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Fine Adjustment Router Circle Jig

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CHJ

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Objective:
1. To make a Circle Cutting Jig for a router that provided for easier hole size correction.

Reason:
1. A need to produce holes to a closer tolerance than required in past projects.

Design criteria:
1. To provide some means of adjusting an attempted hole size by fractional amounts with some degree of certainty.
2. to ensure Router was as rigid as possible on jig without resorting to “screw through” fixing.

Materials used:
1. 6.25 mm Acrylic sheet. (beech ply or MDF would be alternates)
2. 3 pieces of Beech.
3. Existing 8mm Router fence Guide Rod and longer fixing screw.
4. 6 small wood screws
5. 6mm wing nuts, washers & cabinet screws, (csk head screws would be fine)
6. 8mm studding with associated plain , nylock , wing nuts & washers.
7. 4mm screw and nut (if needed) for pivot point.


DSC00389.jpg

Typical jig used in past, not easy to adjust to tight tolerances without a lot of trial and error.

DSC00385.jpg

Component parts sized to suit small Bosch Router.

DSC00386.jpg

The only fiddly bit of assembling router to the Jig. The need to ‘jack’ the router along the thread with the locating/adjusting nuts in place.
Not to onerous once a technique is developed. (note the spanner holding front nut clear of pillar whilst turning studding with end wing nut)

DSC00388.jpg

The finished Jig with router fitted.

Holes sizes covered (using 10mm cutter in picture) 40mm to 340mm.
Smallest disc cut is 20mm. (using 10mm cutter)
One flat on the adjusting nut is approx 0.2 mm. on Radius (0.4mm on diameter)
(8 MM Thread has pitch of 1.25mm / 6 = 0.208)

Method of use and observations:
Set intended hole size as normal using callipers on pivot stud and cutter.
Cut a test hole to partial thickness in scrap material.
Adjust locking nuts to increase/decrease hole size, (remember to release & lock off router thumb screw rod clamps before & after adjustments)
Retry cut and check dimension as required.


Some points on construction to note:
The use of a wing nut on the studding as an adjuster nut was necessitated by insufficient clearance on the router base plate for a plain nut; it proved a blessing in the end as it acts as a self locating jack nut.
The hole(s) for the pivot bolt are counter bored, one above & one below base plate to accommodate a nut if required and an alternate location for the bolt whilst under Router base. (I needed to bolt jig to vertical surface for one job)
The wooden strip along the length of the jig is positioned such that it is a firm fit against the side of the router, thus taking up the slack in the guide rod holes and adding to the overall rigidity.
The guide location holes in the Beech end pieces were marked by passing a pointed 8mm rod though the router guide holes and marking the wood strips placed alongside, the end pieces were subsequently planed along their base to achieve a good fit between the router and the jig base.
The offset position of the two removable bolts on the beech end piece is deliberate; this avoids any chance of miss assembly. (never try to align holes symmetrically on a jig that has to be disassembled for use, s*d’s law says it will fit the wrong way round at some point.) besides it saves having to measure accurately!
The use of a nylock nut as an anchor for the studding may cause some problems getting it to this location (no locking left if threaded from far end, and difficult to thread on in reverse direction) use two plain nuts instead.

A tip that might prove useful.
If you have set this or any jig up to produce a hole or disc size that may be needed in future, cut yourself a template in a piece of scrap material at the same time for subsequent use with a Bearing guided cutter.
 

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Aragorn

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Excellent jig, and useful style of write-up.
BTW - Why do you tend to need such accurately sized circles?
 

DaveL

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Chas,

Good looking jig you have made there, I have used a bought one for all sorts of jobs. :D I should have done something like yours but got lured into buying one. :oops:

Oh expect Tony to be along with his own improved version soon. :wink:
 

CHJ

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Aragorn":2ubia9ty said:
Excellent jig, and useful style of write-up.
BTW - Why do you tend to need such accurately sized circles?
In one instance to produce near airtight fit of pipe work through a bulkhead.

In another instance it was to make access holes and associated tight fitting blanks. (In Acrylic)

And in a current project where I am producing the equivalent of a router mounting plate with a set of matching adaptor rings.

More an engineering requirement than a craftsmans woodwork need.
 

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