The plastic tsuba is mounted on a bokken with a file or knife.
Adjustment is necessary because the Bokken are partly machine made, but the hand finishing creates a one-of-a-kind piece. Therefore, even the Bokken of a production series differ slightly in shape and diameter from each other. A plastic tsuba, on the other hand, is always the same size. The opening is widened so that the tsuba fits the bokken. You can use a knife to do this, but using a file or dremel is much safer.
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Proceed as follows:
Slide the tsuba over the bokken as far as possible without much effort. If the force is excessive, unsightly dents will appear even on the hardest wood. Now the tsuba rests directly on the wood in only two places. You can easily find these spots by looking to the side or by shaking them slightly. Make a note of them or mark them with a permanent pen as shown in the following video. File these areas down and repeat the procedure until the tsuba can be slid into the grip area.
You now have a tsuba that fits your bokken perfectly. The tsuba is also protected with the tsubadome.