Boring A Long Tube

QCTP's have a smaller foot print, the tool carrier hangs out in space only supported by the dovetails in the post, and those dovetails that are prone to rocking. Those fortunate to own an Aloris probably never notice problems, dunno, I can buy a new machine for the cost of an Aloris...

A boring bar held in a 4 way will be applying force directly or near directly down onto the cross slide. The wider foot print and vertical pressure make them more stable.

It is just simple physics, a 4way tool post is just by it's nature more rigid.
Their downside is that they are just enough of a hassle to set up correctly, that they tend to get set up almost correctly more often. QCTP are just so convenient that any bad is out weighed by the good. Plenty of folks stick with the 4way though
 
I am well aware of the pros and cons of a 4-way tool post, having used one extensively. However, I disagree that it is the best way to hold a boring bar. This is not just about the stability of the tool post itself. It is about how well the bar is held in the tool holder.

A round bar held in a slot that locks the bar down with screws provides contact at two very small spots on the bar provided that you shim the inside of the slot to bring the screws on center. You must also shim the bar evenly to bring the tip of the cutter 0.005-0.010" above center. In addition, the screws may, and often do, displace the bar, causing misalignment. Given that the tangential forces experienced by the bar tend to twist the bar, this arrangement is the worst way to hold a boring bar.

The most rigid method of holding a boring bar is in a holder that completely encases the circumference of the bar in a finely finished and hardened bore. Ideally, this holder will be 3-4 times the diameter of the bar being held. Screws are then used to tighten the circumference of the holder to rigidly hold the bar in place, thereby reducing vibration and the possibility of the bar moving as it cuts. An example of such a holder is the Aloris 4D; it is not a coincidence that this holder for 3/4" diameter bars is 3" long, finely finished and hardened.

It is well known that a QCTP can easily sustain the cutting forces encountered while boring. I own an Aloris AXA post that holds a 3/4" bar under full extension quite well but even my little OXA post can handle a 3/8" carbide bar at full extension, 4" deep in a hole. The issue when boring is not just the downward (tangential) pressure like it is with external turning. It is the tangential and radial forces that we have to contend with and a good holder is the best way to achieve that.

Don't take my word for it. See this article by Dr. Uwe Heinrich of Micro 100:
http://www.ctemag.com/pdf/2005/0512-Boring.pdf
 
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If you don't want to line bore it, find a piece of rod 1"-1-1/2, doesn't really matter what grade but the bigger the better. mill/grind, angle grind/file one end to fit your tool holder making sure the center of the bar is close to the screws of your tool holder. Cut off bar to whatever you need . Drill hole at 30-45 degrees parallel to the tool holder base in end of bar to suit a piece of square HSS tool steel. No need to square off the hole. Put a set screw in top of bar to hold tool. Sharpen tool to suit and cut. Fine tune on scrap first.
John.
 
mikey said:
I am well aware of the pros and cons of a 4-way tool post, having used one extensively. However, I disagree that it is the best way to hold a boring bar. This is not just about the stability of the tool post itself. It is about how well the bar is held in the tool holder.

A round bar held in a slot that locks the bar down with screws provides contact at two very small spots on the bar provided that you shim the inside of the slot to bring the screws on center. You must also shim the bar evenly to bring the tip of the cutter 0.005-0.010" above center. In addition, the screws may, and often do, displace the bar, causing misalignment. Given that the tangential forces experienced by the bar tend to twist the bar, this arrangement is the worst way to hold a boring bar.

The most rigid method of holding a boring bar is in a holder that completely encases the circumference of the bar in a finely finished and hardened bore. Ideally, this holder will be 3-4 times the diameter of the bar being held. Screws are then used to tighten the circumference of the holder to rigidly hold the bar in place, thereby reducing vibration and the possibility of the bar moving as it cuts. An example of such a holder is the Aloris 4D; it is not a coincidence that this holder for 3/4" diameter bars is 3" long, finely finished and hardened.

It is well known that a QCTP can easily sustain the cutting forces encountered while boring. I own an Aloris AXA post that holds a 3/4" bar under full extension quite well but even my little OXA post can handle a 3/8" carbide bar at full extension, 4" deep in a hole. The issue when boring is not just the downward (tangential) pressure like it is with external turning. It is the tangential and radial forces that we have to contend with and a good holder is the best way to achieve that.

Don't take my word for it. See this article by Dr. Uwe Heinrich of Micro 100:
http://www.ctemag.com/pdf/2005/0512-Boring.pdf
Click to expand...

I never said the 4 way was the best.
I merely said that by it's very nature it is more rigid.

Boring bars for 4 way posts have a square sleeve for clamping, or have flats ground onto them.
As for deflection, if you are deflecting a flat or un-sleeved 3/4" (or even a 1/2") bar in any meaningful way, they you are way past proper torque.
840788.jpg 18E950_AW02?$mdmain$.jpg
(the set on the right is a Micro 100 set)

I am not sure what your point is. We all know that Aloris and other style QCTP's do the job for boring, and we all know that shimming is a drag (I even pointed that out). None of this changes the points I made above. As for a carbide bar, the OP was pretty clear that he did not want to spend the money on a piece of tool steel for just this job, I think it is safe to say that a carbide bar is also out of the question.

As for Dr. Heinrich, all I can say is be wary of a person who wears a lab coat and writes sales brochures. Those of us that have math degrees know that all we have to do is write a little bit of symbolic math and every one goes "OK, I take your word for it because you are obviously smarter than me." Any time I want my boss to leave me alone, I just start babbling about PDE's or the pros and cons of different ways of arriving at R^2 and she goes away. Seriously, I can just picture a conversation between a shop foreman and a machinist using Dr. Heinrich's math:
Boss: "Uhhhh, Bob, what are you doing?"
Bob: "Setting up the machine for boring."
Boss: "You need a calculator for that?"
Bob: "I have to calculate the modulus of deflection so I know how far above the centerline to set the tool."
Boss: <blank stare>
Bob: "See, the tool will flex down when I start the cut. Not only that, the tool will twist, so I also have to calculate the optimal depth of cut so the tool comes in line when it is under load."
Boss: "Ummm..."
Bob: "I am having a bit of trouble with this though. The optimal depth of cut does not work out with the chip load of my tool when I try to figure feed and speed."
<pregnant silence>
Bob: "I was about to call Dr. Heinrich at Micro 100 and see if he can help me get this straightened out."
Boss: "You're fired."
I suppose that a CNC machine dedicated to high production output of a single part would be set up like this. But this is a hobby forum, and we tend to use the "cut and adjust" method.
We can spend all day discussing clamping length, smoothness of clamping surface, etc. So we clamp our 3/4" boring bar into a 3" long holder full of confidence that we have a rigid set up. Then we drop the tool holder into a tool post that only has about a 2" diameter contact ring with the cross slide. A chain is only a strong as the weakest link.
A 4 way tool post is substantially larger than a QCTP, and has substantially more contact area with the cross slide. typically, you can also put a much larger tool into a 4 way than into a QCTP. Now, I agree that putting a round tool in a square slot will give poor results some of the time, but this is not the fault of the tool post, it is the fault of the user for not using the right tool.

Personally, I despise the 4way, and use an Aloris style QCTP. I am just not a blind to the fact that I gave up some rigidity for convenience. I have no experience with a real Aloris. Maybe it really is as rigid as a 4 way. I will probably never know, I can buy a used lathe for the price of an Aloris set.
 
mikey said:
I am well aware of the pros and cons of a 4-way tool post, having used one extensively. However, I disagree that it is the best way to hold a boring bar. This is not just about the stability of the tool post itself. It is about how well the bar is held in the tool holder.
Click to expand...
I use a (shop made) 4-way, not having money for fancy stuff. However, for my larger boring bars I just use a simple split block that entirely replaces the tool post. It's as easy to make as the boring bar itself and while not perhaps not as convenient as an Aloris I can't imagine how anything more complex could be as rigid.
 
Guys

I made it and it is done now. The long 1/2" boring tool that I made worked OK. I mounted the aluminum tube on the 3-jaw chuck and centered it to make sure that it ran true within 1 to 1.5 thousandth at the far end. Run out near the chuck was not readable or under 0.001". Ran the lathe at its slowest speed (I am not sure if low speed was proper but I thought low speed would give me time to react to any "accident"). Mounted my very special boring tool in 4-way tool post and plunged only 1/2 thou for each pass. Sure, I had to be very patient because each pass took a very long time. In the end after about 30 minutes of running time the tube was ready. My accessories fit properly. The 2" eye-pieces fit well. I am happy at the outcome.

I want to thank everyone for your truly great suggestions. I was lost without them.

A couple of lessons learnt (by me).
1. I should do the boring operation before I work on the outside of the tube. I had to be extra careful because I kept it to the end which was a mistake. Next time I will do it first.
2. Next time I would also make an aluminum collet ring for gripping the tube in the chuck. That would make the operation much easier and risk free. I did use some aluminum shims between the jaws and the aluminum tube. These shims may have created some runout.
3. I also managed to "get to know" my 3-jaw chuck better. It has a runout of under 0.0005". Made me feel good.

Thank you all,
Prasad
 
kd4gij said:
1/2" boring bar 5 1/2" deep .050 per pass.
Click to expand...
Hi Martin

True but I could not risk it. I had the tube gripped lightly in the 3-jaw chuck. A little extra force could have moved it. Also all I had to do was enlarge 0.008" in diameter of the tube. I did not want to risk wrecking the piece.

I will, for sure plunge deeper when I do it on another tube for my next Crayford focuser. In fact I will be making one to replace a crappy plastic Chinese R&P focuser on an iOptron 80mm f/5 refractor soon.

Thanks
Prasad
 
I didn't think you would take that deep of cut for your project. light cuts and a good sharp HSS bit and you should be fine. My bar probably a little more ridged than mild steel. Just posted to show 1/2 can work. Good luck. looking forward to seeing how you make out.
 
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