Vertical shear bit
From the front side.. notice the curved cutting edge. This clearance is required so that it does not rub on the top and bottom of the bit. Just like the side vertical shear bit, it hits only on the center.
Last edited by BryceGTX on Sun Feb 21, 2010 12:33 am, edited 1 time in total.
Now here is a critical secret of the face bit. It must cut at a point ABOVE the centerline. It cannot cut below the centerline. If the point of contact of the bit is below the centerline, the effect is the metal is moving away from the cutting edge instead of toward the cutting edge. In such a case, the metal just smears.
I did a number of tests moving the point of contact above, below and at the center line. It is quite apparent when it is cutting below the center line. It makes a mess of the face. Just slightly above or at the center, it dramatically shifts to a georgeous finish. At the center cutting depends on the vertical angle, DOC, radius of cutter and feed.
I have never seen such a dramatic change in cutting action as this bit above and below center. One significance is what happens when I don't have a center hole as this piece has. It probably will leave a very slight nipple.
This picture shows the point of contact above center and it also clearly shows the required radius of the cutter. Just like the vertical shear bit for the side, this bit creates very fine straight swarf. It is quite happy to cut 0.001 DOC.
I did a number of tests moving the point of contact above, below and at the center line. It is quite apparent when it is cutting below the center line. It makes a mess of the face. Just slightly above or at the center, it dramatically shifts to a georgeous finish. At the center cutting depends on the vertical angle, DOC, radius of cutter and feed.
I have never seen such a dramatic change in cutting action as this bit above and below center. One significance is what happens when I don't have a center hole as this piece has. It probably will leave a very slight nipple.
This picture shows the point of contact above center and it also clearly shows the required radius of the cutter. Just like the vertical shear bit for the side, this bit creates very fine straight swarf. It is quite happy to cut 0.001 DOC.
Chances are, if I want to turn a bar to a very accurate diameter, I want to create a very accurate hole. So I want a vertical shear boring bit. Here are the results of the boring bit compared to a standard boring bar. Again, this is HR 4140. The same piece. I sliced a section off the end so I could bore through it.
The top picture is a standard bored hole. It shows a few machining marks. I could do better, but this hole is not bad. The bottom picture is using the vertical shear boring bit.
In the bottom picture, there are no machining marks at all. It is probably as good a hole as I could hope to find as a turned hole.
The top picture is a standard bored hole. It shows a few machining marks. I could do better, but this hole is not bad. The bottom picture is using the vertical shear boring bit.
In the bottom picture, there are no machining marks at all. It is probably as good a hole as I could hope to find as a turned hole.
Last edited by BryceGTX on Sun Feb 21, 2010 3:26 pm, edited 2 times in total.
I wanted a boring bit that I could use in either my lathe or my boring head for my mill. So I created the boring bit from 1/2 inch high speed steel.
The first step is to turn the HSS blank down a bit, then turn a taper on the end of it. This picture shows I turned about a 6 degree taper. This taper is for clearance.
The first step is to turn the HSS blank down a bit, then turn a taper on the end of it. This picture shows I turned about a 6 degree taper. This taper is for clearance.
Last edited by BryceGTX on Sun Feb 21, 2010 2:58 pm, edited 1 time in total.
Next, I finish turning down the shank. This operation is not so nice because there is very little support of the HSS blank compared to its diameter. The end result should look like below. The large end is 1/2 inch diameter to fint into my boring head for the mill or QCTP holder for the lathe.
If you want to turn a HSS blank, you will need a very good cutting tool. I use TCMT 21.51 Micro 100 indexable bits. Cheap bits will chip very quickly on HSS.
If you want to turn a HSS blank, you will need a very good cutting tool. I use TCMT 21.51 Micro 100 indexable bits. Cheap bits will chip very quickly on HSS.
Last edited by BryceGTX on Sun Feb 21, 2010 11:19 pm, edited 3 times in total.
Next step is to grind the vertical shearing edge. It is simply a 10-15 degree flat on the end of the boring bit. After grinding it, I hone it on my stone.
Last edited by BryceGTX on Sun Feb 21, 2010 3:00 pm, edited 1 time in total.
Now we mount the vertical shear boring bit on the lathe as a simple boring bar. The bit must be aligned so its shear edge is vertical. I aligned this by eye.
Last edited by BryceGTX on Sun Feb 21, 2010 3:11 pm, edited 1 time in total.
Next, we align the boring bit so it is centered in the hole. So its cutting edge is parallel with the centerline. Easiest way is to look down the centerline of the bit and adjust its height in the QCTP holder. This seemed to be a bit important. If the bit is a bit high, it does not cut quite as well. I didn't try it too low. I aligned this by eye.
From this picture, it should be clear why I used a round HSS blank. The shearing edge must be circular so it touches the hole at only one point. Now one could easily create a circular edge on a square blank similar to the circular edge I created on the face bit. So don't think a circular blank is required. I used circular so I could use it in my boring head on my mill.
From this picture, it should be clear why I used a round HSS blank. The shearing edge must be circular so it touches the hole at only one point. Now one could easily create a circular edge on a square blank similar to the circular edge I created on the face bit. So don't think a circular blank is required. I used circular so I could use it in my boring head on my mill.
Last edited by BryceGTX on Sun Feb 21, 2010 3:13 pm, edited 3 times in total.
Then just run the bit through the hole using slow feed and slow rotation speed. I used 150 rpm and 0.0025 per rev feed. As with the other vertical shear bits, this bit will cut a 0.001 DOC all day long in the 4140 creating a very fine finish.
The swarf will be the same as the other vertical shear bits. It will be relatively long straight wisps. The swarf will be pushed into the hole in front of the cutter and will come out the back as shown in the picture below.
A key point is that this produces such a fine finish, that any swarf rubbed by the cutter into the piece will affect the finish. So the best bet is to blow air into the hole to keep the swarf away from the cutter.
Some points about all of these shear bits. The cutting operation creates straight thin wisps of swarf with a 0.001 DOC. If you get very small shards or powder or it smears instead of cutting, there is probably something wrong with your setup or grind.
Lastly, if you have never tried any of these shear bits for fine finishing or accurate turning, you are in for a pleasant suprise when you do. The finish reminds me of a grind finish. Only one last bit to create. I'll do that tonight.
The swarf will be the same as the other vertical shear bits. It will be relatively long straight wisps. The swarf will be pushed into the hole in front of the cutter and will come out the back as shown in the picture below.
A key point is that this produces such a fine finish, that any swarf rubbed by the cutter into the piece will affect the finish. So the best bet is to blow air into the hole to keep the swarf away from the cutter.
Some points about all of these shear bits. The cutting operation creates straight thin wisps of swarf with a 0.001 DOC. If you get very small shards or powder or it smears instead of cutting, there is probably something wrong with your setup or grind.
Lastly, if you have never tried any of these shear bits for fine finishing or accurate turning, you are in for a pleasant suprise when you do. The finish reminds me of a grind finish. Only one last bit to create. I'll do that tonight.