A question about centre drilling

[Robin GrahamParticipant @robingraham42208]

[Robin GrahamParticipant @robingraham42208]

I am making a sheet roller following G. H. Thomas' design published in the October 1976 ME. He gave the following advice for drilling the rollers before turning between centres.

"Face the end; put a suitable centre-drill in the tail-stock chuck with its two cutting edges in a horizontal plane, i.e. one towards the operator and one towards the back. Put a piece of plain bar or any blunt instrument in the tool-post; start the centre-drill into the end of the bar and then bring up the blunt ended “tool” and cause it to bear with slight pressure on the centre-drill whilst it is being fed in. Ease off the pressure gradually as you finish feeding the drill."

I'm sure that this must be good technique or GH wouldn't have said it. But can anyone explain why it works better than just whacking the centre drill in?

Robin.

[BazyleParticipant @bazyle]

A centre drill will wobble if there is any looseness in the tailstock barrel or chuck, and may be off-centre anyway. Pushing it over makes it work like a small boring bar taking up all the slack so it starts off with a true hole that might be a little bigger than the centre section of the drill.

[John ReeseParticipant @johnreese12848]

I have not heard of this technique used with a center-drill. I know it is used with twist drills if the drill tends to shift off center when starting. Essentially it forces the drill to bore a true cone as it starts. If the center-drill were not perfectly aligned with the spindle centerline The technique would prevent an off center start.

[Robin GrahamParticipant @robingraham42208]

Thanks for explanations – I get it now. I think that this is one of those 'real life' techniques which can be hard to come by without a practical background in mechanical engineering. Maybe I should should read more GHT.

Robin

[Neil WyattModerator @neilwyatt]

You should only need to do that if your tailstock is worn or misaligned.

Neil

[Robin GrahamParticipant @robingraham42208]

Hmm. In the article GHT said his lathe was aligned 'beyond reproach'. But he still thought it worth doing, and found he could get get a centre within 0.00015" of the true middle. That raises questions in my mind, but all I really want to do is make some rolls, I doubt that a few microns one way or  the other is going to make any difference,

Robin

Edited By Robin Graham on 30/04/2019 01:49:04

[Mick B1Participant @mickb1]

Posted by Neil Wyatt on 29/04/2019 23:14:28:

You should only need to do that if your tailstock is worn or misaligned.

Neil

Even then, if you lock the tailstock barrel when you reach a suitable depth, and pause to let all cutting finish, you should still have a true enough centre for all practical purposes without having to use toolpost sidestress.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 29/04/2019 23:14:28:

You should only need to do that if your tailstock is worn or misaligned.

Neil

.

Given GHT's perfectionist approach … I suspect that he was more concerned about the symmetry of the centre-drill than his tailstock.

Doing as he suggests starts the drilling with only one flute in action: Which comes close to the watchmaker's technique of 'catching the centre' with a graver.

MichaelG.

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