cut. Plus I'm wondering how accurate the speed chart is on a import machine especially if it has a 50Hz motor.

Given a RPM the depth of cut and feed rate is more important. Unless you are roughing something out with a roughing cutter.
(I like to keep my roughing cutters in good condition too.)