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Old 01-15-2011, 11:41 PM
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Default Trouble with a Mini-Lathe Controller

I'm not expecting much response to this thread. I started this thread to "vent" some frustration and to pass along some information not found elsewhere on the internet. Perhaps it can help others.

As many of you know, I bought a Cummins mini-lathe (the label says it is a "Pro-Tech") when I decided to start making bench dogs. That's been 2 1/2 years ago and the lathe has been doing fine. No problems other than lots of re-alignments which is normal.

So New Years Day I'm trying my luck at "power tapping" with a new 1/4-20 tap. I tapped around 10 holes successfully. I go to hit the F/O/R (Forward/Off/Reverse) switch and put it in off when it I hear a bang. The F/O/R switch has gone bad and seems to be jammed in the forward position. I also notice other things in the shop have stopped and the 20 amp circuit breaker for the shop outlets has tripped off.

So I remove the F/O/R switch and the motor controller board. Since the switch is jammed and bad, I decide to open it up and see what happened. The switch arm inside is bent, causing it to jam. But there are no signs of arcing to explain the popped circuit breaker. Since nothing in the switch is burnt or arced, it is safe to assume something in the controller went bad and caused the circuit breaker to pop. That would most likely be the output transistors, aka MOSFETS.

Most of us with mini-lathes or mini-mills know that Little Machine Shop (LMS) is our friend. Not only do they carry parts and tooling but they also have technical information too. So this is the first place I run to for information on the controller. They have some wiring diagrams so I print one out and verify how the controller is wired (actually interfaced) before removing it. Ironically, they don't have one that matches my controller so I make my own diagram. After reading around their site, I'm lead to believe ALL controllers use the same MOSFET transistor. I order a new switch and the MOSFETS.


DC Power Supply

The first order of business is to get the lathe operational so I can complete 3 orders waiting on me. The motor is 110 VDC so why not run it off of a DC power supply? I find an old 48VDC power supply. This voltage is used by phone companies and phone equipment so they are quite popular. At the phone company, the 48 volts usually comes from batteries and, just like your 12 volt car battery actually puts out 13.5 volts, these batteries and power supplies actually put out 54-56 volts. For the parts I make, I run the lathe RPM pot about half way so this power supply voltage is perfect. The power supply only provides 3 amps (6 amps would be better) but that is plenty of power unless you're taking heavy cuts. A 24 VDC power supply could be used for 1/4 speed and 12VDC for 1/10 speed. You can buy any of these supplies on eBay for $20-50. You might even find a 110 VDC power supply on there.

I wired my 48 VDC power supply so it gets its 110 VAC through the lathe's power cord and emergency switch. This way you still have the safety of the cut off switch. It is possible to use the F/O/R switch also by soldering the power supply output to the switch. Do not keep the motor controller wired in place if you do this, you can do serious damage to the controller. Since my F/O/R switch was bad, I installed a different switch on the power supply to accomplish the same thing.


Parts Arrive

My order from LMS arrives and the first thing I notice is the MOSFETs are 3 times the size of the ones on my controller. In electronics, this is not normal. I call LMS and sure enough, NOT all motor controllers use MOSFETs. In fact, any controller that has a part number starting with XMT does not use MODFETs, they use thyristors. Now why didn't LMS say so with the description of their MOSFET or in the wiring diagrams that they provide on their website. Very disappointed with LMS at this point. Sure, they offer to refund my money if I ship the MOSFETs back. The only problem is I will have spent more in shipping fees than the refund. And, LMS does NOT carry the thyristors. They recommend sending your board to Gerling Laboratories, along with $50, to get your board repaired.

I went to Gerling's website and checked it out. I have nothing against them and would probably charge the same rates as them if I was in that business. I've spent 30 years doing electronic repairs and I know 95% of all repairs are the active components (transistors, IC chips) and generally at the input or output stages. So before I spend $50 needlessly, I'm going to replace the thyristors myself. The controller board has 4 heat sinks. The thyristors are mounted to the 2 heat sinks at the very back of the board and to the right. The other 3 heat sinks are for voltage rectifiers, D8020L, which you can replace but cost more and are harder to find. I looked on eBay and there are several sellers for the BT151-500R thyristor (or SCR). Just like the MOSFETs, there are 2 of these on your motor controller and it's wise to replace both at once. The shipping costs more than the thyristors so I bought 4 of them so I will haves spares. By the way, you can look for the MOSFETs (part number IRFP460 or STW20NK50Z) also and you can get it much cheaper than LMS.


Try Again

My thyristors arrive and I replace them. I put the motor controller back in the lathe and it works. I moved the RPM control to max and checked the motor voltage. It was at 98 VDC so I adjusted the voltage to 116 VDC (normal voltage for the motor). On the XMT-1125 controller board this voltage adjust is the one up towards the wiring connectors. It is normal to have to adjust it after replacing the thyristors.


Static Sensitive Hype

MOSFETs are static sensitive and the thyristors are not. It doesn't hurt to take static precautions anyway since other components on the circuit board might be static sensitive. My opinion and experience tends to say there is lots of hype over the static precautions. In my 30 years of replacing thousands of CMOS chips, I've never worn the "static wrist band" and I've never blown a chip. Now, I'm in the Southeast USA with high humidity and static sparks are a rarity. If I was in Arizona or New Mexico I might consider wearing one. The object of the wrist strap is to keep you constantly grounded. The resistor for the strap is to keep static current small enough that it won't hurt you or arc to the component. With some simple safety precautions, you won't need it. Nowadays, most of the static sensitive components have some safety components built into them to help prevent static issues.

First, we're talking about the static shock you receive after dragging your stocking feet across the floor and touching a door knob, or petting your cat or dog, or combing your hair. Okay, so don't do these things when working around static sensitive stuff. Once you start working, don't walk around, stay seated so you don't build up any static charges. Don't walk around with the sensitive device in your hand, keep it in the special bag/box that's designed to dissipate the static. Find something close to your work area that is grounded - that you know is connected to the AC outlet ground and is metal. After you sit down at your work area, touch this grounded item and touch it every few minutes and before handling the sensitive device(s). The tip on most soldering irons are grounded nowadays but I wouldn't touch it to ground until you measure the tip for any voltages. If there are no voltages on the tip, touch the tip to your grounded item for just a brief second before soldering on the circuit board or the component.

These simple steps are all that is really necessary to work with these components 99% of the time. It's what has worked for me for 30 years. If you are working for NASA, or around an explosive atmosphere, or a very critical "life support" circuit then get the wrist strap and adhere to all safety issues.
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Old 01-16-2011, 02:40 AM
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Default Re: Trouble with a Mini-Lathe Controller

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Originally Posted by Qwas View Post
The other 3 heat sinks are for voltage rectifiers, D8020L, which you can replace but cost more and are harder to find.
If you have an odd-number of heat sinks, it tends to suggest you have a brushlless DC system (A.K.A. a 3-phase permanent magnet motor).

I didn't follow your whole posting, but if you treat one of these as though it was DC, it won't work. A simple DC motor with PWM control is not very complex and can run from straight DC power with a variable voltage.

On the other hand, a brushless DC system requires 3-phase input that is artificially generated in the controller. If you suspect you have a brushless DC system, try connecting the motor directly to a VFD with the VFD's voltage parameters limited to the ratings of the motor.

How many wires are connected to the motor?
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Old 01-16-2011, 03:11 AM
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Default Re: Trouble with a Mini-Lathe Controller

It has 2 wires going to the motor and the motor has brushes.

There might be a 4th rectifier that I overlooked. I was more interested in the thyristors.
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Old 01-20-2011, 04:40 PM
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Default Re: Trouble with a Mini-Lathe Controller

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Originally Posted by Qwas View Post

So New Years Day I'm trying my luck at "power tapping" with a new 1/4-20 tap. I tapped around 10 holes successfully.

Steve, If you aren't using helical spiral taps get some -- day & night difference when power tapping.

These simple steps are all that is really necessary to work with these components 99% of the time. It's what has worked for me for 30 years. If you are working for NASA, or around an explosive atmosphere, or a very critical "life support" circuit then get the wrist strap and adhere to all safety issues.
I don't do much electronics work anymore but I always looked at wearing the wrist straps like wearing a helmet when riding a motorcycle -- an once of prevention is worth a pound of cure.
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Old 01-20-2011, 07:28 PM
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Default Re: Trouble with a Mini-Lathe Controller

This was my first attempt at power tapping. I wasn't sure if the mini-lathe could handle it (it's handling it with aluminum, steel may be too much). I've heard other recommend the spiral taps and I will have to get some. Power tapping is a definite time saver.

I've had other electronic techs around me with the same opinion of the wrist straps as yours. Nothing wrong with the extra prevention.
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Old 01-20-2011, 08:03 PM
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Default Re: Trouble with a Mini-Lathe Controller

Quote:
Originally Posted by Qwas View Post
This was my first attempt at power tapping. I wasn't sure if the mini-lathe could handle it (it's handling it with aluminum, steel may be too much). I've heard other recommend the spiral taps and I will have to get some. Power tapping is a definite time saver.

I've had other electronic techs around me with the same opinion of the wrist straps as yours. Nothing wrong with the extra prevention.
The spiral taps will also require less power.
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Old 01-21-2011, 07:11 AM
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Default Re: Trouble with a Mini-Lathe Controller

Just don't confuse a spiral tap with a spinal tap
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Old 01-21-2011, 09:22 AM
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Default Re: Trouble with a Mini-Lathe Controller

Also, don't try to hand tap with a spiral tap -- they break very easily when not supported by a machine spindle.
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Old 01-21-2011, 04:06 PM
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Default Re: Trouble with a Mini-Lathe Controller

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Just don't confuse a spiral tap with a spinal tap
Funny thing is I did a Google search for "spiral tap" hoping to find some pricing and the first listings were about wrestling where it looks like the loser was receiving a spinal tap. I quickly changed the changed the search to "spiral taps"

Ron, that's the first I've heard of the spiral flute taps not being good for hand use. Thanks for the input.
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Old 01-21-2011, 04:52 PM
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Default Re: Trouble with a Mini-Lathe Controller

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Originally Posted by Qwas View Post
Funny thing is I did a Google search for "spiral tap" hoping to find some pricing and the first listings were about wrestling where it looks like the loser was receiving a spinal tap. I quickly changed the changed the search to "spiral taps"

Ron, that's the first I've heard of the spiral flute taps not being good for hand use. Thanks for the input.
As you can imagine the helix causes the cross section of the tap to be thinner -- you can get away with hand tapping with them in a pinch but it's bad practice.
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