A friend of mine has asked me for some help with an old surface grinder he bought. It works fine, except for the electromagnetic chuck. It's an old machine.

http://img153.imageshack.us/img153/9066/schematicc.jpg < the schematic in question.

Now, here's the problem. The half wave tubes are blown. I know that I can just use a couple of stud mount diodes to replace them, but I'm unsure of what kind exactly to use, and how to mount them ( heatsinks,etc).

Any help or ideas would be appreciated.

 

tubes pre-date me...cant help

but heres a bump.

 

You can use diodes, but without knowing the current & voltage rating of the chuck it's not possible to give a type.

Are there any ratings on the chuck or the supply transformer?

Another thing to be aware of is that the timer circuit may be damaged if you simply substiture the rectifiers.
You should also add a delay-On timer so the output of the rectifiers is not connected to the timer for about 30 seconds, to allow the timer tube filament to heat up.

Alternatively, you could use a new timer to replace the tube circuit.

 

The first thing I would do is try to find replacement tubes. From the look of the documentation, I'd say the thing was made in the 1950's; it may not be hard to find replacement tubes if you can still read the numbers on the tubes.

It appears the purpose of the time delay circuit is just to hold off switching the DC output on through the relay for some adjustable period of time. The tubes and the time delay circuit could likely be replaced by an adjustable delay on relay (easy to find for $50-$75) and two suitable diodes. The only unknown is the current output, assuming the peak DC output is 230 volts (hinted at on the drawing). The maximum current could be estimated by the maximum current rating of the tubes, assuming you know their type. If the voltage is indeed 230 VDC, I'd recommend 400 PIV diodes that are rated to handle current 20% over the maximum rating of the tubes. They'll need suitable heat sinking.

Before buying parts, however, it would make sense to measure the open circuit AC voltage across the rectifier tubes to make sure you get diodes with the proper voltage rating. Obviously, this type of work is for someone experienced with line-powered stuff, so don't attempt it unless you know your skills are appropriate. Since the equipment is older, you'd also want to carefully review and measure all of its safety features such as proper AC grounding, wire insulation in good shape, hipot testing, leakage testing, etc. If you're not sure what to do here, hire a professional, as the operators' safety is at stake. Be aware that grinding can be done with e.g. water-soluble oil, so there may be unexpected paths to ground. If I was going to use the grinder & chuck, I'd probably rewire everything I could at the very least (and still do hipot and leakage testing). I'd also worry about the transformer's and chunk's insulation and isolation.

As far as the heat sinking requirements, this will be dictated by the power the diodes have to dissipate, which won't be known until the current and voltage requirements are nailed down. If this is in a machine shop environment, then it's possible a nice chunk of scrap jig plate could be used. Otherwise, some heat sink extrusion material can be purchased and used.

 

If the tubes are something like 80's ,4pins, 2 small 2 big, as I remember dimley, output is less than 1/2 A.

 

I took a look in my handy-dandy, 1946 printing of the Sylvania Tube Manual and found a match for the schamatic symbol. The type 81 is a 4 pin, half-wave rectifier glass tube.

Max DC current rating is 85mA(per tube), AC plate voltage is 700V RMS max. PIV is 2000 Volts. Filiment is 7.5 Volts.

The rectifier curve chart suggests it is intended for applications where the voltage out is in the 800 to 500 Volt DC range.

Each tube could be replaced with 2 of 1N4007 in series(no heatsink required). Also put a, 500Ω to 1KΩ, 10W resistor, in series with each pair of diodes to simulate the plate resistance, which causes approximately 90-100 Volt drop across the tube at full load. The resistance is an estimate, so you can adjust the value to get the correct output voltage.

The 230V shown on the schematic is on the screen grid, which is usually operated around half the plate voltage.

What is a surface grinder, and why does it need a timer on the chuck?

Just curious,
Ifixit

 

The schematic symbol is a generic tube diode - you cannot derive ratings from that.

The last grinder magnetic chuck I worked on was 220VDC at 16A

You need to look for info on the transformer and chuck - any markings or figures.
How big is the transformer? Just the size will give a rough idea of power rating.

Edit: The 230V on the diagram is not a voltage reading. It refers to the location of C2 on '230V DC Units'.

Can you tell how big the original rectifier tubes were & if they had an anode cap connection (flylead with a clip).

 

I was a machinist for 45 years, the timer is to lower the magnetism when you want to remove the metal part from the chuck. The newer controllers change polarity back and forth rapidly to do this. the old ones, like this one, switched the magnet off and on to lower the magnetic hold.

 

220VDC times 16 Amps equals 3,520 Watts. Seems like alot.
A tube to handle 16 Amps would be the size of an oil drum... if they even exist.

However, it is fun to speculate while the OP is away investigating

Have fun,
Ifixit

 

The magnet is to hold the job - which could be anything from a large casting to small thin items like ice skate blades - flat down to the grinder table.

The table is effectively smooth steel, with flush lead or epoxy-filled slots to separate the magnetic poles while mainitaing a seal to keep coolant out.

The parts must not move on the table while a large grinding wheel is passing back and forth across them & cutting the metal.

The 220V 16A one is eight feet diameter, by the way.
The size of this particular one may give an idea of it's power requierement, assuming it's proportional to area.