Monday, 19 January 2009

The Girls' guide to Cutting and Grinding.

Hello. Lorna here.

This morning I had another welding job.
I thought it would be a good opportunity to write a little guide to angle grinding.

Step one, gather all your stuff together.

I have a small 115mm angle grinder because it is more manageable, the huge ones would be too heavy to direct and hold steady over longer cutting projects.
I have my sheet of steel that was lying around in the engine room. It's 3mm mild steel so doesn't take many passes to cut.
A clamp to hold the sheet steel in place.
Work bench to rest everything on.
Thin cutting discs for metal. These are 1mm thick so that they cut neater edges and kick out less dust, and are just easier to handle. You'll go through a few though.
A flap disc for cleaning up the steel's surface. It goes onto the angle grinder.
Goggles to protect precious eyes from sparks.
Gloves. I use my welding gloves just because they are so robust. They protect your hands from sparks, spinning discs and sharp metal edges.

Now time to mark up the steel with the pattern you want to cut. Some people mark with soapstone, or pencil, but when you have so many sparks and steamed up goggles it can be difficult to follow a line, so I like to make a mask the right size and use spraypaint to mark out the edges. Another tip- don't rely on tape-measures for working out what size to cut your metal to. I like to use scraps of wood or card that I can cut down to size and make sure they fit into the gap perfectly. So many shapes that you think are rectangles turn out to be irregular polygons.

Now time for cutting! You want to hold your angle grinder disc down, and at 90 degrees to the metal. The body of the angle grinder will be pointing directly upwards. Hold it with two hands and run it along your marker out line. You'll be cutting a shallow groove with your first pass, as straight as you can. And don't forget to go past the end mark a little too. You shouldn't be forcing it or that's how you'll get a wobbly line, it's pretty much the weight of the angle grinder doing the work and you just easy it along and direct it. If you are happy with the groove then you can make a second and third pass along it. If you are holding it straight above then it should follow the groove and you'll need less stearing. When you have a nice deep grove, hold it tighter and work it back and forth along the line. Be very aware that sometimes it can jerk when it breaks through to the other side so you should never stand behind or in front of the line you are cutting. When you've cut through the metal the whole way along, switch the grinder off and move the clamp so that you can cut the other side.

And just cut the other long side and finally the short edge.

I always cut it a bit longer so that I can take the metal to the spot where I am going to weld it to make doubly sure that it isn't too short. I then mark out the excess to chop off and either cut it using the cutting disc or grind it off using a grinding disc depending on how much needs getting rid of.

So that's it, all cut out and trimmed to size. Because I'm welding it into place, it also needs to be polished down to fresh bare steel to keep a clean Arc. For this I use the flap disc- a disc that goes onto the angle grinder with lots of pads of tough sandpaper. This is also perfect for smoothing off any sharp metal edges for the long edge that won't be welded.

Here's the spot where the metal plate will go. Between the two U beams that support the window at the bottom. At the moment water has been dripping through into the boat where the window reaches the hull. What this plate does is take the drips and channels them outside.

And voila! Here it is! All painted and perfect.