Need a perfect 8mm hole - to ream or bore?

[BillTodd]

The great advantage of boring in the lathe is getting the OD and ID concentric.

Why not make a boring bar? The picture of the ones attached are simple steel rods with a cross hole, to take the cutting tool, and a threaded axial hole to fix it. The cutting tools are ground from broken reduced shank carbide PCB bits.

The smallest one shown here is 5mm Diameter, used to cut the 2" deep, 5/16" ID, LH acme thread for my steady rest support arms.

[ptjw7uk]

Only real problem is how you going to measure your progress in boring the hole!
Simplest way is 7.5mm drill and a 8mm reamer just remember to withdraw reamer from hole whilst the bar is turning, never turn reamer in opposite direction as it will enlarge the hole nad possibly damage the reamer.

Peter

[irving2008]

The problem with drilling and reaming is there is no guarantee that the reamed hole is concentric as drills cut an oval hole and the reamer may end up off centre.

For small holes like this the best way to check is with the shank of a drill set... a 7.9mm drill (checked with a micrometer) will do as a test... although the stepper shaft is the ultimate test! A bore gauge will also do (6th one down)

[HankMcSpank]

Yes, you need one like this 5mm head on a 6mm shank (top one on page). If your normal lathe tool is bigger than 6mm then you might want to make up a boring bar holder (or buy one) to hold the cutting edge at the right height.

Ok, I'm seriously beginning to doubt my ability to visualize things here - that boring bar you linked to has a round shaft, but boring bars mount into the tool post? (which needs flat/square sided tools) - therefore there must be a boring bar holder with square edges in the equation?

Bill...thanks for the advice about making one, but I'm far too novice at this to think about making a boring bar at this stage (I don't even know how to use one let alone make one!)

Pete - re measuring the progress through the hole - well my mini lathe has DROs I figured it'd just be a matter of zeroing the DRO as the boring bar enters the hole and then taking it thru in until the DRO reads 35mm - that way I know I've cleared the hole? (or again am I missing something here?)

Thanks for your help thus far - I really wish someone had made a youtube video about boring out a whole. (it's no doubt a breeze once youve done it once, but never having even seen the tools I need, it's a little bit hard for me to visualize right now!)

[irving2008]

Ok, I'm seriously beginning to doubt my ability to visualize things here - that boring bar you linked to has a round shaft, but boring bars mount into the tool post? (which needs flat/square sided tools) - therefore there must be a boring bar holder with square edges in the equation?

Thats correct... depends what size tool your lathe takes... could be as simple as grinding a flat on the bar, or a piece of square material drilled out 6mm with a locking screw. You can also get flat sided boring bars but not usually for that small a diameter hole although Chronos do one (second from bottom)

Pete - re measuring the progress through the hole - well my mini lathe has DROs I figured it'd just be a matter of zeroing the DRO as the boring bar enters the hole and then taking it thru in until the DRO reads 35mm - that way I know I've cleared the hole? (or again am I missing something here?)?

I think Pete was refering to the hole diameter, but if you have a DRO then its easier, just dont rely on DRO for final diameter. Hole depth is as you say... assuming the centre hole in the chuck will clear the bar...

Found this video, its a start, there is a better one I have seen but can't find it right now

[HankMcSpank]

Thanks Irving - some really helpful input you've given there.

So it seems that I'm going to have to shell out a bit of cash ....& here was me buying a lathe to save a few bob!

Audio Andy has kindly offered me the loan of a small boring bar - so unless Ebay turns up a cheaper option to the Chronos Route (having to buy a boring bar holder as well as a suitable boring bar is frankly just taking the p***!), then I'll likely take Andy up on his kind offer.

Many thanks to all who chimed in!

[ptjw7uk]

HankMcSpank I think you have just began to realised that getting the lathe is just the start, there is an seemingly endless variety of tools and attachments for the lathe. For my boring bar holder I used some square stock that would fit in the tool holder put a large drill in the 3 jaw chuck and pushed the metal against the drill, then drilled and tpped 2 holes perpendiculer to the hole to hold the bar in place voila, one holder.
My biggest problem is in finding suitable stock to make things usually for a few £'s more I can get the article ready made at times it dont make sense in that the finished article can come from china but a bit os steel jst down the road costs an arm and a leg plus postage!

Keep up the learning process
Peter