Practical answers on using circle cutting guides and roller guides to get cleaner, more accurate gas cuts.
Why does the height of the nozzle matter when I use a guide?
The distance between the nozzle and the plate controls how well the preheat flames and cutting oxygen work together. If the nozzle is too close, you get grooves and deep drag lines because part of the preheat flame burns inside the cut and deflects the oxygen stream. If it is held too high, the preheat loses effectiveness, the top edge becomes rounded or beaded, and the face can be slightly bevelled.
A guide that keeps the nozzle at a steady height removes one of the hardest things to do freehand. Set the height to the torch and nozzle maker's recommendation, then check it again if you change nozzle or plate thickness.
How do I know if I am moving the torch around the circle at the right speed?
Look at the drag lines on the cut face. A good cut has regular, only slightly sloping lines. If the lines lean well backwards and slag clings to the underside, you are moving too fast and risk losing the cut. If the lines are vertical and the top edge looks rounded or beaded, you are moving too slowly, or using too much cutting oxygen for the nozzle.
A circle guide helps because it keeps the radius fixed, but you still control the speed, so aim for a steady, even pace all the way round. Speed depends on the thickness and the nozzle, so use the maker's chart for your equipment as the starting point.
How do I start a circle cut in the middle of a plate?
You normally pierce the plate first. Heat a spot inside the circle until it is at ignition temperature, then open the cutting oxygen gradually and hold the torch steady until the jet goes through. Slag and molten metal can build up and be thrown back at the nozzle, so follow the torch maker's piercing method and wear full eye and face protection.
From the pierce hole you cut out to the line of the circle, then use the guide to cut round it. Piercing is slower with propane than with acetylene, and thicker plate takes longer to pierce, so allow time and do not rush it.
Does the fuel gas I use make any difference when using a guide?
It can. Propane has a lower flame temperature than acetylene, so piercing the start hole takes longer, although cutting speed once under way is about the same. Propane also tolerates a greater nozzle-to-plate distance, which reduces the risk of molten metal splashing back onto the nozzle and causing a backfire.
The nozzles are different too. Acetylene and propane need nozzles designed for the gas, so make sure the nozzle in the torch matches the fuel gas, and set the guide height to the maker's advice for that nozzle. Check the instructions for your torch before changing from one fuel gas to the other.
What does a circle cutting guide do?
A circle cutting guide turns the torch into a compass. A pointed pivot pin sits in a centre-punch mark at the middle of the circle, and the torch head swings round it at a fixed distance, which is the radius. Cutting a clean circle freehand is very difficult, even for experienced operators, because you have to follow the line by eye and hand.
Using a guide means the radius stays constant, so the circle is more accurate and the edge is smoother. A roller or similar support at the torch end usually carries the weight and helps keep the nozzle at a steady height. You still pierce a start hole and control the cutting speed yourself.
How do I set the radius on a circle guide?
First mark the centre of the circle with a centre punch. On most guides you then slide the pivot along the shaft, or slide the attachment along the torch tubes, until the distance between the pivot pin and the cutting nozzle equals the radius you want, and lock it in place. Many shafts have markings to help.
Remember that the radius is measured to the cutting oxygen hole, not the outside of the torch, so check against a rule or a scribed line on scrap before cutting good material. Also allow for the width of the kerf, because the finished hole will be slightly larger, or the disc slightly smaller, than the radius you set. Always follow the instructions that come with your guide.