Forstner bit extension bar?

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Sachakins

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Looking for extension bar for 10mm (0.395") shank forstner bits.
Most of the ones I've found are for 3/8th shank bits (0.375")
Worried that the 0.020" will throw the bit off centre for drilling on the lathe.
Need it to extend by minimum 4" preferably 6" as been asked to turn a few vases/twig pots.

Any suggestions for suppliers, or do you think the 3/8 will be ok?
 

Eric Roy

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Looking for extension bar for 10mm (0.395") shank forstner bits.
Most of the ones I've found are for 3/8th shank bits (0.375")
Worried that the 0.020" will throw the bit off centre for drilling on the lathe.
Need it to extend by minimum 4" preferably 6" as been asked to turn a few vases/twig pots.

Any suggestions for suppliers, or do you think the 3/8 will be ok?
So far the only source I have come across for the 8/10mm shanks is Axminster - and it is not a cheap option!

…also seen these on Amazon at lower cost but not sure of the quality?
 

Sachakins

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Yes, but it does look very flimsy, compared to this style
1652630135830.png
 

Sachakins

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Axminster
Amazon
Quality woodworking

Not sure if these are what you are after, first three from google.
Thanks, the fisch and axminster one are the same I think. Don't look to strong to me and at 330mm too long.
I would think that size is going to wander off easily.

The other one is a good option though, thanks.
 

Phil Pascoe

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A forstner has short sides, best suited for shallow accurate holes, I would expect it to drift no matter what over a depth no matter which extension used. I would bore the initial hole with a good quality bit then take it from there - if the bottom of the hole drifts a couple of mil does it make any difference?
 

Ollie78

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I got one years ago so I can't remember where from now ( I realise this is not super helpful).
Anyway the point is it has grub screws on both sides to adjust the fit to centre. It works pretty well. Once the bit is engaged a little slop won't hurt the cut.

Ollie
 

Sachakins

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A forstner has short sides, best suited for shallow accurate holes, I would expect it to drift no matter what over a depth no matter which extension used. I would bore the initial hole with a good quality bit then take it from there - if the bottom of the hole drifts a couple of mil does it make any difference?
Unfortunately in this instance it will matter, as glass tubes are being fitted also. But otherwise yes you are correct.
 

Orraloon

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I have used a few bit extensions and like said the all will likely be somewhat off. Best result is drill as far as you can with the bit itself to start strait then employ the extension to get to final depth. Clearing the waste as you go. I also found it a good idea to grind a small flat on the drill shank side for the grub screws to seat on.
Regards
John
 

Richard_C

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I have an extension, OK but far from perfect, does drift a bit. Very frequent clearing helps.

If you go slightly oversize for the tube and a bit deeper the drift doesn't matter and it's easy to get the glass out for cleaning. You can then turn a contrasting tight fit ring or a "lid with a hole" to press in and hide the rim of the glass.

Where do you get your glass tubes?
 
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