Mounting lathe tools at center height / operator error?

This topic has 8 replies, 5 voices, and was last updated 14 July 2011 at 19:25 by WALLACE.

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13 July 2011 at 19:17 #71745
Paul Fallert
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@paulfallert28101
I have frequently read “Always set the tool at center height”.>>

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I also found in a modern textbook a formula for setting a lathe tool. It said: increase the height of the tool over center by 1mm for each 25mm increase in diameter if the work diameter exceeds a certain minimum. Another author wrote, “reduce the tool below center when the work diameter approaches a smallish diameter”. Sounds like the same advice from a different vantage point.>>

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I recently turned a 75mm long by 25mm dia. leaded steel work-piece with a tool ground as for General Purpose Turning (as per G.H. Thomas’ Manual). The tool was sharp and I first took a facing cut and adjusted the tool to eliminate the “pip”. The area to be turned was only 25mm long and the work extended 30mm from the 3-jaw chuck. All was going well, but after reducing the diameter to about 13mm, I experienced the tool cutting irregularly and the required pressure on the cross-feed had increased. After checking that the tool was still sharp and still on-center (using a 150mm rule), I raised the tool and it completely stopped cutting. Then I lowered the tool and it began to cut again. AND, the tool was still very sharp. I took a light facing cut and the tool left a “pip” typical of a tool below center. But it WAS now cutting nicely, albeit below center.>>

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I should mention that my lathe is a heavy 10”x24” with the tool mounted in an EMCO brand Quick Change Tool Post. I have checked the adjustment of the spindle bearings and they are tight. No detectable spring in the carriage/saddle. If the tool is cutting, the work turns parallel.>>

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I generally use the facing-off technique to set the tool height. I figured that this put the tool at center height if there is no “pip”. Now, I am thinking that cutting on the side of the tool while facing, is not the same as cutting with the end tip of the tool, as when turning.>>

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When I ask experienced lathe hands how they set their tools, most have suggested facing-off until no “pip” and declaring that this provided a “dynamic” on-center tool height or they suggest using the 150mm ruler. There is a certain amount of indifference in their response as though no one else has these sorts of problems. Must be “operator error”. And indeed, it may be.>>
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13 July 2011 at 19:17 #5581
Paul Fallert
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@paulfallert28101
Should lathe tools be mounted at center height?
13 July 2011 at 21:15 #71751
Tony Pratt 1
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@tonypratt1
This subject has been debated ad nauseum on various forums with a myriad of opinions. I would say set your tool on centre height and if it doesn’t cut the tool geometry is not right in some way, where the problem is would be difficult to say without seeing the tool. Moving the tool above or below centre height is altering the top rake angle so I would look at that first. You are not alone in experiencing poor finish on work, we all get it from time to time and again there are many reasons for it, a search on this forum should point you to the relevant threads.

Tony
13 July 2011 at 22:05 #71757
Harold Hall 1
Participant
@haroldhall1
You do not say Tony to what accuracy you mean by centre height, precision not being guaranteed. I deliberately set my lathe tools a couple of thou low as by doing this I can be sure they are not above. If above then they will start to rub at smaller diameters.

At very small diameters, say 1mm, of course centre height is more critical, but even more critical that it is not above centre.

A few thou will have only minimal effect on the cutting angle presented to the workpiece, so can be ignored.

Haroldl
14 July 2011 at 08:11 #71772
mick
Participant
@mick65121
It could be many things:

Are you using inserted tooling, pre-ground or hand ground cutters.

Is the tool overhanging the tool post.

Is the compound slide over extended.

Is the depth of cut too great.

Are you feeding by hand or with the machine traverse (perhaps the feed is too course)

One thing is for sure ALL tooling must be at centre height, the only exception is a trepanning tool, where it should be set slightly below.
14 July 2011 at 08:23 #71773
Harold Hall 1
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@haroldhall1
Can you quantify Mick what you mean by centre height, ie to what degree of accuracy and what method do you use to achieve it?

Harold
14 July 2011 at 11:27 #71782
WALLACE
Participant
@wallace
There was an exellent idea for a device to set the height a few years ago in MEW – it involved a scribed bit of clear perspex and a mirror. Thought it was almost genius at the time – not that I’ve got around to making one though.. . .

Beware the facing off method for carbide tools – if it’s a bit below, it’s a very good way to remove the tip of a brand new insert !

W.
14 July 2011 at 17:40 #71804
mick
Participant
@mick65121
I set tools by eye against a back centre, then refine it by turning to the centre of the bar untill the turning pip disappears, this is quite good enough, not so easy with parting tools as the component will fall away before the centre is reached. One more thing to add to my list of possible causes, is speed, this is V important. To calculate speed use:

Cutting speed of material X 12

Pi X diameter of work piece

Too slow will tend to produce ragged surface finish. Too fast will burn out HSS tooling, its all a question of balance, as to how the machine feels when your turning.
14 July 2011 at 19:25 #71807
WALLACE
Participant
@wallace
The other way is to find a cheap and chearful dial guage at a boot sale that’s seen better days and make a fixed height guage. One where the pad comes out of the back is probably best as you can look down on it while adjusting.

Or if you’re loaded ,buy a new one !

W.

Edited By WALLACE on 14/07/2011 19:25:47
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