|Chris TickTock||27/08/2020 13:53:53|
|622 forum posts|
This is almost an embarrassing question to ask, nonetheless I think I will, so forgive me for my stupidity in advance.
Question I need to turn down a 9.5 mm Dia, 75mm long piece of EN1A steel in my Sherline lathe to mostly 4mm. I have put it in the chuck then thought should I center drill the end or not. Basically is there a rough guide to what can be held in a chuck without the other end being supported. There surely is a table somewhere saying e.g. 3mm Dia only 4 times its Dia sticking out unsupported?
Or is this another instance of its up to the machinist, which is all well and good once you have sufficient experience of machining.
Edited By Chris TickTock on 27/08/2020 13:54:36
|Former Member||27/08/2020 14:05:25|
[This posting has been removed]
4136 forum posts
Your small hobby lathe will always do better with tailstock support
It's up to you to decide when it's not necessary
|Steve Neighbour||27/08/2020 14:11:17|
|74 forum posts|
I'm sure I have read somewhere that as a general 'rule of thumb' no more than 3-4 x the dia should protrude from a chuck 'unsupported' or you risk distortion when turning !
so a 9.5mm stock should as a 'general rule' be no more than 28-35mm of unsupported length
I'm sure one of the site 'gurus' will comment soon !
6674 forum posts
Rule of thumb, up to 3.5x diameter sticking out of the chuck unsupported should be OK. Over 5x diameter is asking for trouble. Depends on the material, so I engage brain if sticking out more than 2.5x. How much of the job is gripped by the jaws is important; a thin rod completely gripped by the full length of the jaws is considerably safer than a fat rod gripped only by the far end of a set of reversed jaws. If it looks or feels dodgy, it is!
As Barrie says, long jobs usually need to be supported by a fixed and/or travelling steady because they bend so much.
I'd follow Barrie's step-at-a-time approach, ideally going down to 4mm in one cut if the lathe can manage it. (JasonB told me this one!) It's because a rod's ability to resist sideways forces depends on how thick it is - a 6mm diameter rod is far more easily bent than a 9mm rod.
I'd also consider holding the rod in a collet rather than a chuck because collets have less run-out. Each time the rod is advanced and retightened in a 3-jaw chuck is an opportunity for run-out to change the axis slightly and leave a visible ring at each step. A really poor chuck will make a mess of it! A really good one might not be a problem, or leave rings that can be quickly polished out.
As jobs go turning long slender rods is tricky. Don't be disheartened if it takes a few goes to get right.
|199 forum posts|
If you use the cut move cut system use the same chuck socket each time and avoid rotating the work
|212 forum posts|
All the general rules about projection from the chuck are OK, but frankly are not the real answer here,
We need more information.
What is the length of the piece of stock you are intending to turn?
Will there be larger diameters on the finished item?
Depending upon the answer to the last question, why are using 9.5mm bar to produce 4mm? Why not start with 5 or 6mm stock?
How accurate do you need to be?
All things being equal why not simply centre drill each end and mount the piece os stock between centres and use a travelling steady?
|Brian H||27/08/2020 15:17:35|
1960 forum posts
You could always try a trial cut of, say 0,1 and then measure along the length to see if the resulting taper due to 'push off' is excessive.
19555 forum posts
The other thing to remember about the rule of thumb is it's the final diameter you are going to be turning that should be applied to the 3-4 x D rule of thumb not what you start off with.
So taking the example given above 35mm stickout for that final pass down to 4mm could mean you have 9D sticking out from the solid support.
Barrie's method work OK, you don't have to take the cut from 9.5 down to 4.25 in one go but you will find you can take deeper cuts as the metal being removd is nearer the chuck than trying to take the same depth of cut in the middle of a long length even if with tailstock support. So for example you may be able to use 1mm DOC doing it in stages close to the chuck rather than 0.5mm DOC along the whole length which can speed up the job.
|Chris TickTock||27/08/2020 15:38:57|
|622 forum posts|
Really helpful Guys...many thanks
|duncan webster||27/08/2020 16:15:03|
2943 forum posts
Running down tool or roller box, but that only works if turning to same diameter all the way. In answer to the quetion, it's a sort of tool and travelling steady built into one unit.
A clever version is shown here **LINK**
but I'm sure I've seen simpler in ME over the years
|William Chitham||27/08/2020 17:57:42|
|66 forum posts|
The question reminded me of this Tom Lipton video: https://youtu.be/thrvEhufJJM
Obviously not directly comparable since he's turning plastic but maybe some of the techniques are relevant.
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