Making Bolt Hole Chamfers with an End Mill on your CNC.

I spent the last 10 years using CNCs without a tool changer. Whenever the job required a bit change normally I thought about how I might be able to do the job using just one bit and some creative use of the CNC software I was using. This is one example.

Projects that needed perimeter or internal chamfers were done most efficiently with a bit change to a V bit.  I have a project that is put together with flat head bolts. The need to chamfer the bolt holes could have been done using a drill press after the CNC was done. Figuring out how to chamfer a hole with the same end mill I used for all the other cuts turned a 2 process job into one CNC  job with no bit changes. 

Vectric.com's Aspire and VCarve programs have a fluting toolpath.  Fluting takes a vector and ramps the bit down or down then up with the bit being centered on the vector. A circular array of 9 (or more) short vectors centered over the bolt hole, using a bit no larger than the diameter of the hole, can effectively chamfer the perimeter of the hole from the bolt head diameter down to the hole edge.  Using a small bit (1/8" or 1/16") may require a greater number of vectors to result in a relatively smooth chamfered edge. The chamfers in this image were made with a 1/4" diameter end mill. 

2 x 2 Hole Array

The slope of the chamfer and the diameter can be easily changed.  Aspire/VCarve also have a circular array layout tool that will use one vector that spans the hole and make any number you specify of rotated copies around the same center. Using an odd number of vectors results in a circle of lines with alternating start points.  Alternating the start points reduces the travel time needed move to each successive vector.  

Vectors Used

The center of the bit used is what follows the line. The depth the bit will slope down is set when creating the fluting toolpath.  In the image above the shaded area is a shadow of the area the 1/4" straight bit travelled over.  The short vectors used are shown in red. The center circle was the hole for the 1/4" bolt shaft. Depth of the fluting chamfer cuts was 1/8".

I made the vectors for one bolt hole, then simply copied them to all the other bolt locations. 

You can find downloadable  .CRV3d and .CRV file of the sample above on Vectric's user forum: Vectric.com

Questions or comments are encouraged!

4D

 

A Domino Slot Template.

Using Festool's Domino floating tenons is usually an expensive upgrade for hobby woodworkers.  The machine to cut the slots can cost over $1000, and the tenons themselves are more costly than traditional dowels made for joining wood parts. 

There are other less expensive sources for the floating tenons.  Attempts have been made to build simpler machines that duplicate the slot cutting action of the Domino machine.

I design and build furniture, and have found that floating tenons come in handy for connecting parts.  I can't justify the cost of a Domino machine, but I do have a small CNC and with it I can make the slots for ready made tenons on most furniture parts.  For joining long boards together I've designed a simple template that can be used with a handheld plunge router using a template bushing. 

A Domino Slot Template

This template could be made using a router table.  I drew up the template for cutting out with my small CNC.  The slots in the sides are for screws that would hold wood guide strips.  The guide strips can be adjusted to center or offset the slot that will be cut by the template. The template can be clamped to the edge of your board to stay in place when being used.  

A 5/8" diameter bushing in a plunge router base used with a 3/16"d spiral upcut router bit is all that is needed to make perfect sized slots for commercial Domino brand tenons or alternatives.  Some plunge bases for routers come with a dust collector shroud to suck away the chips as the slot is being cut. 

Plunge Router with Template Bushing

  This template is for 6mm x 20mm tenons. 40mm length. 

Slot has been Cut

You could make templates for each available size of tenon. All that needs changing is the size of the center hole. With floating tenons available in large quantity packs each size template would get plenty of use. The guide strips could be removed and used on whatever template plate is needed. A stack of different template plates would cost far less than the cheapest Domino machine you can find out there. 

A Good Fit

Once the template has been set up for the material, making several slots along the edge is relatively easy.  It just requires undoing the clamps, sliding the template to new position, then clamping it in place. Modifying the template for thicker or thinner material takes no longer to do than adjusting the fence of a Domino machine for the same job.

It is also easy to change the slot hole so you can use 5mm or 6mm bits to make the tenon slots.

For metric bits

If you use Vectric.com's VCarve or Aspire a files for these templates can be found on their forum HERE. 

Comments and Questions are encouraged!

4D

     

Tapering French (Sliding) Dovetail Joints for Ease of Assembly

Tapered French Dovetail 

French dovetail joints are a good way to connect critical cabinet parts.  A straight dovetail slot is easy to cut with a straight edge, router and a dovetail bit.  The mating male end of the joint takes a little more care to size right.  I've used a router table with the same dovetail bit and a tall fence to trim off the sides of the mating part end until the dovetail end fits snuggly into the slot. 

On wide boards a snug French dovetail joint can be difficult to slide in place. Friction builds up the farther into the slot you go. Getting the joint closed occasionally requires the pull of a bar clamp or tap of a wood mallet. The tendency of wood panels to bow or flex a little can add to the difficulty.  

Tapering the routed slot is fairly easy.  Make a straight slot against a straight edge first. Then move the leading end of the straight edge over 1/16" or so. Make note of how far you moved it.  A penny (1 cent) measures almost 1/16" thick (.06" rather than .0625"). I usually moved the straight edge end 1/16". 

To taper the male dovetail end a shim as thick as the amount the straight edge was moved is needed.  I tape a penny (tape + penny=1/16") or laminate sample chip to one edge of the mating piece just above the height of the dovetail bit. As the board slides past the router bit that shim holds one end away from the bit 1/16" to effectively taper that cut. The other side is cut parallel to the face. Care needs to be taken to match the side of the tapered slot to the tapered side of the joining piece. Cut on the wrong side the joint will still slide together, but be slightly crooked when in place.

CNC cut Dovetailed Side

CNC cut Tapered Slot

Using the CNC to cut the tapered slot is easy. The CNC cut tapered slot can be centered so there is no chance of ending up with a crooked panel. A vector that starts at the beginning of the slot, runs to the end of the slot, then returns to the beginning 1/16" away from the start point is all that is needed.  A profile toolpath centered on the line with cutting depth set for one pass for the dovetail bit will cut the tapered dovetail slot. 

Vector for Tapered Dovetail Slot (blue)

In the drawing above the green circles represent the diameter of the dovetail bit at beginning, middle, and end of the vector. The red lines show the width of the intersecting board. Blue is the vector needed and it is extended outside of the board by half the diameter of the bit.  This lets the bit drop down to cutting depth before it enters the edge of the board. To ease the stress on the dovetail bit I usually run a 1/4" down-cut spiral end mill on the same vector to clear out the slot before the dovetail bit runs to undercut it.

Vector for Male Dovetail End

Cutting the mating end requires being able to clamp the mating board vertically and level under the CNC spindle.   Making the toolpath to move the dovetail bit around the mating end requires some careful drafting.  You need to know the side angle, diameters of the dovetail bit, and depth of the dovetail cut.  Offsetting the male toolpath vector from the vector used to make the tapered slot will ensure a matching taper.  The amount you offset that vector is the necessary detail to come up with.  See the diagram above. One half of the bit tip diameter plus 1/2 the bit diameter at the cutting depth is how far you should offset the vector. I use a section view of the dovetail bit to find the cut depth diameter of the bit.  

Almost together. Still loose.

Alternately you could simplify the CNC toolpaths to duplicate the result of doing it without a CNC.  This is easiest if the dovetail isn't blind, and runs completely though the board. Two vector lines, with one slightly angled, to run the dovetail bit down on-the-line. To make the mating dovetail you'll need one pass down one side of the board, and an angled pass down the other side of he board. Placing the vectors for these requires knowing the bit tip diameter and the diameter at the cutting depth on the bit.  

With all CNC cut joinery the precision fit can be hard to nail down.  Using the same vectors and precise offset leaves no room for error or glue. I prefer to do the male side of the joint first, then the female slot.  I leave the slot side on the CNC to check the fit before unclamping it.  If the joint doesn't close completely you can reset your X axis (or Y axis depending on how you clamped up the board)  by a few thousandths and run the dovetail bit toolpath again.  I find .003" is a good amount of offset between sides of the joint for a good snug fit with room for glue. With sliding French dovetails it should take just a tap or two of a wood or plastic mallet to close the joint completely.  I tweaked my sample until my joint took two taps to close and will take the same two taps to free it up. 

Snug and Tight when completely together.

Comments and questions I encourage and welcome. 

4D