There are a lot of videos and articles out there on how to improve the accuracy of a miter saw. The best one – by far – was written by former finish carpenter and Collins Coping Foot inventor David Collins for ThisIsCarpentry. It’s a lengthy process that will pay for itself in spades quickly, especially if your saw isn’t already in near-perfect tune (if you’ve never adjusted it chances are it’s not as accurate as you think). Having your saw’s owners manual handy will help guide you to adjustment points that are specific to your model.
Here are Collin’s tips:
Because some of the tune-up requires cutting and checking sample boards, Collins recommends starting this process with a fresh blade. He likes a thin kerf crosscutting blade with 60 teeth or less, which is what comes on most new saws.
Check the Table
In lieu of the machinist’s straight edge or surface plate, you can use a good framing square. Place the straight edge down on top of the table and use paper of various thicknesses to check for irregularities.
There are two ways to flatten a table: by scraping it, or with a press. If you have the tools and skills to scrape, that’s a good way to go. Pressing should be done in small increments and with care; cast aluminum can break. Or, the safest bet would be to take the saw to a tool repair shop. Chances are your saw’s table is flat from the factory.
Here, trim carpenter Gary Katz checks the table on his Bosch miter saw:
The Fence is Next
Once the table is flat it’s time to check the fence, which is the most common adjustment point that hinders accurate cuts.
Use the framing square as a straight edge, and some very thin paper (telephone paper is a good choice) to find any gaps. This is an adjustment where pinpoint accuracy is key, so take your time.
Watch Katz check and adjust the fence on his Bosch miter saw here:
Calibrate the Miter Gauge
According to Collins, a cut that’s out of square even as small as .005 in. over about 10-in. would yield a miter joint on casing that would be visible from four feet away. For this reason, he takes great care in calibrating the miter gauge. He starts by squaring the blade to the fence. Depending on the saw, this can be done by either moving the fence or moving the miter scale. This is a multi-step process that involves making sample cuts and measuring their accuracy with a caliper (ideally).
Taking the time to follow this entire process will ensure precise cuts, greatly reducing the “cut until it fits” method which can waste a lot of time.
Read the article (it is extensive though lengthy), and David’s full bio here.