As I was working on my Hodgson Radial, I needed to accurately size the crank journals, but I didn't want to risk tapering the ends as usually happens when polishing with sandpaper. So I made myself a set of external hones (photo of my set on the left), based on a set that I remembered seeing on Ron's Model Engineering and Model IC Engines website interestingly named the Nikapena Hone.
Ron Chernich in Australia, of which Ron's Model Engineering website is named after, put me in touch with the Nikapena's creator, Nick Jones from the UK. Nick obligingly sent me copies of the sketches he made when he created his hones, and the photo to the right shows his hone set.
More searching of the web finally turned up a photo of a Delapena hone which Nick used as his inspiration. While the Delapena company is still in business in the UK, sadly the external hone is no longer made. Model Engineers' Workshop in the June/July 1997 issue described a kit built Delapena Hone Clone, but as far as I can tell, the kit is also no longer available.
The beauty of the Delapena style hone is in its fluid looks and the ease in which it fits your hand. This I did not want to change, but being an Engineer, I was sure I could "improve" upon the previous designs.
One thing I wanted was a method to provide a hard stop so I could accurately size multiple parts without making adjustments, and I wanted to minimize the number of fasteners and external protuberances that could snag a delicate hand.
A few nights spent on the CAD system resulted in the design on the left. Honing pressure is controlled by the spring, and the graduated dial provides a hard stop to limit the material removal. The axles are retained internally resulting in a clean exterior. The solid model was done in AutoCAD 2010 and is free to use.
Since most of the work on these hones will be in the fixturing and setup, it makes sense to machine more than one hone while you are at it. Each of these three 7075-T6 aluminum blocks will make both halves of one frame for a total of three hones.
The various axles are pieces of 6mm and 10mm ground drill rod. At this point, the axles for three hones have been rough cut to length.
The axles are all faced to 32mm long and chamfered slightly on each end. A 90° v-groove 1mm deep is then added to the center of the three main 10mm axles.
To make sure the groove was centered, I use a collet stop in my 5C collet, plunged the groove to depth, flipped the part and then plunged the groove again.
Three of the small axles are tapped M4 through, and three are drilled for a close fit on an M4x50mm set screw.
Next, six 10mm x 10mm x 10mm cubes are made from steel stock. I had some 416 stainless hardened to HRc35 so I machined the blocks from this as accurately as possible then drilled a clearance hole for the M4 setscrew through centered on one face.
The next task is to square up and size the frame stock to 107mm x 40mm x 32mm. Accuracy is important, but your main goal here is consistency so that each piece of stock will work the same in the various setups.
With the three pieces of stock machined to size, we'll start putting in the holes while the pieces are still easy to clamp.
Here's how the two frame halves fit into the stock. As you can see, the holes in one half are the mirror image of the holes in the other half. We'll use this bit of information to cut our setups in half.
With a vise stop setup, I'm setting my zero on the center of the 32mm stock width, and the edge of the 107mm length.
I'm starting with the 4mm clearance holes for the anvil retention screws. Move back 8mm from center and 36mm from the end and center drill the first position. Rotate the part 180°, flip it 180°, and spot the second hole. Repeat on the other two blanks.
Move 8mm forward from the center, and repeat the above procedure until you have four holes spotted on each blank.
With the retention screw holes spotted, drill them M4 by 10-15mm deep by repeating the above procedure.
You can now drill and tap M4 for the main axle setscrew along the centerline and 11mm from the end. Note that there is only one of these holes per blank and you don't want to drill any deeper than 16-17mm.
Now, flip the block over to work on it's 40mm side and set your zero on the back edge of the part.
Begin by drilling and reaming the two 10mm main axle holes following the above drill-flip process. These holes go all the way through and should be a nice slip fit on your 10mm axle.
Repeat the process for the two reamed 6mm adjustment axle holes, and finally the two drilled 6mm saw blade clearance holes.
At this point your blanks should look like this. The one side not shown should have the two retention screw holes but not the axle set screw hole.
To make the sawing operation a little easier, I printed out copies of my stock plan and glued them to the stock.
The blocks are then split at the bandsaw. Don't remove any of the waste at this point. We'll use the orthogonal surfaces for location in future operations.
Here, I'm using an 8mm endmill exactly on center to rough out the adjuster block pockets. Once the pocket is roughed to depth, I'll step over to make a 1mm climb cut to finish one side.
Without moving the cutter back to center, the part is flipped and the finish cut is made on the other side. This insures that the slot is centered in the blank.
On your first slot, you'll need to work up the 1mm dimension, flipping and cutting the other side and perhaps repeating this process a few times to insure a nice slip fit on your 10mm cubes you made previously. Make sure your cubes rotate freely in the slot when installed on the axle.
Next cut the anvil pockets. Since we have not made the anvils at this point, the actual width is not critical, but it is important that all of the anvil pockets be the same width and centered on the retention screw holes.
At this point, you can head back to the bandsaw and remove the excess material from around the profile. Leave about a millimeter for cleanup.
We'll need a couple of bushing to hold our parts on the rotary table fixture we'll be using later. From some scrap make one bushing 10mm in diameter by 15mm long with a 6mm clearance hole through, and a headed bushing with a 10mm diameter with a 31mm long shank and a 6mm through hole. The head on the bushing should be 16-18mm in diameter and both of these should be a nice slip fit in the 10mm axle holes.
This next image looks a little complicated, but it's rather simple. At the top of the image, each color segment on the profile corresponds with a single cut and also with the same color profile on the bottom which shows the position on our rotary table fixture and the endmill starting location.
This is the actual rotary table fixture. The plan presented right is also available for download.
Preparing the Rotary Table Fixture
Arc #1 - the blue arc in the plan
The first cut is finished. Set your depth to cut just above the fixture surface, not below like I did. The small amount of "flash" left on the part is easily removed with a file and by not cutting the fixture you retain a nice flat surface for subsequent cuts.
Arc #2 - the magenta arc in the plan
This is the finish cutter position for arc #2, ready for unloading the part.
Arc #3 (small end radius) - the yellow arc & straight cut in the plan
Arc #4 - the red arc in the plan
Since this a concave cut, I found it easier to plunge the cutter to depth at this point and then finish the rotary cut.
Arc #4 - continued
Just one more end to shape
Half of our parts are going to need clearance cuts for the hinge knuckles so we can't use the headed bushing any more on the large end. Here I've mounted the short bushing
Arc #5 (big end radius) - the cyan arc in the plan
Left Cheek Cut
Left Cheek Cut - continued
Right Cheek Cut
With all the rotary table work finished, re-mount the vise and indicate it in. Working with the half of the parts that did not receive the cheek cuts, rest the flat bottom of one of the hone frames on the bottom of the vise and center the table on the width of the part. With a 14mm endmill, rough out the clearance for the center knuckle.
Using the same procedure we used on the pivot slots, open up the knuckle slot until the half with the cheek cuts moves freely in the slot with the axle inserted.
Dry fit all the pieces and make sure each of the hone bodies opens and closes smoothly with no drag or catching. Any sloppiness in the hinge will affect the hone's ability to hold close tolerances.
The geometry of the hones is such that for every millimeter of movement at the adjusting screw, the diameter change at the honing point is about 0.4mm. With a M4x0.7 screw for our adjuster, that comes out to 0.28mm change in diameter. So, if we engrave our indicator with 28 graduations, each division should be about 0.01mm on the diameter.
I knurled and tapped one indicator but did not part it completely from the stock. With the stock set up on the rotary table (or dividing head if you have one), I'm using a "V" cutter to engrave the divisions about 0.2mm deep.
With the engraving finished, it's back to the lathe to finish parting off the indicator and prepare the next one.
Repeat the above as needed. When all indicators have been engraved, add an M3 tapped cross hole for a soft-point setscrew to lock the indicator to the adjusting shaft, and finish with a light lapping of both sides of the indicator dial.
The adjustment knobs are made from brass and we begin by knurling. After the knurling is complete, a blind M4 hole is tapped in the end of the adjuster.
Next, the neck of the adjuster is turned down to approximately 1mm smaller than the ID of the spring. The springs I used were Lee Spring LCM095E03M.
A quick chamfer to knock the sharp edge off the knurls.
The part was then chucked on the adjuster neck, faced and chamfered, and a recess was cut with a 4mm ball end mill.
The only thing remaining are the bronze anvils and the aluminum stone holders. There are a few pieces missing in this shot, but I cut enough stock for three sets of anvils and stone holders.
After sizing all the pieces (adjust the widths as needed to snugly fit the pockets in the hone bodies), I began cutting the v-grooves in the anvils. Try your best to get these centered in the blocks. The small and medium anvils are cut at 90°.
The large anvils are cut at 120°.
I'm using Boride Abrasives 1/4" x 1/4" die finishing sticks for my stones, so I'm grooving the stone holders to fit.
The M4 tapped holes are added for the button head retention screws.
The last step on the stone holders and anvils is to stamp or engrave the identification codes. I'm using some 3/32" stamps to mark mine.
I used a small diamond cutting wheel to cut the die polishing sticks into 32mm lengths for my hones. Since I made three sets of holders, I can mount up some 240, 400, and 600 grit stones. The stones are held in the stone holders with 5-minute epoxy.
Here are the completed external hones. This photo show the hones with a previous iteration of the adjuster knobs. These have been changed to the knurled version presented above.
To assemble the hones, Loctite an M4x50mm long set screw into each brass adjuster knob. Assemble the two frame halves with the hinge pin and with the frame fully open, insert the M4x6mm cone point setscrew to lock the hinge pin in place.
Insert the threaded small axle into one side of the lower frame, through one of the 10mm cubes, making sure the threaded axle hole lines up with the hole in the cube, and into the other side of the frame. Do the same for the unthreaded axle in the upper frame half.
Slip the spring over the adjuster knob set screw and onto the neck of the knob. Insert this assembly through the upper axle and pivot block assembly, and thread it into the indicator dial making sure the graduations face up. Continue to thread the indicator dial and begin threading the adjuster shaft into the threaded lower axle. Once the adjuster shaft is threaded into the lower axle, Continue threading the adjuster up the shaft until the spring begins to compress.
The amount of compression in the spring sets the honing force. You will want to play with different amounts of preload based on the hardness of the abrasive stones you are using. Once you find a setting you like, insert a M3x5mm soft point setscrew into the cross hole in the indicator and lock the indicator dial in place.
To use, insert the appropriate stone holder and anvil into the frame, slip the frame over your shaft and turn the adjuster know until the anvil and stone just touch your shaft. With your shaft rotating, move the frame side to side if needed and slowly turn the adjuster knob to begin removing material
To stop and check your work without disturbing your size adjustment, place your thumb in the adjuster knob depression, and your index and middle fingers underneath the axle in the upper frame and push down on the adjuster knob. This will open the frame slightly to remove it from the shaft. Reverse this process put the hone back on the shaft to continue honing where you left off.
With a new stone, you may need to true it before it's first use. To do this, insert the stone holder with the mounted stone along with the appropriate anvil into the frame. Using a pencil, rub the surface of the stone, and then proceed to lightly hone a test shaft. Remove the hone frame from the shaft and check the pencil marks on the stone. Wherever the pencil mark has been removed is a high spot on the stone. Use a small diamond file to remove some additional material from this area, and repeat this process until the pencil mark is evenly removed across the entire stone surface when test honing.
Once this truing is done on a new stone, it should not be required again until the stone is replaced.
A nice tool deserves a nice case don't you think? Happy honing!
CAD Files Used On This Page (AutoCAD 2010 Format)
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