trepanning?

Grant If you are not to worried about the waste on the outside or the width of the cut, my standard practice was to make a cut down as far as practical then move out about two thirds of the width of the tool(or in depending on if it was inside or out trepanning) and machine down to the level of the first cut then down the same distance as the first cut, and repeat until finished. If the tool is stalling, A- its too wide or B- it is rubbing on the sides and lacks clearance. A 1/4 was probably way to wide. I used a lot of Iscar treppaning cutters but thats not an option to the home machinist unless he has deep pockets. You could get away with grinding the 1/4 down to an 1/8 wide. I would have thought twice about using a 1/4 wide cutter even on the 30hp Lodge & Shipley I used to run(mainly because of the noise it would make :-)

James Crombie

Well, on all lighter lathes, and in most circumstances, a wide chip is what makes things difficult. Only a totally stiff setup including the boring tool itself will allow you to just plow in with a boring tool. So, you often have to use a thin tool, and take a "walking" approach to it. By that, I mean cutting on the left side until it chatters, then cut on the right side of the groove until it chatters, then back to the left, etc. You can often get through the part like that without making the groove much wider than the tool. But, the face of the tool plus both flanks cutting in is just too much for most boring setups. One trick I sometimes use to allow the tool to fit the groove better is to set it so it has half as much above the centerline as below it. This minimizes the rubbing of the bottom of the tool on the groove, and doesn't change the effective top rake a whole lot.

I'm not sure what material this was, or what speeds the SB has in back gear. You want to use a fairly aggressive infeed on any material that can work harden, but you want to use a modest feed to prevent chatter. That is a conundrum, of course. I would probably do the 3" in direct drive rather than back-gear speeds on the lathes I have. The 5" dia may well require back-gear speeds on all lathes.

Jon

Oh, my gosh, the 1/4" was the height of the cutter, NOT the width! The width was probably about a sixteenth or so. I did do your trick of widening the gap on the waste side, that's how I managed eventually. I probably screwed up grinding the cutter. Fun morning, though, and eventually the part turned out great. I haven't done much precision HSS lathe bit grinding for a few years, need to get my chops back together. Thanks!

GWE

Trepanning = Black Magic! Did you burry chicken guts in the back yard during a full moon? I did see a real trepanning tool once. I have to do 4" holes in 3/4" D-2 every so often and I DREAD IT! But the slugs are worth it, they come in handy too often. My advice is: lots of HP, a number of tools ground with care and empathy, lots of ridgidity and lots of coolant.

I figure you need at least .021" of side clearance at the bottom of a

1/4" high cutter on a 3" circle. I'd bet your cutter was dragging in the trench.

Tom If you do the D2 thing with any regularity then it would probably pay in time and effort to invest in a carbide face grooving tool similar to this

have used this and similar Kennametal tools. How many hours do you spend grinding the tools :-) We used to make a lot of sleeves for the gearbox on a P&W PW100 turboprop. We first tried using bar stock 2024 but there was to much stress in the bar and the sleeves would taper 5-10 thou after parting off. So we tried plate instead and would use one peice of plate to make multiple sizes of sleeves. We would rough machine the OD, then trepan the ID, finish the OD and then partoff. Then repeat for the next size down etc. The tools saved us tons of time and material.

James Crombie

Tom Gardner wrote:

You're RIGHT. I was calculating diameter when I should have been calculating radius, no doubt I didn't have enough side clearance. That must have been it. Thanks, Don!

GWE

MDF, ie paperboard, is a likely cause. A mere hint of chater would cause trouble.

Hul

Grant Erw> I had a piece of 1/4" plate 6" square, and I needed a 5" circle with a 3"

I have used this and similar Kennametal tools. How many hours do you

Thanks! Exactly what I want!

I like my AXA tool holder on my Atlas 10 inch lathe, but every time I part something off or attempt somthing like trepanning, I look at the tools sticking off to one side and wonder about rigidity and leverage. Then I get out the old boring bar holder that I made years ago where the boring bar goes through the center of the hold down bolt and so provides all the rigidity I'll ever get out of that lathe. Then I make a tool and holder for that set up. This not to say that I am any good at trepanning. But thanks for posting the question. I already got some good tips from some of the posts!

My approach would probably be to torch-cut the part out about 1/4" oversize (assuming that the material isn't going to be hardened by the torch), then chuck as needed to turn the OD. Recently a guy gave me a set of real good hole saws that go up to almost 3 inches. They make short work of holes, although you still would have to bore your 3 inch hole to an accurate finish.

Okay, so I would really cut the part out 1/2" bigger! I was jsut trying to impress you guys with my torch skills.

Pete Stanaitis

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Grant Erw> I had a piece of 1/4" plate 6" square, and I needed a 5" circle with a

Geez, that should have been obvious! There should have been metal wadded against, and maybe even welded to, the bottom of the cutter. If nothing else, there'd be alot of heat and a polishing or smearing effect on the side of the cutter, telling you you need more relief. I do this all the time by "feel", applying the cutter for a bit, and if things don't seem to be going well, I can pull the cutter and immediately see where the relief is insufficient.

Jon

Yes, except I ground it for .019" side clearance, and so needing .021" wasn't quite so obvious. Even at 5" it didn't work quite right, so maybe the tool got thicker back a ways from the cutting tip. Anyway, for SURE next time I'll grind it more carefully.

GWE

Greetings Grant, When I am trepanning plate I try to set things up to cut from both sides of the plate. I grind the tool on the front at a slight angle such that the tool will break through on the large diameter. First the plate is grooved half way through, then turned around and grooved until the tool breaks through. This leaves a disc that is supported concentrically in the hole by a knife edged web. The advantage of this method is two fold. First, the tool only needs to be 1/2 the length of a tool that goes clean through the total plate thickness. Second, and maybe more important, when the disc is cut free from the plate it can't get jammed against the tool and break it. This method works well in the lathe and on the mill. Cheers, Eric