How hard can it be to drill a hole?!

[cjs3]

Although I have been of a practical inclination for many years (decades) it is only recently that I have started to build a stationary steam engine. (Heritage models Stothert & Pitt Beam Engine). I've done electronics, furniture restoration, house restoration, built a busker organ, restored a player piano - all those sorts of things - however the precision required in model engineering is another thing altogether. I reckon it is summed up in the effort required to drill a decent hole.

We have all been surrounded with drills and have made holes in things for years but it is quite illuminating how much effort is needed to actually drill a quality hole. That is, one that is of the intended size; true, circular and in the correct place! It seems that one might achieve some of the above parameters but getting them all in the same hole is not as easy as might be initially thought.

Am I alone in having difficulty in using a centre drill in the lathe? I wish to drill a hole in the end of a prepared mild steel bar in the lathe three-jaw chuck but try as I might I find that however I endeavour to start I seldom get the centre drill to run true - I can invariably see the slightest wobble. I've tried running the lathe at slow speed, high speed, lots of lubricant, no lubricant, gentle feed, high feed. In almost every case I don't feel confident that I am 'on the mark'. If the centre drill is 'off' the subsequent increases in drill size not surprisingly follow the error.

Being new to 'precision' working I would welcome any hints on what I am doing wrong.

[dickdastardly40]

CJS,

God knows I'm not perfect though I'll try to assist, firstly I need to make some assumptions:

Your work is securely held in the 3 jaw and faced flat.
The centre drill is correctly ground or new and you are using the same end each time.
The centre drill is firmly held in the tailstock jacobs chuck of which the morse taper is clean and the barrel of the tailstock is clean so it is true and correctly fitted.
The minimum of the drill is protruding from the chuck.
The tailstock has not been 'set over' by a previous user of the lathe for cutting tapers and is the correct tailstock for your lathe.
The tailstock barrel is not out of the casting by almost its entirety
The 'mark' you are aiming for is the epicentre of the previously faced flat.
The job hasn't been removed from the chuck between processes. (3 jaws are not as repeatable in accuracy as you may like).

If the above are all 'correct' put a centre drill into a piece of scrap just enough for the larger cutting tip to cut and the small diameter portion is 'submerged'.
Change drills for a larger diameter than the centre mark and drill a hole, watch to see if the longer drill wobbles as it cuts.
Swap ends on the centre drill and try again.
If it does both times it sounds to me as though the tailstock is out of alignment or the lathe bed is not square to the headstock or there is play in the headstock bearings

With the 3 jaw installed see if you have any play in the headstock bearing.
Install a dead centre in both the head and tailstocks bring the tailstock gently to the headstock and see if they touch bang on. Rotate the spindle and see if that makes any difference .
Turn a piece of bar unsupported by the tailstock with two collars of about 2" about 2" apart on one pass and measure to see if the diameters are the spot on the same.
Turn a long piece of bar supported by the tailstock and measure for a taper aftr a full length cut.
Borrow a parallel test bar and check with a dial test indicator.

Personally for centre drilling I use about 800 RPM and always use coolant for all drilling, opinions vary on this. The feed in on the drill is enough to stop chatter, do not force it.

Let us know how you get on, good luck

Al

[paul]

cjs3: You're not alone as my scrap-box will attest - on my last two engines I scrapped numerous parts because holes weren't in the right place or not perpendicular to the required surface.

One thing to check other than Al's extensive list - is the end of your bar really flat? It's possible to have a 'pip' or even a 'bump' in the middle of the bar if your tool height isn't just right when facing and as a result even a centre drill might wander.

Oh welcome to the forum BTW! ;D

[brozier]

cjs3,

What are you comparing the hole to? The circumference of the rod?

Have you machined the rod all over? Most Mild Steel bar will not be particularly circular in section.

Try facing the end and then take a few light cuts along the bar (use self act if you have it) you will probably find the first few cuts are not continuous all of the way round the circumference. The cut makes a distinctive noise as it does this.

Once you have a continuous cut the bar will be truly circular and you should find your centre drill produces a hole that is central.

Even if your tailstock is offset slightly fairly long drills will bend to follow the centre drilled hole. Though it's better to adjust setover as mentioned above.

Centre drills (Slocombe) don't bend so they will always be in the centre of the rod, It's just the centre might not be where you think it is!

Twist drills on the other hand will wander as the flutes allow them to flex so if you want an accurate hole you have to bore it.


Cheers
Bryan

[jasonb]

Also check that your drill chuck holds true particularly if its a cheap import, if one jaw is off it will push the drill bit to one side.

After turning the end of your bar flat, put some marking fluid on the end or felt pen, offer up the center drill while rotating the chuck by hand, if the point of the drill leaves a small "ring" rather than a "dot" something is out of line.

Jason

[baggo]

George Thomas describes a method of accurately centre drilling stock in his construction notes for his bending rolls.

Quote:

"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.
The method described will produce a true centre even on lathes that leave a lot to be desired and readers might be interested in the results of a test carried out today on my own lathe, the alignment of which is beyond reproach. A 10 inch length of 1 inch diameter P.G.M.S. [precision ground rod] was set up exactly as described above and, after facing the end, a #2 BSS centre-drill was carefully fed in to produce a centre about 3/16 inch diameter. The work was then supported on a fixed centre and all support from the steady-rest removed. I obtained an indicator reading (total) at 1/8 inch from the end of the bar of .0007 inch. The steady-rest was replaced and a larger centre-drill fed in a little way under side pressure as recommended after which I obtained a T.I.R. of .0003 inch, in other words the centre was within one and a half tenths (of a thou.) of its true position. I have no doubt that it could be brought closer than that. "

I assume that the sideways pressure on the centre drill prevents the tip wandering.

John

[paul]

John, I can see how that would be useful to keep the hole produced parallel to the lathe axis but it wouldn't necessarily mean the hole was properly centred would it? "start the centre-drill into the end of the bar" in the right place sounds like cjs3's problem - my bet would be a tiny pip on the end of his bar. But what do I know?! ;D

[dickdastardly40]

Some thing else which occured to me, ensure the tailstock is locked in position before you start drilling and it is extended enough that the taper ejector in the barrel is not pushing the chuck out.

Al

[cjs3]

Thanks all for the range of useful comments. I'm afraid the day job will take my time for the next few days so it'll take me a little while to check but I'll report back in time. I suspect that as the lathe is a far eastern import starting with the tailstock alignment may be the place to concentrate.
I also think that time is a factor. I could be a better pianist if I practised more (or at all!); I could setup an oscilloscope quicker if I did it more; I could program quicker in Basic if I did it more and yes I could drill holes better if I did it more. I really don't have time to go to work!

[klank]

This thread has been one of the best I have read - from a beginner's perspective.

I too have had the same problem drilling a hole using tail-stock chuck. Always wandered a bit off centre - I assumed it was the drill flexing or something.
I was too embarrassed to post for help - seems such a stupid problem and be thought of as a fool for asking a dumb question in front of so many.

CJS3 - thank you for having the sense to put the question here - the answers and advice have been really helpful.

For my sins I have a "Chinese" lathe (Chester DB7) - all I can afford on limited means.
Having read the advise here, I did a quick and dirty check with head/tail stock centres and sure 'nuff, the result was bad.
I put matters right with the tail stock set-over adjustors and thought why not check the headstock chuck/mandrel backplate (bolted on "cheapo" type chuck).
I clocked it, and found two high spots. Machined these off very slowly and carefully and clock is now good.
I also clocked a "test bar" in the three jaw chuck jaws - a rod which came out of an old dot-matrix printer - read somewhere these were (usually) perfectly true.
Results were a bit off, so fitted up my Dremel in a clamp for the tool post and ran a small grind stone up and down the open jaws with the chuck turning quite fast - just enough bite to make a few sparks each time.
Re- clocked and result was much better.

Yes, I know "Eastern" lathes seem to have a reputation, and yes - possibly warranted. I have never had the experience of using any other lathe to compare with my little one (my first ever lathe) and so must accept what it offers for the price I could cope with.
Sure, a Myford or better would be wonderful but the cost is way beyond my limited resources - so I shall stick with it on what I've got. (Work happily progressing slowly on my Stuart 10H).

I have also learned - "don't be too proud to post a seemingly dumb question" - thanks to all those who freely proffer such genuine help and advice.

[brozier]

Klank,

As one of my old lecturers once said "The only stupid question is the one you don't ask."

Good tip about the test bar I have a broken laser printer in the garage must get round to disassembling it.

Sound like you have got your lathe running really nicely. One thing to point out is that a self centering 3-jaw chuck relies on the integrity of it's scroll for accuracy even top quality ones wear unevenly. So although your chuck will now be very accurate for the diameter you ground it at there will be some variation over it's full range.

For ultimate accuracy you will need to use a chuck with independent jaws (most commonly a 4 jaw chuck) and a DTI.

I won't get into the Chinese lathe vs Myford argument. The important thing is you are using the lathe to produce something and enjoying it...

Cheers
Bryan

[circlip]

I reiterate Bryans' advice Klank, NOBODY knows everything. Regards Ian.

[klank]

Thanks for the kind comments.

Never thought about the scroll accuracy - hmmm - hope for the best I suppose.

Best wishes
Peter

[cjs3]

Thanks everybody.

Sure enough when I checked with centres in both head and tailstock that indicated the heart of the problem. I adjusted the tailstock to remove the 'setover' (cracking the yellow paint along the joints indicating it was as received from the factory) and am pleased with the improved results. For anybody else with the same machine (Machine Mart CL300) you might not have seen the following site www.mini-lathe.com/Default.htm which has quite a lot of material related to this machine albeit under the american brand name.

[steam4ian]

G'day CJS

I recently wrote in another post.
The Yahoo 7x12 group is not bad. See sites mini-lathe.org.uk/ and www.gadgetbuilder.com/index.html .

Enjoy your little lathe, I enjoy mine.

Regards,
Ian

[dickdastardly40]

CJS

I'm glad the advice worked, all you ever need is only a few keystrokes away as somebody either has the answer or can point you towards it.

Look forward to seeing any work you may like to post.

Al

[paul]

May 22, 2008 11:22:05 GMT cjs3 said:

I adjusted the tailstock to remove the 'setover' (cracking the yellow paint along the joints indicating it was as received from the factory)

What did you do exactly? I have the same lathe and there doesn't appear to be any way to make lateral (crosswise) adjustment to the tailstock...

[steam4ian]

G'day Paul

I am surprised at your comment if you have a C2 or C3 7" lathe or equivalent.

On my lathe the tail-stock body is mounted on a foot which runs on the bed. The foot has a transverse tongue which mates, well almost, with a groove in the body. One or two screws low in the body facing towards the head stock clamp the body to the tongue. There is a further clamp screw accessible from under the tail-stock. If you slacken these screws off the body slides across the foot.

Look up the links I have posted to get more info. One worthwhile mod is to move the bottom screw to the upper side of the body so you can do adjustments without having to take the TS off the lathe.

Regards,
Ian

[cjs3]

Paul

I removed the tailstock from the bed and just loosened the single allen screw that is visible on the underside. Ensuring that the slides were completely free of swarf I replaced the tailstock without securing the clamp nut. The gentlest tapping with a small hammer was sufficient to correct the setover in a matter of moments. Then gentle removal of the tailstock to tighten up the allen screw again before rechecking the alignment. Finally I checked with the clamp nut tightened and was relieved to find that the alignment did not change. Finally I fitted the chuck in the tailstock with a centre drill in place and studied the alignment of that against the centre in the headstock.