How to make your Perfect Dovetails - Spacer Method

A perfect dovetail?

The picture shows some dovetails made using the "Perfect Dovetails" method. This is actually the joint at the front of a drawer from a chest of drawers. All the drawers were made using the technique, which proved to both more accurate and quicker than traditional handcut methods.

This page describes the 'spacer' method. Please note that this method will not work unless your boards are exactly the same width with exactly square ends and parallel sides. However, with that proviso, it works pefectly well (and the joint shown here was made using it).

The materials required for this are very simple:

  • Some scrap hardwood and/or MDF.
  • Some coloured masking tape
  • ...and, of course the wood for your joints

For tools, you need:

  • A bandsaw that tilts both ways (if yours doesn't tilt, you can make a jig - see Lonnie Bird's book or ‚Äčthis website).
  • Simple hand tools, e.g. a plane, some sandpaper, a drill
  • An accurate method of measuring - ideally including a vernier gauge
  • Cutting gauge or wheel gauge (better than an ordinary marking gauge for cross-grain marking)
  • Chisels (size depends on joint size), files / rasps, coping saw or fret saw (to remove waste)
  • Router and table for lapped dovetails only (optional - otherwise some hand cutting is required)

The instructions provided here take you through the entire process of creating your perfect dovetails. It is recommended that you read these through completely before starting so that you have a good idea how it all works. After that, you can dip in and out as you need to. There is also a simplified instructions sheet to keep handy in the workshop. The general approach is illustrated by the picture of the bandsaw cutting the pins with the bandsaw table tilted to the left. Note the labelling of the spacers and the board.

Cutting pins

Before you start

Accuracy is the secret to great dovetails, whatever method you use. The "perfect dovetails" method relies on two important things before you start:

  • an accurate cut from the bandsaw
  • accurately dimensioned boards

As regards the bandsaw, any decent quality bandsaw should be up to the task so long as it

  • has a two-way tilting table,
  • can be set with the fence either side and
  • has been properly set up and maintained
  • and
  • has a good sharp blade in it.
With short cuts in fairly thin pieces of timber, such as for dovetails, blade wander is not such a problem as it can be for deeper and longer cuts, but nevertheless you do want to be as accurate as possible and a smooth cut is important, so use a new blade with, say, 4tpi for larger boards or 6tpi for smaller ones.

As regards the boards to be joined, when using the spacer method, they must be exactly the same width and each board must be of an even thickness. (The importance of having exactly equal width boards cannot be underestimated. A 0.5mm difference between the board widths will translate to a 0.5mm error in the relative sizes of pins and tails. This is because the spacer method references off both edges of the boards). Fine tune the widths to within 0.2mm of each other by either planing or building up an edge with strips of tape. The edges should all be square and smooth. Don't forget to make the boards fractionally longer than the finished length to allow for trimming the joint after it is complete (a typical trim allowance might be 0.5mm in which case allow an extra 1mm length if the board is jointed at both ends). Also, if you are making multiple joints then make sure that similar boards are exactly the same dimensions as each other. But you would do all that normally anyway, right?

Having got the basics right, you now need to know exactly the dimensions of your boards. I advise working in millimeters throughout as they are a useful scale for most dovetail joints and the design tool takes decimal inputs, not fractions, but if you want to work in cubits, feel free.

You also need to know the kerf of your bandsaw blade. This is quite critical, so it is worth measuring it accurately as follows:

  • Make a board from scrap timber that has smooth and parallel sides and is 4-6" (100-150mm) wide. Measure this width accurately (to the nearest 0.2mm) using a vernier gauge. (See picture)
  • Make 4 or more lengthways cuts - i.e. cutting into 5 or more pieces - as straight and cleanly as possible.
  • If the sawn edges are rough you can clean up them up minimally to remove the worst of the sawn roughness (e.g frayed edges which prevent the pieces fitting back together), but don't go overboard. It is better to use a really good blade which leaves a clean cut.
  • Now put the pieces back together and clamp them well, then re-measure the width. The clamping of pieces is to simulate the tightness of fit of the required joint, so no gaps should be apparent.
  • Subtract the new width from the old, divide this difference by 4, or however many cuts you made and that is your saw kerf. (See picture)

Finally, before you start the design tool, label the edges of your boards "A " and "B " so that they all match. For drawers, I normally label the top edge "A ". (see picture above).

Kerf testing - before cut
Kerf testing - after cut

Design the dovetails and calculate the spacers

Now it's time to plan your perfect dovetails and see what they will look like. Put all your measurements into the "design tool" calculator form, namely:

  • The saw kerf you calculated earlier
  • The width of your two boards (which should be the same)
  • The thickness of each board (which may be different)

You need to decide how many tails you want, but you can always change this if you want to. You also need to decide the dovetail angle to use. I recommend using a whole number of degrees (which makes it easier to see on your bandsaw table tilt gauge). 8 degrees is about 1:7 which is a good all-purpose angle. Put these details into the design tool calculator. By the way, it is perfectly possible to use a dovetail angle of zero to make a finger joint, but please note the comments in the "Tweaks" section.

Now design and enter the pin widths. The dimensions in each case are the widths at the base of the pins (the widest part). It makes life easier if you set this to be just a fraction more than the width of one of your chisels, so that chopping out the waste between the tails is simpler (i.e. the width of a chisel plus a saw kerf is good). One nice thing about the "perfect dovetails " method is that you can make narrow pins in the traditional style, without being constrained by router shanks. Make them as wide or narrow as you like, you can always change it in the design stage. Of course, they will need to be wide enough to allow the tails to be cut (i.e so that the narrow end of the pins is a bit more than the saw kerf), but the calculator will let you know if you go too narrow.

You also have to specify the widths of the half-pins at each edge of the board. These will normally be a bit wider as you don't want them to break off when fitting (they have nothing the other side to support them). You may also need one of the half-pins to accommodate a groove for a drawer bottom. Talking of drawers, you may want to have the back of the drawer a different width from the sides (to accommodate the bottom of the drawer running underneath the back, say, and/or to reduce the top of the back to allow for easy insertion of the drawer). You can do this by specifying a cut-away for the pin board on either or both edges. If you use my convention of the top being side "A" and the bottom being side "B", then you will, for example, set the "Cut-away on Edge 'B'" to be the distance from the bottom of the drawer sides to the top of the groove in them which takes the bottom piece. (Note that either the pin board or the tail board could be cut down after making the joint, but it is usually the pin board that is affected).

Finally enter the "trim allowances ".

  • The "end trim allowance" (when positive) allows the tails to protrude slightly so that they can be neatly trimmed. If you are making a lapped dovetail joint (see later notes), then enter a negative end trim allowance equal to the amount of the covering thickness required on the pin board.
  • The "fitting allowance" allows you to trim the cut sides of the tails and pins slightly without causing gaps, giving a much more accurate finish to the joint. For "workmanlike" rather than "show" joints, a fitting allowance of zero may be used and the joint should fit straight from the saw. Otherwise, 0.3mm will give an excellent finish without adding too much work (this equates to removing 0.15mm from each side).

Now click "submit " and the calculator will draw your joint for you as well as calculate the spacers that you will need. You may get a number of warning messages if the calculator doesn't quite like your specification. Hopefully these are self explanatory. If it can draw the joint then it will do so and this might help you to understand any error messages and how to fix them.

Don't forget to put the name of your joint in the box at the top of the calculator. If you are logged in then all the details will be saved. Otherwise they will not be saved and you will need to print them before navigating away from the page.

All about the spacers

Look at the "spacer widths " table in the calculator. You will see that there are two columns, labelled "Edge A to fence " and "Edge B to fence ". If the joint is symmetrical (i.e. the half-pins on each edge are the same width) then these columns will be identical, otherwise the last spacer will differ. In either case, there are the same number of spacers as there are tails.

You will need to cut spacers of the specified width, ideally from a stable hardwood or MDF. You will also need a jig for making the tails, cut from a similar wood.

To cut the jig use the pattern illustrated in the picture - make it about 10" (250mm) long and with a diagonal section at the angle you have chosen for your tails and the "notch" at right angles to this. The dimensions are not critical, but the angle (and the right-angled notch) does need to be accurate.

Jig

To make the spacers:

  • Make them from a similar thickness board as the jig and as long as the cut-out diagonal section of the jig.
  • Cut them as accurately as you can (a table saw with a fine blade is best, but otherwise finish with a plane and abrasive paper stuck on a flat surface). In particular, ensure that the edges are smooth and parallel. If the widths are very slightly off then this will result only in slightly different sized tails - the joint will fit so long as the spacers are numbered and always used in the same order.
  • Except for the last spacer, fix a "cap" to the top of each spacer as shown. It is easiest to fix one cap piece across them all and then cut to separate - see picture.
Caps
  • Write the number on each spacer - it is important to use them always in the same order, in case of slight differences in width.
  • The last spacer (or pair of spacers if the edges are different) does not have a cap, but has a notch cut at he same angle as the tails. There are two ways (at least) of doing this.
    1. Make a small "filler piece" (length specified by calculator). Place this in the notch of the jig and place the spacer behind it (see picture), adjusting the fence so that the blade just brushes the corner of the spacer. Now remove the filler piece, move the spacer into the notch of the jig and cut the notch in the spacer.
  • Filler
    Notch2
    1. Or: Cut the notch in the corner of the spacer in a similar fashion but without using the filler piece. Now drill a 3.5mm hole in the top end of the spacer and insert an M4 bolt (waxed) and screw in until it protrudes by the amount specified for the length of the filler piece (see picture). The advantage of this method is that it allows for small changes to improve the fit and also means that the spacer can be used for other projects (which might require a different sized filler piece). Do make sure that the bolt is close enough to the inside (flat) edge of the spacer to fit behind the cap on the adjacent spacer.
    End spacer with bolt for adjustment
  • Label the end spacer(s). If there are a pair of different spacers label them clearly "Edge A towards fence" or "Edge B towards fence".
  • Write the width on the bottom ends of all spacers, so that they can be re-used in other projects if required.
  • Prepare the boards

    If you haven't already done so, label the boards clearly on the face side (outside of the box or drawer). When using the spacer method, the face side will be uppermost throughout the bandsaw work, for both pins and tails. Also make sure that the edges are labelled "A" and "B" so that you can keep them aligned as you work (and so that you use the right spacers, if the end spacers are different). I find it best to put labels on masking tape as it is clearer and does not mark the wood.

    Double-check that the boards are exactly the same width and that this is the same width that you have entered into the design tool. Also double-check that the edges are parallel. (Measure twice, cut once!)

    Mark the shoulders on each board, using a cutting gauge. For the tail boards, the width of the shoulder must be the thickness of the pin board plus the trim allowance as specified in the calculator. For the pin boards, the trim allowance is up to you. Mark the pin board on both sides and mark the tail board all the way round. If you put coloured masking tape on before marking the line, then peel it away, you will have a clear edge to cut to (see picture).

    Masking tape cut for shoulder line

    Do a test

    Now, particularly if this is the first time you have used the "Perfect Dovetails" method, I advise that you make a test joint. Use two short pieces of wood the same thickness and width as your real boards and work through the whole joint-making process. (Label the board edges in the same way as the real boards). It's a good idea not to remove all the waste, particularly at the edges of the tail board, since then you can use your test pieces as a template to easily check your fence setting when you cut the real boards. (see picture)

    Test joint

    Note that on the test pieces in the picture I have done two joints - one for the front and one for the back of a drawer. The joint can be test fitted as shown without fully removing the waste on the tail board edges.

    Cutting the pins

    Now at last you get to make the joint. We will cut the pins first. Place the bandsaw fence to the left of the blade and tilt the table anti-clockwise (when viewed from your normal position - i.e. fence lowest) by 8 degrees (or whatever angle you have chosen). Place all the spacers against the fence (leave out the angled jig - that's only for cutting the tails). Make sure the spacers are in the right sequence - lowest nearest the fence. They should all fit neatly in place, caps overlapping. The last (end) spacer has no cap and should be placed so that the notch is not next to the blade. If there are two types of end spacer, because the jont is asymmetric, then use the correct one - i.e. "Edge A against fence" if the board you are going to cut will have edge A nearest the fence (remember that you will be cutting the pin board with its face - labelled - side uppermost). Now adjust the fence so that the blade just touches the end spacer (a good test is to run the blade in the wrong direction, by hand - if it lifts up the spacer then it is too close). See picture. Remove the end spacer.

    Setting the fence to cut the pins

    Place the pin board, face side uppermost, with the correct side nearest the fence, into the notch formed by the cap of the spacer (made available after removing the end spacer). It is a good idea to leave the end spacer, which you just removed, on the bandsaw table to remind you which edge should be nearest the fence. Also, if you made a test joint, you can use this to check that the cut will be in the right place. Now slide the whole assembly forward (see picture), cutting down to just meet the shoulder line (you can use a stop if you wish, but I tend to do it by eye). Remove the outer spacer and repeat for the next cut. Repeat until the last cut is with no spacers.

    Cutting the pins

    Next, move the fence to the right of the blade and tilt the table the other way (clockwise) by your chosen angle (again the fence will be lowest). Place the spacers as before (counting the numbers from the fence) except that your end spacer may be different because now the other edge of the board will be nearest the blade. Adjust the fence, remove the end spacer and proceed as before (test as necessary with your test piece).

    Cutting the tails

    To cut the tails, place the table level and leave the fence on the right (assuming that you just cut the pins, that's where it will be). Place your angle jig against the fence and place the spacers (in sequence) against it. Use the appropriate end spacer (for whichever side of the tail board will be nearest the fence) but this time place it so that the notch is next to the blade. Adjust the fence so that the blade just touches the notch, but does not rub against it (this is to ensure that you do not make the joint too loose). See picture (move the assembly forward to ensure the notch does not bang the blade).

    Setting up the tails

    Remove the end spacer and cut the tails similarly to the pins, removing one spacer at a time. Always slide the whole assembly - in particular, do not just slide the board back after cutting as it will jam and possibly damage the tail.

    Cutting the tails

    Move the fence to the left of the blade and repeat the procedure (but with the other end spacer, if it is different).

    Multiple joints

    This section describes how to streamline the process when making, for example, a set of drawers. This makes the production of set of drawers a relatively quick process - and all with "Perfect Dovetails". Reading this section is optional, particularly first time around. It is assumed that you know how to cut the pins and tails as described in the earlier sections.

    Let's assume you are making a set of drawers. You will probably have two types of joints - one for the fronts and one for the backs, so you will need to do two calculation sheets. You will therfore also have two sets of spacers (although you may have some spacers the same size which are shared between the sets - see the tweaks section on how to achieve this).

    First cut the pins. With fence left and table tilted anti-clockwise at the dovetail angle:

    1. Cut all the back pin boards using the "back" spacer set (remembering to use the correct end spacer depending on which edge is nearest the fence).
    2. Cut all the front pin boards using the "front" spacer set.
    With fence right and table tilted clockwise at the dovetail angle, repeat as above (but with the other end spacers, as appropriate).

    Now cut the tails. With fence right and table level:

    1. Cut (one side of) the back tails of all the drawer RHSs using the "back" spacer set (this might be, say, edge A against the fence).
    2. Cut (one side of) the front tails of all the drawer RHSs using the "front" spacer set (this would then be edge B against the fence).
    3. Repeat for the drawer LHSs, but note that the spacers for the other edge will be needed in each case.
    With fence left and table level:
    1. Cut the back tails of all the drawer RHSs using the "back" spacer set (this might be, say, edge B against the fence).
    2. Cut the front tails of all the drawer RHSs using the "front" spacer set (this would then be edge A against the fence).
    3. Repeat for the drawer LHSs, but note that the spacers for the other edge will be needed in each case.

    You need a certain amount of presence of mind to do this. Avoid distractions. Label the ends of the boards "Front" and "Back", depending on the type of joint. Then just remember to check before you cut that (a) you are using the corrrect spacer set and (b) you have used the correct end spacer, depending on which edge of the board will be nearest the fence. It is a good idea to double check with your test piece before cutting.

    Removing waste and fitting the joints

    Mark all the waste before cutting. The space between pins can be mostly cut out with a bandsaw, if you wish, but do cut with the face side down so that you do not inadvertently cut the pins themselves. Remove the remaining waste with a coping saw or fretsaw, then with a chisel, as for hand-cut dovetails. See here for an excellent guide on removing waste.

    Test fit the joint. It will probably be a little tight in places, so carefully trim with a sharp chisel as needed. Again, there are plenty of guides available on how best to do this - for example this one.

    Drawers, and boards of unequal width

    Typically drawer backs will be narrower than the sides, so that the bottom, running in grooves in the sides, can go underneath the back. Also the top of the back may be lower than the sides by a few mm, with the back end of the sides feathered to match. This section describes how to produce these, including cutting out the waste correctly.

    1. As described in the opening section, start with the boards the same size. If this is not possible, make up the narrow boards to the correct width using strips of wood attached with glue or double-sided tape (they will be removed later).
    2. Enter the amount of the cut-out for each edge in the calculator. Make sure that the bottom half pin is wide enough to include the groove. The groove will need to fall within a tail at the front, so do a calculator for that too and check that it will work.
    3. Make all the cuts as usual.
    4. Do not remove the waste at the bottom edge of the drawer sides (tail boards).
    5. Cut the groove on the insides of the drawer sides, using your router, plough plane or whatever. N.B. if the ends of the front tails are to be exposed, make sure that the groove is stopped short of the front.
    6. Cut an exactly similar groove in the back pin board. (You will need a similar groove in the front board too, so you may as well cut that at the same time).
    7. On your bandsaw, cut the bottom off the back pin board, using the top edge of the groove as a guide - i.e. cutting just to the waste side of the top of the groove.
    8. Use the top of the groove to reference a mark around the shoulders of back end of the tail board (i.e. the back end of the drawer side).
    9. Mark the waste and remove. The resulting half pin on the back should then locate above a "finger" containing the groove on the side.
    10. The top of the back pin board is simply trimmed as required and the sides feathered to match.

    Lapped dovetails

    The method described above is only designed to produce through dovetails that are, to all intents and purposes like the hand-cut ones, just quicker and more consistent. If you are making drawers then you may not want the dovetails to show on the face (on the other hand you may be very happy to display your "Perfect Dovetails" in true "Arts and Crafts" style). In this case there are three alternative options:

    1. Make an overlay drawer, using a separate overlay board on the front; or
    2. Rebate the edges of the drawer front by at least as much as the thickness of the sides and add a cockbead to give a flush drawer; or
    3. Use lapped dovetails on the front.

    If you are determined to use lapped dovetails then:

    1. Plan the dovetails as usual, but enter a negative trim tolerance in the spreadsheet, equal to the thickness to be left in the front board.
    2. Cut the tails on the bandsaw as per instructions.
    3. If you do not have a router with a table, then mark out the pins either:
      1. in the traditional way, by laying the tail board on the pin board and marking with a knife; or
      2. using the spacers as if you were making a through dovetail, but only making a very shallow cut to act as a guide for a dovetail saw.

        If using method (b), do not cut deeper than the trim tolerance you have allowed for the pins. Remember, this can be any amount you want - it does not need to be the same as the tails trim tolerance, which in this case is negative. So you could plan to trim 1mm from each end of a pin board, giving ample to make a clear guiding cut. Also use a dovetail saw (in step 4 below) with a kerf as close as possible to, but no wider than, the bandsaw kerf.

      3. Cut out the waste in the traditional way. Don't forget to mark the top edges of the tails first and do not cut above this line, especially if using method (b) above since the guiding cuts will extend above it

    Cutting lapped dovetails with a router (jig-free!)

    It is much quicker if you have a router (with the right bit) and table; the best way is:
    1. Using a piece of scrap the same width as the real pin board and at least as thick as the tails, cut the pins on the bandsaw as for a "through" dovetail joint. The tops of the pins may get cut off, but don't worry (so long as they are as deep as the lapped joint requires). Check the fit - you can clean up a little, but you want it to be tight.
    2. Now use your scrap piece to set up your router bit and table. Use a moveable fence square to the main fence. Set the main fence to the required length of cut and set the bit to the required depth.
    3. Set the moveable square fence to the right of the bit (to ensure that the bit rotation acts against the fence rather than away from it) and place your "test" piece next to it, adjusting so that the bit just touches the inside of the required pin; clamp it in place using a fence stop to secure against the force of the rotating bit (see picture).
    4. Check that the bit does not quite cut the pin on the scrap board, then rout your real pin board. I also suggest you check the depth and length of the cut on your scrap before routing the main piece. I adjust the length of the cut so that it will undercut slightly at the back of the joint but stop fractionally short on the inside fence, leaving a simple clean-up with a chisel
    5. Repeat this process, setting the bit against each side of the "test" pins, then cutting the real piece. If the tails are wide, you will also need to make cuts in the centre between pins.
    6. Cut back to the shoulder and clean out the corners (or round over the insides of the tails, according to your preference).
    Using a test piece to set up lapped dovetails

    Fancy tails, re-using spacers and other tweaks

    Once you are comfortable with using the "Perfect Dovetails" method and the "Design Tool", you will realise that you can start to tweak the results to get some interesting and useful results. Before discussing the main types of tweak, it is worth mentioning what happens if you change the dovetail angle. You can do this quite easily by altering the value in the calculator. It is easy to see how the new angle looks, so that you can choose an angle which is aesthetically pleasing (but bear in mind the performance of the wood). You can also put in an angle of zero to make a finger joint. However, note that if you do this, there is an important change to the spacers: The last spacer does not require a notch and no angle jig is required. However, when cutting the tails, you need to add a thin shim (thickness equal to the saw kerf plus the fitting allowance) in place of the angle jig.

    There are two possible types of tweaks you can apply:

    1. Modify the spacer widths. This is useful if you wish to re-use spacers from another joint. Bear in mind that this will change your pin widths.
    2. Change the sizes of individual tails (and therefore the corresponding pins). This allows you to make a huge variety of decorative joints. If you are doing lapped dovetails, you can make the base of the tails just a fraction wider than a forstner bit - to make removing the waste between the pins easier.
    In each case the tweaks are applied interactively in the calculator and you can see how the joint will look.

    Modifying the spacer widths: Click on "Tweak spacer widths" at the top of the table of spacer widths. Note that this only lets you change the widths of the main group of spacers and then the end spacers separately, so that the tails will still all be the same size. Put the new sizes into the form. When you click "Apply", the main input form will be updated for the new pin sizes as well as updating the spacer widths and the diagram of the joint. If you click "Clear" then the tweak form will be cleared, but the input form is unchanged, so there is no "Undo" function - you will need to re-input the original pin sizes or spacer widths. Use the diagram to check your tweaks make sense (if they are really daft you will get error or warning messages).

    Modifying the tail sizes: Click on "Tweak tail sizes" at the top of the table of pin and tail dimensions. This operates slightly differently from the spacer widths tweak. Here you enter the amount by which to increase or decrease each side of each tail. Click "Apply" to see the results. Note that individual spacer widths will be changed to enable the joint to be made. To clear all tweaks, click "Clear" then "Apply". Note that the tail tweaks are relative to the main inputs and basic spacer widths - if you tweak the tails and then tweak the spacer widths, the tail tweaks will still be applied relative to the new basic inputs.