Machining a 70mm hole in steel

I think i may know the answer but thought i would see how you guys would do it.

the hole needs to 70mm dia is a 65mm block of steel

have both lathe and mill. Have drills up to 22mm and boring tool and bars.

70mm dia hole in a 65mm block should be interesting

Roy

Edited By roy entwistle on 22/12/2019 09:30:00

Gas axe.

No, just allowing my agricultural background to take over.

Depends on how big the 65 mm (which I assume is material thickness) block is, also depends on how accurately you want to keep the hole diameter and surface finish -

First choice - quickest and most accurate - 4 jaw in the lathe, pilot drill, boring bar. First reserve first choice substitute face plate for 4 jaw.

Second choice if the block is too big to swing - milling machine and boring head. Slow and less accurate, or at least less reliably accurate in my hands, than the lathe

Third choice (getting rougher by the minute) pilot drill, holesaw in a mag drill, from both sides. Pushing the limits of the technology, but it'll go.

Fourth choice , yes the gas axe

Fifth choice - sub it out to someone with bigger machinery that I've got

Season's greetings all

I had the same thought. It would be better if the question gave some more detail. I ASSUME 65mm is the thickness. The answers would be very different between a block 80mm square and a block 1 metre square. (Unless he has a VERY large lathe and mill.)

Les.

of course 65mm thick

made a start. up to 32mm dia using mill.  boring bar next

making a pillar drill table holder that will grip around the colum

Edited By petro1head on 22/12/2019 10:37:30

There are two obvious solutions, although I don't think one can automatically say one methed is faster and/or more accurate than another. It will depend upon the equipment available. One, drill and bore in the lathe. How quick will depend upon the largest drill available. My largest drill is 1-7/8", which still leaves a fair amount to be removed with a boring bar. Two, drill a series of thru holes, which slightly overlap, on a circle. Of course that's a lot easier with a DRO that has a bolt circle function. Once drilled the centre can be tapped out leaving a few tens of thou to be cleaned up with a boring head. With method two you get get keep the core in the offcuts bin as well.

Andrew

When I did a similar job, albeit a larger hole in thinner (2" ish I think) material I drilled a circle of barely overlapping holes and knocked the blank out. Drill 3/8" or so diameter leaving maybe 1/16" to clean out at the thinnest point sounds about right. Job for a 4 facet drill really but a pilot hole most of the way through kept the drill straight enough for me. Final bit was done with the bigger drill alone so I filled the holes with cutting lube and made sure the drill was sharp. A decent pillar drill on slow speed helped as the drill was really a bit large to go through without a pilot hole.

Attacked the ruff hole with a big half round file to take the pointy bits off before finishing on the mill. Rotary table and decent size cutter for me but a boring head will get you there. A bit more slowly.

Clive

Thanks for the thoughts.

this will take quite a while but actually very therapeutic. Radio on plenty coffee and fags

got the lathe at low speed, about 300rpm, any higher and it would vibrate out the hut and just taking 1mm cuts

hey ho, back to work we go

Up to 51mm, down tools for lunch

Re both hole-saws and chain-drilling:

You might, in fact almost certainly need at that depth, to use a hole-saw from both sides, but not for the finished size whatever it says on the saw's box! (Unless the hole does not need to be accurate: a hole-saw is not intended as a precision tool.)

Saw appreciably smaller diameter, blunt the core's edges and pop it in the "come-in-handy box"; and bore the hole to finish either on the lathe or milling-machine (boring-head).

Chain-drilling: I have had to use it occasionally but the "teeth" are very unkind to the finishing-machine and tooling, so it is best to remove most of the ridges' bulk by filing or grinding. You can of course also keep a chain-drilled core in the "C-I-H-B" - but at least blunt the nasty fangs first or the core will hide, ready to ambush your rummaging hand!

Boring-heads: not intrinsically less accurate than a conventional boring-tool in the lathe, but a lot slower to use. I the option is available, I would suggest the most accurate would be a hefty boring-bar between centres.

Shouldn't take any time at all with 1mm cuts.

Was thinking rotabroach but it wont cut through 65mm thick unless I started smaller and went up in diameters to make a pocket and then cut through with a second pass. Finish off with a boring head.

You would think so, however even though i am using a 16mm boring bar, i seem to get quite a bit of deflection

Edited By petro1head on 22/12/2019 20:55:20

The Smart & Brown model A will just take 20mm in the toolpost as long as the back has a flat put on it to enable the screws to reach the centre line. One day I might make another top for the rear parting toolpost to take a left hand 25mm bar and run in reverse.

What lathe do you have?

Edited By old mart on 22/12/2019 21:15:33

Warco Wm290

The boring bar shouldn't be deflecting that much. What insert are you usng and where did it come from? Also what feedrate is being used?

Even if the bar is deflecting it isn't a problem. As you get near to final size take a couple of spring cuts without moving the cross slide before measuring the bore.

Andrew

That 16mm bar is projecting over 4 diameters and there is the flex in the toolpost and the compound to add. I don't think I would have the nerve to go for more than a 1/2mm cut.                                                                                         Inserts for stainless are sharper than steel types and might be worth trying, and for finishing with 0.002" or less doc, the aluminium type are very good, and cheap Chinese ones are getting better all the time.

Edited By old mart on 22/12/2019 21:44:07

Bought it from Arc Here

Kind of what I thought, when I get close will use slowest feed rate and minimal cuts

Edited By petro1head on 22/12/2019 21:47:29

The trouble with inserts for steel is they only cut nicely with deep cuts, when you start creeping up at the last bit, the surface finish drops dramatically. That's when the shiny sharp aluminium grades come into their own, still cutting with 0.0005" doc.

To be honest not too bothered about the finish for this job but I get your point