Warning: AAW Article on how to add reverse to a lathe

[Randy Smith]

The December 2011 AAW Magazine has an article on how to add reverse to a mini-lathe.

While it has a warning in the text of the article, the warning is not firm enough. Please understand that is you make this mod to an electronic variable speed lathe, YOU MUST NOT CHANGE THE SWITCH POSITION WHILE THE LATHE IS TURNING!!!!!

Doing so will blow up the electronic speed controller and the lathe will not work except after some expensive repairs.

Just trying to keep friends from doing something damaging to their lathes.

Merry Christmas!!!

 

[Tim L]

Thanks Randy. I read the article and thought about it in the back of my head. Prolly won't ever do it, but I will remember your warninng.
Thanks again, Tiim.

(edit: Not for the shopsmith in my avatar. I have a Jet 1014 I upgraded to VS and a HF 95607 VS I may eventually try that on.)

 

[Betty Scarpino]

Randy,
There are two other concerns that need to be addressed as well. John Giem (editorial advisor for the journal) is working with comments that Bruce Perry sent to me earlier this week to formulating a safety announcement. John's safety note will appear in the February issue, be posted to the AAW website, and shared on this Forum.

The concerns Bruce pointed out:

1) Make sure the grub screw that holds the hand wheel is tight. It can come loose when reversing the lathe. (Same for set screws on faceplates and chucks, but we are supposed to know that already.)

2) In Bruce's email to me: ".... apparently there is power stored in the rectifier board, so that shutting down with the off position on the reversing switch, then turning off the power at the primary control, can, if you then move the reversing switch to forward or reverse result in a couple of turns from a lathe that is clearly "off", with a hand in the wrong spot this could be damaging. I turn the lathe off with the main switch, then with the Forward/Reverse switch (or set it back to forward), to make sure I don't find this bit of reserved voltage at the wrong moment."

3) And to corral together the three safety concerns: Do not change the switch position while the lathe is running.


Anything else from our very knowledgeable members? I continually appreciate the wisdom shared by all! In this particular case, thank you Randy, Bruce, and John.


Betty Scarpino, Editor, AW

 

[Bill Boehme]

Betty,

I think that concern #2 may be unfounded. Although it appears to be based on a logical extension of how the power supply for a piece of electronic equipment operates, I don't believe that it is applicable in this case. Before a warning is announced, it would be prudent to evaluate the circuit diagram and also to actually perform real-world testing.

I do agree that concern #1 is valid. There is a potential for component failure within the controller depending on both the circuit design and robustness of certain components. My gut feeling feeling is that the controller may behave as a very expensive fuse to prevent the chuck from unscrewing itself. 😀

Installing a reversing switch for single-phase AC induction motor is far less problematic. I did this for my Delta 46-715 lathe. The thing about adding reversing capability to this type of motor is that nothing happens if the switch is flipped while the motor is running it will just continue running in the same direction. in order to reverse, the power to the motor has to first be turned off and the motor allowed to come to a full stop.

 

[Betty Scarpino]

Betty,

I think that concern #2 may be unfounded. Although it appears to be based on a logical extension of how the power supply for a piece of electronic equipment operates, I don't believe that it is applicable in this case. Before a warning is announced, it would be prudent to evaluate the circuit diagram and also to actually perform real-world testing.

Thank you, Bill. John Giem is currently researching the problem. I'm sure he'll read what you wrote and take note.

I took two electricity/electronics classes in college. The first time I tried to replace a broken on/off switch, the darn thing blew. Happened a second time. I now avoid anything to do with wiring .....


Betty Scarpino, editor, AW

 

[waltben]

Almost eight years of using my Stubby reversing it by rotating the speed dial to zero, stopping the piece, flipping the switch to reverse and then turning the speed dial up has never caused a problem. I do NOT turn the main power off - in fact this will turn the VFD off and not provide any braking (which even my old Eurotherm does), meaning it takes more time to stop the piece. I'm about 90% certain any damage to a VFD is caused by a piece rotating in the reverse direction of the one it wants to turn. Sort of like not stopping your car's motion before putting it in the opposite gear.

 

[Bill Boehme]

Almost eight years of using my Stubby reversing it by rotating the speed dial to zero, stopping the piece, flipping the switch to reverse and then turning the speed dial up has never caused a problem. I do NOT turn the main power off - in fact this will turn the VFD off and not provide any braking (which even my old Eurotherm does), meaning it takes more time to stop the piece. I'm about 90% certain any damage to a VFD is caused by a piece rotating in the reverse direction of the one it wants to turn. Sort of like not stopping your car's motion before putting it in the opposite gear.

The situation is different for variable frequency drives and three phase motors such as used on your Stubby and most other large lathes (except for the Nova DVR). If your lathe uses DC injection braking or resistor or chopper/resistor braking there is no problem with reversing the motor (other than locking down the chuck). The scenario is actually no different than normal acceleration/deceleration ramping as far as the electronics and motor are concerned. A braking resistor is the optimal solution since the stored mechanical energy in the motor gets dumped into a resistor and dissipated as heat. With DC injection braking, the energy is dissipated in the motor field winding which heats up the motor slightly, but as long as this is not done to excess, the heating should not be a problem. If you routinely turn huge pieces weighing well over a hundred pounds and stop frequently to inspect the work, then resistor braking should be considered to minimize motor heating. I do not know of any woodturning lathes that use resistor braking although most drives that operate in the sensorless vector mode have the option of adding the resistor module.

With DC motors and variable speed modules, the problem is that sudden reversing can cause a very large current command since ramping he speed is not a feature of these simple devices nor do they incorporate braking.

 

[Gynia]

... I do not know of any woodturning lathes that use resistor braking ...

The Oneway 2436 VFD offers a breaking resistor option.

 

[john lucas]

I'm on my 3rd VFD lathe and all 3 could be turned to reverse with the lathe running. It simply slows down and then starts turning in the other direction. In fact, on all 3 of them if you start the lathe with something blocking the wood (like the tool rest, not that I've ever done that) the lathe will simply shut itself off.

I put reverse switches on two Jet mini lathes but I never did reverse the lathe without hitting the stop switch so I can't say what would happen if you did.

 

[hockenbery]

Personally the few times I want my mini- lathe running backwards, I would take one step and be on the other side.
My Fisch doesn't have any stuff in the way on the reverse side.
Tool rest works on either side.
If I'm sanding and don't need a tool rest i go from full speed forward to full speed reverse with one walking step. No on off required.

To use a hollowing tool meant for reveres use, you reverse the hollowing tool first.

It can be fun to add features.

Al

 

[Bill Boehme]

The Oneway 2436 VFD offers a breaking resistor option.

Yes, you can get braking resistors for almost any sensorless vector VFD.

What I really meant was getting it as standard equipment. I was contemplating getting one to use on my Robust because the extremely high heat this past summer along with planning to do very large hollowforms. On the other hand, it is easy enough to just hit the main power switch and let the lathe coast to a stop. That is actually a very good way to prevent the chuck/faceplate from unscrewing.

 

[Odie]

When I changed over to variable speed on my Woodfast lathe, the Minarik speed controller instructions gave explicit warnings about reversing direction while in motion. Damage could result.

OK, I was informed and everything went well until one day when I was reaching for the run/brake switch and accidentally hit the reverse switch, which was in close proximity and directly under. Although the rpm's were relatively low, the results were dramatic.....but I was lucky to not sustain damage. I can easily see how doing this would eventually cause something to break, or go wrong.

So that this never happens again, I've rigged up a little block of wood on magnets that covers the reverse switch. Now, there must be a deliberate action to remove the wooden cover before the reversing switch can be actuated. (It's a little difficult to see, but the the modification is directly under the red colored switch on the right side in the picture provided.)

At one time, I was a crew member on a military aircraft with armament systems. In order to activate the armament, it was necessary to flip a hinged cover off the switch.....a deliberate action. Maybe some lathe and speed controller manufacturers would be wise to think of mounting their reversing switches in such a manner.....might solve the issue for good.

ooc

 

[Fred Belknap]

The forward/reverse switch on my lathe is a 3 position switch. To go from one to the other you have to go through off. 🙂