Chris,

40mm O.D and 20mm I.D. is a stock size for "Hollow Bar"

Beyond that … we would need to know the accuracy required before anyone could give practical advice.

MichaelG.

I'd drill from both ends. It helps if you make sure that the drill has a good straight start so I would bore the hole, say 16mm diameter and then put a 16mm drill down as far as you can reach, then turn end for end and repeat. If you can't get your 40mm bar down the lathe spindle then I guess you will need to hold the bar centred in a fixed steady. If you can use e.g. an 18mm drill before the final 20mm then this will probably help as well.

That's the way I've just drilled a deep hole down a new spindle for my Flexispeed. OK, that was only 1/2" OD, 9/32" ID  but the principle and relative length of drill was much the same.

Actually, you could probably drill all the way from one end, 160mm is not terribly deep for a 20mm drill but again I would make sure it had a good start by boring first and then step drilling.

HTH

Rod

Edited By Roderick Jenkins on 10/09/2014 23:56:31

Hi Chris,

please bear in mind that the bar you are using may not be round or parallel so .05mm may not be achievable which ever way you do it.

Ifi were making this item and needed to get it as accurate as possible i would do it as follows and yes it is a bit long winded but …

start of with the material a little oversize 2-3 mm or so would suffice.

Set up in a four jaw chuck and get it running as true as you can

centre drill then drill through to 17mm and bore to size or even better is to bore to a size a little under and ream.

for the best accuracy i would make up a mandrel and mount the piece on it so i can finish turn the OD concentric with the bore.

Another difficulty would be measuring the bore at the chuck end and this is why i would allow a few spring cuts or ream so the bore is parallel and round.

thats my version anyhow

Ian

For your spec,the only option is to gundrill, then put it between centres and then turn the out side true to each end of the hole.

If you want to drill it, like above, predrill and bore for about 1.5 diameter long, in your case 20mm hole X 30mm deep. Then with a chisel point sharpened drill, 175 rpm at .05mm to .06mm feed per rev feedrate, about 8 to 10 mm per minute, and 2 to 3 mm per peck distance. This will give you as good a hole with a conventional drill as can be achieved. I just put the drill just straight down, no pre drilled smaller hole. Have the work piece setup with a steady to support the front . Then after drilling through 1/2 way do the same again from the other end.

Neil

What was the outcome and how did you go about the job?

Just curious.

Neil

I have the latest model engineers workshop, but did not see any such article.

Neil

I don't have access to that.