Line boring vs Drilling vs Boring bar

[DJShoe]

My first post here so I'll try to get all the details out first.

Med sized machine - lathe 14x40, 2.5hp
Machine has a MT3
Material 6061 alum extruded tube ID is 35mm/1.377"
OD is 57mm/2.25"
Length is 355mm/14"

I need to bore out the ID another 5mm/0.19685" for all 14" and it needs to be repeatable.
Sending out is not an option.

I watched Keith Fenner on youtube and saw him line bore and looked like it would do the job. But I wanted to get some other opinions. I have a 39mm twist drill bit but it's MT5, and I don't have a boring bar big enough or tuned/dampened.

So I either make a boring bar for line boring, buy another monster twist bit and recut the taper, or go all out and get, try to get, a dampening bar with the right size head. I'm just concerned I should just get a dedicated machine instead of that last option.

I'm leaning towards line borning.

Dave

 

[Gordon Heaton]

Dave, you're talking twist drills and measurements to 5 decimal places! First question anyone here will ask before offering advice is: What are your tolerances? Tolerances will need to include concentricity too. A lot of extruded aluminum tube is woefully off-center, so drilling will get you a bigger hole but neither straight nor centered.

 

[DJShoe]

Hi Gordon,

I just wanted to get as much out there as I could, knew I'd forget something. The tolerance will be somewhere around .01. The ID will be broached to finished ID. And once the ID is sized, prior to broaching, the OD will be turned between centers, was planning on capping the ends. I might change the operation to OD then ID. I'm still working it out in my head. If I line bore, then OD first to get a good grip on the clamp I'll have to make to hold the piece to then do the boring. And there is certainly enough material there to bring them both concentric.

 

[Gordon Heaton]

Ok, turning OC centered on ID takes care of all that, but are you broaching a shape other than round? Or were you possibly referring to reaming? I'm not sure how much wander you'd get with an ordinary twist drill but it might be worth a couple of tests to find out. A line-boring setup can be fitted through the spindle, allowing the stock to be mounted in the chuck in the usual manner with the boring bar pushed through by the tailstock (pulled by the carriage). For the equipment I have (all manual) that's probably how I'd do it for speed and repeatability. A boring bar that size and length would be easy to make and you wouldn't need any exotic material or heat-treat for the bar itself.

 

[Mtndew]

IF you decide to try a twist drill, make sure it has more than 2 flutes.
3 or 4 flutes will do better in tubing.

 

[michiganbuck]

If the OD was turned first between centers and then one end set true in a 4 jaw chuck with the out end in a steady the bore would run true to the OD. Agree that I don't know if that is important.

The way I set a steady is to hold the part between centers and bring my steady up to .001 (or what) shim.

Yes, nothing wrong with turning the OD last if you know that the bore will run close/straight enough to still have stock on the OD to make size.

QT Op: (The tolerance will be somewhere around .01) .01 mm (.0004)

Relieving the ID so to only have a portion of the finished size/shape might be an option (if the part design would allow.)doing that design would/could be accomplished with the use of the 4 jaw and the steady.

Tough would be measuring the chuck held end, if it was held in a chuck.

 

[henrya]

We really need a little more information. How many will you make? What is the desired OD? How are the ends finished?

I’d buy a boring bar that takes high positive inserts and bore the ID while the tail end is supported in a steadyrest. Then make a mandrel that gives a good wring on by hand fit set up in the chuck and tailstock center and turn the OD to size.

You need the boring bar anyway. I have 8 or 10 bars from tiny up to 1”. I use them a lot. Your bore is long to diameter. I’d get a 1 1/4” bar with sharp inserts and take light cuts to minimize taper.

 

[Overland]

Ok, turning OC centered on ID takes care of all that, but are you broaching a shape other than round? Or were you possibly referring to reaming? I'm not sure how much wander you'd get with an ordinary twist drill but it might be worth a couple of tests to find out. A line-boring setup can be fitted through the spindle, allowing the stock to be mounted in the chuck in the usual manner with the boring bar pushed through by the tailstock (pulled by the carriage). For the equipment I have (all manual) that's probably how I'd do it for speed and repeatability. A boring bar that size and length would be easy to make and you wouldn't need any exotic material or heat-treat for the bar itself.

Gordon,
Not understanding your line-bore thinking.
Are you suggesting holding the bar in either the toolpost, or tailstock, with some sort of busing in the spindle, behind the chuck ?
Thanks,
Bob

 

[PackardV8]

When you say "repeatable" how many parts?

When you say "buy another machine" what is the budget?

FWIW, that line boring operation could be done on an automotive cam tunnel line boring machine. I saw one go for cheap at an auction recently.

jack vines

 

[kenton]

I'd be looking for a used 40mm or 1.562 shell reamer. Ream from both ends if necessary.

https://drillsandcutters.com/spiral-flute-shell-reamers/#/filter:size_facet:1-9$252F16$2522

Make your own arbor if you want to.

 

[Gordon Heaton]

. . .Are you suggesting holding the bar in either the toolpost, or tailstock, with some sort of busing in the spindle, behind the chuck ?
Thanks,
Bob

Yes, that exactly! I've done it in the past when I needed a similar process. I made the boring bar with a step-down suitable for passing through a bushing at the back of the spindle and held the other end in the MT3 tailstock. Since the bar is stationary there isn't any whip and I got good results. Now, I read the tolerance in post #3 to be .01 inches, so if he meant .01 mm it might not work. The inch tolerance seemed reasonable for a pre-broach hole.

 

[henrya]

Hi Gordon,

I just wanted to get as much out there as I could, knew I'd forget something. The tolerance will be somewhere around .01. The ID will be broached to finished ID. And once the ID is sized, prior to broaching, the OD will be turned between centers, was planning on capping the ends. I might change the operation to OD then ID. I'm still working it out in my head. If I line bore, then OD first to get a good grip on the clamp I'll have to make to hold the piece to then do the boring. And there is certainly enough material there to bring them both concentric.

.01 inch or mm? And in what dimension and how straight does the hole need to be?
You have to do better to get a decent answer.
Try harder.