Some Dumb Boring Questions

Newbie has some dumb (for anyone who knows about boring) questions. I am using an off-set boring head (APT) with "spoon type" cutters on a vertical mill to drill a number of odd size holes (eg .342", .543", etc), all between .3" and .75"; some shallow, some 1.3" deep, in carbon steel FS 1141 (heat treatment: normalize @1200F air cooled, hardened @ 1525F, oil quenched & drawn @ 700F) w/ Rockwell hardness @ A60-70. 1) Do these boring cutters plunge into the steel like a center-cutter?..., or do they need a pilot hole to enlarge?...would it be good practice to run a slightly smaller endmill down through the center anyways? 2) Would you suggest boring these holes out in one pass?...or more than one pass with increasing diameters? If so, what is a good incremental increase per pass? 3) The surface where the hole will go is irregular, ie. not flat: within the hole I want to bore the surface probably has up to three different levels, and I have parallel holes partly within the target hole. Does this present a problem for these standard boring cutters I am using? 4) Does anyone have any suggestions re: rpm and quill feed rates for what I'm describing? Without knowing any better, I would tend to use 300-400 rpm and feed by feel. 5) My mill (Bridgeport Series 1) has an auto quill feed which I have not used nor know much about. Should I be using that mechanism either automatically or manually?...or is the standard quill feed lever handle just as good?

As you can tell, I only know enough to be dangerous. Any advice would help me lower the danger factor. Thanks for looking!

BTW if any of you have helped me out in the set-up phase two weeks ago, I got this 25 year old machine up and running great! Shimming with steel plates and a rubber anti-vibration pad under each corner did the trick...and the VFD phase converter from that outfit in Brooklyn, NY for my single phase source seems just perfect; and Harold the tech was above and beyond: we got it running at 6:30 Friday nite over the cell phone while Harold was stuck in NYC traffic.

I am missing one of the 6 copper electric contacts in the motor reversing switch, just a ~3/8"x1+" piece of copper, no tools needed to remove or install. What's a good source for parts?


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Start with a twist drill or center-cutting end mill, and take it fairly close to the final diameter, if possible. You might want to leave .020" on the wall of the deep holes, in case the drill wanders.

I usually can handle .010" per pass in a Bridgeport, a lighter machine might need a shallower cut.

Probably not.

Figure the surface speed in FPM for the diameter and materials. Then, calculate RPM from that. HSS on steel would be 60 - 100 SFPM, carbide could be

150 - 600 SFPM. For .3", 100 SFPM comes out to 1300 RPM, for .75" it is 510 RPM.

The power feed gives a consistant feed per rev, and you should use it to get the smoothest holes. You don't really have to use the auto cutoff if you are going to be watching it.


Reply to
Jon Elson

Spoon type? Could you provide a pointer to an image? Do you mean the ones with a reduced intermediate shank, and a wider cutting edge (offset to one side), perhaps with a brazed carbide insert?

You want to cut it *after* hardening? I don't know offhand, what final hardness will result with that draw cycle, but it would probably be a lot more difficult to cut than doing the same for the same steel annealed, and hardening afterwards.

If these are what I think, I would never have thought of running them without a pilot hole. The stem is a bit to skinny to take the forces involved.

Absolutely! Drills take the most material in one pass, a center-cutting endmill is probably the next. Your boring head is designed for removing relatively little at a pass.

If you start with a small enough pilot hole (e.g. just a bit skinnier than the width of the tool) you will want to take many passes. As an example, I have a 1/2" diameter solid carbide boring bar (nice when you need long extension on a lathe), and the carbide insert sticks out perhaps 1/16" on one side. I tend to drill a 1/2" hole for starting, and bore just large enough to clear the overall diameter of the bar plus the insert's extension (say just a bit over 9/16", plus a bit to clear the chips, so perhaps a 5/8" hole on the first pass.

Are those cutters HSS or brazed carbide? Carbide does *not* like interrupted cuts. Nor do the rather skinny necks of the cutters which I *think* you're describing.

400 RPM with a 0.750" hole works out to 942 SFM, which sounds high for HSS on a carbon steel, though it might be reasonable with brazed carbide boring bars -- if the are not too hardened in the necks. I've seen those break right at the skinny part with what I considered a fairly light load.

Automatic feed will give a better surface finish. Be sure to set the automatic trip so it stops before the tool hits something other than the workpiece -- like the vise or the machine table.

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First off -- when you are using a VFD, you really should *not* be interrupting the current between the VFD and the motor. Far better to use the contacts of the original switch to command the VFD to run forward or reverse. That way, you won't need to replace the missing contact, all you need is a SPDT contact and three wires to the VFD.

Good Luck, DoN.

Reply to
DoN. Nichols

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Oops -- I forgot the conversion from inches to feet in the above. Make that 78.5 SFM. :-) Go with the HSS, unless your heat treatment makes the workpiece too hard. :-)

Enjoy, DoN.

Reply to
DoN. Nichols

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