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Variable speed in reverse

DPittman

Active Member
#1
I have tried to do some research but have not been able to find an answer so I ask here...

My variable speed lathe has only ever ran in reverse at half the rpms it will do in forward. I don't understand why that is? Is the an electrical reason for this? The motor is a brushed DC motor.

I sometimes would like a few more rpms when I'm running in reverse and using a parting tool.
 

John Conroy

Well-Known Member
Premium Member
#2
What kind of lathe is it? Does it use a tumbler gear for reverse? Depending on the lathe design the gear ratio may be different in reverse than when the spindle is driven forward.
 

DPittman

Active Member
#3
What kind of lathe is it? Does it use a tumbler gear for reverse? Depending on the lathe design the gear ratio may be different in reverse than when the spindle is driven forward.
It is a Craftex brand 10x22 lathe. The tumbler reverse mechanism is used only for carriage reverse and not the spindle. The actual motor speed is reduced when running in reverse.
 

RobinHood

Active Member
Premium Member
#4
I would guess it’s in the motor control circuit. The DC motor itself does not care which direction it runs at what speed.

I found a wiring diagram for a CX706 (is that your lathe? Maybe similar?)

B02A1811-E60C-4A53-9B8C-7AC67484F397.png

There is a 1/2 speed signal going from the “Filter”to the “R/F Switch”. (item 1). I would swap leads 4 and 6 on the motor (item 2). If now the spindle is turning at full speed in the reverse direction while the F/R switch is in the FWD position and at 1/2 speed while in REV (spindle is turning in forward direction), then it is not the motor. It would be the control, perhaps by design?
 

DPittman

Active Member
#5
My lathe cx700 is the previous version of the cx706. My wiring diagram is a tiny bit different I think but haven't had a chance to study it yet. I suspect it is also purposely wired to be at half speed but I can not figure out why. Thanks for your assistance!
 

Bofobo

Well-Known Member
#6
I just read through your manual, As the forward and reverse is a switch rather than the lever as with a smaller version (7x—- ) where an added gear 1:1 reverses the rotation. Before you try to reverse any power wires, inspect the motor. (if its brushed) make sure the brushes are directly opposing and not offset. This can cause problems, on one hand its not the most common motor but it is a chinese machine so anything is possible
 

DPittman

Active Member
#7
I just read through your manual, As the forward and reverse is a switch rather than the lever as with a smaller version (7x—- ) where an added gear 1:1 reverses the rotation. Before you try to reverse any power wires, inspect the motor. (if its brushed) make sure the brushes are directly opposing and not offset. This can cause problems, on one hand its not the most common motor but it is a chinese machine so anything is possible
My motor is a brushed version and they are opposite of each other (180 degrees)
 

RobinHood

Active Member
Premium Member
#8
if its brushed) make sure the brushes are directly opposing and not offset.
Yes, excellent point. In an offset situation, reverse speed could be kept lower to preserve the brush life.

@DPittman : since yours are 180* it should not matter and FWD and REV speed could be the same. Maybe on early serial# lathes of your model, the motor was different and brush life was a concern...
 

DPittman

Active Member
#9
Ahh very inte
Yes, excellent point. In an offset situation, reverse speed could be kept lower to preserve the brush life.

@DPittman : since yours are 180* it should not matter and FWD and REV speed could be the same. Maybe on early serial# lathes of your model, the motor was different and brush life was a concern...
Ahhh very interesting! I did not know that about the brushes. I had been having some problems with them and replaced them a while ago. I wonder if running it in reverse is somehow hard on them if they are in fact at 180* to each other?
 

RobinHood

Active Member
Premium Member
#11
My variable speed lathe has only ever ran in reverse at half the rpms it will do in forward.
The CX700 manual does not have the basic wiring diagram (as posted above) the newer CX706 manual shows. If the 706 is any indication of how yours may be set-up, then that 1/2 Speed signal is probably the answer why you get only 50% of your forward RPM while in REV.

Someone with experience (another owner perhaps?) on those CX series lathes should be able to confirm that for you.

My lathes and mill either reverse by the AC motor turning backwards or by 1:1 reverse gearing in the headstock. Thus FWD and REV rpm are identical.
 

Tom O

Active Member
#13
Ahh very inte
Ahhh very interesting! I did not know that about the brushes. I had been having some problems with them and replaced them a while ago. I wonder if running it in reverse is somehow hard on them if they are in fact at 180* to each other?
On my Craftex mill the rear brush heats up and fuses with the cap making it next to impossible to remove, Busy Bee gave me another motor seeing as it was under waranty the fix I used to combat that was to put in brass shims between the screw in cap and brush so they won’t fuse together. I also 3D printed a baseplate for a fan that sits on top of the motor housing.