Step 7: Undercutting + Chamfering

Picture of Undercutting + Chamfering
Time for the undercuts.  This is very similar to roughing the bores.  This time around, we want power feed on the X axis, as we won't be doing much on the Z axis.  Note that the X axis feed is about 1/3 to 1/2 of the Z axis feed, you can test this with a dial indicator on the carriage and watching how the cross slide moves when the power feed is turned on (example, if the feed is set to 0.010, and the cross slide only moves 0.005, then you know that it's 1/2.  I'm sure you can guess what It would move if it was 1/3. Easy stuff.)

1) Put your undercut tool in, set it to centre height, touch it off on the face of the cube with paper, just like we did last step with the boring tool. (Picture # 1)

2) Move it to the side, set up the dial indicator again, yet this time move  to full depth (0.625 + 0.003 for paper), and set the  indicator to 0.

3) Now we need touch off on the edge of our bore and set the cross slide hand wheel micrometer collar to 0 (since we know that the bores have to be 0.200 and 0.350 bigger).  Move the tool to depth (we only cut to 0.615, so the dial indicator should be at 90) and move the tool to touch the edge of the bore.  I just turn the chuck by hand, and bring it closer and closer till the tool JUST scrapes the edge.  We're going to be cutting here, so don't worry about ruining the finish or anything.  You could try and use the paper again, but it's an awkward place to get to, and makes it more complicated then it needs to be.  Once you touch, set the collar to 0.

4) Now we can turn on the lathe (you can bring the tool out, we know where it needs to go.) Move the tool into the correct depth, and turn on the power feed, so it begins facing off the back to leave a nice finish.  Watch the collar, so you know that when it passes 0 you're at the edge.  Turn off the power feed and continue by hand.  Make your undercut now, by expanding the diameter 0.200.  Once you get to 0.200, pull the carriage 0.010 back towards you, and bring the tool out.  This is so you don't scrap back along the nicely finished face you just made.  Pull the tool out, and your first undercut is complete! (Picture #2)

5) Do the same thing for the next size.  Touch off on the face of the cube, move in 0.316 (0.313 + 0.003 for the paper), set 0 on the dial indicator,  touch off on the edge of the bore (1.500 bore), set 0 on the cross slide collar, turn on the lathe, cut the depth, power feed face cut to 0, hand feed the undercut so the diameter becomes 0.350 bigger, retreat the carriage 0.010, and bring out your tool.

6) Chamfering time, to give nice edges on your bores.  You can make these chamfers as big or as small as you like.  Mine are about 3/64.  For the outermost cube, you can do it on the mill when you are making the cube, or just do it with a file.  For the inner cubes, follow along here.

7) Use the countersink to put a chamfer on the 9/32 hole first.

8) Using the 90 chamfer tool we made, set it to centre height, and chamfer the other two bores.

9) Voila!  Side 1 of your cube is complete!  That wasn't so bad, was it?  All you have to do now is repeat the same operations on the other 5 sides.  I find it best to do opposite sides (Imagine a dice, do side 6 and 1, 2 and 5, 3 and 4.).  I think that it keeps the most material in the right places, and its easy enough to dial in each time.

- When you get a few sides done, you have to balance the shims so that they cover the holes, yet leave enough room to dial in
- Always work on the smallest cube first, then the middle cube.  Work from the inside out.

For the final side:

10) You have 5 sides done, the cube is looking excellent, you've finished the bores on side #6, now you're ready for undercutting. STOP!!! Here we are switching things up a bit.  As soon as you undercut, the cube will fall apart.  So we need to chamfer first.  Chamfering the biggest bore is the same as always, but chamfering the middle bore and countersinking the 9/32 hole need to be done a little differently.  You're going to be facing these two surfaces off (removing 0.010), so make the chamfers a little bigger then the others, so when the surfaces are faced, they become the right size.

11) Now proceed as normal, cut to your depth with the undercut tool, begin power feeding the face, when you reach the edge of the bore (0 on the collar) TURN THE MACHINE OFF, LEAVE THE POWER FEED ON.  I learned this the hard way.  When you try to turn the undercut, as soon as you reach the final diameter, the cube will fall off.  Now, when your machine is spinning at 600 RPMs, that little cube begins bouncing around inside and smacking into your tool and gouging the heck out of itself, ruining all the nice finishes you've put on it.  My idea to solve this was drop the RPM to as slow as it can go, and get ready to stomp on the brake as soon as the cube fell off. My teachers idea was much better, so we'll be using it here.

Now you should be touching the edge, ready to undercut 0.200. Just start spinning the chuck by hand.  Since you left the power feed on, the cross slide will continue moving along, and you can make your cut this way.  Now, as soon as you hear/see the cube fall off, you can stop spinning.  No ruined finishes!

Note:  Sometimes the cube will not fall off right away, even once you get to the 0.200 undercut (or 0.350).  This should be fixable, its just the slight errors accumulated, and there may be a few thousandths left on some of the corners, holding the cube on.  Just use something soft to smack that cube out.  I used a piece of wood and hit it with a hammer a few times. (my middle cube got stuck on one of my prototypes).  When you are cutting the undercuts on side #5, you'll be able to see if they will be big enough to break the corners.  You should be able to see the corners free floating, ready to fall apart when side #6 is cut.  If the corners are still attached, then you messed up somewhere.  Recheck your calculations.

12) Do the same thing for the next undercut, use the "hand feed".  Power OFF, power feed ON.  Spin that chuck!  It's really not so bad, you may think that it'll be horrible spinning the chuck by hand, it's really not, but if you want you can just use power feed to get it close, then spin it by hand for the last little bit.  Or try my stomping on the brake idea.

Now you have your very own beautiful Turners Cube!  Not one built by a robot (CNC), but one built with your own flesh and blood and manual power (hopefully not too much flesh and blood!)

gfish1 year ago
You can also make a set of step-cylinders which nest into the bores. These hold everything in place as you do the final undercuts, and generally reduce the risk of damage from clamping forces.