Boring the outside of a cylinder

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Boring the outside of a cylinder

Post by Torch » Tue Oct 26, 2010 6:39 am

I want to make a part where a small (1/2" - 3/4") aluminium cylinder will nestle snugly against a 2-3/8" aluminium cylinder at an angle (about 45°, but I haven't measured it exactly yet). The larger cylinder is a hollow sailboat mast, so it ain't going on the table!

It seems to me that the best way to do this is to mount the small cylinder in a fixture at the required angle and mill a groove along the outside using a 2-3/8" boring head. I don't have one of those just yet. What about using a fly-cutter as a boring head for this operation? Or is that just asking for trouble?

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Lew Hartswick
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Re: Boring the outside of a cylinder

Post by Lew Hartswick » Tue Oct 26, 2010 10:25 am

You will have a devil of a time getting the bit set to the right radius. But if the bit is ground
for cutting in the axial direction rather than the radial direction it possibly could work out.
Maybe I'll try it sometime :-)

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Re: Boring the outside of a cylinder

Post by TimTheGrim » Tue Oct 26, 2010 2:46 pm

First...Make sure your machine is isolated form it's power source while doing this.
Put a T-nut in a slot off to one side and bottom out a short hold-down stud so it's tight enough to stay in place and be as close to vertical as you can get it.
The stud can't be all-thread it has to have a full diameter section. A sawed off bolt could do too.
Measure the OD of the stud. Let's say it's .500 in some commercial type clamping sets.
Dial indicate your zero over the stud and set your dials or DRO.
Put on some Dykem or paint the stud with a marker pen.
Move the table off to a point that equals the sum of your two radii, ie. 1.1875 +.250.
Put your fly-cutter in the quill with the bit clamping screws just snug enough to allow you to tap the blank end of the bit to position it.
Having a new bit and good screws helps with this part.
Bring the quill down so you can swing the cutter and tap the bit until the cutting edge just marks the OD of the stud as it swings by.
Tighten your bit holding screws and check again to see if it moved and needs readjustment.
There is your cutter that will do your 1.1875 Radius/2.375 diameter.
Certainly it won't be accurate to 4 decimal places as the measurement examples I used but it will be pretty darn close and work fine for what you're trying to do.

Now all you've got to do is position your workpiece in the vise or fixture so that everything clears.
The real limitation when using a fly-cutter for this type of cut is how far away from the quill the cutter is and how long the part you need to machine is.

Post some pics when your done. :D
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'96 Birmingham mill, Enco 13x40 GH and Craftsman 6x18 lathes, Reid 2C surface grinder. Duro Bandsaw and lots of tooling from 30+ years in the trade. Now I boil oil.

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Re: Boring the outside of a cylinder

Post by david5605 » Tue Oct 26, 2010 3:48 pm

Why not use one of the tube coping calculators on the internet and just make a pattern you can trace on the tube and then grind/file away? Done in 5 minutes and no fixturing, etc.

Here is one such calculator...

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Re: Boring the outside of a cylinder

Post by Torch » Tue Oct 26, 2010 7:35 pm

Ok, I tried it -- after reading Lew's suggestion and before reading Tim's. I touched up the bit as Lew suggested. I don't bother with a chip breaker on a fly-cutter so it didn't take much reshaping, and now it will cut both ways.

I actually did something similar to what Tim suggested, but I just used the side of the vice as my reference point for finding centre, offset the table the required amount and slid the bit in the holder until it just touched the vice before tightening. High precision was not really required for this particular job -- the mast is a aluminium extrusion that's been flexing and swaying for over 35 years now. Two digits of precision is probably overkill!

Anyway, the fly-cutter did what I needed it to do. I made a really simple fixture to hold the cylinders -- just a scrap block of steel squared off with a 1/2" groove milled down the centre deep enough that the 3/4" cylinder rested on the edges. I set it at the required 53° and clamped it all in my magic jaws vice, ball end down. I kept the cuts shallow -- 0.030" and the speed down around 200 rpm. (Final 10 thou pass was done at 400, for a nicer finish) The parts nestle snugly along the mast at the right angle.

I'm sorry, I didn't get any pictures. It started to rain just as I finished milling the bolt hole slots, so I tossed them in the cuddy and battened down the cockpit cover -- I don't want dampness in there over the winter.

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