tool tip height

Hi all, so being new to lathe work i get the feeling i'm missing something. The problem being if i set my tool tip at dead centre height so it removes to pip while facing, the finish i get is terrible. cutter seems to just bounce and dig in alot but if i set the cutter down say 0.5/0.75mm i get a nice clean cut but then i end up with a pip. Where am i going wrong? lathe is a comet vs with c-tipped tools tho i have tried hss and get the same results. Help!

Edited By mark mc on 05/11/2012 09:26:58

firstly Hi and Welcome

Tool height .. OK there are differences of opion here but I favour at center height ..

OK if your tool is low WHILE CUTTING you will leave a pip.

If High then the tool tip isnt the only point of contact and the tool nose below ( in the relief part of tool) is rubbing the work.... heat , poor finish etc.

I say while cutting because the tip will "sink" under load (however little , it will sink) and this is why the tool is right for facing but not turning say ( lower loads in facing) also tool shapes can differ for facing and turning

thanks jason, guess i'm just going to have to experiment abit more

It seems to me that the tool may not be clamped rigidly, perhaps not flat underneath or too much overhang. Colin

Edited By colin hawes on 05/11/2012 16:16:44

Lowering the tool will increase the front clearance - perhaps you need a greater clearance angle? However I suspect the main problem is lack of rigidity. Deflection of the toolpost/topslide will lead to the tool moving forward and digging in. Lowering the tool will reduce this effect.

Make sure the gibs are properly adjusted to eliminate play and that the toolpost is firmly clamped down.

Russell.

Hi Mark,

Is your machine new or pre owned? The tool should cut well if it is sharp, on centre and not too high a feed rate. I have been machining for many years and prefer well sharpened HSS although I realise many prefer carbide tooling, usually indexable with replaceable tips.

Here is an example of some turning on a old Boxford using Hand ground HSS tools.  The saddle was hand fed.  The tool was perfectly on centre height, you can click on the pictures for larger view:

There must be something wrong with your technique or your machine set up or even the material you are using, try to get some free cutting mild steel or aluminium rather than any old stuff from the scrapbox You should eliminate possible problems one at a time. If you can, get an experienced machinist to demonstrate, if not you could join a local club or try to find a local evening class, they still exist in some areas.

Let us have more information.

Best regards

Terry

Edited By Terryd on 05/11/2012 17:35:49

Hi Mark, get as close to centre height as you can that's your reference, then try all the other suggestions above.

Tony

Hi

I have a Comet vs and the first thing I found was that the toolpost would not stay put, the area on the cross slide round the post bolt was raised, the area round the toolholder underneath round the hole was also raised and the silly little handle wouldn't do up tight enough which meant that the tool not only tipped on a cut but also rotated away from the job.

after recessing the underneath of the toolholder with a large countersink bit in the drill press and making a new longer handle for the toolpost ( I used a nut and a spanner to start with) it now cuts perfectly.

BTW if you got a set of tipped tools with the lathe, chuck them in the bin, they are useless.

Roy

Thanks all for the replies, at first i thought it was to much over hang movement in the tool as well so i made a riser block and removed the compound slide this helped abit. I'm based in northern ireland so finding a machinist to watch is a problem. they don't seem to exist here any more!. I'm using glanze tipped tools so they should be ok?

The steel I'm using is old stuff from my work which has been turned before, old shafts from large bins usually 60mm dia by about 5 feet long. the shafts wear out brake, bearing goes so they replace them tho i'm not sure what type of steel it is.

I'll try the gibs and see what happens and try and get a few pictures up to you see what i mean.

Again thanks all

Check this out Mark

Hi Mark welcome
I use mainly HSS and after grinding the tools to the required angles. I always stone mainly the rake angle . a slight radius would help Theres several ways to set tools on the centre have a look through this forum . Also on tools for my little hand shaper gives a great finish
Nobby

Edited By Nobby on 05/11/2012 22:45:00

Thanks chris i never knew about them tho they are quite far away from me and driving thro belfast makes me cringe, hate the place. Nobby i've been thinking of making a Tangential tool as i have some hss but am abit rubbish at shaping the angles but i think i can easy make the tool holder and the tools are just ground at a 45. so I'm going to make one and give it a try, i'm not to bad with the mill but the lathe is getting the better of me. Also chris you say the tips where abit rubbish and to be honest I'm thinking you maybe right. Anyway off to the workshop/mancave to try out yor tips thanks again everyone for the help.

Edited By mark mc on 06/11/2012 08:29:26

Also i seen this over at model engine maker, very funny.

**LINK**

The Glanze indexable tips are fine for roughing but you should by some better tips from Greenwood/Cutwel/Sumitomo/ArcEuro for real work.

You must be on centre height (tolerance here is critical, parting tools better than +/-0.1mm for example)

Hi Mark,

The 'old shafts' you mention may be higher carbon steel which is a PITA to turn on a relatively light machine. They would turn ok using a large professional machine with proper carbide tools, but a hobby machine, I'm not so sure. I would suggest getting a bit of free cutting steel of a known composition, leaded is even better if not to be welded, and trying that.

The shafts are probably EN8 (080m40) , a medium carbon steel for shafts etc, while a good machining free cutting steel is EN1 (230m07). That may be one of your problems.

Best regards

Terry

Might seem a silly question ,but does your lathe spindle lift due to slack when machining? A lever under the chuck would show up bearing wear.I don't know this lathe but spindle end float could also be a cause of poor performance. Colin

If Terry is correct, and the shafts are EN8, then that is most likely your problem. I've found that EN8 in particular needs to be run faster than other steels to get a good finish with insert carbide tooling. Fast means the swarf coming off dull red, ie, hot! See this link for some detailed numbers on turning EN8:

**LINK**

Regards,

Andrew

so i did a little test to day to show what i mean. from left to right we have hss .5 below dc next on dc then indexable tips on dc and again .5 below dc

the steel may well be to blame too, i'll invest in some en1 and try again