Hi Diug,

You could chain drill it and then trim up the rough circle on the lathe. Alternative method could be laser cutting unless you are out in the pine forests !!

Mike.

Hello Doug,

Probably the easiest is to use a holesaw to cut out the bulk, maybe 3 inch diameter if you have one that size. You should be able to cut through from one side and I recommend backing the job in the chuck with something sacrificial, chipboard of similar.

Turning speed between 50-100 rpm and use coolant on the saw teeth, backing out frequently to clear chips

The alternative already suggested is chain drilling, cutting the between hole links with a cold chisel and cleaning up afterwards. Slow but just as effective.

Regards Brian

Trepanning will work on the SB. Slowest speed possible, ie 5-12 rpm max in back gear and largest spindle pulley. This will keep cut surface speed below 150 feet per min. which is a good max guideline with HSS. Grind the tool as a narrow parting tool with 2-5 deg clearance each side and on the front. On the inside-side, grind slope on the tool bottom toward centre to clear the slot wall. On the outside side, grind slope on the tool also bottom toward centre to clear the slot wall. Top rake about 5 deg front high back low as usual. Use tool sharp and at exact centre height, with lots of oil.

KBC Tools have trepanning tools on their website for hints at the shape to grind HSS to.

I have done similar jobs on my SB 9B.

Good luck, JD also in Canada (near Toronto-where are you Doug)

I don't doubt that a lot of machinists regularly trepan out large diameter holes, all I'm suggesting is, if you haven't attempted it before a half inch thick piece of MS plate is not exactly the best place to start.

I usually do it the other way round to what you all seem to be suggesting. Mount the work on the cross-slide, large angle plate screwed into tapped holes, usually a bit of thick plywood screwed to the angle plate and the work screwed and clamped to the lot. Align flat with the face-plate. Mount the cutting tool in a suitable holder bolted to the face plate. It just seems more rigid and the inertia of faceplate helps. Slow speed for big holes, I made an extra large V pulley to sit over the existing one and get about 50 rpm on my DB8.. Made the angle-plate from a bit of 6"x4" steel angle. Accuracy is not usually problem.