LumberJocks

Reply by JBrow

  • Advertise with us

Posted on Circle Jig help

View JBrow's profile

JBrow

1368 posts in 1528 days


#1 posted 05-07-2018 03:48 AM

Karda,

It is difficult to say what went wrong. Several factors occur to me that could affect accuracy, aside from an inaccurate measurement. The first two deal with geometry and the remaining with setup and technique.

The first could be from taking the radius measurement after the pin was installed and thus taken from the outside wall of the pivot pin. The radius measurement should be from the center of the pivot pin. If this is the problem then the radius of the disk would be larger by the radius of the pivot pin.

Second is that the point at which the blade begins cutting does not form a line that is perpendicular to the blade and intersecting the point at the center of the pivot pin. This could result in a radius longer than expected.

Third potential cause could be related to the blade. A ¾” wide blade cannot cut a 5” radius. Assuming a 5/8” or narrower blade, the blade could be flexing due to a dull blade, excessive feed rate, inadequate blade tension, and/or roller guides set too high above the workpiece.

Lastly, if the jig moved slightly during the cut, the radius could be off a little, but then the resulting disk would not be perfect round and would have been noticed.

One idea for correcting the jig without extensive reworking of the jig is to recut the kerf so that the pivot pin is 1/16” closer to the blade. This should result in the 10” diameter you are after. Moving the pivot pin closer to the blade by 1/16” would begin with clamping the jig to the band saw table in the position that resulted in a disk that was 10-1/8” diameter. If the jig has been repositioned since last used, the jig could be re-installed and a test cut made in some scrap and the resulting radius measured. In this case the thickness of the shim (below) would have to be adjusted to match the error.

Next a shim such as a long piece of scrap cut to a thickness of 1/16” can be held against the edge (opposite from the band saw column) of the band saw table while a fence (a straight piece of wood) is clamped to the underside of the jig so that the fence is snug against the shim and the shim is sandwiched tight between the edge of the table and the fence. With the fence clamped in place, it can be screwed to the jig. Once screwed in place the fence will be the thickness-of-the-shim away from the edge of the band saw table.

The jig can be un-clamped and removed from the band saw, and the shim set aside. The kerf in the jig can be re-cut by keeping the fence screwed to the underside of the jig firmly against the edge of the band saw table until the cutting edge of blade reaches the line from the center of the pivot pin. The fence can then be used to clamp the jig to the band saw table.

If the fence extends a few inches beyond the edge of the jig, engaging the fence and keeping it firmly against the edge of the band saw table may cutting the new kerf little easier.


DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics

HomeRefurbers.com