Circular saw

From DIYWiki
Jump to: navigation, search

What is it?

The circular saw probably needs little introduction. It is a saw designed for rapid and accurate cutting of sheet materials and timber with a flat surface. It uses a spinning disc shaped blade to do the actual cutting, and features a number of design elements to keep the operator safe and to make the tool easy to handle and use.

Types?

Circular saws are almost always dedicated machines these days. There was a time that a small saw attachment was a available for some brands of drill, but these have all but disappeared from the market due to their many limitations.

This FAQ is about hand held saws. Note however that the circular saw also has a number of other common forms such as the chop saw or mitre saw, radial arm saw, and table saw etc.

The main thing that differentiates the tools, is size and build quality.

Saw Size Guide

Size Max Cut Limitations

Comments

Under 6" 35mm to 45mm Limited depth of cut and a small motor limits these saws to sheet materials that are not too thick. Cut speed is also less than larger saws. This class of tool is now less commonly found as a mains powered tool, although is quite common as a cordless tool. The range and choice of blades available for it are not as good as other sizes. The main advantage is light weight and smaller physical size which may be useful in some circumstances.
6 - 7.5" 65mm Good for most sheet materials, but usually unable to cut 3" thick timber in a single action. 7 1/2" saws are probably the most common size and will tackle most jobs with ease. There is a very good range of blades available in this size.
8" and above 85mm Big and heavy, may be overkill for most of the jobs you will need it for the 8 and 9" saws are typically the largest hand held saws that you will find. These will do all the jobs expected of the smaller models, but also have the advantage of significant cut depth of over 3" typically. Blade range is also very good.

Safety

Circular saw blades are sharp, big, and spin fast. If you come into contact with one you can be assured of very easily ending up in a big mess. A circular saw should hence always be treated with respect. Care should be exercised even when picking one up since some models are very easy to pick up and switch on accidentally in the same action (the on button is positioned right where you naturally want to grab it!). Even after turning the saw off, the blade can continue to rotate for several seconds.

You should ensure that your work area is well laid out with space to move round the work piece, and the work is well supported and will not move or parts of it drop off onto you while you are cutting. You want both hands on the saw as much of the time as possible, and not trying to balance or catch falling sawn material.

Many of the better saws are good at directing dust into an organised stream of material that can be collected with a vacuum extractor. The dust is relatively coarse and hence does not usually pose a serious inhalation hazard apart from when cutting materials prone to fine dust generation like MDF.

Ear defenders should be warn at all times, since even the smallest saws are loud with much of the sound energy concentrated into a narrow frequency band. This will dull your hearing very quickly, and prolonged exposure will result in permanent hearing loss.

Gloves and eye protection are a good idea to protect you from flying wood chips and splinters.

Never use a saw that has a defective guard, and never attempt to disable or remove the guard. The guard should move out of the way easily as you start a cut, and should spring back the moment you finish a cut (or drop the saw!). If you need to move the guard away from the blade manually (for example when making a very fine cut close to the edge of a work piece, always use on the knob on the top side of the saw rather than reaching under the saw close to the blade edge to do this.

It is wise to use a saw from a supply protected by an RCD, since cable cutting incidents are not uncommon.

Supporting the work

In order to work safely and effectively it is important to make sure that what you are cutting is well supported, and not likely to slip or fall as you cut. Not only will this improve safety but it will also give better quality results. The most difficult material you will normally need to handle are the large 8x4' sheets. One workmate is never going to be enough to hold this safely while you cut it up.

Additional supports like lightweight trestles, or fold away roller stands are invaluable for keeping things well supported.

A quick and cheap solution is to simply lay the board across a few spare battens. If you set the depth of cut to just a little more than the board thickness then you can safely saw right over the battens without fear of cutting right through them (although you will take a small cut out of them each time).

Quality

The single most noticeable difference between the budget and high end tools is the rigidity of the tool. For the best quality of cut and smoothness of operation the saw sole plate and other bracing mechanisms needs to be a solid as possible. Low end saws will typically have a pressed steel sole plate, and better tools will have cast steel or aluminium sole plates. Blade tilt locks want to be rigid and resist bending to one side or the other, because any flex here will be reflected in the squareness of the cut. It helps if they are marked with an accurate angle scale.

Blade depth locks also allow the saw to be set for a shallower cut than its maximum. Again better ones are more rigid, and better marked with an accurate depth scale.

Vibration from the saw should also be very low, this not only reduces operator fatigue, but enhances cut quality as well.

Less visible differences will be in the quality of the bearings and their resistance to dust ingress. Also higher end tools will be rated for continuous operation, whereas the lower end ones will need to be rested periodically to prevent overheating.

Blade quality varies greatly. As its hard to judge them by appearance, look at brand and number of teeth.


Included Accessories

As a minimum most saws will come with a rip fence and a spanner for blade changing. The rip fence allows the saw to be guided along the edge of the work piece for when accurate cuts are required. Note however than fences are limited in length, and hence typically restrict guided cuts to within 12" of the edge of a board. Note also some rip fence designs are better than others in terms of rigidity and dealing with small cuts. Most saws should have a dust port ready to accept a suitable vacuum nozzle. Some better saws may include a sole plate cover to prevent marking the surface of fine wood finishes.

Features

Some saws will have soft start to reduce the kick at switch on. This is more desirable the bigger the saw gets. A saw should have plenty of easy to grip handles and knobs that the operator can use to guide the saw. A good guide on the sole plate makes for easy alignment of the blade with the actual cut line. Don't be too swayed by gimmicks like laser line generators since these are rarely accurate enough. A blade brake is well worth having since it will arrest the rotation of the blade quickly and hence reduce the risk of an accident.

Cordless saws

For general use a mains powered saw is a much better bet, however, cordless saws are now available. These are usually restricted to the smaller sizes. Note also that run time will be very restricted unless you buy a high end model with high capacity batteries. Capacity = Voltage x Ah.

Care also needs to be taken to ensure that the freedom from a mains flex does not lead to you attempting to work in an unsuitable location! Even cordless tools will take fingers off and slice through major arteries and tendons in no time!

Types of cut

Many sawing tasks will involve cutting sheet materials like ply, MDF, or chipboard. These generally require a general purpose blade and no particular special precautions (over and above the general safety measures detailed above).

Cross Cut

Sometimes however you will want to cut real timber. A "Cross cut" is one made across the grain of the timber. This is the easiest type of cut to make in real timber. For these cuts it is often better to substitute a blade designed for this type of cut. It will have finer teeth and cause less tear out from the top surface of the cut.

Rip Cut

A "rip cut" is one made along the grain. This requires a blade with larger and more aggressive teeth, and will use more power to make the cut. When rip cutting it is important that your saw is fitted with a correctly adjusted riving knife. This is a metal tab that should follow closely behind the blade in the cut. Its purpose is to ensure that should the cut close up for any reason, the material can not pinch the back of the saw blade. Pinching the front of the blade tends to pull the saw harder into the work piece and possibly stall the motor. Pinching the back tends to throw the saw away from the work (and toward you!)

Non through cut

A non through cut is one that does not cut right through the material. This may be because the saw does not have enough depth of cut for the task (see "Cutting thicker materials" below), or you are using the saw to cut a rebate or grove in the work (possibly using several passes)

A bevel cut

Most saws allow the angle of the blade with respect to the sole plate to be altered. This allows non square cuts to be made that will put a bevelled edge onto the cut material. Note that the maximum depth of cut is reduced when bevel cutting.

Plunge cut

Not to be attempted by the feint hearted! This is where a cut needs to be made starting in the middle of a board and not at an edge. A typical example may be when removing a section of sheet material flooring in a room. The saw will need to have the riving knife removed, the blade guard fully retracted, and the saw either balanced on its leading edge ready to start the cut or be placed flat on its sole plate with the blade retracted to its highest setting. The saw is then started and the blade slowly plunged into the board to the required depth.

The "tilted forward" method allows for the blade depth setting to be preset (handy when you want an exact depth of cut (e.g. floorboard depth)), but presents more risk to the operator since the sharp edge is exposed.

Its easy for the saw to end up being thrown, and flying spinning circular saw blades aren't a good thing. Grip the saw very firmly, and only apply light force to the workpiece. Low power cordless saws have a real advantage for plunge cutting, they tend to stall rather than throw.

Cutting thicker materials

It is possible to cut material that is thicker than the maximum cut depth of the saw by the simple expedient of cutting twice - once from each side. While not ideal it does extend the versatility of even the smallest saws.

Types of blade

Most saws will be supplied with a general purpose blade. These are as their name suggests suitable for most tasks including cross and rip cutting in timber as well as cuts in many composite or man made boards. However they are a bit of a compromise, and frequently you can obtain dramatically better results from even cheap saws by the expedient of changing the supplied blade for a better one. If you find yourself needing to do may cuts of one particular type (i.e. cross or rip) it may be worth buying a blade designed for the task.

Most blades these days are Tungsten Carbide Tipped (TCT), these give the most wear resistance and will stay sharp longer. They can be sharpened, but this is often not a DIY task and a specialist sharpening service may need to be sought. The TCT tips are also easily damaged by hitting embedded nails other hard objects in the wood. Having a spare "old" blade for situations where you can't be sure the wood is free of nails can be handy.

High Speed Steel (HSS) blades aren't common now. They have much lower maximum cut speed, and blunt relatively quickly. They need repeated sharpening during their life, and occasionally setting. Don't put an HSS blade in a saw designed for TCT, its speed capabilities will be well & truly exceeded.

There are also specialist blades available for occasional cutting of plastics, aluminium and other soft metals. Some are even designed for cutting timber with nails in.

Rebating blade sets (or dado sets) as they are sometimes known are not suitable for use with a hand held saw.

Jigs and Guides

There are a number of after market jigs and guides that can be bought for use with a circular saw. Many are designed to help guide the saw, and to speed up the process of making accurate straight cuts. You can also get mitre guides designed to help accurate manufacture of matching 45 degree mitres.

One of the most useful accessories is the home made saw guide or saw board. These are quick and simple to make, but greatly ease the process of marking out and cutting sheet materials. They also have the added bonus of reducing tear-out at the top surface of the cut, and protecting the top surface of the material from scratches caused by the saws sole plate. Details for making one can be found here.

Second Hand Tools

Not much to watch for, other than making sure all the safety features work and are in place. Work on the assumption that the first thing you will need to do is to buy it a nice new blade and you should not be too disappointed! One thing to check is the standard angle of cut really is 90 degrees (use a small set square) - this should eliminate both saws made to poor tolerances, and those that have been mistreated.