question about drilling

[gog64]

Apologies in advance, I'll explain this badly and get all the terms wrong!

I've got a cheapish new Axminster (craft) lathe, which I'm chuffed with, but I'm making holes larger than I intend. For example, I've got a work piece attached to the headstock (normally in a chuck) and a drill chuck on a morse taper mandrel in the tail stock. If I have a 40mm forstner bit in the drill chuck, my hole comes out 41.8mm. Same bit in my pillar drill results in an exactly 40mm hole. If I offer up a smaller drill bit to the workpiece it looks like it's wobbling slightly. Of course that's just my eyes as the workpiece is spinning, not the bit! Everything is tightened up, so my guess is that the tail and head are not aligned perfectly. Is this likely to be the case, or am I missing something? If this is the case, can I do anything about it, or is it likely to be the limitation of a budget machine?

TIA!

 

[marcros]

Have you checked alignment with a centre in the headstock and a forstner bit in your drill chuck?

 

[Eric Roy]

I have a similar Axminster lathe and they are limited in accuracy due to the difficulty in getting good alignment of tailstock/headstock as already noted, but also due to in-built eccentricities.

Use of a dial-gauge allows you to measure the run-out on the spindle itself, on the spur drive or chuck etc etc. When you add up all the small potential inaccuracies of the spindle, taper, drive, chuck mounting, these can combine to give the sort of results you are seeing - although probably nowhere near as large as you get? I would typically see around .005 inch “wobble”.

It is also worth checking the tapers for dirt and damage. Either may cause misalignment and off-axis mounting resulting in wobble.

 

[CHJ]

Sounds like typical tailstock misalignment. But having said that it might be a combination of tailstock, Chuck jaws & drill shank alignment.

Does the alignment get worse the further the tailstock quill is advanced?
(Usual indication that it's just the tailstock)

If not then if the headstock can swivel, check it's alignment with the bed, fit a long straight shank in you Chuck & check it by eye with the bed. (Also the misalignment of your drill will be greater with a longer overhang from the Chuck than on a turned piece closer to your Chuck jaws.)

 

[Dalboy]

Other than the alignment issue already mentioned is to try and make a centre hole using a small stub drill bit so the centre of the Forstner bit enters that keeping everything in line, or use a skew chisel just to make a small divet for it to start. Drill bits if over long may try to move slightly off centre due to the grain structure.

 

[gog64]

Many thanks for all of the above pointers, really helpful. The headstock can rotate, so with hindsight I should have checked that first. Sure enough, it has an enormous amount of "slop" in it's position, but now I realise that I can align it with the bed of the lathe and hopefully that won't move when in use. The tail stock has much less slop, but I can get it to align centre to centre with the head stock by twisting it when tightening the tailstock. Of course that doesn't help much as I have to move the tailstock from that position! As the headstock is now aligned with the bed as accurately as I can get it, my guess is that I've found the limitations of this level of lathe. Plenty good enough for now for a duffer like me as long as I understand those limitations.

A follow on question if I may?

When turning bowls, I am holding the workpiece at the headstock side with a screw held in chuck (clubman sk100) jaws. I bring up the tailstock with a standard 60 degree live centre to support the workpiece. I can turn the outside of the bowl just fine and make a dovetail recess for the jaws. I should probably say that I flatten the base and make it very slightly concave so that the bowl will sit nicely before cutting the recess. At this point the bowl appears to be spinning perfectly to me.

I then reverse the bowl. I make the dovetail recess very slightly deeper than the jaws, so the the base sits perfectly flat against the jaws. I normally tighten out into the dovetail (though I get the same issue if I'm working the other way around on a smaller piece). I then turn the lathe on and the workpiece is slightly out of balance. In other words, the base is spinning fine, but the rim of the bowl is not spinning in alignment with the base. Sorry, that's probably a poor explanation!

Any tips on what I'm doing wrong here? We're only talking about a small amount here, so in reality I can happily go on and finish the bowl. The rim ends up a mm or two out of balance with the rest of the bowl, which people don't notice, but it is frustrating to me!

 

[Dalboy]

If you make the bottom of the tenon too deep it may bottom out in the chuck it needs to be a fraction shorter as well as that a small flat next to the tenon for the jaw faces to push against may help.
Hope this LINK works but some very useful information.

 

[gog64]

...and then I found this:

https://www.axminster.co.uk/axminster-l ... 2mt-102427

sounds like it will be very useful for setting up for drilling.

 

[Robbo3]

It could just be wood movement, especially if you've gone in for a cup of tea before remounting.
All Stewart Batty's videos, as in Derek's link above, are well worth watching.

 

[Phil Pascoe]

Use these - https://www.ukdrills.com/hss-drills/centre-drills
if you start your hole accurately you have a good chance of its staying accurate.

 

[gog64]

Thanks again for all of the suggestions.

The 2mt to 2mt bit of machined steel (AKA the alignment tool from Axminster) has sorted me out. I really can't complain at all now. Lock the tail stock (i.e. very little wiggle room), align the headstock (which can rotate on my machine, i.e. loads of slop in the position relative to the bed) to the tailstock using the alignment too. Lock down the headstock (with a mallet, otherwise the blessed thing WILL move) and the job's a good un. Very happy with drilling now, pretty near accurate.

I hope that helps someone else!

 

[Phil Pascoe]

I would advise anyone with lathes with swivel headstocks not to rely on any marks or stops provided for alignment. Mine aligns with a pin through a hole in the headstock ......... which is as much use as a chocolate kettle, the centres are about 3mm off with the pin in place. I aligned it then put a cut across the head and the base with a very fine disc in an angle grinder so now I just line the two marks up, forgetting the pin.

 

[James Thomsan]

Once check the model of Axminster lathe, as most of them are limited in achieving accuracy due to the difficulty in getting precise alignment of headstock/tailstock. However, there’s one more issue of in-built eccentricities that might be the reason behind this problem.

If you’re up to solve this, you can use a dial gauge to check the alignments and also check for the presence of any dirt or damage in the tapers. Many a time, it’s also a reason for misalignment and off-axis mounting resulting in wobble effect.

Further, to solve the issue permanently, you can also practice the suggestion given by @gog64 in the above comment of setting up an Axminster lathe alignment centre 2MT. I hope this helps.

 

[RickG]

...and then I found this:

https://www.axminster.co.uk/axminster-l ... 2mt-102427

sounds like it will be very useful for setting up for drilling.

Interesting tool. But another idea is to have a live-centre in the tailstock, and a chuck on the headstock before the headstock is clamped in place. Bring the tailstock up to the chuck and close the chuck on the front of the live centre, this will centre the headstock on the tail stock. Clamp the headstock in place and you have it centred....

Well, it's a theory. I'll try it later.

You could also try clamping the headstock chuck around the forstner bit, as long as you don't need to remove your work from the headstock to do this if it's all nicely centred in the chuck.

The problem I have is, in order to remove a drive centre from the headstock, I need to swivel the headstock to get a knocking bar in there. I think I need to make space and get a shorter rod.

Thank you all for posting. It's got me thinking.

 

[Robbo3]

Might be quicker to bring up the tailstock centre to the hole in a faceplate or perhaps just a dimple in a turned piece of wood that is kept solely for this purpose.