Maybe a silly idea, could you stand the sheet on spacers on the table and bore it from the bottom face to the top?

You can get RH and LH boring bars, and even double-ended ones. The type supplied with the head are correct for vertical operation in the boring head and for most lathe-work.

I've run my boring head in reverse and not had the drawbar come undone on a mill with an MT spindle. If the drawbar is tightened enough for the taper to grip properly, it shouldn't unwind.

Possible Perspex contributed. It's on my list of difficult materials. It needs a sharp cutter and it's important to make sure the plastic doesn't melt and stick. I once ran a jig-saw through a sheet of Perspex only to find the two halves welded back together behind the blade.

I wish I knew a way of tightening drawbars scientifically. I do mine by feel, first hand-tight, then spannered to close up with two fingers, plus a 1/4 to 1/3 hard turn to nip up. Seems to work well enough – the taper releases after a single sharp tap, and doesn't spin. But what on earth I mean by 'spannered to close up with two fingers' is probably only meaningful to me – pretty unhelpful!

I suggested in a recent thread that R8 might be more likely to slip than MT2 because the R8 taper has a smaller surface area. No proof though! Perhaps Ian has hit the problem in the real world?

If I bored lots of big holes I'd buy a Left Hand boring bar.

Dave

If the OP is boring plastic, then home-made tools from silver-steel rod is plenty good enough. Can be sawn, bent, turned, filed and milled to any shape you wish. Ground after hardening. Cheap as chips.

Hi Ian, when you place the ACW boring bar in the side holder of the head and rotate it through 180' to get the cutting edge facing down, wouldn't it become a CW tool? You can then spin the boring head clockwise and do your cuts.

Will try to draw a diagram, maybe it's clear to me or maybe I'm missing something

You could make a tool holder that fits in the side and takes the standard tools to hold them further out but oriented the same as the standard holes in the bottom of the head.

Martin C

Only thing with that type of extension on a light machine is the out of balance will make it jump off the bench unless you can run very slowly without loss of torque.

I did consider the balance but it is for a thin piece of plastic so low speed for a short while time.

Martin C

Now we have started looking at alternative holders, ChrisB' sketch reminds me of a style of holder I have seen that has two projecting pins that locate in the two vertical holes in the head and somehow puts the cutting edge horizontal.

I have a hazy memory that either the bar itself or a separate insert then fixes to this piece. I wonder if that was aimed at assisting with the balance issues. I cannot remember where I have seen it though.

OK. Found something similar at:

https://www.homemadetools.net/forum/improving-boring-heads-56968

As DC31K's picture suggests below is a Wohlhaupter UPA2 head with a side bar extension fitted to put a boring bar into a position more suited to the job in hand. This set up was to externally machine the snout casting of an Eaton supercharger to allow a reduced diameter pulley to fit (20% upspeed on the charger).

Easy to make from a square bar of silver steel.

Funny how we spend so much time 'toolmaking' to do the jobs we need to do!