Making small dovetail slides.
My little X1 mill is not very happy milling dovetails. It can be done but the depth of cut has to be quite small and it takes a long time.
I needed a dovetail slide for an enhancement to my spark erosion apparatus. Rather than cut the dovetails out of solid I decided to fabricate them. The end result, shown above, worked vey well.
The slide above consists of five main parts. The base plate is a length of 6 x 50 mm flat 43 mm wide. To this are bolted the two sides of the dovetail. The other parts are the slide itself, in the centre, and the gib strip adjacent to the left hand dovetail.
I needed a dovetail slide for an enhancement to my spark erosion apparatus. Rather than cut the dovetails out of solid I decided to fabricate them. The end result, shown above, worked vey well.
The slide above consists of five main parts. The base plate is a length of 6 x 50 mm flat 43 mm wide. To this are bolted the two sides of the dovetail. The other parts are the slide itself, in the centre, and the gib strip adjacent to the left hand dovetail.
The angled cuts were made using a 1.5 mm slitting saw. A piece of 6 x 50mm flat is gripped in a tilting vice at an angle of 30degrees from the vertical. The slitting saw is then used to cut the left hand dovetail piece. Once this has been cut the headstock is lowered by 3.5 mm to cut the 2mm thick gib strip.
To cut the middle part of the dovetail slide, the slide, the steel must be turned over in the vice so that the next cut faces the opposite way to the first two cuts. It is very important that once it is turned around the the cut is exactly parallel to the previous cut. This was done by butting the previous cut up to a headstock reference plane and tightening the vice. The final cut with the slitting saw is then made. The final right hand part of the dovetail is then cut off the steel with a band saw. Once all the parts have been cut then the edges of the dovetail pieces are cleaned up with a file and small flat (circa 0.25 mm) is filed to remove the sharp edges.
Using a slitting saw to cut the parts for the dovetail has many advantages. First since the saw is thin and cutting tangentially to the work the forces tending to bend the work are very low. This allows cuts to be made even when the stock protrudes a long way out of the vice (as in the photograph). Secondly, it is very easy to make the parallel cuts required. Thirdly, cutting of one piece at an angle creates a matching angle on the stock left in the vice. Thus the angles match precisely. A fourth advantage is that the surface finish on the edges is very good.
Assembly of the dovetail is quite straightforward. Line the left hand dovetail parallel to one edge of the baseplate and clamp them firmly together. Mark the positions of the 4mm screw holes, centre punch and drill through with a 3 mm drill. Separate the pieces and open out the holes in the base plate to 4 mm and countersink them on the back. Drill the dovetail piece to 3.3mm and then tap M4. The left hand piece is the firmly bolted to the base plate. Now assemble the gib, the slide and the righthand dovetail piece to the base plate. Insert a slip of 80 gsm paper between the gib and the slide. Push the right hand dovetail piece firmly against the other pieces and clamp it firmly into position. Mark out punch, drill and tap the righthand dovetail piece as done previously for the left hand piece. Screw the right hand piece to the base plate. The slip of paper is inserted to provide a little clearance between the gib and the slide.
The slide should move freely on the base plate with the gib in position.
To cut the middle part of the dovetail slide, the slide, the steel must be turned over in the vice so that the next cut faces the opposite way to the first two cuts. It is very important that once it is turned around the the cut is exactly parallel to the previous cut. This was done by butting the previous cut up to a headstock reference plane and tightening the vice. The final cut with the slitting saw is then made. The final right hand part of the dovetail is then cut off the steel with a band saw. Once all the parts have been cut then the edges of the dovetail pieces are cleaned up with a file and small flat (circa 0.25 mm) is filed to remove the sharp edges.
Using a slitting saw to cut the parts for the dovetail has many advantages. First since the saw is thin and cutting tangentially to the work the forces tending to bend the work are very low. This allows cuts to be made even when the stock protrudes a long way out of the vice (as in the photograph). Secondly, it is very easy to make the parallel cuts required. Thirdly, cutting of one piece at an angle creates a matching angle on the stock left in the vice. Thus the angles match precisely. A fourth advantage is that the surface finish on the edges is very good.
Assembly of the dovetail is quite straightforward. Line the left hand dovetail parallel to one edge of the baseplate and clamp them firmly together. Mark the positions of the 4mm screw holes, centre punch and drill through with a 3 mm drill. Separate the pieces and open out the holes in the base plate to 4 mm and countersink them on the back. Drill the dovetail piece to 3.3mm and then tap M4. The left hand piece is the firmly bolted to the base plate. Now assemble the gib, the slide and the righthand dovetail piece to the base plate. Insert a slip of 80 gsm paper between the gib and the slide. Push the right hand dovetail piece firmly against the other pieces and clamp it firmly into position. Mark out punch, drill and tap the righthand dovetail piece as done previously for the left hand piece. Screw the right hand piece to the base plate. The slip of paper is inserted to provide a little clearance between the gib and the slide.
The slide should move freely on the base plate with the gib in position.
This shows the back of the baseplate with the M4 countersunk screws securing the two dovetail pieces
Mark and centre punch the position of the three M3 gib screws 3 mm in from the outside edge. Clamp the gib into position and carefully drill the three holes M2.5 until they just penetrate into the gib. Remove the gib and slide and tap the holes M3. Clean up the inside face of the left hand dovetail piece using a fine triangular file.
Replace the gib and slide and insert the three gib screws. These should have a 60 degree conical point to locate into the dimples in the gib.
Replace the gib and slide and insert the three gib screws. These should have a 60 degree conical point to locate into the dimples in the gib.
Work the slide back and forth. Loosely tighten the gib screws and check that the slide moves freely. If it does not apply some felt tip marker ink to the slide surface and determine where it is rubbing. With a fine file gently relieve the high spots until the slide moves freely. Tighten the gibs a little more and repeat the test and if necessary adjust a little more.
The final operation is to mill all the peripheral edges. The two sides should be milled parallel to the right hand dovetail and the other two sides at right angles to it.
The final operation is to mill all the peripheral edges. The two sides should be milled parallel to the right hand dovetail and the other two sides at right angles to it.