Bowl lathe advice...

[ScottPerz]

Hello,

I just found the website and it's great to see a woodturnung forum with active posting. :cool2:

I've been turning for about 15 years now and have great old American Woodworking Machinery Patternmakers lathe. I mostly turn open formed bowls and platters, but I'm currently limited to 16". Up until now that's been fine but I'm itching to try some bigger projects.

I'm in the process of accumulating pieces for a bowl lathe that I would like to make. What I plan on doing is using the headstock off of a 9" South Bend metal lathe (complete with back gears for slower RPM). This will be mounted to a steel base filled with sand, and to that will be mounted my toolrest assembly (or I may opt to use a free standing one at first). I'm thinking of using a variable speed DC motor, but am unsure of the HP I will need. For now, I can't imagine turning anything much larger than 24" (maybe 30") if I get daring.

Is 1HP large enough? Should I be looking at 2HP instead? Also, I'd like to hear any opinions on my choice of headstock. I chose this option because it was already made and ready to go (and because I have it). Maybe it's not strong enough?

Thanks for any tips/advice you may be able to offer.
ScottPerz

 

[Bill Boehme]

Welcome to the AWW forum, Scott. You will find lots of very qualified turners here who will be able to help you with any questions that you may have. My limited experience tells me that 1 HP is not enough if you want to turn larger bowls. I would suggest a minimum of 2 HP. Wood lathes generally run at higher RPM that metal lathes and, because of the difference in drive ratio of the two types of lathes, more torque is needed at the motor to provide the necessary spindle torque for turning wood.

Bill

 

[ScottPerz]

Thanks Bill,

As far as the RPM of the metal lathe headstock versus that of a wood lathe, would you need the higher rpm for large turnings? What is the maximum RPM suggested for a 20" diameter platter? Would it be higher than 1200RPM?

 

[john lucas]

Scott
I would normally say that 1 horse would not be enough because I've turned a 29" bowl with 1 1/2 horse and it definitely needed more power. However I didn't have all the gears that you have on the metal lathe. I would say if you already have the motor to give it a try and upgrade later. If you don't have a motor then get at least 2 horse. If your going to use a DC motor remember that you loose power at the slower speeds. In this case I would either use the gearbox of the metal lathe and plan to change speeds often or I would opt for an even bigger motor.
A better way to go on a bowl lathe would be Variable Frequency Drive with a 3 phase motor. They run off of single phase 220 and still have plenty of torque at really low speeds.
On bowl 12" and larger you don't need over 1200 rpm unless you just want to. I think a lot of turners turn this size faster but it's not necessary and is certainly more dangerous.

 

[dkulze]

Scott,

Have you considered hooking up with your closest AAW turner's club as a source of advice and a chance to try out some larger lathes to get a feel of need for power, etc? Where in Central Mass would you be located? There's a club out of Worcester and ours is in Woburn near Boston.

Dietrich

 

[ScottPerz]

John:

Yeah, I was thinking that gearing would definately present more load to the motor. Currently I do not have a motor, so my options are open. I guess will start looking for a 2HP+ DC motor and controller next.

A friend of mine suggested a treadmill motor, but I can't believe that such a small motor can produce as much continuous HP as they claim on the label.

Do you need a special 3PH motor to work properly with a VFD? How do the VFDs and 3PH motor's compare to the DC and DC Controllers cost wise ? As if I need to tell you, I'm trying to do this the least costly method.

Dietrich:

I live in Dudley, about 20 minutes from the Worcester Craft Center. I went to couple of meetings a couple of years ago, and was meaning to sign up, but never got around to it. I will look and see when the next one is scheduled and try to make it. I did enjoy going.

Thanks everyone.

 

[Bill Boehme]

.....A friend of mine suggested a treadmill motor, but I can't believe that such a small motor can produce as much continuous HP as they claim on the label.

Do you need a special 3PH motor to work properly with a VFD? How do the VFDs and 3PH motor's compare to the DC and DC Controllers cost wise ? As if I need to tell you, I'm trying to do this the least costly method.

If you use a DC motor, it will develop its peak torque at zero speed (i.e. at stall) and the torque decreases linearly with increasing speed until the theoretical maximum speed at which point, the output torque is zero. The base operating speed of the DC motor is usually defined as the speed at which maximum horsepower is produced and at that speed, the output torque is roughly half of the zero speed torque. DC motors are normally not operated much above base operating speed.

You are probably right about the claimed horsepower of treadmill DC motors unless you are talking about the motors used on the big treadmills found in health clubs. The claimed HP on home treadmills is sort of like routers that claim a 3 HP motor, yet calculations show that the best they could possibly do is a little better than 1.25 HP.

If you want to use a variable frequency drive, you are correct in assuming that not just any three phase motor would be acceptable. Variable frequency drives generate three phase output using high-speed switching transistors with a pulsed output to drive the motor. This high frequency switching of power to the motor's windings causes high voltage spikes (several times greater than the applied voltage) on the power leads to the motor which can break down the insulation between adjacent wires in the motor's windings. A lot of newer motors have insulation that can withstand the voltage spikes and there will often be a label on the motor that says something like "Inverter Ready". The important thing to look for on the nameplate is the insulation class. Class H insulation is fully compatible with inverter operation. Class F insulation is also compatible with inverter operation within stated restrictions. Another thing to be mindful about is that when operating a TEFC motor at low speeds, there will not always be sufficient cooling from the fan, so those motors will usually have a stated minimum speed which is often 25% of base speed. Also, if the motor is not specifically intended for inverter duty, the maximum speed should not be much more than base speed. AC motors that are specifically designed for variable frequency operation over a wide speed range will often be blower cooled (TEBC -- i.e. a separate fan motor that runs at constant speed is used to provide cooling). I have a 1.5 HP Baldor TEBC three phase motor rated for variable frequency duty from zero speed up to 6000 RPM.

There are different types of variable frequency drives for AC motors. The cheapest is usually known as Volts/Hertz or something similar. Their output power and torque tend to poop out at low speed and they usually don't have the speed range of better controllers. The next step up is known as sensorless vector and they normally provide full torque below base speed and full horsepower above base speed. The fanciest controllers, known as vector controllers, use position feedback so that they can operate from zero speed up to the motor's upper limit with full performance.

Three phase AC motors usually cost a lot less than quality DC motors, but that is usually offset by the high cost of AC variabled frequency drives. The cost of variable frequency drives continues to drop as their usage continues to increase. You can often get reasonably good deals on the surplus market for both DC and AC motors and drives.

Bill

 

[pfduffy]

DC Motors

What a great block of info on the classes of motors and things to watch out for when replacing the "engine" of the lathes we all use. Thanks!

 

[ScottPerz]

WOW... thanks

You obviously know your motors!!!

Thanks a lot for the info and I will keep you all posted! When I get a chance I'll post pictures of my progress.

Oh, one more thing... My first option for a headstock was going to be some big 2 1/2 - 3" ID pillow blocks I have with a custom machined spindle. Based on the two options I'm looking at (above or the metal lathe headstock I have) the premade headstock is definately the easier of the two, but I'd like to know I will be wasting my time rigging that up.

Any opinions?

 

[Mark Mandell]

Scott,

I understand your idea to use the geared headstock, but once you go to VS drive on your motor and controller those gears would seem to be TOAB since the motor and speed will be controlled by the POT. The metal lathe needs them for precise control of speed for both the spindle and the feed which all run off of the same motor; not so in a wood lathe which is a very simple machine after all. Have seen some monster bowl lathes set up just with pillows & spindle bolted down to a steel beam and concrete (not sand) base. If you're going to a bigger motor [argh,argh ] make sure that your pully/belt "system" is adaquate to transmit the power of your motor set. Bigger lathes are using the poly-v or 2 or3-belt drives because a single belt (I'm told) won't carry much more than about 1.5 hp.

M

 

[Steve Worcester]

When you start looking at turning large, think slow. Most rough outs I do in larger sizes are probably less than 60 or so RPMs. The HP rating is important but the torque is what will really matter. Start looking at how larger lathes are designed to get some ideas for making your own. Look at VB, Oneway, Powermatic, Nichols, Stubby, etc.

Larger pieces you have to start thinking about surface speed. A one inch item at 1000 RPMs has much less surface feet per minute than a 20+" Item at 1000RPMs. It becomes a safety factor also. A small item off the lathe would be comparable to brushing away a gnat vs. the 20" item that is the spinning equivalent of a buffalo.

I had a 18" roughed out dry walnut bowl come off the lathe because i brushed the speed knob and sent it to high speeds. Hit the banjo, exploded and hit two opposite walls (20' away) and went through the dry wall in the ceiling. Had to make some mods to the speed controller after that

 

[mkart]

home made bowl lathe

I have been in the process of making a home brew bowl lathe for about 3 months now. I am using a 2hp 3 phase motor I bought used for $75 and a VFD I got free from my father in law. Its the same drive used on the Stubby. Currently I am using a wood frame bolted and weighted heavily with a spindle in a pair of pillow blocks. Just realized my spindle is not true so now I am looking for a machinist to help me true the spindle and fit new bearings. I do use it for roughing out large bowls and have not had any problems other than being about a 64th out of true. It can easily handle a 18" by 7" bowl blank with my supernova chuck. An early picture can be found in my gallery but it has changed since. I am just playng around since I had the drive already and was itching to use it. I would be interested in seeing others attempt at the same thing if there are any pics out there. I hope to turn wall hangings when I work out the kinks.

 

[rsteinle]

RPM's - Bowl Size - Rim Speed

When you start looking at turning large, think slow. Most rough outs I do in larger sizes are probably less than 60 or so RPMs...

Larger pieces you have to start thinking about surface speed. A one inch item at 1000 RPMs has much less surface feet per minute than a 20+" Item at 1000RPMs...

Here is a table listing RPM's, and Rim Speed for various size bowls...
Rule of Thumb - RPM Speed Range for Turning Bowls
Bowl Size - In.......RPM-Speed........Speed - Ft/Min........Rim Speed - Mi/Hr
Diam.....Circumf....Low.......Hi.........Low...........Hi..........Low.........Hi
1_______3.1.......3000......3000.........785.........785........8.9........8.9
2_______6.3.......3000......3000.......1,571......1,571......17.8.......17.8
3_______9.4.......2000......3000.......1,571......2,356......17.8.......26.8
4_______12.6.....1500......2250
5_______15.7.....1200......1800
6_______18.8.....1000......1500
7_______22.0.......857......1286
8_______25.1.......750......1125
9_______28.3.......667......1000
10______31.4.......600.......900
11______34.6.......545.......818
12______37.7.......500.......750
13______40.8.......462.......692
14______44.0.......429.......643
15______47.1.......400.......600
16______50.3.......375.......563
17______53.4.......353.......529
18______56.5.......333.......500
19______59.7.......316.......474
20______62.8.......300.......450
21______66.0.......286.......429
22______69.1.......273.......409
23______72.3.......261.......391
24______75.4.......250.......375
Above Figures are based on "Rule of Thumb" formula from Oneway
RPM's x Bowl Diameter ==> Should be Between 6000 and 9000
Results in Constant "Rim Speed" for any Size Bowl
Note - Speed Listed or 1" & 2" assume Max Speed of 3000 RPM's

This is my first post... I'm still learning...
PS - How do you post a Table...?
It converted "Tabs" to "Single Space"; Then I converted Single Space to "many spaces", it compressed that to a "Single space"; then tried - ....'s
There must be an easier way...?

 

[ScottPerz]

Chart question...

This information is definately helpful, but they're assuming that the blank is balanced... correct?

 

[Steve Worcester]

This information is definately helpful, but they're assuming that the blank is balanced... correct?

I would take it all with a grain of salt and a wheelbarrow to carry your, um, bravado.

When demonstrating, I am often asked how fast I am turning, usual response is lathe wobble minus about 50 RPMs. You can turn at any speed low enough to cut wood and give you a clean surface. The only reason you may need to speed it up is to have the air gaps move by faster.

 

[Bill Hale / The Wood Spin]

3 hp

Use a 3 hp My pref. is AC if you want to afford it.
I have 1 Vic with 2 hp DC and an Oneway 3 hp AC and just like the feel of the AC better.
If you are going to turn large as you say I feel the 3 hp is a must.

Good turning and be safe
Bill in Casa Grande, AZ.

 

[Kudzu]

Scott,

I understand your idea to use the geared headstock, but once you go to VS drive on your motor and controller those gears would seem to be TOAB since the motor and speed will be controlled by the POT.

I have been looking a lot at Variable speed options and from what I gather there is one advantage to a geared head.... I take that back there are two.

From what I read a DC motors torque is lower at low speeds. If you turning big items the gearing allows you to run the motor at higher speeds while still keeping spindle speeds down. Plus, gearing also has the advantage of torque multiplication.

 

[Jeff Jilg]

Use of html inside a post is not possible. There is a hack to turn it on for the whole forum, but it creates some security and usage concerns. So I will not be installing that ability.

The best way to make a table is to use Courier font and basically insert spaces between your rows so that the information displays as you would like. Example:


5 15.7 1200 1800
6 18.8 1000 1500
7 22.0 857 1286



But that is not a perfect technique

 

[Mark Mandell]

I have been looking a lot at Variable speed options and from what I gather there is one advantage to a geared head.... I take that back there are two.

From what I read a DC motors torque is lower at low speeds. If you turning big items the gearing allows you to run the motor at higher speeds while still keeping spindle speeds down. Plus, gearing also has the advantage of torque multiplication.

Ok, be that way. Get technical on me.

We have a few EE's floating on the Board, but I get the impression that DC motors have been replaced by 3-phase AC and matched drives. That's not to say anything bad about gearing or more than one size pulley. My Stubby has 2 speed ranges for just that reason; torque. In truth, however, I've never moved the poly-v belt to the high range.

Mark

 

[Bill Boehme]

From what I read a DC motors torque is lower at low speeds. If you turning big items the gearing allows you to run the motor at higher speeds while still keeping spindle speeds down. Plus, gearing also has the advantage of torque multiplication.

See my earlier posting about motors. DC motors reach their peak torque value at zero speed. From that point, the available torque decreases linearly (for PM type motors, or square-law for wound-field motors) with increasing speed. That is one of the primary reasons that until very recently, control system electric motors were exclusively DC motors. They have been largely replaced by a hybrid design known as brushless DC motors. The field winding is excited by a synthesized three-phase speed command and the rotor uses rare-earth magnets instead of the squirrel-cage winding found on AC induction motors. They have the low rotor inertia advantage of DC motors and brushless advantage of AC motors.

Bill

 

[Elizabeth Woodturner]

Turning Large Pieces

I would take it all with a grain of salt and a wheelbarrow to carry your, um, bravado.

When demonstrating, I am often asked how fast I am turning, usual response is lathe wobble minus about 50 RPMs. You can turn at any speed low enough to cut wood and give you a clean surface. The only reason you may need to speed it up is to have the air gaps move by faster.

I love the bit about the piece being "BALANCED" That probably aint gonna happen. The key to all this is to SLOW IT DOWN however if you are on a suicide mission faster is better. If you guys can manage a fixed steady of some substance it might save you one day. I am posting a photo of a table base with table top that we turned. The table top was really something and I wouldn't suggest turning it but ask a turner to make a round table and see what he come up with!

 

[TurnedAround]

Is there a link to that photo?