CHAPTER 10 Half-Blind Dovetail Procedures

Half-Blind DovetailProcedures

CHAPTER 10

Chapter Foreword In these instructions for using the LeighDovetail Jig, we have recommended usingcertain cutters and board sizes just becausethey are easy to work with. When you havecut some practice joints and gained con-fidence in your ability to get the results youwant, use the half-blind cutter selectionchart on page 160 to plan the dovetail rout-ing you need for your own projects.Note: Only the 7⁄16" OD guidebushes andthe four cutters listed opposite may beused for half-blind dovetails. See “HalfBlind Cutter Selection” on page 160 fora full description on how to select theappropriate cutter.

IMPORTANT!The most misunderstood aspect of rout-ing half-blind dovetails is how the cutterangle affects the Depth of Cut setting.Theoretically, there is only one “perfect”depth of cut for a given cutter that willproduce properly mating Pins and Sockets.Therefore, raising or lowering the cutterduring test cuts is done only to achievethat one “perfect depth” that is determinedby angle of the selected cutter. Any otherdepth will produce a joint that is too looseor too tight.Please read this carefully and note the spec-ified Depth of Cut for the different cut-ter angles shown.

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10-1 Half-blind Dovetails:➀ Pins➁ Pin sockets➂ Half-pins➃ Half-pin sockets➄ Tails➅ Tail Sockets The pins fit in the pin sockets. Joints almostalways end each side with half-pins.

10-2 Important! Read This About Depth of Cut.• Depth of cut must be as specified when using any of the four cutters illustrated above.

• Raising the cutter above its specified cutting depth will result in loose joints and may dam-age the jig, cutter and/or guidebush. A lower setting will result in tighter joints that may notfit together.

• Minor Depth of Cut adjustment will change the tightness of joint fit. See 10-14 to 10-16to find out why.

• Half-blind PINS and TAILS are routed with the same dovetail cutter and must be at thesame Depth of Cut.

• Choose one of the four 1⁄2" [12,7 mm] diameter dovetail cutters shown above, and check cut-ter selection on page 160.

• Use only 7⁄16" [11,1 mm] outside diameter guidebushes.

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10-3Let’s cut a single test joint. You will need onerouter, a 7⁄16"[11,1mm] guidebush, eithera No.120-1⁄2"[12,7mm] 14° dovetail cut-ter or a No.101-1⁄2"[12,7mm] 10° dovetailcutter or the 8mm shank equivalent. The 80series cutters cut too deep for 3⁄4"[20mm]boards (see the HB cutter selection charts onpage 161). You will also need several piecesof 3⁄4"x5 1⁄2"[20x140mm] x8"[200mm] orso long, and some 1⁄ 4"[6,35mm] x 1⁄ 2"[12,7mm] hardwood strip for bridge pieces.

10-4Clamp the spacer board in the rear clamp.

10-5Mount the finger assembly on the supportbrackets in the HHB TAILS mode, flaton the spacer board, scales set on the thick-ness of the tail board (3⁄4"[20mm] in thisinstance).The HHB TAILS scale is always set atthe tail board thickness.(The scale increments above 1-11⁄2 "[25-38mm] are for use on sliding dovetails - seechapter 14.)

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10-6Measure and mark a line on the inside faceof the tail board ➀ to the working depthof the cutter to be used as per 10-2 thischapter .Clamp this test tail board in theleft front clamp, against the side stop withthe top edge flush under the guidefin-gers, and the inside face i of the drawerside away from the jig.

10-7Unlock and raise the finger assembly sup-port brackets slightly so that the fingerassembly is about 1⁄16"[2mm] ➀ above theboards. This will allow easy movement ofthe guidefingers.

10-8The following joint design is only a sug-gestion for this trial. It has a typical and tra-ditional even layout of pins, with half-pinsat each edge. The Leigh jig, however, allowsfor an infinite variety of joint designs, andboards of different thicknesses can be joinedto each other as shown in this illustra-tion. Before attempting joints of asym-metrical design, please see chapter 13.

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10-9Ignoring the extreme outer guidefinger nextto the scale (which just supports the router),loosen enough of the adjacent guides togive the required tail layout. The half-pinguidefinger position illustrated will give ahalf-pin socket profile like that shown (dot-ted lines).

10-10If the gaps between the guidefinger tailsare wider than about 1⁄8"[3mm], mark offand cut some pieces of hardwood bridgestrip ➀ to fit into the slots in the ends ofthe guidefinger tails. Make sure the 1⁄4"[6,35mm] dimension ➁ on the strip is snug,so the bridge pieces will not fall out. Be sureto make them slightly shorter than the dis-tance between the guidefinger shoulders,so the guidefingers will not be forced apartwhen you put in the bridge pieces.

10-11Remember to tighten any loose guidefin-gers. Lower the finger assembly back ontothe spacer board and workpiece. It musttouch the workpiece or the depth of cutwill vary and the joint won’t fit. The scaleshould be set on the tailboard thickness, inthis case 3⁄4"[20mm].

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10-12Attach the 7⁄16"[11,1mm]guidebush to therouter securely.

10-13Mount either the No.120 or No.101 (orthe 8mm shank equivalent) cutter to therouter.

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10-14 Adjusting Joint TightnessHere’s why the depth of cut ➀ changes thefit in half-blind dovetails. Increasing ordecreasing the depth of cut does not affectthe pin socket width ➁, but does affect thewidth of the pin ➂ that goes into the sock-et ➁.

10-15Note that decreasing the cutter depth ➀makes the pin ➂ narrower while the pinsocket ➁ stays the same width, producinga loose fit.

Decreasing the cutter depth (i.e. raise thecutter into the router) produces a looserfit.

10-16Increasing the cutter depth ➀ makes thepin ➂ larger while the pin socket ➁ staysthe same width, producing too tight a fit.

Increasing the cutter depth (i.e. lower thecutter) produces a tighter fit.

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10-17Remember to follow all safety precautionswhen routing.

10-18Adjust the cutter height until the cutter tipis level with the marked line ➀. For the firstlight cut move the router from right to left.Make sure you control it firmly, becauseit is driven in this direction by the cut-ter. Only the tip of the cutter should becutting on the first cut ➁. This back orclimb routing leaves a very clean shoulderin side grain.

10-19Now rout in and out from left to right fol-lowing the guides and bridge pieces to routout the pin sockets, leaving the tails.

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10-20Remove the test tail board, then clamp ascrap board in the front of the jig so thatthe top edge projects above the top faceof the jig by about 1⁄8"[3mm]➀. This willkeep the scrap piece below the path of thecutter when routing the pin board.

10-21Remove the spacer board from the rearclamp. Place a test pin board in the left rearclamp against the side stop, fitting its frontend edge flush against the vertical scrappiece, with the inside face i of the draw-er front away from the jig body. The pinboard is now positioned with the edge tobe routed flush with the jig’s front face, cor-rectly registered for the scale readings.

10-22Rotate the finger assembly to the hHBPINS mode and set the HB PINS scaleequal to the tail board thickness (i.e., the samesetting as for the tails: in this example,3⁄4"[20mm]).Both HB pins and HB tail scales are alwaysset to the tail board thickness.

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10-23Make sure the finger assembly is flush andlevel on top of the pin board.The guidefingers must be touching the pinboards or the depth of cut will vary andcause poor joint fit.

10-24If you have difficulty leveling the fingerassembly on a narrow workpiece, place aboard the same thickness as the pin boardunder the other end of the finger assembly,but not in the rear clamp.

10-25Rout out the waste between the pins. Routeach space from left to right. Do not backrout on end grain. If the cutter enters onthe right side of the opening there will bea very strong pull to the left, so...

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10-26Rout each opening in at least three or fourpasses, left to right.

10-27Remove the pin board and test the joint forfit. If the joint is loose, as shown here, youneed to lower the cutter by the same amountas the gap at the bottom of the pins ➀ (whenthe pins are pulled against the socket sides).If the joint is too tight, raise the cutter slight-ly. Test again. You cannot rout the sameboard twice with a dovetail cutter, so usetwo fresh board ends for each test.

10-28Keep the test tail board that fits well, andmark it with the number of the cutteryou used to rout it. For quick set-up nexttime, clamp this tail board in the jig as adepth-of-cut gauge ➀ to show how far tolower the cutter.

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10-29When you have the proper tightness of fit,check the flushness. The tails should beunder flush to the pins by no more than1⁄64"[0,3mm] to allow for cleanup. The con-centricity of the collet and guidebush ondifferent routers will affect this tolerance.

10-30If the tails stand out from the pins, setthe h HB PINS scale away from theoperator by the amount required.

10-31If the tails fit in too far past the pins ends,set the h HB PINS scale toward theoperator by the amount required. Theseadjustments for “flushness” are made onlyin the hHB PINS mode.

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10-32To make a box, rout all four ends of the tailboards, keeping the inside face i of the tailboards away from the jig.

10-33Rout all four ends of the pin boards keep-ing the inside face i of the boards awayfrom the jig. Note: When making drawers you may preferto use through dovetails on the rear corners.

10-34 End-on-End DovetailsWhile you have the router set up and thecut depth correctly adjusted for half-blinddovetails, it is a good time to try end-on-end dovetails. These are made in the samemodes as half-blind joints, but the boardsare both routed vertically in the front clamp.End-on-end joints may be up to 3⁄4"[20mm]thick. (See page 95 for end-on-end dove-tail procedures.)



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Documents

CHAPTER 10 Half-Blind Dovetail Procedures