Line boring question

[David KParticipant @davidk77137]

I am nearing the stage of line boring my Gypsy casting, I have done all the set up on my Myford saddle and intend to first drill the hole up to 14.5mm then bore out to the correct 5/8 inch [ 15.8750mm].

My question is , what diameter round bar should I make the boring bar out of.

If I use 14mm would that be enough clearance for the cutter or do you think I should drill through 15mm and use a 15 mm bar.

The casting is approx. 7 inches long with 5 main bearings to bore through.

[David KParticipant @davidk77137]

[BazParticipant @baz89810]

Would have thought 11-12 mm dia would be adequate over that length.

[JasonBModerator @jasonb]

I'd go as thick as possible to avoid chatter, have a look at the Benz thread over on MEM where there were issues and how it was suggested to use a bar with the cutter off to one side.

Remember for a 7" casting bar needs to be 14" plus if cutter is placed in the middle.

[duncan webster 1Participant @duncanwebster1]

This might be a daft idea, but unless you want to bore all the bearings at exactly the same diameter (ie without moving the cutter bit) could you have several cross holes in a shorter bar and move the cutter along.

You could also use a bar nearly up to the drill size but file flats where the tool pops out to leave some space for chip clearance

Holding one end of the boring bar in a 4 jaw rather than between centres makes the whole setup stiffer as well, just don't take the boring bar out till you've finished.

[David KParticipant @davidk77137]

Hi , my lathe has not got reverse so is there a way of sharpening a cutter to run in both

directions, I was thinking a single point with a rounded tip. This is so I can use a shorter

bar and reverse it to do each half as suggested in the Benz thread

[Howard LewisParticipant @howardlewis46836]

My advice would be to ensure that all the Main Bearing parent bores are cut in one operation, (This has been the practice in Industry for MANY years ) with one tool. In this way, they will be in line, and the same size.

Setting multiple tools to the same size will be difficult, so why introduce extra complication and risk, needlessly?

As Jason says, make the boring bar as large a diameter, and as short, as is practical. This will maximise stiffness, (less tendency to whip/deflect ) and accuracy, as well as reducing the risk of chatter. Rough out with a drill, and finish to size with a stiff Boring Bar. (If need be, set it up on consumable material, ready for use after the roughing operation )

Howard

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