Is there a chart for drill hole dia vs. boring bar size?

[William AyerstParticipant @williamayerst55662]

Hi all,

Another boring operation and more frustration. I worked out that my 8mm shank cutters have the cutting edge 280 thou above the bottom surface, so needing a shim of 33 thou to bring it up to the expected 5/16" – but it still bottomed out.

JasonB, where can I find those bars with the holes for inserting HSS tooling, please? I'm trying to avoid carbide inserts, really.

Where can I find out the height of the cutting face above the bottom surface, for boring tools?

[JasonBModerator @jasonb]

William, you can just grind a bit more off your tool so it clears a smaller bore, can you post a photo of what you have got just to be sure.

The bars can easily be made just a hole for a round tool bit and a tapped hole at 90degrees for a grub screw.

Used to be that most of the ME suppliers did a set of 4 – 1/4, 5/16, 3/8 & 1/2" shanks but now mostly insert types. Looks like Tracy have some, I assume the size is for the tool bit so you may want to check with them the size of the 1/8" one which may just be the right height for you.

The ones with the black shanks are from one of the 4 piece sets and have had a lot of use

Plus a few homemade ones

I've only known heights to be published for insert tools

[William AyerstParticipant @williamayerst55662]

Jason, thank you so much for the help on this – boring isn't as straight forward as I thought!

I ground off the bottom of my 8mm HSS square threading tool to act as a boring tool for the 10v standard – it worked (just) but there was a huge amount of flex in it.

The 8mm HSS square boring tool won't fit ( presumably minimum of 17mm dia starting hole, unless the bottom is ground)

The 8mm shank brazed carbide tool has the cutting edge at about 7mm from the bottom surface, so I shimmed it with 1mm packing underneath, but the steel underside of the tip was still fouling the work. Am I OK to grind that down too, in theory?

I found a tool in the box of previous owner's toolbits which has been ground to a stubby boring tip. It's 3/8" square but the cutting face is pushed down to roughly centre height, maybe a hair above. Given the nature of the boring I gather this is OK (it seemed to cut much better than anything else I've tried so far) – it just adds to the rake angle. I'm currently turning brass so that's OK but I clearly will need a better solution for steel and CI.

I'm sure I saw some of those long, round-shanked HSS boring bars (i.e. fourth from front in your first pic) somewhere, but I can't find them again. I've asked Tracy to confirm on their holders – and will keep looking – or make my own as you have shown!

Edited By William Ayerst on 31/07/2021 09:43:32

[JasonBModerator @jasonb]

Chronos and ARC do the bent HSS ones.

Yes you can grind more off but will get more flex

[Clive FosterParticipant @clivefoster55965]

I'm sure you will be unsurprised to hear that there are a few subtleties concerning holding round boring bars that can seriously affect how well the system works.

The big red mount in Jasons video link is impressive but, unfortunately, not a terribly sound design for DIY folk. The thick walls of the clamp are less than flexible so the bar needs to be a good fit in a high quality bore for the screws to clamp it securely. Basically stiff sliding fit. Needs decent boring tools to create a good enough bore which is very chicken and egg as far as this thread is concerned. I've fixed several clamps of this style for various non machining applications where users (and designers) have been over-optimistic on the closing ability and stripped the screw threads. Thread inserts and a bit of judicious machining so the holder can deform around a slightly smaller bar sorts them.

Given that any deformation means the holder bore is no longer quite round you are looking for contact planes similar to three or four jaw chucks if things are to be stable. My die head holder works rather like a four jaw chuck. The flexure planes being defined by the ends of the bolt carrier bores and the welds along the back onto the mounting bar. Being relatively thin the carrier cylinder can deform a little around a smaller rod but those are where the hard contact occurs. I made it a stiff sliding fit but it could work with anything that doesn't rattle.

The split square holders are nice and effective but the clamp force must be applied on the split side of the boring bar centre line. Theoretically best if outside the boring bar diameter altogether but that is generally impractical unless the bar is tiny. Given typical component dimensions overlap of up to maybe 1/4 bar diameter should be fine. If unsure a few hard side to side tugs will verify that its stable. I saw one where a slightly too small bar had been fitted into a holder that just managed to fit into the toolpost slot. The clamp bolts were significantly towards the solid side from the centre line of the bore.

Remarkably wobbly!

Fortunately I stopped for a re-think and further rooting around in the tooling stock for something more suitable before attempting a cut! Knowing how things ought to be is not complete protection from the "Aha, I'll use that!" moment.

Clive

[Author]

Posts