Reaming vs. Drilling Holes In Tube Sheets

[Fender]

I am working on building a steel boiler with copper tubes. The tubes will be 1/2" type K copper, with an O.D. of 0.625". If I use a 16 mm drill for the tube sheets (16mm = 0.6299") this would yield about 0.005" oversize on the holes. Some clearance is a good thing, although I recognize that a drill is not as precise as a reamer. A drill would also give a somewhat rougher finish than a reamer, which may (or may not) be desirable to key in the tubes after rolling (?) I would appreciate any comments on this.

[Harold_V]

I am working on building a steel boiler with copper tubes. The tubes will be 1/2" type K copper, with an O.D. of 0.625". If I use a 16 mm drill for the tube sheets (16mm = 0.6299") this would yield about 0.005" oversize on the holes. Some clearance is a good thing, although I recognize that a drill is not as precise as a reamer. A drill would also give a somewhat rougher finish than a reamer, which may (or may not) be desirable to key in the tubes after rolling (?) I would appreciate any comments on this.

Keeping in mind I am not a boiler builder (but I do have extensive experience in drilling, boring, and reaming holes).
Welding of the tube sheet is likely to distort the drilled holes in proximity of the weld(s). Also, drills have a nasty habit of cutting less than round holes, which may or may not be straight, or on size. While reamers, too, can yield similar errors, they're reduced drastically, so the end result would be more reliable, plus any distortion of hole diameter due to welding would be eliminated.

If you must drill and not ream, don't drill the hole with the target diameter drill. Drill undersized, then open the hole with the proper sized drill after the boiler has been welded. That's not as good as reaming (or boring), but it will yield more consistent hole size, and likely a better surface finish.

Harold

[Greg_Lewis]

Ken Schroeder, who has built perhaps a dozen boilers, told me that he just drills the holes and rolls in the copper tubes. He says he's had no problems with this. It sounds like it should make sense as the soft copper should be forced into the holes by the roller. I'd be sure to anneal the tubes before rolling.

[Fender]

Thanks for the answers so far. Next question: whether drilled or reamed, should the holes have a slight chamfer on either side of the tube sheet? I guess I should have asked that also, but the question didn't occur until now!

[Greg_Lewis]

I didn't chamfer mine. I'd think you'd want the tube and sheet to get a good grip on each other.

[Trainman4602]

Dan your holes are drilling oversize because your drill is ground longer on one of the cutting edges. The rounder you can make the holes the better. an out of round condition will eventually cause a leaky flue.

[Fender]

Dave, I actually haven't bought either a 16mm drill or reamer. I have a 5/8 (0.625") reamer and drill, but nothing oversize. Probably will wait until I have the tubing available to measure before deciding what to get.

[Trainman4602]

Dan
If I where you I would rough drill the holes about a 1/64 smaller then the finished size and set the boring bar to the finish size and just take one finish cut walk over to the next one and take the cut repeat until you have them all don't mess around trying to drill them to finish size the boring will give a nice finish with good concentricity as well.

[John_S]

The two boilers I've built both were drilled undersize prior to welding. After welding they were reamed out to size to fit K copper flues. I did not chamfer the hole, but rather used a sandpaper disc to deburr the inside and outside faces of the tube sheets before rolling the tubes in. End result was a nicely rolled copper tube with no leaks.

[Chris Hollands]

Dan I did the exact thing on my challenger boiler (72 holes times 16 mm), bought a brand new good quality 16mm taper shank drill , and drilled and chamfered them nothing else using my bridgeport mill , as advised to me by the boiler inspector at the time ( Phil Gibbons ,you may remember his injector making articles on this forum), no issues and then rolled the copper tubes .
Using a soild machine like a bridgeport mill with the tube plate bolted down is also very advisable to get accurate results .
The boiler was tested to the Australian boiler code , at the time I was looking at possibly running at 150 psi so the boiler had to be tested to 2 times working pressure for the first test , so 300 psi ( I know some people will say something about stressing the boiler etc ) no issues .
Yes the Australian boiler code is a max of 100 psi but we were looking at higher working pressures at the time .

[Andy Rafferty]

After all the welding ream the holes 5/8 you will faint when you see how out of round K wall is slightly deburr the holes so the flues press in without scoring the tube wall. place all the copper tubes in the freezer and run them out to the tubesheet one at a time. Have a rod in the tubesheets to help align the tube. The sweat forming on the cold tube helps lube the tube have a wood dowell ready with a shoulder to drive the tube into position. get them all in and start your rolling. No Problem.

[Greg_Lewis]

By the way, John Bailie designed a nice d.i.y. tube roller that works great. If you want drawings and info, send me a p.m. with your email address and I'll forward it on to you.