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Need ideas for project

I repair older machine tools. I am currently trying to fix a problem on an older Mazak turret lathe. It has an old stepper relay that is mechanically worn out and nobody makes new stepper relays. The lathe has 8 tool stations on a turret. When the CNC computer control program calls for a tool change a signal is sent to a solenoid to unclamp the turret and rotate to a different tool. This operation works fine.

The CNC computer control also sends a +24 VDC signal to the stepper relay to energize the relay coil and cause the wiper to advance to the appropriate output (there are 8 outputs, one for each tool station) as feedback to the computer control as to which tool station is currently active.

I want to replace the stepper relay with something more modern, and am thinking of possibly using a multiplexer circuit. I have looked at some multiplexers but it seems they use 3 inputs to determine which output is selected. Different combinations of high signals on the 3 inputs determine which of the 8 outputs is selected. I am dealing with 1 input and each time it goes high (+24 VDC) the next output needs to be selected.

Any ideas would be greatly appreciated.
 

JimB

Super Moderator
Most Helpful Member
My first thought is that you need a counter.

A three bit binary counter will count up from 0 to 7 (8 steps), you can then use the multiplexer to convert the binary count to a 1 to 8 count to actuate the turret.
Logic circuits cannot provide enough current to drive relays/solenoids, you will have to use a driver transistor of some kind on the outputs to the turret.

JimB
 

rjenkinsgb

Well-Known Member
Look at a 4017 or 4022 CMOS ICs, those are a combined counter/decoder.
Data: http://www.ti.com/lit/ds/symlink/cd4017b.pdf

The outputs are low current but you can use such as a darlington driver array chip to boost them so they can switch 24V relays.
A ULN2803 would be suitable with the counter operating on 5V:

You may need a debounce circuit at the input as mechanical contacts often give several pulses as they close, when seen at logic speeds (when things can count at megahertz frequencies).

If it's operating from a solid state PLC output then just a resistive divider and a series resistor from that to the logic input should be OK; eg. 3k9 and 1K0 for the divider then a 10K or higher from the divider junction to the IC clock input.
 
I really appreciate the advice. I don't know how to use a 3 bit binary counter for this application because there is only one input. Every time a new tool is called for the CNC control sends a 2 second +24 VDC signal to the stepper relay. I don't have to send outputs to drive solenoids for the turret. That function is being handled just fine. I only need to send feedback to the CNC control to tell it what tool station is currently on.
 

JimB

Super Moderator
Most Helpful Member
I don't know how to use a 3 bit binary counter for this application because there is only one input.
That is OK, there is only one input to a 3 bit binary counter, but there are three outputs ie three bits.
But lets not get hung up on that...

The suggestion by rjenkinsgb to use a combined counter decoder is a good one, simplifies the circuit a bit.

JimB
 

Les Jones

Well-Known Member
Does the stepper relay replacement have to remember it's last position when power is removed or does it need to reset to a fixed starting position (0 ?) when power is applied ? If it does need to remember it's last position when power is removed then a small microcontroller may be a better solution as it could save the last position in it's internal EEPROM when powered down.

Les.
 

Pommie

Well-Known Member
Most Helpful Member
Do you know the current requirement of the turret? If not too high then a 4017 driving a UDN2981A might be a good combination.

Mike.
P.S. is there another PNP darlington that is readily available.
 
The turret rotates to another tool just fine as is. I need to replace the stepper relay which only provides feedback to the CNC control as to what tool is now operational. There is a wiper on the stepper relay that is connected to +24 VDC and this wiper advances to 1 of 8 output contacts (each one representing one of the 8 tools). An absolute pulse coder would work but there is no place to mount one.
 

rjenkinsgb

Well-Known Member
Note that you can still get hold of Ledex switches; some places on ebay have "new old stock" ones.
eg. https://www.ebay.co.uk/itm/LEDEX-ELECTRO-Mechanic-Stepping-Switch-12-Way-Shorting-28VDC-Part-No-8111-1-pc/152096416303

They take standard kit-type rotary switch wafers so you can make the contacts up as you wish.

For 8 way you would have to add a wafer and pulse circuit to step it past the four unused positions, any time it got there.


Or uniselectors - again, there are several on ebay:

As with the Ledex switch, use contacts beyond the needed ones to cycle it back to the first contact position.
 

Pommie

Well-Known Member
Most Helpful Member
The 4017 can only supply a tiny current - not enough to drive your turret. You need something like the chip I posted above to supply a higher current at 24V. However, that might still not supply enough - are you able to measure the current that the turret takes?

Mike.
 
The turret is driven by a separate circuit and that works just fine. The stepper relay I'm trying to replace just supplies feedback to the CNC as to what tool is currently being used. So maybe the hot set up is the 4017 driving the UDN 2981A. Thanks Mike.
 

unclejed613

Well-Known Member
Most Helpful Member
i had a bit of an "off the wall" thought when i read the first post. a lot of old pinball machines used stepper relays for the digit counters. replace the 120V coils with a 24V coil, and you're in business...
 

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