By Keith Mitchell
Recently I purchased a wedge type quick-change toolpost for my lathe. The toolpost is of course only the start. Now you need multiple toolholders to get the benefit of a quick-change toolpost. Depending on which one you select these toolholders cost from $25 to $50 in the import grade. Knowing that I would not be satisfied with less than a full complement and being unwilling to spend the money on this many toolholders, I set out to make my own. Afterall, on initial inspection, it amounted to only a block with a dovetail slot, a straight slot and a few drilled and tapped holes. I purchased a 60-degree dovetail cutter and proceeded to make some chips.
I had purchased two toolholders so I measured the dovetail slot in each and attempted to replicate it in the new part. My first attempt produced an unusable part when I machined the dovetail too large. Both the width and the depth of the slot determine the fit of a dovetail. I found it to be difficult and time consuming to accurately measure the dovetails while they are being machined. The most accurate procedure to measure width of a dovetail involves placing two rods or balls of known diameter in the dovetail slots and measuring between them. It quickly became apparent that a sizing gage would be very helpful in machining the dovetail.
I set out to make a male tapered dovetail gage. The gage I made had two sides. The smaller side will pass completely through the completed dovetail and is used to warn when you are approaching final dimension. The large side will wedge part way through a completed toolholder dovetail and is used for the final sizing. The 0.020" taper over the length of the gage was achieved by machining one side of the dovetail to full depth then installing shim stock at opposite corners of the gage block in the mill vise to cause the taper. 1-1/2 X 2" aluminum stock was selected for the gage. CRS would be better since it is not as susceptible to dents and dings during storage and use. I had my gage block hard anodized to make it less susceptible to damage. For this type of work I find it easier to mount the work in the vise and find the center of the piece. Then work from the center out. The straight side was machined to full depth first. Then the shims are installed and the taper machined. I used a purchased toolholder to check the gage as I machined the tapered side. Of key importance is the fact that the outside corners on the dovetail are not sharp corners, they are radiused or broken. As the gage is machined you must make sure the corner breaks are maintained. Use a file or periodically replace the dovetail cutter with an end mill to break the corners. If the corners are not adequately broken, the gage will not fit until it is seriously undersized. Go extremely slowly as you approach final size. I made the last few passes with a 0.002 cut. Ideally the final sizing gage will fit the sample dovetail when it is inserted about halfway or a little more.
Once the gage is completed, you are ready to start machining the toolholder blocks. The attached drawings are for a BXA or a 200 series size toolholder in the import brands. Other sizes will require different dimensions. I used 1-1/2" X 3" CRS for the tool holder blocks in the photos. I have also used a 2" X 6" X 12" slab of 4130 for toolholders. Since making the dovetail cut is a time consuming operation, I like to start with enough material to make about four or five toolholders, usually a 6 to 7-1/2" length. When you size the material make sure you allow for the saw cuts and cleanup machining which will be required later. My rule of thumb on saw cuts is to allow at least 0.050" additional material to clean up each saw cut if your saw is pretty well setup to cut straight. That's 0.100 for saw cuts on both sides. The dovetail slot is cut in the long axis. Then cut the block on the bandsaw into the separate toolholders for finishing. I machine the center slot with a straight 3/4" end mill first. The dovetail cutter I use is a1-3/8" diameter HSS, so the 1.400" slot width will clear the dovetail cutter and breaks the inside corners sufficiently to clear the gage. The initial cuts with the dovetail cutter can be a little aggressive since not much material is being removed. As the dovetail develops, the depth of each pass should be reduced. As final dimension is approached, use the gage often. The last cuts are made with a feed of about 0.001 - 0.002" per pass. I can tell you from experience, it's a sick feeling to make the final cut and find that the gage block drops right through. I've sucessfully salvaged some blocks cut oversize by tig welding one side of the dovetail, however, it's easier to get it right the first time. MAKE SURE THE CORNERS ARE ADEQUATELY BROKEN! Otherwise the gage block may not pass due to interference from the sharp edges. If this happens you will get a false indication from the gage. Also make sure the chips are well cleared so they don't interfere with a good reading from the gage. I keep a 1" chip brush handy to clear the chips.
I like for my toolholders to close within 1/4 turn. This ensures the toolpost handle is not in your way when it is closed. To achieve this I usually machine the toolholders slightly undersize and then do the final fitup by hand using a file and emery cloth after they are cut into individual holders.
Once the main block is machined, I bandsaw the individual toolholders from the block. Do a cleanup operation on the saw cut ends to square up the block. I like to use a flycutter for this operation since it will cover a large area in minimal time. Also with a flycutter two blocks can be machined at the same time by putting them in the milling vise back to back. A shot on the belt sander with 120 or 180 grit will remove the tool marks.
To make toolholders for 1/2" tooling place the block in the milling vise to cut the slot for the tool. I use a 5/8" wide X 1/2" deep slot for most of my lathe tool blocks. It works for a number of tools. If you wish to use 1/4" or 5/16" HSS toolbits they can be used by making a carrier from 1/2" square stock with a suitable slot milled.
Once the tool slot is cut turn the block in the milling vise to drill the holes for the height adjustment screw and the tool set screws. I used a 3/8-24 thread for the height adjustment screw. The fine thread makes it easier to get the height set accurately. In addition, the knurled height adjustment nut is not very thick and needs the fine thread to get adequate thread engagement. The tool holding setscrews are also 3/8-24 to keep it simple to drill holes and tap.
The knurled height adjustment nut is made from 1" diameter 12L14 rod. When turning, I like to have plenty of material to grip in the chuck so I start with a piece about one foot long. Like the toolholder blocks I like to make several of these at a time. Put the stock in a three-jaw chuck in the lathe. I usually cleanup the surface with some Scotchbrite either by hand or with a disc in a 1/4" angle head die grinder. Face the end. Center drill and tap drill a hole for the 3/8-24 thread in the center of the rod in the lathe. Chamfer the outside end about 0.050". Using a cutoff tool layout the number of knurled nuts you plan to make. I usually set up a dial indicator on a magnetic base to space the nuts for cutoff. Bring the cutoff tool up even with the outboard end of the stock. Step off the width of the nut plus the width of the tool. At each location make a cut about 0.100" deep. Return to each location and cut a chamfer on both sides of the cutoff tool slot to match the end chamfer. Set up the knurling tool. I used medium diamond wheels in the knurling tool. Put the lathe in backgear, apply cutting oil and knurl all of the nuts in one operation. If you try to knurl first then cut-off, I find the edges of the nuts are upset slightly and are not as nice to use.
After knurling set up the cutoff tool again. Re-enter the cutoff tool slots and cutoff each nut. If you end up with a burr on the free side, install a short piece of 3/8-24 threaded rod in the stock remaining in the chuck. Install the nut on the rod so the threaded rod is slightly underflush. A few light facing cuts will cleanup the burr.
Assembly is pretty simple. The height adjustment screw can be a long setscrew; however, a piece of allthread works as well. The height adjustment screw is Loctited into the toolholder block.
The drawings show a cutoff tool, a toolholder for 1/2" shank tools and a boring bar holder. All of the boring bar toolholders are made for 3/4" boring bars. For smaller boring bars I have a split sleeve to reduce the diameter. The extra bushing with the face key is used with boring bars to index them in the holder. The diameter of the center hole is adjusted to fit the boring bar. A friend has a toolholder which he really likes that takes 5C collets.
I'm considering having a gunsmith friend blue my toolholders for the final finish if I can catch him when his tank is hot. Now you can have a separate toolholder for every tool and get maximum benefit from your quickchange toolpost. No more swapping tools between toolholders.
I have seen another technique for making dovetails I may consider trying. This technique uses a built up method. Two rails are machined with the dovetail angle. These are silver soldered to a block at the correct spacing to make the dovetail. I have not tried this so I can not tell you if it would work. If I give it a try I'll let you know. Many thanks to George Carlson for processing the drawing that accompanies this where it loads quickly and is legible.