Flat bottomed hole with a boring bar - technique?

All, I need to open up a blind, drilled hole, which has the drill taper at it's base.

I've got a small insert type boring bar, with a triangular insert.

Is the technique to go down the middle, not cutting the side, move out to size then repeat until the taper is gone,

or,

do you go down the bore cutting the side, until you hit the taper, then move inwards to the clear bore?

or,

what's the technique?

It's a small boring bar, so I can quickly get into a situation where I'm rubbing the side of the insert.

I've got a depth stop, so I can consistently cut to a set depth.

Thanks.

really depends on hole and bar size, if there is not much room you may find when going down the middle that the back of the bar hits first so usually best to come from the bore and then move bar towards ctr and keep repeating that.

Understood, thanks.

If I needed a flat bottomed hole and it was less than, say, 12mm or so dia. I'd probably use either a slot drill of matching diameter for relatively shallow holes or a home-made silver steel D-bit for deeper holes.

If all I want is a drilled hole (not to bored accuracy) I just grind a drill bit square across the tip. Drill with a conventional drill to size and then use the modified drill to square the hole off. (If doing this on a drill press freehand it's helpfull to have the modified drill part way down the hold before switching on)

regards Martin

Hi DR_GMGN. How critical is the hole and depth as you could easily flatten the bottom of a hole with a flat end drill. You just grind the drill with a flat end and set the depth.

David

Make sure you have the drill vice fastened securely as the modified drill bit WILL grab & try to throw your vice across the workshop due to no chisel point on the tip.

Thanks all.

Doesn't modifying a drill bit then mean the drill needs re-grinding to do normal holes?

Just seems like if I've got a boring bar that will do the job I might as well use that rather than modifying something.

Just modify a standard drill & buy a new one for 'ordinary' holes, a truly flat bottomed hole is tricky to produce & best avoided in the design stage. You can try pecking away in the bottom of your hole but it is quite nerve wracking.

Tony

Last time I wanted a flat-bottomed hole I just used a slot drill!

John

Yes but it's just another approach dependent on circumstance. I have some hundreds of old drills so it suits me. No one was suggesting you used your best set. If it's already set up in the lathe and there is a suitable boring bar to hand then use that.

regards Martin

If you do want to have a go with a boring bar it's got to be small enough for the tip to start in the centre when you get near to completion, just a heads up.

Tony

Slot drill does not produce FB hole.

Bottom is going to be convex.

OTOH boring bar will produce such an animal (or something very close to).

Martin

Edit:

I would go around a problem presented by OP by frist relieving taper with sharp tool, shallow cut going down to bottom and then facing bottom with boring bar and final 2-5 thou cut while boring bar is retracting.

It can also be found (at least on my lathe) that truly parallel hole (or somehing very close to one) is produced by inverted bar cutting on wall farther from operator.

Normally such a hole would have lower diameter on entry than on the bottom (assuming lathe is set properly, eg to turn bar in such a way that diameter grows towards right end of bar).

However minute deflection of bar is compensated for by doing so and hole ends up parallel with carful, shallow finishing cut.

Martin

Edited By Martin Dowing on 12/03/2021 23:50:21

What? How does that work? Lathes are usually set to turn parallel are they not?

Not really.

There is someting like 0.5 - 3 thou per foot taper.

Myford specificaion says about "up to 3 thou per foot".

It is usually "less" but inspector would pass such a lathe as well (my is turning a taper about 0.5 thou per foot).

This taper is set in such a way that diameter of turned bar increases towards tailstock.

This implies that hole turned with front tooling would resemble lets say Morse tapper (of course not near this steep).

With rear tooling the opposite is true.

Lathe which would turn bar *narrowing* towards tailstock would be sent for rebuilding (headstock re-allignement) by inspector - not acceptable.

Comparable rule applies with facing - lathe is designed to give slight concavity with front tooling.

Martin

Oh dear I think another large 'can of worms' has just been opened.

Tony

No worms harmed in the making of this post.

Myford User Manual says a 6" test piece should measure "the same diameter at both ends" when adjusting the bed "levelling".

Myford inspection sheet allows 0 to 6/10ths of a thou over 6" misalignment between headtsock spindle and bed, toward the direction of the tool bit. 0 would be the value you ideally aim at. 6/10ths would be the maximum error you are allowed if you fail to achieve the ideal of  0.  This would then be followed by the above levelling process to obtain 0-0 on the actual turning test.

Conelly in "Machine Tool Reconditioning" allows 8/10th of a thou per foot variation but allows it to be in any direction. I think Schlessinger is something similar.

So over a 1" deep hole that would be about 8/100ths of a thou of taper, or less. Roughly.

Nowhere I have seen says a "properly" set up lathe is set to turn bigger at one end than the other. I have difficulty trying to imagine why you would deliberately set a lathe up to do that.

Edited By Hopper on 13/03/2021 13:44:26

Edited By Hopper on 13/03/2021 13:45:14

Lathe set to produce tailstock end of bar larger in diameter than a headstock one will also produce Morse socket like bore at opening with front tooling and it is a preferable situation if piston entering bore meets gradually increasing resitance (easier to rectify) than large resistance at he entry followed by dropping in a bit further down a bore.

About these and other practices (including up to 3 thou taper per foot) i was told by a guy who was working few decades in Myford, N'gham, while it still existed.

According to him ML7 with 3 thou a foot of taper would still be passed by inspector albeit most of assembled lathes were doing much better than that, typically ~ 1 thou a foot.

Martin

Edit:

Perhaps discrepancy of our data is related to bed straighness versus assembled lathe straightness.

Edited By Martin Dowing on 13/03/2021 17:09:06

You have not stated how deep the hole is, what diameter it is or why it needs to be flat at the bottom so all these options are speculations. Here are two more options to consider.

Drill deeper than you need to be, turn, face and part off a suitable slug and put it in the bottom of the hole with some suitable Loctite.

Mount the part on a rotary table with the hole over the centre of rotation. Using a smaller end mill or slot drill than the hole required mill out the bottom of the hole by offsetting the rotary table as required and rotate it 360°. This is the way CNC uses small mills to create larger holes, flat bottom is easy this way.

Martin C

Thanks guys.

Sorry - context: I got a Hemmingway kits saddle lock lever for the lathe (for the sake of £10 including the handle it seemed ok).

One part is a 'bucket' type spacer that fits around the lower end of the lever:

0.75 " o/d

0.6" i/d

0.4" overall depth

0.3" bore depth (flat bottom)

and there's a 5/16" through hole through the base.

Instructions state:

- Hold in chuck

- Drill 5/16" dia. x 1/2" deep, then 1/2" dia. x 0.3" deep.

- Bore 1/2"dia. hole to 0.6" dia. with flat bottom

- Part off to 0.4"

So that's what I need to do - the flat bottom is so that it fits snugly against the spigot at the end of the lever. It's basically a big counterbore.

I know its not critical, but at this stage it's still good practice for me to follow a simple drawing and try to achieve what's specified as well as I can.

So it's clear what needs drilling and what needs boring - which makes sense - but I was wanting the best technique in terms of do you enlarge the 1/2" bore incrementally, and at the end of the bore go inwards to form the flat bottom, or start at the middle, or what?

The 0.3" drill depth obviously has to be to the tip of the drilled cone, otherwise it would always be a coned base.

BTW this is the smaller of the two boring bars I've got:





Thanks again all.