Digital RPM readout......do you use it?

[odie]

My digital readout quit working on my Jet 1642 but I don't really miss it. I do a lot of multi-axis turning so I use an RPM just 'short' of the vibration RPM. Having a variable speed control is a big plus!

I "get it", Brian......your meaning about "just short of the vibration RPM". This is a great bit of knowledge for fine tuning your turning skills.

I find it necessary to return to a specific predesignated RPM, during multiple starts and stops of the lathe. For this, some might find a DRO helpful, but they may have to rely on memory, or need to write the rpm down for reference. This wouldn't work well for me, since my memory of such things can't be relied on......so, for my own purposes, I have a small "pointer" with a magnetic base that I place around the circumference of the rpm rheostat knob......this makes it easy to return to the specific rpm I'm looking to duplicate without the need to remember it.......

Yes.....variable speed is an absolute necessity. I went for a long time changing belt on the pulleys to change spindle speeds......man, I'm sure glad I don't do that anymore!!!!!

-----odie-----

 

[musky]

Never have had a read out. On my latest lathe, Robust, that I have had for about ten years, I just sort of Know where I want to be on the dial, or just start a little slow and turn it up till I think it is about right.

 

[Mike Adams]

So, what am I missing here?

Your lathe has pulleys, but other lathes do not? My Nova 2024 (ca. 2011), for example, is digital. I look at the readout after I've cranked something up enough to vibrate. Then I bump down a bit to run smooth. Started on a PSI lathe with pens and small dishes and never ever shifted the belt from the factory position. I prefer setting speeds digitally.

 

[Roger Chandler]

Odie, the part of your question on rpm readout..."do you use it?" Well, I think it depends on how one learns and on what equipment he/she learns on. I am basically self taught, and bought my first lathe [craftsman 15" vs] to turn table legs. It had a basic rpm indicator around the speed knob [potentiometer] that was labeled with the rpm at that particular point on the dial. My next lathe was an 18/47 model, with a readout of rpm's, and then my next lathe, a Delta midi had a label affixed to the underside of the pulley door that tells the rpm range on each belt setting, and the speed knob had markings from 1-10 for each desired speed in the range the belt was on.

My last two lathes both have rpm readouts on them, and yes I do use them quite a bit, especially for repeating that sweet spot on rpm's for a particular piece, and for sanding speeds to get the best result, which varies some from each species of wood I'm turning. So yes, I appreciate the rpm readout, just like I appreciate air conditioning on my vehicles........they would work fine without it, but the ride is more enjoyable with it!

 

[Brad Winesett]

I added a DRO to my 3520A and for me it is indispensable. I refer to mine all the time. It is especially important when I use my texturing tools as I have found optimal RPMs that work best for the different cutters. It is also helpful when I help my sons turn so that I can give them a reference point. Could I learn to live without it, probably. But, I would be disappointed buying a new lathe that didn't come with one.

 

[Tom Gall]

I find it necessary to return to a specific predesignated RPM, during multiple starts and stops of the lathe. For this, some might find a DRO helpful, but they may have to rely on memory, or need to write the rpm down for reference. This wouldn't work well for me, since my memory of such things can't be relied on......so, for my own purposes, I have a small "pointer" with a magnetic base that I place around the circumference of the rpm rheostat knob......this makes it easy to return to the specific rpm I'm looking to duplicate without the need to remember it.......

To Odie and others.....what am I missing here? I assume(d) that all or most lathes worked the same way. My old General 260 has a small dial to set the variable speed....it has a small white dot to indicate motor speed from slow to fast (I think of it as 12 o'clock (off - which I rarely use) to 11 o'clock as fastest. RPM's depend on the pulley selection. The other dial is a three step.... Forward-Off-Reverse. The lathe always returns to the pre-set speed of the potentiometer. All other lathes I've turned on work similarly even if the buttons/switches are different looking. Are you (and others) using the speed dial to turn the lathe on and off?[

 

[odie]

To Odie and others.....what am I missing here? I assume(d) that all or most lathes worked the same way. My old General 260 has a small dial to set the variable speed....it has a small white dot to indicate motor speed from slow to fast (I think of it as 12 o'clock (off - which I rarely use) to 11 o'clock as fastest. RPM's depend on the pulley selection. The other dial is a three step.... Forward-Off-Reverse. The lathe always returns to the pre-set speed of the potentiometer. All other lathes I've turned on work similarly even if the buttons/switches are different looking. Are you (and others) using the speed dial to turn the lathe on and off?[

Howdy Tom.......You could keep the potentiometer at the preset speed. I've never done that, because I've always felt it's not a good idea to resume by powering up with a surge of power suddenly. (That is up for discussion for those who'd like to comment on this practice.)

My Minarik controller has four controls.
> power
> brake/run
> potentiometer
> forward/reverse

What I normally do is use the brake to stop the spindle (recommended in the manual). Then I return the potentiometer back to zero rpm. When I power up, I return the brake to the run position, and then bring the potentiometer up to speed gradually.

I'm open for comments.......

-----odie-----

 

[Doug Freeman]

With a new piece I start slow and ramp up, but once I have set the speed and starting cutting, and I need to shut the lathe off, I start the lathe without changing a thing, let it ramp to the speed setting. Thats for short time periods. If I shut off power to the lathe, say for the night, the Galaxi looses the setting anyway, so I start slow just in case something changed. Did the same with my old lathe that didnt loose the setting.

 

[odie]

@Bill Boehme .....The above exchange with Tom got me to thinking.......

Here's a question for you, Bill......since you are an expert with electrical applications.

A standard constant speed motor usually has a capacitor start, and this is a boost of energy for start-up purposes......correct?

OK, so what about our DC variable speed motors......they do not have capacitor start......correct? Is there a disadvantage to start-ups with full power......or, could that be considered an advantage for lathe applications?

-----odie-----

 

[Tom Gall]

Howdy Tom.......You could keep the potentiometer at the preset speed. I've never done that, because I've always felt it's not a good idea to resume by powering up with a surge of power suddenly. (That is up for discussion for those who'd like to comment on this practice.)

My Minarik controller has four controls.
> power
> brake/run
> potentiometer
> forward/reverse

What I normally do is use the brake to stop the spindle (recommended in the manual). Then I return the potentiometer back to zero rpm. When I power up, I return the brake to the run position, and then bring the potentiometer up to speed gradually.

I'm open for comments.......

-----odie-----

Hi Odie.....I've been doing it this way since I bought my lathe in 1996 - no problems! My General only has a 1hp motor so I don't know if that changes the equation, but I would doubt it. My lathe also brakes automatically when I turn it off....no separate control for braking. It also has a main power on/off buttons that I only use at the end of a turning session.

I have a Minarik controller on my Vicmarc 100 (also 1996) mini-lathe with a 1/2hp DC motor which I operate the same way.....no problems. There is a slight delay when I turn it on but I just assumed that all DC motors operate that way. I'm not an electrical engineer.... or any other kind of engineer for that matter.

 

[odie]

Hi Odie.....I've been doing it this way since I bought my lathe in 1996 - no problems! My General only has a 1hp motor so I don't know if that changes the equation, but I would doubt it. My lathe also brakes automatically when I turn it off....no separate control for braking. It also has a main power on/off buttons that I only use at the end of a turning session.

I have a Minarik controller on my Vicmarc 100 (also 1996) mini-lathe with a 1/2hp DC motor which I operate the same way.....no problems. There is a slight delay when I turn it on but I just assumed that all DC motors operate that way. I'm not an electrical engineer.... or any other kind of engineer for that matter.

Yeah, Tom......I'm not very knowledgeable about electrical things either. I'm interested in Bill's opinion here......he always has input on electrical things, and is handy in the forums. I guess, as long as you're not experiencing any problems in keeping the potentiometer set to the designated rpm, it should be ok......we all just get used to doing things in a particular way that works for us.......nothing wrong with that!

-----odie-----

 

[Bill Boehme]

@Bill Boehme .....The above exchange with Tom got me to thinking.......

Here's a question for you, Bill......since you are an expert with electrical applications.

A standard constant speed motor usually has a capacitor start, and this is a boost of energy for start-up purposes......correct?

OK, so what about our DC variable speed motors......they do not have capacitor start......correct? Is there a disadvantage to start-ups with full power......or, could that be considered an advantage for lathe applications?

-----odie-----

A single phase AC induction motor has two sets of windings. The main winding is canned the "run" winding. The other winding is called the "start" winding. The capacitor is connected in series with the start winding. The start winding and capacitor are only connected for about a half second during motor start up and then a centrifugal switch located inside the motor breaks the electrical connection to the start winding. The start winding has done its duty and doesn't do anything until the next time that the motor is started.

So, why do we need a start winding in a single phase AC induction motor anyway? The short answer is that the start winding and capacitor tells the motor which way to turn as well as helping the motor accelerate to full speed. If the motor had a faulty start winding, open capacitor, or open circuit centrifugal switch then it would just sit there and hum when power is applied ... not knowing which way to turn. The capacitor creates a "phase shift" in the start winding. A phase shift in this instance is basically a time shift of about 3 or 4 milliseconds of the voltage in the start winding. This small difference in timing between the voltage in the run winding and the voltage in the start winding is enough to tell the motor which way to turn. When the motor speed reaches about 75% of full speed the centrifugal switch opens which disconnects the start winding and start capacitor from the circuit.

A DC motor is a completely different creature. They have a commutator ring and carbon brushes to supply power to the armature windings. The result is a rotating magnetic field in the armature that is constantly chasing after the magnetic field in the stationary past of the motor. Soft starts on DC and universal motors are beneficial in that it eliminates the typical high inrush current during start up that creates a lot of heat in the motor.

The speed of a DC motor is proportional to the applied voltage. A speed control for a DC motor is fairly simple since it basically is just a variable voltage power supply. On the other hand, an electronic speed controller for a three phase AC induction motor (usually called a VFD or inverter) is quite complex since the motor speed is proportional to the frequency of the AC power to the motor.

 

[stu senator]

In a standard commutator DC motor the start up current is determined by the resistance of the winding. This develops maximum torque at that voltage. This torque has to overcome the initial friction as well as rotary inertia of the rotating mass. As the motor accelerates the current drops as the speed increases due to the back emf developed. (another story). The higher the friction and the inertia of the load the slower the acceleration and the longer the high current (heat) has to develop. These motors are normally made with steel lamination cores and have a high thermal mass that prevents overheating. However many start stops in a short time could be a problem.

If leaving the machine for a break it would be safer to turn the speed down so if something has changed it will be obvious at the start and slow acceleration. This would be true for any lathe.

There are many power supplies for industrial DC motors that have circuits that can keep a motor at near constant speed with varying load but I don't think that is the type usually supplied.

Stu

 

[Mark Jundanian]

Another vote for DRO of RPM.

 

[Don Wattenhofer]

I recall teaching stick shift ,, the wantabe driver did not have any feel for the clutch....What finally clicked for her was to watch the rpm s to ease the clutch out, within 1/2 hour she was vastly improved, and within a week hell on wheels

Who was watching the road?

 

[Don Wattenhofer]

I have one but it is only a reference to tell other people how fast I am going after I have determined the speed by feel.
One reason they are put on new lathes might be that the cost of the tach is the least of any of the accessories they can put on the machine.

 

[Charlie Knighton]

Don.....it was at stop go on flat or hill....green light once she got the feel of clutch he'll on wheels

 

[Donovan Bailey]

I don't use it for rough work (bowls, spindles, etc.), however, I do use mine for precision work particularly that I have done before (chasing threads, coring/important, some hollowing tasks, etc.).

 

[Joe Sheble]

Why do phones play music, video, and games? Why do toaster ovens come with a digital temp display when a regular knob works just as well? Why do televisions have apps? Why do some refridgerators have internet connectivity? Why ask why?

Just because a lathe has a digital RPM display doesn't mean you have to use it, it's there for those that want it and others can just ignore it. It's pretty much the same thing with any new feature on any product.

 

[odie]

However many start stops in a short time could be a problem.

Overheating problem, Stu? Over a couple hours, I might start/stop 25 times for progress inspection......this a problem?

-----odie-----

 

[Lamar Wright]

I might start/stop 25 times for progress inspection

Same here Odie, I start and stop many times when turning to inspect my work and never had a overheating problem.

 

[stu senator]

<Overheating problem, Stu? Over a couple hours, I might start/stop 25 times for progress inspection......this a problem?>

Odie, not a problem just the engineer in me stating a fact to be considered in an industrial setting where a machine is running an on/off cycle. I can't see a turner having this problem with a standard DC motor.

Stu

 

[William Rogers]

The original question was about Digital Read Out. I had that on my Laguna, but not on my Robust. I don’t need digital, but like some indication of the speed I’m turning in certain situations. The Robust basically has analog readout. The more I use the Robust I am getting familiar of the speed of the dial position. I had an 11” bowl let go one time. This is when I was new to turning and didn’t know the 9000/6000 rule. The bowl had some hidden defect and I was turning beyond the recommenced speed, although that speed seemed ok. Anything over 10” I double check the speed I”m turning. I also like to know the speed when texturing. So I don’t necessarily need digital, but want some indication of my speed.

 

[odie]

This is when I was new to turning and didn’t know the 9000/6000 rule

What is the 9000/6000 rule?

-----odie-----

 

[Doug Freeman]

Dia of the wood in inches x rpm, 6000 for roughing, 9000 for finish cuts. Starting point “rule of thumb”. Based on surface speed the edge is actually cutting. Adjust from there based on numerous other variables. Dont know that a 1” dia piece needs 9000 rpm, but it gets more accurate as dia increases.

 

[odie]

Dia of the wood in inches x rpm, 6000 for roughing, 9000 for finish cuts. Starting point “rule of thumb”. Based on surface speed the edge is actually cutting. Adjust from there based on numerous other variables. Dont know that a 1” dia piece needs 9000 rpm, but it gets more accurate as dia increases.

Interesting......Now that you mention it, I believe I've run across that before, but just ignored it because the variables are too numerous to make up a rule like this.

For instance, the vibration tendencies of various rpms according to one specific wood blank spinning on one specific lathe. Things to consider are if the wall thickness were 1 1/2", 1/2", or 1/8".....or height of the bowl were 1", 2", 4", or 6".....or the shape had inward slanting walls, or outward slanting walls.....or differences in grain patterns, obvious defects, or species......etc,etc,etc......

In other words, a rank newbie might need some reference point to come up with an appropriate spindle rpm, but a seasoned turner probably develops a kind of "instinct" for a more correct rpm.

Then there is the difference between the maximum safe speed, and the best speed to accomplish a specific task.....

-----odie-----

 

[Dave Hulett]

I use the speed readout on my DVR-XP constartly and furthermore I use the customizable preset speeds everytime I turn it on .

 

[William J Smith]

I use the digital read-out primarily for those unique speeds that aren't normally used -- certain speed ranges for various texturing tools, thread turning jig, drilling, sanding, etc. I turn a lot of different products, which often require somewhat different speeds. Sometimes I find myself wondering why I'm not turning the way I expect until I look at the digital readout and realize that I'm turning at about half the speed I'm used to. It isn't always enough to say "turn as fast as you feel comfortable" when different turnings dictate different speeds.

 

[William Rogers]

Interesting......Now that you mention it, I believe I've run across that before, but just ignored it because the variables are too numerous to make up a rule like this.

For instance, the vibration tendencies of various rpms according to one specific wood blank spinning on one specific lathe. Things to consider are if the wall thickness were 1 1/2", 1/2", or 1/8".....or height of the bowl were 1", 2", 4", or 6".....or the shape had inward slanting walls, or outward slanting walls.....or differences in grain patterns, obvious defects, or species......etc,etc,etc......

In other words, a rank newbie might need some reference point to come up with an appropriate spindle rpm, but a seasoned turner probably develops a kind of "instinct" for a more correct rpm.

Then there is the difference between the maximum safe speed, and the best speed to accomplish a specific task.....

-----odie-----

Odie, don’t totally agree. I'm not necessarily a newby. However I turn many different types of turnings. Minuatures, hollow forms, basket weave illusion, small lidded boxes etc... If I mainly turned one type then I could adapt to instinct. As far as texturing, the speed is important and is not the turning speed you use for material removal. If I predominately just did one type of turning, sure I would develop an instinct. Many times I will go quite a few months between turning a types.

 

[John Torchick]

I don't have a digital readout. I just count the RPMs real fast. My lathe is set at the highest belt position. I do have VS and use the slower speeds for some finishing steps.

 

[odie]

Odie, don’t totally agree. I'm not necessarily a newby. However I turn many different types of turnings. Minuatures, hollow forms, basket weave illusion, small lidded boxes etc... If I mainly turned one type then I could adapt to instinct. As far as texturing, the speed is important and is not the turning speed you use for material removal. If I predominately just did one type of turning, sure I would develop an instinct. Many times I will go quite a few months between turning a types.

Yes, I can see that......thanks for your input, William........

What works for me, might not necessarily work for everybody......or anybody but me, for that matter! .....

-----odie-----

 

[Tom Gall]

Odie, don’t totally agree. I'm not necessarily a newby. However I turn many different types of turnings. Minuatures, hollow forms, basket weave illusion, small lidded boxes etc... If I mainly turned one type then I could adapt to instinct. As far as texturing, the speed is important and is not the turning speed you use for material removal. If I predominately just did one type of turning, sure I would develop an instinct. Many times I will go quite a few months between turning a types.

I assume you are saying that when texturing you like to use a slower speed. True. But a digital readout tells you the rpm's....not the "surface" speed needed when using texturing tools. So, unless you are always texturing the same distance from the spindle center you will have an infinite number (well several anyway ) of rpm readouts to keep track of......i.e. - texturing a box lid vs. texturing a bowl rim. Just my 2¢.....you get what you pay for!

 

[Doug Freeman]

Interesting......Now that you mention it, I believe I've run across that before, but just ignored it because the variables are too numerous to make up a rule like this.

For instance, the vibration tendencies of various rpms according to one specific wood blank spinning on one specific lathe. Things to consider are if the wall thickness were 1 1/2", 1/2", or 1/8".....or height of the bowl were 1", 2", 4", or 6".....or the shape had inward slanting walls, or outward slanting walls.....or differences in grain patterns, obvious defects, or species......etc,etc,etc......

In other words, a rank newbie might need some reference point to come up with an appropriate spindle rpm, but a seasoned turner probably develops a kind of "instinct" for a more correct rpm.

Then there is the difference between the maximum safe speed, and the best speed to accomplish a specific task.....

-----odie-----

Like I said, its a rule of thumb starting point, and adjust from there for numerous other variables. You apparently turn similar woods at similar diameters and have memorized where you want rpm, and you “know” the rpm based on the dial settings. Great that it works for you.

Reading through the answers, if the question was “ do you use a speed indicator” vs specifically a digital rpm readout, there would be a lot of yes’ and possibly “0” no’s. Belt position and dial position are really no different than a digital readout - indication of rpm.

 

[William Rogers]

I assume you are saying that when texturing you like to use a slower speed. True. But a digital readout tells you the rpm's....not the "surface" speed needed when using texturing tools. So, unless you are always texturing the same distance from the spindle center you will have an infinite number (well several anyway ) of rpm readouts to keep track of......i.e. - texturing a box lid vs. texturing a bowl rim. Just my 2¢.....you get what you pay for!

I said I use a different speed when texturing than I do for material removal. Not the same speed for all sizes when texturing. Haven’t a clue to what you mean by “you get what you pay for!"

 

[Tom Gall]

I said I use a different speed when texturing than I do for material removal. Not the same speed for all sizes when texturing. Haven’t a clue to what you mean by “you get what you pay for!"

Your reply (#69) to Odie was a little unclear to me, that is why I started my reply with....."I assume you are saying.....". The topic was about a digital read out (which makes it easy to return to the same rpm's) - which I think would be useless in your different speed(s) for texturing. I totally agree. Sorry if I misunderstood what you were saying.

Re: "you get what you pay for!" ..... relates to "Just my 2¢" ..... aka my thoughts/opinions on the topic.

 

[William Rogers]

Your reply (#69) to Odie was a little unclear to me, that is why I started my reply with....."I assume you are saying.....". The topic was about a digital read out (which makes it easy to return to the same rpm's) - which I think would be useless in your different speed(s) for texturing. I totally agree. Sorry if I misunderstood what you were saying.

Re: "you get what you pay for!" ..... relates to "Just my 2¢" ..... aka my thoughts/opinions on the topic.

Ok

 

[Bill Boehme]

Like I said, its a rule of thumb starting point, and adjust from there for numerous other variables.

I don't think of the rule of thumb as a starting point, but rather a "do not exceed red line". My starting point is usually zero.

I don't have a digital readout. I just count the RPMs real fast. My lathe is set at the highest belt position.....

Unless you need the high speed, it would be more advantageous to use the slowest pulley range that allows the lathe to run at the speed that you want. While this isn't an issue for small diameter spindles, it is important when turning larger diameter pieces. The reason for using lower speed ranges is that the motor work output (horsepower) is proportional to motor speed for an electronic variable speed system, regardless of whether the lathe uses a DC motor or AC motor.

 

[Tim Tucker]

When I had a Powermatic 3520B...I recall looking at the DRO occasionally. But, when I went to the Robust AB....I never really look at the dial. I just "feel" the speed as either too slow to achieve a good cut...or too fast because of vibration. While it might be "fun" to have a DRO...I can't say that not having one has depreciated my ability to safely and effectively turn wood. I feel that I am a bigger impediment to the process than a digital read out could ever be.....
But, ain't it great that we all have choices and ways to find our own comfort zone? For me part of the creative process is the elimination of distractions so one can focus on the project. If you find that you "need" a DRO...you can add a tachometer to your lathe. There are parts available that are generic.

 

[John Torchick]

I don't think of the rule of thumb as a starting point, but rather a "do not exceed red line". My starting point is usually zero.



Unless you need the high speed, it would be more advantageous to use the slowest pulley range that allows the lathe to run at the speed that you want. While this isn't an issue for small diameter spindles, it is important when turning larger diameter pieces. The reason for using lower speed ranges is that the motor work output (horsepower) is proportional to motor speed for an electronic variable speed system, regardless of whether the lathe uses a DC motor or AC motor.

I'm aware of the dynamics but I don't turn anything bigger than a pen blank right now.

 

[Tom Albrecht]

I added a DRO to my Robust AB after several years without it. My Robust Scout came with it. Do I use it? Very rarely-- mostly only when duplicating something experimental and I'd like a reference point.
Otherwise it can be helpful when teaching.