Tail stock indicator.
Question from a newbie here.
Are there any instruments for measuring the depth of a drill hole made with the tail stock of a lathe? I mean while you are drilling the hole.
How do others read the depth of a blind hole cut with a lathe beyond simple reading the lines on the tail stock?
Has someone made something to do this accurately?
Model Engineer Mag, HSM and probably others have had articles on this, but it usually means mounting a DTI on the tail stock and clamping a stop on the barrel for the pointer to rest against. Kind of clunky at best. You could mount a small digital scale or caliper in a similar fashion if your lathe is big enough to handle it.
mag base and a dial indicator
Some lathes come with or are modified to have a micrometer collar like the cross slide. Mine came with it OEM. That gives you fairly precise offsets, but you need a starting point for depth. Then there are the TS mounted DIs and even HF electronic scales.
Tail stock indicator
If you can make a plate to fit behind the drill chuck and extend up, you can then use a dial indicator mounted on the tailstock itself. You can then zero the indicator either when the drill point makes contact. If the tailstock shape doesn't lend itself to mounting the indicator easily, extend the tailstock plate either to the front or rear wherever it is easiest to position the indicator.
To measure the true depth of the hole, you might want to calculate based on the drill geometry just where the outer edge of the bit lies.
When I am drilling a blind hole to a predetermined depth I use the division markings on the tailstock quill. My handwheel is marked in thousandths but I don't completely trust that because it's hard to determine where the outer cutting edge actually enters the hole.
I mentally compute the depth using the scale on the quill and double check it with a depth mic if it needs to be within a few thousandths. If only fraction dementions are required then the scale on the quill is adequite.
It's only ink and paper
How about one of those cheap 4" or 6" horizontal linear digital readouts? Fix one end of the scale to the barrel, the readout to the body. Inches or metric, zeroable wherever you like...
A micrometer dial on the tailstock is very handy to have, imo. Then, like others have said, its just an issue of figuring out whether the depth is where the point of the drill stops or where the actual diameter stops since the bottom of your hole won't be square.
Usually i end up measuring the depth to where the point of the drill stops and then use a two-flute endmill to square the bottom just because i've needed to make a bunch of "buckets" recently for a project.
Otherwise, make a clamp for the tail stock quill. You can't retract it all the way without ejecting a taper anyway, so put the clamp on the excess bit and then read with a dial indicator and mag base off of that plate. Just make sure that you can loosen the clamp and slide it enough forward while the drill chuck/drill bit etc is in the quill so you will be able to retract the quill and eject the accessory.
Fellows, a drilled hole is usually measured from the outer edge of the drill bit, not the tip of the drill bit. It is nearly imposible to determine the point when the outer edge of the drill starts cutting in the hole. if you can do that you are truely a genius.
Only shallow holes could be flattened with an endmill and a flat bottom is very seldom needed.
It is very rare when the depth of a hole is spec'ed on a drawing in anything more than a fraction of an inch.
If you are using a dial indicator or digital reader to measure how far the drill entered the hole I suggest you measure the real depth and I think you will find you are not drilling to what the indicator is showing. You'll be close but not exact so get over it.
It's only ink and paper
True - but as i said i've just made over 120 buckets ranging from .25 in diameter to .80 in diameter and from .5" long to 1.5" long all requiring a precise depth and square bottom. The amount of "webbing" at the bottom of the bucket had a tolerance of +/- .001
<edit> my point was to give David a method to precisely measure depth from the tail stock in case he had a special need for it, since those do come up. Furthermore, all though the measurement is given from the outer cutting edge of the drill, so long as you know the point geometry it isn't all that difficult to get a precise depth as Alan pointed out.
Last edited by Fasttrack; 01-11-2008 at 01:28 PM.