Thread: Half-Blind Dovetail Bits

As far as I'm aware, there's no such thing as a uniquely "half blind" D/T bit. A dovetail is just that: governed by its shape & geometry.

Top (shank) bearings are occasionally used to guide a D/T bit through a jig's "fingers" as an alternative to a router-mounted guide bush. They're essentially interchangeable means of guidance.

The governing factors in how a machined dovetail's male & female parts....... erm...... "dovetail" is the bit's geometry, depth of cut & bearing or guide-bush determined offset.

There's other fairly comprehensive sources of quality D/T bits out there. Many of them particularly jig & even timber thickness specific. Titman Tip Tools & Trend Tooling are both likely (UK based) sources. The former exports both small & large orders, The latter is both locally & Home-country represented. Let Google be your friend in your search.

Of course, the other logical (& local) source is the excellent quality Oz-produced Carb I Tool range. Also available from a myriad of local & interstate sellers either by shopping locally or on the Interweb.

My own experience is limited to old-style Elu/DeWalt half-blind jigs, & more versatile, eccentric & idiosyncratic jigs from Akeda, Trend & Prazi (Chestmate), so please take any information that I impart with a grain of salt. Your own particular specific jig may very well differ radically from those I've used.

This is a half-blind Leigh router bit but has no bearing and which made me think it was made specially for a half-blind dovetail jig. Thanks for your reply Ratbag; good to hear from you.

https://www.carbatec.com.au/joinery-...l-cutter-14deg

Kidbee

On a slight tangent ...

How about teaching the kids to cut dovetails without a router jig? Not only will you save a bunch of money, but the kids will come away with a serious skill. And it is dead easy. Let me know if interested, and I'll link a couple of articles that show the short cuts.

Regards from Perth

Derek

Is the 818.142.11 on this list suitable as the blurb at the top says it comes with a bearing when a "b" is on the end of the P/No. Just the cutting length might be a bit short.

If you fit a guide bush to your router baseplate you can use a cutter with no bearing. The guide bush does the same job.

I agree with Derek regarding teaching them to cut by hand, perhaps you could consider splitting the class up? Those who are truly interested could cut by hand and those who are there to make up the numbers can cut by machine.

Originally Posted by Kidbee

Leigh has a half-blind router bit that claims to have a 8mm shank, which is odd, but it is a slightly different size and no bearing.

Leigh has pretty much standardised on 8 mm shank bits for their DT jigs.

Originally Posted by Chief Tiff

If you fit a guide bush to your router baseplate you can use a cutter with no bearing. The guide bush does the same job.

That’s how my Festool dovetail jig works.

Originally Posted by ian

Leigh has pretty much standardised on 8 mm shank bits for their DT jigs.

Do they supply a special Collett to fit the router? I have only ever used 1/4” and 1/2” bits.

The Leigh jigs that accommodate adjustable spacing use 8mm shanks or 1/2" shanks for the larger bits. An 8mm shank has a significantly greater shank cross section than a 1/4" shank and is hence more rigid and less prone to deflection or chatter when cutting. The bits are used with specific template guides that Leigh supply in the jig bundle or extra bit bundle, and they supply mounting adaptors to allow you to fit their template guides to specific brands and models of router. They also include a 1/2" to 8mm reducer with the jig bundle. 8mm is a non standard size for a collet in US based gear, but not uncommon in Europe. I have an 8mm collet that was supplied as part of the package for an Aldi router, and have purchased an 8mm collet and nut set for my Triton JOF from England.

Really for the jigs you have on hand, you need to know the tip and shank diameter, the overall diameter of the bearing, and the angle of the slope on the bit so that you try to match the bit from another source, just buying any dovetail bit and fitting a bearing or using it with a template guide would be relying a lot on luck to get a tight joint.

Originally Posted by Kidbee

Do they supply a special Collett to fit the router? I have only ever used 1/4” and 1/2” bits.

Most manufacturers of better quality 1/4" machines will also produce 8mm collets to fit these machines too.

The main advantage of 8mm bit shanks is stiffness. Much D/T jig cutting involves shear cutting of end grain in both tails & pins. This is hard on any machine & bit, with most jig manufacturers recommending powerful routers fitted with freshly sharpened bits.

As TCT bits always have "shallower" or "duller" sharpening geometry to the cutting edge, the router (& bit) must work harder than if fitted with HSS. The harder, more brittle (Wolfram Carbide tipped) cutters' edges last longer, but won't cut as well as the "sharper" HSS. Woodrat jigs, requiring at times extremely long endgrain cuts in hardwood timbers, recommend big 1/2" routers of 2000w+ & HSS cutters for this very reason.

The main advantage of 1/4" shanks is the ability to fit bushes or bearings to the smaller shank "above" the cutter. This eliminates the need for a guide bush, allowing far superior & safer through-base vision of the cutter at work & chip clearance whilst engaged in/on the fingers of a D/T jig.

Despite what others may claim, just about any guide bush guidance system can be substituted for a bearing guided cutter. Guide bushes, despite their inherent disadvantages of inferior vision & chip clearance (blocking airflow from the routers' blower), allow generally smoother progress as there's a smooth, even, fixed guide bush outer edge providing guidance rather than a frenetically spinning bearing outer ring, which can generate extreme heat & is always (particularly if abused or poorly maintained) subject to seizure & premature failure.