Quick advice needed - enlarging a long blind hole

[laukejas]

The hone sounds like a workable solution with sections of hacksaw blades as cutters. Go slow and vacuum/blow out cuttings regularly. Having a bit hang up with a high torque low rpm drill is hard on the wrist. Hope it works out OK; if not the water will be there for another day.

Alright, so following this advice, I fashioned myself a tool from a car cylinder hone and 3 pieces of hacksaw blade. Really messy, but the best I could do with the few metalworking tools that I have.

Tried it out on a scrap piece of pipe - it does work, but VERY slowly. I'd say it takes around 10 minutes to take off 0.5mm from a single section of the pipe. And like Gonzo said, it does not seem to cut cylindrically, more like an ellipse. I am not sure if it is just repeating the existing ellipse of the tube (like I said, it is not perfectly round to begin with), or making it worse. In any case, I guess this is kind of workable, but will take ages. Also there doesn't seem to be any way to make a depth stop for it so that it doesn't damage the bottom of the pipe.

Those spring hones will not cut a cylindrical hole. It will be uneven. A boring bar of some kind in the only way to do that. Also, pvc is a soft material that will clog any honing stone; they are at most 80 grit. A hole saw will bore that in maybe half an hour. You don't need any giant drill. A regular cordless will work. If the hole saws available are the ones with small teeth for metal, you can keep them from clogging by grinding three or four relief grooves on the cutting edge.

This is actually a sound advice - cutting some of the teeth off a hole saw. It makes sense, by removing the teeth I can fine-tune how much it cuts, reducing the torque requirements. I will get a 83 mm teethed hole saw tomorrow and try again. Pity I didn't buy it today - I told my sad story to the hardware store advisor, and he said that hole saw will absolutely not cut concentrically, but will wander off mid-pipe and cut through the wall. Now that I think about it, it seems to me that perhaps he was talking out of his hat... Hole saw should definitely be self-centering, assuming I can start the cut properly.

 

[wet feet]

Your advisor was correct about the risk of the holesaw deviating from the ideal path.The only way I would expect to avoid such an outcome would be to use the pilot bit -or a plain rod-and run it in a guide disc as I described previously.I would still not expect a battery drill to make rapid progress or to complete the job without some battery changing.Again,I would recommend larger guide discs on the necessarily long extension,both just above the holesaw and close to deck level.You ought to mark the shank of the extension at deck level and this could easily be done by placing the extension and arbor in the tube without the holesaw in place,When you see the mark at deck level-stop drilling!

 

[seasquirt]

Do you have coarser teethed hacksaw blades, like 18 Teeth Per Inch or less ? They would cut more deeply; or copy a suggestion above, and cut out a few teeth from each section, at different points, so the small teeth you do have can bite a little deeper. They hold each other up, and there is minimal spring pressure on the cutting edges when shared by many small points.

The tube being out of round is like an internal cam, and so the blades will follow the shape, and bottom out cutting deepest at their maximum outward velocity. Just keep at it, and test the mast frequently for fit before it is too loose; being too loose in the middle doesn't matter, but the ends do matter for mast stability.

If you do get a hole saw, try it upside down first, if it fits without binding, and it might be self guided following the tube.

 

[laukejas]

Your advisor was correct about the risk of the holesaw deviating from the ideal path.The only way I would expect to avoid such an outcome would be to use the pilot bit -or a plain rod-and run it in a guide disc as I described previously.I would still not expect a battery drill to make rapid progress or to complete the job without some battery changing.Again,I would recommend larger guide discs on the necessarily long extension,both just above the holesaw and close to deck level.You ought to mark the shank of the extension at deck level and this could easily be done by placing the extension and arbor in the tube without the holesaw in place,When you see the mark at deck level-stop drilling!

I wonder though, why would a hole saw (I mean, such as this one, but with some teeth removed like Gonzo suggested) run off-course? Seems to me that the path of least resistance should be in the perfectly centered position, no? Also, the teeth are slightly bent outward to provide clearance for the arbor so it doesn't rub against the wall of whatever it is cutting, but that clearance is very tiny, so that should also help with centering, wouldn't it? Of course, if my first assumption is wrong, then yeah, deviation might build up over time. I will definitely make guide discs. It might be difficult to get them to the required tolerance though, as 3D printing won't work (plastic will melt from the shaft/drill bit rotating in it), and making it from plywood accurately enough might also prove challenging. Not only the diameter has to be perfect, but the concentrically of the disc and it's center hole must be perfect as well. Drilling a hole in plywood, even with a pilot hole and clamped down on a drill press can still lead to a small drill bit wandering, and in this case even a tiny deviation could put the hole saw at a small angle. Or maybe not, I'm not entirely sure.

EDIT: I just realized that I can use that same holesaw to make a perfectly concentric guide disk from some plywood, and if it's undersized - maybe wrap a couple of turns of duct tape on it to expand the diameter just enough. And if it's oversized... Not sure, maybe mount it on a drill and turn it down a little as a makeshift lathe.

 

[laukejas]

Do you have coarser teethed hacksaw blades, like 18 Teeth Per Inch or less ? They would cut more deeply; or copy a suggestion above, and cut out a few teeth from each section, at different points, so the small teeth you do have can bite a little deeper. They hold each other up, and there is minimal spring pressure on the cutting edges when shared by many small points.

The tube being out of round is like an internal cam, and so the blades will follow the shape, and bottom out cutting deepest at their maximum outward velocity. Just keep at it, and test the mast frequently for fit before it is too loose; being too loose in the middle doesn't matter, but the ends do matter for mast stability.

If you do get a hole saw, try it upside down first, if it fits without binding, and it might be self guided following the tube.

I don't have it, but I can get it. Did you mean to say that with this internal cam behavior, the ellipsicity gets worse over time? Or should it be just a regular offset? I'm not entirely sure how that velocity change plays here.

 

[comfisherman]

Oh wow. Thats a lot more complex than the method we used with the hone. We just took emry cloth in a rough grit and wrapped it around the existing hone paddles. Through some electrical tap on the top and bottom and went hog wild. Was essy to rotate when it gummed and the hone just kept steady pressure.

 

[laukejas]

Update: I tried a hole saw on a scrap piece of that pipe. Gonzo was right: for whatever reason it does not self-center, even when the entire arbor is in the pipe. It definitely needs guides above and below. I tried that trick by using a disc cut out by that very same saw, with some electrical tape around it to create an interference fit, but that wasn't sturdy enough and the saw still went off-center. I guess the disc has to be both very precise and very rigid. Also it seems that by the time the disc above the saw engages, the saw will have wandered already - it is very prone to that. On the up side, even with all the teeth intact, the torque isn't that bad, and the cutting speed is significant. If only I could make proper guides, the entire job could be done in 10 minutes. Not with my dewalt screwdriver though - it started smelling funny after just 10 seconds of drilling, so I won't try that again with it.

 

[wet feet]

As i wrote in post #47 you should use a disc on the pilot drill of the holesaw that is a snug fit in the tube to keep the cutting centred.Extra guide discs above the holesaw,to the correct hole size will also help.You were warned about the drill and now you know why.

The hone with springs pressing outward will tend to remove a similar amount from the entire path of rotation but I doubt that it would make any noticeable difference to the performance of the boat.You may well need to hone the enlarged inner surface to lose ridges or irregularities at the end of the job.

 

[laukejas]

As i wrote in post #47 you should use a disc on the pilot drill of the holesaw that is a snug fit in the tube to keep the cutting centred.Extra guide discs above the holesaw,to the correct hole size will also help.You were warned about the drill and now you know why.

The hone with springs pressing outward will tend to remove a similar amount from the entire path of rotation but I doubt that it would make any noticeable difference to the performance of the boat.You may well need to hone the enlarged inner surface to lose ridges or irregularities at the end of the job.

Yeah, I know, I was just testing how it all behaves to get a sense of how accurate this whole setup needs to be to work. Turns out, very accurate. I will try to make these discs. Getting a snug fit might be difficult, because the pipe isn't perfectly round, and the diameter that the hole saw leaves behind seems to be a tad above 83mm, more like 83.2. Difficult to tell, since the surface is pretty rough. Honing it afterwards might be quite challenging, but maybe the honing tool I tried before could work. A lot of testing ahead.

 

[BlueBell]

With a dial gauge you can measure and remove the oval by hand-sanding it vertically to finished dimension, no?

 

[trip the light fandango]

Be aware that heat is how you bend PVC so part of your process must attempt to minimise any local increase in temperature. Heat will normally cause expansion so vertical removal of plastic from top to bottom working around the tube, as a clock 12 then 6 ,9 then 3 etc may be the best approach until nearly finished.

 

[wet feet]

With a dial gauge you can measure and remove the oval by hand-sanding it vertically to finished dimension, no?

Actually,no.You might increase the dimension,but you have little reason to believe that the increase in diameter will be concentric and for the purposes of this task,the enlarged hole needs to be both concentric and of the necessary dimension.Which is why we have recommended the use of guide discs to determine the path of the cutting device that is chosen.

 

[BlueBell]

wet feet you have misunderstood me
I am suggesting a finishing technique to reduce the oval
Thanks for your feedback though

 

[fallguy]

You can also file every other outward tooth to reduce the torque and help with starting.

 

[wet feet]

wet feet you have misunderstood me
I am suggesting a finishing technique to reduce the oval
Thanks for your feedback though

Since you didn't explain how you would ensure concentricity and axial alignment,I couldn't see what would be usefully achieved other than local enlargement.Simply increasing a dimension is of little use if the result is a banana shaped bore having a diameter that can't actually accept the mast from top to bottom.If a diagram should have been attached,adding it now would be helpful.