As Neil says stitch drill it out first, if you get the holes spaced right it won't need much more than a tap with a big hammer and you get to use the bit in tne middle for something else.

Presumably the outside dims are too big for your lathe?

In case it isn't ******* obvious, practice on something smaller first. When I acquired a small boring head, it took a bit of experiment to figure out how to get the desired result. Once tamed, good results, even on an SX2P, which is not known for the finesse of the Z-axis.

Ron, good evening to you.

At risk of asking a whole lot of questions you may have already addressed:

(1) What are you going to turn said boring head with? (Question of sufficient torque to cut material at 50 mm radius)

(2) What material are you cutting (same question as (1))

(3) What tolerance can you accept on the nominal finished diameter of 100 mm?

(4) How good (rigid and repeatable) is your boring head?

I've played this game, and (as Neil says – I paraphrase) you could die of caffeine poisoning waiting to get to the finished size, my experience with holes this big is that it is very difficult not to end up with chatter at the tool, which in turn jeopardises your ability to hit a given size. I also found that it was all too easy to stall the cutter in the work, which of course chips the edge.

Using a carbide tool solved some of the surface speed problems I encountered trying to do the job with a HSS cutter, but then it is a lot more difficult to adjust the cutter geometry than with HSS.

The reason for querying the quality of the boring head you envisage using is that you gets wot you pays for in terms of rigidity and settability (if that's a word).

If you've got a socking great milling machine which will run at a suitably low speed (say about 20 rpm for HSS) and a nice Wohlhaupter boring head then I'll wind me neck back in. If you've got a mini mill and a Chinese £20 boring head, well……

Look forward to hearing you've managed it.

Simon

Hummmmmm

Thanks for your quick reply Ron, not sure how to break the news to you, and I'd be grateful if Jason and or Neil would chip in, but I think you are beyond the realms of SX2P capabilities, and stand a good chance of wrecking the motor/drive. You're going to be running the machine at bottom speed for an indefinite amount of time, and, from what I've read on this site, this is territory where these machines are not happy. Having said that I have no direct experience of this range of machines, so if someone wants to say I'm talking nonsense please do!

However, sorry to be the party pooper, but that's just the first problem.

The good news is that you are looking at cutting ally. At least that lets you push the cutting speed up towards the capabilities of the machine.

However I do worry about your wish to make a set size to a couple of thou. My experience of trying to set one of these boring heads to a given size is that they are lip service to the idea of boring rather than walking the talk. I have two ostensibly the same, but they are made differently and it makes a big big difference to the serviceability of the head. I estimate that the repeatability of setting is about +/- 10 thou, or 0.25 mm, you are going to struggle to hit a size to a couple of thou. One of the heads I have here has a captive gib built into the dovetail, I can set that to about +/- 5 thou, the other is the really cheap and nasty version, it looks the same but there is no gib and the setting screws bear on the side of the dove tail. On a good day with patience I can set this to about +/- 20 thou! It's rubbish! Hopefully Arc' don't sell these!

As you say, by all means try it and see how you get on, but be sceptical about the carbide tools you get as part of the kit. They look pretty enough, but to get good results the cutting edge needs to be at the centre height of the hole – in other words the point of contact of the cutting edge must be on a diameter of the hole. Some of these tools are made wrongly – the tool is located in the head on a diameter of the head, so the top face of the cutting edge needs to be BELOW the centre line of the tool, so that when it is mounted in the boring head you can twist the tool to give some top rake and bring the cutting edge up to the centre line. If the tool is too thick tilting the tool to give some rake pushes the top face of the tool way too high, and the cutting edge is actually trying to cut with negative rake. You won't get good results with this situation, even in aluminium.

Having been a douche of cold water, what about a more positive approach? Where are you in the country? If you fancy a trip to West Gloucestershire I'm pretty confident that my faithful Centec will stand a good chance, though of preference I'd stick it in the lathe and bore it out the old fashioned way. What's the overall size of the part? and the dimension of the hole centre to the furthest edge/corner?

Best of luck, Simon

When I have cut out discs for aluminium gears, I have used Starret/Bosch hole saws. Rather cheaper system than rotabroaches. Can get you close to finished size.

Yes, on both counts, although minimum rpm is 30.

However, I do have an el cheapo Soba boring head that I use on the Bridgeport. You can hit a thou or two but it takes some fiddling. As mentioned the tool geometry can be poor as bought and will rub rather than cut. Mine came with HSS toolbits that needed a serious regrind. It is essential to lock the boring head slide during every cut. A right pain, but still essential. Like boring in the lathe take spring passes when nearing size. To get any sort of proper usability you'll need to change all the socket head grub screws for proper US/UK ones.

Another question; how are you going to measure the hole to a thou or two?

Andrew

Certainly cheaper, and fine when they will do the job. I have often found chip clearance a potential problem with holesaws when used on thick material, often necessitating frequent stops to clear and add lubricant. When, as with gear blanks, the middle rather than the hole is the object, I find it helpful to arrange things so that the kerf runs out at one or more points thus allowing the chips an easier exit.

Apart from the fact that the Rotabroach type of cutter has flutes for chip clearance it has the added advantage that it cuts closer to size and, further, a pilot is not necessary so that the resultant slug has no central hole.

It was actually directed at Ron, but I forebore from saying calipers were the wrong answer! Turns out he's machining to fit an existing part, which is fine, as long as he doesn't get the part stuck. It's quite easy to get a part slightly skew when checking fit wherein it jams and won't come out.

Andrew

It must be boring being perfect.

Assuming 100fpm for HSS in steel the 'correct' speed is around 95rpm. I don't know what the minimum speed is for the SX2.

Andrew