Undersized Plywood router bits

[William Chain]

I'll keep it short and sweet - am I better served picking up a set of undersized plywood bits (7/32", 11/32", 15/32" and 23/32") or the equivalent metric offerings (6mm, 8mm, 12mm, 18mm)? Just doing the math seems that the metric ones are closest to the actual dimension. Please advise me. I don't want to get into a brand debate, just advice on proper sizes.

[Nick Decker]

To me, the problem is that there is no undersized "standard" for the plywood itself, metric or imperial.

[William Chain]

The borgs seem to be busy selling imperial (or at least its labeled as such and seems to measure up to the quoted thickness), but everything I've gotten there has huge voids and few laminate layers. I started buying from a dedicated plywood supply house and everything they sell is routinely double the number of (thinner) laminates, zero voids, but is all metric. The greatly increased quality from the plywood supply for a trivial price difference keeps me going back there. Seems the metric bits would be a better match, can anyone speak to this experience?


To me, the problem is that there is no undersized "standard" for the plywood itself, metric or imperial.

[Charles Lent]

I bought a set about 10 years ago, but never used them because the plywood that I get never seems to be close enough to these bit sizes. I made a jig like the one in post #22 of this link https://www.routerforums.com/general-routing/60785-dado-jig-vs-exact-size-bit-3.html. You use a straight flush cutting bit with no bearing and guide the router base against the upper rails using the same under size straight bit every time that you used when first making the jig to trim the lower pieces. The jig can then be closed around the board to be installed in the dado and locked at that width. Then the jig is placed in position on the board to receive the dado and clamped in place. You then cut the dado by running the router against one side of the jig and then return against the other side of the jig. A perfect fitting dado no matter what the thickness of the board that will be installed into it with no math required in the setup

Charley.

[glenn bradley]

I bought a set about 10 years ago, but never used them because the plywood that I get never seems to be close enough to these bit sizes.

This was my experience as well. I have standardized on an undersized bit and a jig that matches the material with two passes as Charley describes.

391438

[George Bokros]

This is the only way to go. As has been said the thickness varies from supplier to supplier and even from the same supplier. The only ply I have found that is true sized in imperial is that with MDF core.

[William Chain]

Thanks Glenn and Charley, I'll have a look at this jig.



This was my experience as well. I have standardized on an undersized bit and a jig that matches the material with two passes as Charley describes.

391438

[Robert Engel]

I measured thicknesses of 3 plywoods sheets I recently bought at HD.

The varied from 18mm to just under 19mm and even different places on the same sheet were as much as .5mm off.

1/4" sheets ran 5mm, not 6, but were pretty uniform.

[Jim Becker]

I have a set from Whiteside and they have been convenient to have over time. But as others have said, the variability in sheet goods thickness is sometimes great...even on the same darn sheet, and especially for "cheap imports".

[Brian Holcombe]

Even really nice B.B. ply has some variation, I’ve found .005-.010” not uncommon, which seems petty until you plan around a measured size.

Most often I am using it as a substrate, so I will veneer it then plane it to an imperial thickness.

I take no issue with metric but most of my tooling is imperial so the odd bit if metric throws a wrench in the works.

[lowell holmes]

I think this gives you a reason to buy both sets. I see absolutely don't see why any self respecting woodworker
would not have both:)

[William Chain]

One expense at a time there chap.


I think this gives you a reason to buy both sets. I see absolutely don't see why any self respecting woodworker
would not have both:)

[Rich Engelhardt]

Re: both

Whiteside makes this conversion (https://www.whitesiderouterbits.com/collections/panel-stile-rail/products/6000x) if you wish to switch from 1/4" to 5.5 mm and back & want to stick close to some kind of budget.

I just bought the 5.5 mm "Shaker stile and rail set" instead of the conversion since I use a lot of 5.5 mm underlayment for jigs, fixtures & cabinet backs.

[Curt Harms]

This was my experience as well. I have standardized on an undersized bit and a jig that matches the material with two passes as Charley describes.

391438

I made a jig like that and use a 1/2 X 1/2 top bearing bit. The bearing runs against the jig's jaws. Set the width of the slot using the piece that's going to fit there, route and perfect fit. The downside is that jig won't cut a dado less than 1/2" wide.

[Rod Sheridan]

Plywood in North America has been made in metric thicknesses for decades, as the NA standard is metric.

The issue is that although metric bits would be the exact nominal size, the sanding allowance on panels would result in variations anyway..............Rod.

[Charles Lent]

I made a jig like that and use a 1/2 X 1/2 top bearing bit. The bearing runs against the jig's jaws. Set the width of the slot using the piece that's going to fit there, route and perfect fit. The downside is that jig won't cut a dado less than 1/2" wide.

That's the advantage of using the jig style that I posted. The edge of the router base rides on the guide and you can use a smaller bit with no bearing, but your first pass through the jig has to trim the edges of the lower level of the jig to match the distance between the bit and router base. Once that has been completed you are ready to make dados, but you must use that same bit and router every time you use the jig. If making very narrow dados as well as wider you may need to have two versions of the jig, one for narrow dados using a small bit and one for larger dados using a larger bit.

I always write on my jigs to indicate what tool and bit is needed to use with it, so I don't have problems a year or more later when I want to use the jig again. My jigs hang from ceiling hooks in my shop ceiling, so I can find the one that I want quickly, but I'm running out of ceiling space.

Charley.

[glenn bradley]

I made a jig like that and use a 1/2 X 1/2 top bearing bit. The bearing runs against the jig's jaws. Set the width of the slot using the piece that's going to fit there, route and perfect fit. The downside is that jig won't cut a dado less than 1/2" wide.

A bearing guided bit works fine. Mine runs a template collar. This allows for smaller bits to be used. I generally use a 3/8" spiral. It also gives me a lot of freedom on setting the depth or plunging multiple passes for deeper cuts while maintaining a constant guide/reference surface.

391521

The collar rides against the step, the bit cuts flush to the edge. Since this is a "version 2" jig (meaning I learned something from version 1 :)) I built it so that the flush edge can be replaced by gluing on an over-sized strip and re-cutting the "zero clearance" edge. I have done this once already due to an errant cut :( into that material.

[Curt Harms]

A bearing guided bit works fine. Mine runs a template collar. This allows for smaller bits to be used. I generally use a 3/8" spiral. It also gives me a lot of freedom on setting the depth or plunging multiple passes for deeper cuts while maintaining a constant guide/reference surface.

391521

The collar rides against the step, the bit cuts flush to the edge. Since this is a "version 2" jig (meaning I learned something from version 1 :)) I built it so that the flush edge can be replaced by gluing on an over-sized strip and re-cutting the "zero clearance" edge. I have done this once already due to an errant cut :( into that material.

Makes sense. And who hasn't made v2.0 or v3.0 of an original jig?