Engraving Utilities
Working with a Dovetail Cutter
By Antares
Apr 14, 2003, 10:48

DOVETAILING

DESCRIPTION

 

Figure 1

Dovetailing is a method of milling slots or channels that have angled side walls yield­ing an opening that is wider at the bottom than at the top.  This type of groove is intended to hold a piece of material whose edges have been bevelled at an angle corresponding with those in the slot.  When  assembled, the mating piece is retained within the channel, but con be moved back and forth.

A good example of a dovetail application, is the master copy type and copy slide used on manual engraving machines.  The copy slide is a dovetailed channel and the type blanks have bevelled edges.  The type is retained in the slide, but can be moved easily for insertion and removal.

APPLICATION

A practical application for dovetailing in the engraving industry is the fabrication of directory signs that have removable in­serts or legend plates.  Signs of this type are generally made using acrylic or flexible engraving stock and can be mode to be as decorative as they are useful.

When using acrylic, products that have both a clear and coloured layer, like the Romark Slickers, con be used to create a wide range of options and effects.  These

materials can be reverse engraved and then paint filled with contrasting colours.  Directories made from flexible engraving stock can incorporate inserts and back­grounds of different colours and con be equally as attractive.

The main sign plate that has the dove­tailed slots should be a minimum of 1/8" thick. The insert strips are generally 1 1 / 6" thick and are recessed the same thickness so that the face of the strip is flush with the surface of the sign.

TECHNIQUE

Creating the dovetailed slot is a milling operation that is done with a 'dovetail' cutter.  This cutter is ground with a nega­tive angle that produces the undercut on the edge of the slot.  The insert strip is cut out using a profiling cutter that has an angle that matches that of the dovetail cutter. Neither procedure is particularly difficult once the basics are understood.

For the dovetailing operation, we first need to create a computer generated logo pattern or an engraving template that provides a series of parallel cutter passes (Figure 1). For purposes of example, let's assume we are making a 1.5'wide slot and are using a 0.25 dovetail cutter.  Since the lines in our logo pattern define the centre of the cut - not the edge - we must subtract the cutter tip diameter from the finished slot width. In this case, 1.5", less the cutter (.250'), equals a pattern width of 1.25".

Since each cut is .250" wide, there would be five passes required to create the full width of 1.25" (1.25 divided by.250 = 5. It is desirable however to have one pass overlap the next, so the logo should be created with at least one more line than the number calculated. In the case of our example, there would be six lines rather than five.  This will eliminate any ridges in the bottom of the cut.

The dovetail cutter cuts wider at the bot­tom than it does at the top, so the cut must be made at full depth.  The logo pattern should be made longer than the width of the plate so that the cutter can be low­ered to the specified depth before it contacts the plate.  After it is lowered, the cutter enters the edge of the material, makes the pass, exits the other edge and repeats the process for the next pass, (Figure 2) It is important to remember that the cutter can only be raised when it is off the plate.

As there is a considerable amount of material being removed, the dovetailing process is slower than normal engraving.  As a rule-of-thumb, cutter speed should be about 10,000 rpm and the feed rate (the speed the cutter moves through the material) should be about 15 to 30 inches per minute depending on the material ­slower in acrylic, faster in flexible, A slot that is 1.5" wide by 10" long can easily be produced in 3 to 4 minutes.

When working in acrylic, it is strongly rec­ommended that a water-soluble cutting oil be used.  It allows for better finishes, faster feed rates and extends cutter life.

With the dovetailing complete, the next step is to make the insert strips.  This is done with a 'profiling' cutter which is sharp­ened at an angle that matches the angle on the groove.  Like the dovetailing op­eration, a logo pattern must be created that will yield a plate of the proper length and width.  A simple rectangle is all that is needed, (Figure 3). As with the dovetail pattern, it is necessary to factor in the cutter tip size when making the calculations.  If we were making a strip that was 1.5" x 1 0" and were using a profiler with a .030" tip, we would add .030" to each dimension yielding a rectangle measuring 1.530" by 10.030". When mak­ing the strips, we are concerned with the inside of the cut, so we add the cutter width rather than subtract it.

There are two methods of profiling, One is to tape the sheet of material to a scrap piece of plastic using double-faced mask­ing tape and engrave completely through the top piece of material.  The strips are removed from the tape backing and are ready to use.

The other method is to clamp or tape the material to the machine table and en­grave only partially through it, leaving a few thousandths of material to hold the plates in place.  The pieces are then snapped apart, however they must be trimmed to remove the thin web that was not completely removed during profiling.

The fit between the strips and the dove­tailed channel should be close enough to hold the strip snugly, but not so tight that it binds when inserted or removed.  The fit can easily be adjusted by altering the width of the insert by adjusting the width of the rectangle pattern a few thousandths either way.

Figure 2