Cutting accurately with a chop-saw

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27 May 2011
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I developed this technique whilst laying parquet flooring described in this thread -Laying reclaimed parquet and am posting this here since the method is applicable to any woodworking where you want a fine fit and can perform incremental cuts to achieve the required size -
In the process of laying 1000's of blocks one tends to become blind to sometimes glaring defects which subsequently required chopping out the offending block and replacing and in my case because I had resized all the blocks and thus they were of uniform dimensions replacing whole blocks was simple, the challenge was the 45 degree cut ones around the perimeter, and this is what I did -
Marking a replacement block by placing it as as far as possible 'in-situ' is error prone due to parallax errors and tbh my poor eyesight, so I would mark the cut line with a knife - I in fact use an old Swann-Morton scalpel which is handy because I am left-handed but fairly ambidextrous and therefore can use the fine blade in either hand and get a pretty close line on the block.
I then set my chop saw to the appropriate angle - I have a cheapo Evolution one from Screwfix, and I lock the slide.
Knowing there is a degree of inaccuracy with my mark I then place the block against the saw fence, lower the blade so the guard retracts and make sure taking account as to which side is the waste I position a blade tip some 0.3mm off the mark on the block, raise the blade, power up and plunge through the block.
Then I take the cut piece and re-try against the hole and usually if I get it right it's oversized but by no more then a mm or two, then knowing that my chop saw has a tct tipped blade the body of which is 0.15mm thick and whose tips are 0.2mm wide. From this assuming zero runout of the blade if I place the block such that the face to be cut is just touching the body of the blade - that is not touching any tct tips that a subsequent plunge cut will take a shaving of 1/2 x (0.2-0.15) i.e. 0.025mm off the block!
In this way you can make pretty accurate repeat cuts as you move closer to the required size. If you need to remove a larger increment then you can achieve a larger cut by applying a small amount of pressure to the block when holding it against the blade body - i.e. you marginally deflect the blade, then after raising the blade and powering up you can plunge down and take a larger but equally controlled cut.
For my use-case where measuring a block size in-situ is difficult to perform accurately this technique allows one to pretty quickly creep up to the required size with little chance of cutting away too much.
I imagine this technique would be useful to others who use their chop saws for perhaps repeat cuts where an end stop doesn't work or is impractical for whatever reason.
I know one could make up a micro-adjustable end-stop to achieve the same level of accuracy however this just works and it's really quick once you get the hang of it!
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