|31 forum posts|
Returning to a 'filed' project, I'm trying to bore a 60mm long hole in an aluminium bar using my lathe's power carriage feed, but I'm finding it's significantly tapered to the tune of around 0.6mm (in 25mm or so) over the 60mm length of the hole, with the end closest to the chuck being the smallest. The lathe is a Denford Viceroy 250, but I don't think that's particularly relevant for this question.
As the bar is around 150mm long, I've brought out my fixed steady to help keep things in place (see picture) but I wonder if my lack of experience in using it is causing the taper I'm seeing.
I've simply used the steady on the outside of the bar (which is perhaps not perfectly round....?) and gone from there - is there a technique/order of setup things up that I should be using in order to ensure everything is lined up properly to give me a non-tapered bore?
The boring bar is of course sticking out quite a long way, but I'm taking deliberately light cuts and in any case, the deflection forces on the bar should be the same at all points along the cut.
|vintage engineer||13/04/2020 11:35:39|
254 forum posts
Depends how you set the steady up as this can cause a taper. I normally set the job up using the tailstock to centre the job then set the steady up. Try running a clock gauge down the outside to make sure it is running parallel.
|Alan Waddington 2||13/04/2020 11:43:42|
|503 forum posts|
Why not true the bar up externally first, at least in the area where the steady sits ?
4654 forum posts
Usually you clamp the steady in position right next to the chuck. Then set the jaws in position. Then slide the steady along to the working position and clamp it down again. This ensures the end of the far end of the bar is held in the correct position and does end up tapered.
Bar does need to be round and parallel to start with though.
Edited By Hopper on 13/04/2020 11:48:12
|1542 forum posts|
What is the finished bored diameter going to be ?
Best to drill the major amount of material before boring, that will allow a larger diameter boring tool to be used and help reduce any tool deflection.
Using the steady is no guarantee the lathe will bore parallel unless it is running on a truly turned surface, even then I wonder does the lathe turn parallel on external turning over the full depth being bored ?
If the lathe does external turning parallel it will be tool deflection causing the taper, check tool exactly on centre height, cutting tip does not drag or rub and is sharp before trying to take light correction cuts.
You may have to adjust speed and feed to get optimum results.
|not done it yet||13/04/2020 11:56:37|
|4744 forum posts|
Your ‘not perfectly round’ won’t help.
If I want to do that sort of thing, I support with the tailstock, and surface until there is a perfect round for the fixed steady to bear on. Usually from chuck right out to the end. Set up the fixed steady on that and it should run as good as it can ever. The thickest boring bar that will fit is always chosen - better for rigidity.
Why are you using a 150mm boring bar stick-out for a 60mm deep hole? It only needs to be 70mm.
Edit: 4 replies up before I got mine posted!
Edited By not done it yet on 13/04/2020 11:58:09
|Nicholas Farr||13/04/2020 12:00:07|
2318 forum posts
Hi YouraT, set the bar up like Vintage Engineer says and then true the area where your steady will be running on while the tailstock is still in position. With long slim boring bars, I've often found that the spring becomes accumulative the deeper you go, i.e. as soon as the bar enters the bore it will have a small amount of spring, but of course there will be resistance to the cut which will make it spring a tinny bit more and can build up like that the deeper you go. To over come this, I take a cut, back out and then take another cut or even two without advancing the tool and then carry on this way until you get to the size you want, Always works for me.
|31 forum posts|
- a number of things to think about - I'll take a look at skimming the stock before setting up the steady (I have spare OD on the stock), and also at setting the steady up right up against the chuck initially, then moving it into position - both suggestions are eminently sensible.
'Not done it yet' - my terminology was perhaps a little vague - the boring bar stick out is only around 70mm - it's the aluminium bar stock that's ~150mm long.
Nick - I'm not sure I understand why cumulative spring might be the case here given the initial drilled hole (as large a drill as I have, but I'll give it a go.
|Nigel McBurney 1||13/04/2020 12:15:54|
717 forum posts
Thats and awful lot of taper, and assuming your lathe usually turns true,the fault lies with steady and spring in the boring bar, The steady pads should always run on a true machined surface,and be careful when tightenind the pad screws they must only just touch the work,too much pressure and the work can be pushed to one side and therefore out of true.plus they must be kept lubricated when the work is revolving ,use some oil and squirt with an oil can.One cause of a severe taper when boring is swarf building up in a " birdsnest "around the tool and the bar and deflecting the bar away from the work .I would be using a bar at least twice the diameter,made from silver steel with a hole in the bar to take the toolbit, If the bit only protrudes a small distance from the bar then swarf quickly builds up ,particularly with materials that produce long strings of swarf ie steel and ali ,also check that the boring tool bit has adequate clearance. A pecularity of the steady is that if the three steady pads are set up so that they are not in in line with the centre line of the lathe,the work will work or "walk "out of the chuck,even if you are convinced that the set up is true keep an eye on the chuck end of work to make sure it is still well in the jaws, I have seen 'Walking" as it used to be called but never seen or experienced work come out the jaws. Usually caused by one pad screwed in too much and deflecting the work off centre and the other pads then screwed in .When working with a steady,lubrication is essential to avoid the pads marking the workpiece,if a near finished work piece has to be set up in a steady then some times it possible to make a collar ,which can be secured with grub screws, the collar is then set up true by either adjusting the grubscrews or machining the collar,to avoid marking the work piece small brass slugs can be fitted under the grub screws.Well theres some info on steadies but I would bet that the bar is deflected by birds nesting build up of swarf,I have seen large buildups of swarf put sufficient pressure on the business end of the bar to actually move the toolholder.
|vintage engineer||13/04/2020 12:34:25|
254 forum posts
On large hydraulic cylinders you will often see steady tracks machined on the outside.
|Fowlers Fury||13/04/2020 12:41:32|
345 forum posts
All sound advice above but one question.....how are you returning the boring bar to start another cut?
If you're using the self-act (or winding out with the lead screw hand-wheel) at the same setting to back out the tool this could be a contributory factor. The safest way is to stop the lathe at the end of the cut, release the carriage and wind out by hand. Now put on your next cut and continue to bore with the self-act engaged. Carry on ad nauseam.
(if you use self-act to withdraw the boring tool and there is slackness between the carriage and the lathe bed shears, the carriage can tilt and take off some more material from the bore on the return. I appreciate I'm inviting abuse for offering this but years back, on an old & worn ML10, I discovered this issue. It was when boring out the rear axle for the Clayton wagon. Eventually after stopping the 'powered withdrawal' I got it parallel).
|5932 forum posts|
0.6mm is a lot. Are you moving the boring bar with the whole saddle or just moving the top-slide? If the latter, it has to be set-up very accurately to cut parallel - difficult. Much easier to move the saddle with the hand-wheel on the apron, because then the tool point always travels parallel with the bed. (Flexing apart!)
|not done it yet||13/04/2020 15:02:02|
|4744 forum posts|
|Mick B1||13/04/2020 15:11:28|
|1610 forum posts|
I think that's key if you suspect the OD's roundness. Quite apart from the taper, your bore might not be round or straight either.
Once that's resolved, look at spring cuts and swarf clearance. I've certainly experienced what Nick Farr described with cumulative deflection or something like it, and that was in freecutting LG2 where there were no practical issues with swarf clearance.
|31 forum posts|
Thanks for the further input guys.
I'm returning the bar to the start by hand after moving it clear of the cut a little and then returning to the original position and putting further cut on for the next pass. The cut is done through the powered saddle - I do the last 2mm or so using the topslide as the Viceroy doesn't have a repeatable saddle travel clutch arrangement like the Boxford, just an 'anti catastrophe' dog clutch.
I'm going to:
I've not seen any tendency to walk out of the chuck, but I'll keep more of an eye on that too.
Likely tomorrow now, but might manage to start something tonight.
|Mick B1||13/04/2020 16:37:04|
|1610 forum posts|
I bored the cylinder casting on one of my engines this way. IIRC I ended up with about 0,02mm taper in a 22,2 dia. x 94 long bore - which was quite good enough to work well.
I could only engage part of the steady with the end flange, to avoid collision with the valve face that you can just see disappearing round the bottom of the casting.
Edited By Mick B1 on 13/04/2020 16:39:11
|old mart||13/04/2020 19:54:01|
|1829 forum posts|
Put a centre in the stock.
With a live or dead centre in the tailstock, take a minimum skim of the od to give the steady a usable surface.
Set up the fixed steady 25mm from the end. Cut a disc of plastic or card to fit on the end to keep the swarf away from the legs of the steady.
Drill the hole close as possible to the finished diameter. Careful depth control needed if the hole is blind.
Use a boring bar of the maximum size to fit the drilled hole. Use a bigger boring bar if and when the hole gets big enough.
Pay attention to swarf build up, you may have to clean out the hole at every pass.
Never expect the top slide to bore parallel holes, always use the saddle.
If the size is critical, make a plug gauge with a stepped end first, when the front end goes in, then you can creep up to the second diameter which should be -0.025mm. An exact size plug will not enter a bore until the bore is oversize.
Edited By old mart on 13/04/2020 19:55:18
|Pete Rimmer||13/04/2020 21:01:56|
|734 forum posts|
If you have two dial gauges put one on the side and one on the top (or bottom) up near the steady rest. Release the steady fingers from the part and turn it by hand tapping it true until the dials read no runout and zero them. Now bring the steady fingers in until all three are touching and the dials read zero, the part will be running true in the steady.
If it's not round on the OD you'll have to stuff a centre in the end and turn it true first.
TBH that boring bar is too thin for that much stick-out. It might well be deflecting as you feed it in under load.
4654 forum posts
That will work too. Just extra work though. If the chuck is holding its end true and you set the jaws on that end before sliding it along , the jaws are set true. Thats the common way of doing it and has been for over 100 years. Quick and easy and works with long jobs that would otherwise need the steady in place to drill the centre hole in the first place.
Edited By Hopper on 13/04/2020 21:25:49
4654 forum posts
Using dial indicators (and why would you use two?) will only show the work is concentric. It could still be out of alignment with the lathe axis, causing a taper. Bit like using an offset tailstock centre.
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