A decent circular saw is capable of producing inaccurate cuts because of a shift in a workpiece, a misplaced straight edge or an unsuitable blade. Even a properly marked hole may end up being off by 3mm due to skidding of a bit. However, this time a tool itself does not become the culprit.
Accurate measurement defines where a piece of work should be done, and having it measured in the right way is important. Ensuring the cut, hole or fixing actually lands there requires taking care of the whole setup, including the tool, the accessory, the settings, the reference and the support.
The blade, bit or driver bit is the part of the tool interacting with the material, so it should be checked first. It is important to have the accessory suited to the material and the job, in good condition and secured as instructed by the manufacturer.
Dullness of a blade, a chipped bit or insecure fitting of an accessory will lead to poor results no matter how perfectly you will have the tool set up.
A great number of tools define the cutting line on the basis of some other element. This can be a mitre saw fence, a parallel guide of a circular saw, a rail of a plunge saw, a fence of a router or a drilling guide.
Make sure all elements you are going to use are properly set, securely fastened and clean before doing any work. A single shaving stuck somewhere between the mitre saw fence and a workpiece is enough to move a properly marked board.
Even a properly marked board cannot help if the tool follows another line. Methods of calibration can differ, but always refer to the manufacturer's instruction.
Setup of such tool parts as depth of cut, mitre and bevel angles, plunge depth, router depth or drill depth stop should be done before starting work, not adjusted halfway through the job.
It is important to double-check that a scale or previous setting is still right. For instance, bevel scales of site saws tend to give slightly wrong readings. A stop knocked somewhere in a van will not tell you about that. Check the setting against the work or an offcut where it is important to have the precise result.
Even if a tool is adjusted properly, inconsistent results can be caused by movement, flexibility or sagging of the material. In most cases, these are the causes of "the saw cut wrong" problems.
A 4.8m length of timber on a mitre saw should be supported at the same height as a bed of the tool, as the weight can lift or twist the piece and move it from the fence. A piece of ply being cut with a circular saw should be supported on both sides of the line to avoid sagging and tearing of the board.
Clamp the work wherever it might move and ensure that the surface underneath it is stable.
Repeatability can be achieved through having a consistent setup, not by measuring every piece separately. Take measurements from the same edge or face each time and keep using the same fence or guide.
Eighth noggins to be cut to the same length can serve as a good example. Measuring and marking each of them relative to the previous one results in accumulating small errors. Setting a stop on one of your mitre saws and positioning each piece against it makes all of them to have the same length, with nothing to read or mark in between.
There are some jobs where the best solution is to use a physical guide.
| Tool | The Guide That Will Help | The Things That Are Important Anyway |
|---|---|---|
| Circular or plunge saw | A straight edge clamped or compatible rail | Rail position, clamping and blade condition |
| Jigsaw | A parallel guide or straight edge clamped | Blade choice, material and steady feed |
| Router | A fence, template or guide bush | Template securing and correct bearing or bush |
| Drill | Drilling jig or guide | Jig position and support behind the work |
The guide is able to help only if it is set up and fastened properly. A rail that moves during a cut makes a straight line, but it is in the wrong place.
In order to start a hole in the right place, the idea is to make sure a hole starts properly and then continue to drill normally. Begin with making a clear mark and choosing appropriate drill bits.
Then, depending on the material, take measures to start drilling properly. A centre punch provides a starting point for an HSS bit in steel. Brad point bits start working from a mark made in timber. In case of tile, starting drilling at a low speed without hammer action stops the bit from skating over the glaze. Also, a drill should be square to the face and the material should be supported from behind. Several holes requiring lining up or matching an angle are best done with a jig.
The same principle is applied to fixings. An appropriate driver bit should be used and it should be neither worn nor loose. Keep the tool aligned with a screw so that it would not come loose or go in at an angle.
The screws of plasterboard should be placed just below the paper without breaking it. Drywall depth-setting noses can be used for that purpose. Consistent depth and spacing make a visible difference in the case of deck boards. Clutch or depth settings of the tool should be tried on a few fixings before doing a long run.
If there is a possibility of having some extra material, then doing a test makes sure that position, depth, angle, guide positioning, finish and fixing depth are all right before touching the workpiece.
This is not needed for every routine job. The most worthwhile tests are done after changing an accessory or setting or before cutting some expensive material like a worktop.
Accuracy is not needed only once. While doing a longer run, stops can be knocked, clamps can loosen, accessories can wear, settings can change and dust can cover the reference surfaces. Material may vary from one piece to another as well.
Checking the finished piece against the required size occasionally can detect a developing problem before running the entire batch.
An expensive tool is not enough to produce accurate work. This accuracy is determined by an accessory, the settings, the reference, the workpiece support and consistency of the setup.
Check the accessory, adjust the settings, support the work, establish the reference, do a test if necessary and recheck the repetitive results. It is not needed for every job, but it allows achieving more consistent results with the tools you already have.
Q1. Why is my circular saw not cutting straight?
A1. Some of the possible causes include poorly positioned guide, movement of the workpiece, worn or unsuitable blade, incorrect setup or technique of feeding. Check this first before concluding that it is the saw that is the culprit.
Q2. How should I set the cutting depth on a circular saw?
A2. The depth of cut should be set according to the saw, blade, material and job. Refer to the manufacturer's instructions as there is no one-size-fits-all depth of cut.
Q3. How can I stop a drill bit from wandering when starting a hole?
A3. It is necessary to have a proper mark, to choose an appropriate bit and to start drilling properly, for example, center-punching of steel or slow drilling on the tile.
Q4. Are guide rails more accurate than cutting freehand?
A4. A properly positioned and fastened guide provides the physical reference that helps achieve better consistency. The setup, including support and condition of a blade, still should be correct.
Q5. Why should I make a test cut first?
A5. This allows checking position, depth, angle, accessory choice and finish on the scrap before doing the workpiece.
Q6. How do I keep repeated cuts the same length?
A6. You should work from the same reference face and use a stop instead of marking every piece. Periodical checking of finished pieces is helpful in case of any changes of a stop.