Can you turn down a forstner bit?

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HGEnco
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Joined: Wed Jul 25, 2007 8:31 pm

Can you turn down a forstner bit?

Post by HGEnco » Sun Jul 29, 2007 1:52 am

Hi All,

I need to bore out a piece of PVC pipe....130 of them approx 4.5" long. I want to find a fast, accurate way to bore them all almost exactly the same. Is it possible to turn down a forstner bit, say 30 mils or so to get an exact size that I need and put it on the drill press? Is this even a viable option? I know I could bore them out on my lathe with the boring bar, but that would be several passes etc... I was just wondering if there was a quicker method....ie the forstner bit question I was wondering about.

I thought about fly cutting, but decided against it because I have to take some big cuts especially on my 2" size tube because the ID is 1.13" and I have to make it 1.84"...(I hear fly cutting can be a little dangerous also)...

I am new to this so these may be dumb questions...but let me know what you think.

Thanks,
HGEnco

Harold_V
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Post by Harold_V » Sun Jul 29, 2007 3:56 am

It would help a great deal if you described the end product. Tolerance on finish size, and finish requirements will dictate how you should approach the job.

As for machining a Forstner bit, I have serious doubts that it would machine well, although it could be easily ground to size with a cylindrical or cutter grinder. Opening the hole with the bit may not leave a surface finish to your liking, and may also heat up and create serious problems in use. PVC pipe becomes very soft at elevated temperatures, which are easily raised at the point of cut. I fear you're going to have to bore them if you want a decent finish and close size control.

I'm of the opinion you're going to have problems with this project. You're left with such a thin wall section that it will be difficult to hold without crushing. Have you considered how you're going to hold them? Also, have you considered what the exterior is going to do when you remove the core? It most likely will not remain as it is.

Harold

thomas harris
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Post by thomas harris » Sun Jul 29, 2007 8:52 am

How accurate does the hole need to be? This is a bit off the wall, but here goes: Could you chuck the forster in a drill, then let the drill take off the needed material from the sides. You would need to watch the heat and possibly debur the edges with something afterwards. I assume the proper grinding equipment is not availible when suggesting this somewhat crude method. I've used this on occasion with hardened material and with careful handsthe grind stays fairly concentric.

Mcgyver
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Post by Mcgyver » Sun Jul 29, 2007 9:54 am

you may have some trouble holding a 2" pvc pipe with an resultant 80 thou wall thickness so that you can take out .7 in once pass - you might have to devise a collet (i call it a collet, but its not really a collet, more a split sleeve made out of steel) that goes in the three jaw so that there was equal pressure all around rather than in three points.

I've cut pvc with a forstner bit, works ok, but i was only cross drilling so don't know what the heat build up would be at 4.5" trying boring with the next size down forstner then second op with a boring bar...you can take a big whopping cut in plastic. I'm not sure why drilling .7" will work but a single point boring cut at 350 won't...maybe because the action is balanced? have you tried boring?

the forstner bit is hardened, so as Harold says you would have grind. with the right equipment grinding a few thou is ok, but 30 thou would be tedious - unlikely the homeshop has the equipment anyway.

if you really want production without that second op, consider making your own cutter from tool steel then hardening it - this is a very common home shop dodge to get the tooling you need. still, if mass production's the goal, my first try would be 1) a slit steel collect to firmly hold the pipe in the three jaw and 2) specially ground boring bar to take it all off in one pass
Last edited by Mcgyver on Sun Jul 29, 2007 10:05 am, edited 2 times in total.

Mcgyver
Posts: 241
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Post by Mcgyver » Sun Jul 29, 2007 10:03 am

here's another idea. vacuum tubing is 2" dia with .067" dia wall thickness......that is very close...could the design be modified to use this cheap available PVC?

thomas harris
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Post by thomas harris » Sun Jul 29, 2007 10:59 am

thomas harris wrote:How accurate does the hole need to be? This is a bit off the wall, but here goes: Could you chuck the forster in a drill, then let the drill take off the needed material from the sides. You would need to watch the heat and possibly debur the edges with something afterwards. I assume the proper grinding equipment is not availible when suggesting this somewhat crude method. I've used this on occasion with hardened material and with careful handsthe grind stays fairly concentric.
Hate to quote myself,but I left out something important. I meant chuck in drill, then use handrill as a toolpost grinder. Hold int against the bench grinder wheel while running the drill.

geraldvowles
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Deep-boring PVC

Post by geraldvowles » Sun Jul 29, 2007 11:16 am

I'm watching this thread with much interest as I just asked yesterday about a similar PVC deep boring requirement (except with heavy wall) on the gunsmithing forum. The response I got was to use what's called a gun-drill. Don't know if that might be a possible avenue for you?
-Gerry
A Legend in His Own Mind

Harold_V
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Post by Harold_V » Sun Jul 29, 2007 1:55 pm

thomas harris wrote:
thomas harris wrote:How accurate does the hole need to be? This is a bit off the wall, but here goes: Could you chuck the forster in a drill, then let the drill take off the needed material from the sides. You would need to watch the heat and possibly debur the edges with something afterwards. I assume the proper grinding equipment is not availible when suggesting this somewhat crude method. I've used this on occasion with hardened material and with careful handsthe grind stays fairly concentric.
Hate to quote myself,but I left out something important. I meant chuck in drill, then use handrill as a toolpost grinder. Hold int against the bench grinder wheel while running the drill.
This has the potential to leave a cutter that has a larger body size than the size at the cutting edge, due to the relief of the teeth. In order to avoid the problem, the entire assembly should be rigid, revolving around a fixed center, with a wheel that isn't capable of grinding deeper when the relief area is presented to the wheel. Very hard to avoid that when grinding by hand. It takes only the slightest amount of corner roundind to result in a useless tool. The cutter would bind in the hole, and create heat beyond an acceptance level.

Harold

Russ Hanscom
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Post by Russ Hanscom » Sun Jul 29, 2007 2:54 pm

Some time ago I needed to reclaim a bunch of PVC fittings. The tool that worked best was similar to a counterbore, or spade drill, with a pilot. For the size you are wanting, you might consider making an aluminum or mild steel body with provisions for four, six, or other convenient number of cutter teeth. Nothing fancy is required since the stresses are low. The pilot seemed to help with reducing distortion of the parts.

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