Lathe Drilling

[Dave HalfordParticipant @davehalford22513]

It's possible the spindle taper is off, but the chuck is OK.

If the tailstock is low it will drill big.

If the tailstock offset loosens when the tailstock is unclamped it will drill big

[DiogenesIIParticipant @diogenesii]

I guess it's a bit of a long shot, but if all else fails you could check that the workholding chuck is holding the stock firmly and not letting it 'walk' in the jaws, maybe only in one particular plane in each rotation.. ..maybe swap the 3J out for a 4J?

[SquarepegParticipant @squarepeg]

Thanks guys. Unfortunately I didn't get as much time in the garage as I'd hoped this weekend but I have made a start last night. I'll work through the advice and hopefully get to the bottom of it or at least make an improvement. Just wanted to say thankyou for everyone's efforts to help and offer advice.

Dave

[old martParticipant @oldmart]

However good your setup is, you will tend to get slightly oversize holes when drilling. I use solid carbide drills for the best size and finish where possible, but because of the cost, my selection is limited and used mostly for hard materials. For a good hole it is best to finish the size by boring and if the hole is smaller then you can bore, then a new stub drill will help. Stub drills are stiffer and if you use one as a pilot to its maximum depth, it helps to keep the larger drill as straight as possible. I had to drill a 12mm tapping size through a steel bar 50mm long and used a new, top quality split point drill. When I managed to get the whole length tapped, the first half of the tapped hole was quite concentric, but from the other end there was about 0.5mm eccentricity.

 Drill chucks may not always be holding the drill in line with the tailstock. If you can get a piece of steel turned to the max holding size of the tailstock chuck, and leave it in the chuck, you can clamp on the tailstock chuck and check for any runout in the MT shank. I have tested several drill chucks for the lathe and mills and remember which ones are most accurate, they vary from excellent to mediocre

Edited By old mart on 01/11/2021 10:10:07

[ZanParticipant @zan]

Sounds like a badly aligned tailstock. Making the drill angled to the lathe c.line Try this

put a straight bar in the tailstock

set up a dti on it at the end

move the tailstock bodily forward, then repeat with the handwheel

do this the dti both horizontal and vertical remove the Chuck, rotate 90 degrees, try again

do the same check with the Chuck in the headstock spindle to check the Chuck

put a lump of metal in the toolpost and centre it and drill it from the headstock. Same result Chuck at fault, while if ok it means tailstock. Correcting any error depends on what it is…..

good luck

[SteviegtrParticipant @steviegtr]

We all look at up & down alignment. But what about side to side. Or even play in the main shuttle. I have watched many youtube video's where the op thinks he is drilling centre. When you watch the vids the drill is wandering all over the place.

I recommend you set up a camera & drill a hole. Slow down the vid & watch it. You may find the problem.

Steve.

[SquarepegParticipant @squarepeg]

Thanks Zan and Stevie, both good ideas that I will try out. I'm going to keep chipping away at this until I've solved it, even if its a combination of errors and takes a while to do.

Thanks again!

[not done it yetParticipant @notdoneityet]

Making the drill angled to the lathe c.line

I does not need the drill at an angle to the centre line to cut crooked/over-size. It can be perfectly parallel to the centre line, but if it is not on it, the hole drilled will either wander or be larger than the drill. I’m not convinced of the lathe’s ability to cut parallel without, and with, tailstock support.

[Howard LewisParticipant @howardlewis46836]

A drill will cut oversize if

1 ) It is ground so that the lips are not of equal length.

This means that the intersection of the lips is not on the centreline.

Consequently, the drill will rotate about a centre point that is not on the centreline of the lathe.

2 ) If the drill is not held on the centreline of the lathe, because the drill chuck is inaccurate, the Tailstock barrel is worn, or the Tailstock is off the centreline of the lathe either vertically, or more likely horizontally, the drill will,effectively be used as a boring bar to cut an oversize hole.

Any of these faults is likely to encourage the drill to wander and produce an out of line hole, apart any flexing of the drill.

As each fault is eliminated, accuracy should improve.

Howard

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