Drilling off-centre with boxford lathe! - how do I correct it?

[raynerd]

Hi Guys

I`m having problems with drilling a centre hole on my boxford lathe, the problem being that I am well off centre. I`ve spent the last couple of hour trying to sort the problem and I`m getting a little frustrated. I have followed all the steps here:

http://www.mini-lathe.com/Mini_lathe/Operation/Drilling/drilling.htm

I ensure that the job is clocked up and true in the chuck and the drill chuck is well seated within the 3mt tailstock, I face up the job and I bring a centre drill up and into the work piece followed by my drill bit. When I remove the job the hole seems true to size (as accurately as I can measure) but well off centre. I`ve put dead centres into the chuck and tailstock, clocked them in and they meet OK. The tailstock doesn`t seem to have any adjustments. .... something just doesn`t add up and I`m currently having to do all my drilling on my mini lathe.


Any advice would be welcome.

Chris

[zeusrekning]

With the two dead centers "clocked" together, spin the chuck. Do the centers stay aligned? To drill off center but have the hole on size almost sounds like the chuck is not centered on the spindle. 

[raynerd]

I checked this again and the problem is now sorted - the holes were enlarged and it was due to the movement of the ram of the tailstock. I have since tightened this up, realigned the tailstock and I`m now back in business. Thanks for the suggestions - next task is to drill holes around the circumference (see new thread).

Chris

[raynerd]

Contrary to my last post, this issue is still not resolved. The ram of the tailstock was moving and I thought it was causing the issue but it wasn`t the only problem.

I am still getting a perfectly true to size hole but off centre   

I have spent a while looking at it today and using the collets that came with the lathe, drilled a few holes which have been perfectly true. I then swapped back to the 3 jaw and discovered that the chuck is holding the job just off centre! So it is an issue caused by the chuck but how can I rectify this? What is the correct order and way to put in the jaws, I really struggle getting them all to lock/draw in at the right time...perhaps it is my fault?

Finally, while I`m talking about drilling I may as well just clarify my drilling method. I am currently facing off, centre drilling with a centre drill and then following it with the jobber. I have noticed that occasionally using a small centre drill it will still wander - what size centre drill should be used for the inital "spot" and what is the correct way to get your first centre spot. 

[kvom]

If the hole is round and the correct size, then the tailstock must be aligned with the spindle.  You proved that as well by drilling with collets.  So either the chuck must be off center or the jaws are not centering or both.

You can likely check the chuck by measuring runout of the sides with a DTI, assuming they are machined.  If the runout is minimal or less than the error you're seeing, then the jaws are the likely culprits.  You don't saw how far the holes are offset, but a damaged jaw or jaws in the wrong slots might be the cause.  Normally jaws will be numbered 1-2-3 and the slots as well.  You would place jaw 1 in its slot and turn the key until the scroll mates.  Then insert jaw 2 and continue to turn the scroll to engage it, followed by 3.

One thing to check would be to close the jaws completely and see where the ends come together relative to a center in the tailstock.

[Bluechip]

Chris

I don't know whether this is relevant.

Nothing to do with headstock 3-jaw chucks, but I had a similar problem with the Myford, so I'll stick it in anyway.
I had a useless Chinese drill chuck with this fault. Don't know brand name 'cos it didn't have one.
Why doesn't that surprise me ?



Took ages to suss out what was wrong, it ain't obvious, but the 6JT socket was not axial. Not by much, but it made a hell of a difference.
Paricularly on small holes.It gave erratic results as the arbor can go in the tailstock at any angle. So, the error differs each time.

Dave BC

[pabird]

Chris, if your 3-jaw is holding the part off center and you suspect that the jaws are incorrectly installed remove them and look for a stamped number on each. Install them in the chuck's slot with the same number. Now this is where I had a similar problem, my jaws were installed correctly but still wouldn't close and meet at the center of the chuck. I found that if I turned the chuck key backwards while putting slight pressure on the jaws with my other hand I could feel the scroll passing each jaw and the jaw would drop slightly. When the scroll passed the #1 jaw I could turn the chuck key in the normal way to close the chuck and the jaws would be in perfect alignment. Hope this helps. Smitty

[Gerhard Olivier]

Chris simular problem mine was that the chuck just could not get any better than 0.84mm runout.  No solution ( that I could think of) other than to use slightly ovesize material and alway skim the surface to cut it concentrc then drill.  Problem with that is all work needs to be done in one setup. I re skimmed the backplate opened and cleaned the chuck checked jaws -  but mine does this with both external and internal jaws.

My conclusion was it is a rubbish chuck?????
gerhard