# 3520A RPM question



## MartinPens (Nov 24, 2010)

Just came into a Powermatic 3520A. This model does not have the digital readout of RPM's. I can make marks and generally guess, but was wondering what other, more accurate, solutions are out there. I've done some searching but haven't come up with anything yet.

Martin


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## maxwell_smart007 (Nov 24, 2010)

An internet search implied that there's a drive speed readout on the back of the headstock (i.e. not viewable from the front), and a chart in the manual that you can use to convert that to RPM...

I don't have time for a detailed search, so I hope that helps you on your way! 

Andrew


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## IPD_Mrs (Nov 24, 2010)

I can't answer your question, but I sure detect a big grin on your face.  Congrats.


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## JerrySambrook (Nov 24, 2010)

What is the necessity of knowing what the rpms are?

As long as you are comfortable working then the actual speed would not matter.

A test was done at the Worcester Center for Crafts a couple of years back, and the front readout was "made inoperable", and then people turned with out knowing what speed they were using.
Then we asked the people how much faster or slower they were turning on the machines at home, and 9 out of 9 said they turned around 1200-1500 rpms back home and that is what they were turning here.

We then let them know that they were actually turning around 1800-2000 rpms, and asked if they were still uncomfortable with the speed.
Only one said yes, and got made, because "the formula for turning speed" is the only way to turn.

They only reason to actually know the speed is so that you can repeat it for a repetitive motion job like threading.

PM me if you want to chat about it, and I will give you my phone

Jerry


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## jusjoe (Nov 24, 2010)

At work we use portable gauges similar to the one in this link

http://www.reliabilitydirectstore.com/Non-Contact-Tachometers-s/88.htm


Hope this is of assistance.


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## JerrySambrook (Nov 24, 2010)

Once again, what is the fascination of knowing what the speed is?


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## IPD_Mrs (Nov 24, 2010)

I don't think we will hear from Martin for a few days. He is having too much fun playing. arty:


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## steeler fan1 (Nov 24, 2010)

MLKWoodWorking said:


> I don't think we will hear from Martin for a few days. He is having too much fun playing. arty:


 

Even without knowing the RPMs.:devil:

Carl


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## Chthulhu (Nov 24, 2010)

JerrySambrook said:


> Once again, what is the fascination of knowing what the speed is?



At higher RPMs, some materials can fail catastrophically. It's a good idea to know what your materials will handle and what speed you're turning so you can stay within bounds.

More important than just RPM is surface speed: for a given RPM, surface speed will increase with the diameter, affecting both surface finish (for metal) and tool wear, as well as stress on the material being turned.

Simply because your test subjects couldn't accurately determine the speeds they were using without a readout doesn't diminish the importance of knowing.


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## Robert Taylor (Nov 24, 2010)

+1 on  digital tach


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## bitshird (Nov 24, 2010)

*Long response*

I have 4 lathes in my shop and soon to get another, Only one has an RPM indicator, it and one other have reeves drives so the speed is reasonably consistent, to the lock in positions on the face below the lever. The Shop Fox which is almost a good lathe is the one with a Tach. It doesn't agree with the indicated speeds buy the speed lever, I find that I can feel when the wood is cutting at a safe optimum, given that this is on a 16x42 inch lathe I find that 600 RPM can be too fast, and this is a heavy lathe, it's all Cast Iron including the base. I don't know of any wood species that has a surface foot per minute spec. in machine tool technology we designated wood and plastic to have a SFPM (Surface Feet Per Minute) factor of 200, with cold rolled or 10-18 steel to have a coefficient of 60 to 70 and tool steels down to 25 to 30.  aluminum were in the 100 range Carbide and some other exotics got down into the low teens and required Diamond cutting bits and mills, so this should give you an idea of the spread of materials in regards to their machineability and turning is just another form of machining, More fun but a little easier and that turning a hundred or two or even three will cause catastrophic failure.
If the wood is balanced, the tools are sharp the operator in control, and not being brazenly ignorant of his surroundings,  Crank it up!!! But on the other hand there are times when 90 RPM may be frightfully fast

Common sense must prevail, just because some wood guru has said the a piece of this or that can be best turned at 1250 RPM.Balderdash I say!! 
OK what is the gross weight of the piece of timber, How wet is it, how are you securing the work piece to the machine, if a person is going to turn wood they need to be comfortable, not worrying about how fast the lathe is spinning, I feel it's far more important to concentrate on keeping tools sharp working as swiftly and safely as possible. 
On Metals there is a given formula for all qualities and grades of Steel brass Bronze and different alloys of all ferrous and non ferrous metals, stipulating how fast or at what SFPM the metal can be removed,. well as how many surface feet per minute can be safely and expediently removed and a good finish provided , if the tools are properly shaped and sharpened. This is also up for grabs when things like the cutter material the coating of the tool and what type or lack of cooling is used. 
but there is still a general rule based on these things, 
Man enjoy the lathe Tachometers are available and should be through Powermatic, but best advice from an old man the builds machines and tools is "Work with in the envelope that you feel comfortable", later when you and your machine get to know each other, see if the machine feels better up a hundred or so . if not then drop back to YOUR comfort zone.
 As an anecdotal note I saw a video done by a guy using a tool similar to my Woodchuck Bowl Pro, I'm not sure but it looked like a big Oneway Lathe, any way this guy is doing the inside of a bowl using a square insert and is really leaning on the tool, a pretty good size guy and he was exerting a good bit of pressure inwards on the tool, it was about a 12 or 14 inch bowl, and I'd guess he was turning around 1600 to 1800 RPM. I had never saw such a shower of shavings come out of a bowl like that.
 OK I had to try my tool, My Woodchuck tool is virtually identical except for the stamped logo.
I was on the back end of the lathe, the tail stock was off and it was a 15 inch piece of Osage Orange I was on a 3520 and honking along showering a yellow blizzard when all of a sudden I hit an inside void in the bowl about 4-1/2 to 5 inches in,and about 5-3/4 or more over the tool rest ,  I couldn't get to the head stock to shut it off and I was holding on for dear life, I weigh 260 and I swear I thought I was going over the headstock, fortunately my friend finally opened his eyes and saw the problem, and shut the lathe off.
You mat be wondering how this answers or even gets near your question , there is one thing that rpm TABLES FOR WOOD can not compensate for and that is plain Ignorance an stupidity, or the type and condition of the tool being used, the wood will tell you where it want's to be cut.
Respectfully
Ken Ferrell


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## MartinPens (Nov 24, 2010)

What a great contrast of information. From "why do you need to know" to some great technical info. All good stuff. I am out of town today and unable to turn, so my big grin will come back tomorrow. I will look into the "why" of why I need to know. I do occasionally wonder about speeds depending on diameter. Thanks to all who commented. Still trying to wrap my head around the last one.


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## MartinPens (Nov 25, 2010)

MLKWoodWorking said:
			
		

> I don't think we will hear from Martin for a few days. He is having too much fun playing. arty:



: )


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## Nate Davey (Nov 25, 2010)

I just read a post on another site where they added an electronic RPM measuring kit.


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## randyrls (Nov 25, 2010)

You can get a portable tachometer from HF
http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html

But there are dedicated tach options made for machine tools where the speed is much more critical for getting a good surface finish and extending tool life.  This can also do Surface-Feet-Per-Minute (SFM) calculations based on diameter and show the SFM directly on the display.
http://mkctools.com/tachoptions.htm

You need to attach a ring of paper to the spindle with alternating black and white bands to drive the tach.  

The easiest way may be to contact Powermatic and see if they have a retro-fit kit for the older lathes.

You look WAY TOO HAPPY!    are your feet touching the floor?????


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## JerrySambrook (Nov 25, 2010)

Chthulhu said:


> JerrySambrook said:
> 
> 
> > Once again, what is the fascination of knowing what the speed is?
> ...



We are talking a wood lathe, not metal working here. In metal working, you are correct, to a point, depending on the opreation.
As for materials that can catostrophically fail due to straight speed, the thinner we make them, the more likely hood of a catastrophy as well, so your theory says we cannot make things thin either. And knowing what the speed of failure is is not going to stop the failure.

SPEED helps much much more than it hurts.
It reduces the aire time that we encounter over voids and while roughing.
It allows many more cuts per inch which will typically give a better finish.
It actually decreases the amount of material removed PER CUT which has a better effect on the material being turned and the tools on a stress factor.
And still, the only reason for knowing a speed is to replicate it for a given purpose, not for just general turning.

My PERSONAL opinion is that people are getting too hyped up on what speed they NEED TO SEE and what can actually work for them.

As for the test subjects, they all stated they felt just as safe at the higher speed as they did when they typically turned.

And once again, as per my original it is SAFE SPEED.  If the machine is waltzing its way across the room, it is unsafe. If it is swaying where it stands, then the finish will not be good.

I think too many people stress they have to have a digital readout without really knowing why, and that the technology phase of turning is really starting to take over in this area.

and as far as knowing what speed the lathe is spinning, he can still get an idea from the dial position without the expense of a digital readout


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## MartinPens (Nov 25, 2010)

randyrls said:
			
		

> You can get a portable tachometer from HF
> http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html
> 
> But there are dedicated tach options made for machine tools where the speed is much more critical for getting a good surface finish and extending tool life.  This can also do Surface-Feet-Per-Minute (SFM) calculations based on diameter and show the SFM directly on the display.
> ...



No, my feet are not touching the floor yet. Especially because I was expecting delivery of a Jet 1642 when this deal came along for virtually the same price! So I canceled delivery on the Jet and was the early bird who got the worm on the PM 3520A. It has less than 100 hours on it. I will check out the tach route. I can't say that I depend a lot on RPM readout, but I did want to pose the question and it is one of those good topics. I am relatively new to turning - 4 years or so, so I am still in the process of getting a "feel for it.". This machine has a whole lot of power! I have been on a VS mini lathe!  So some of the rationale for knowing RPM is due to the new range of speed and power. I'm getting into larger bowls, would like to get a bowl saver system and I'm scheduled for a day long class on hollow turning in December. There is so much I do not know. It seems like there is a wisdom in knowing as well as feeling. I like a combination of both.

Martin


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