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Home Made Rear Toolpost Issue

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old mart04/04/2021 21:03:01
2914 forum posts
184 photos

I have Kennametal and Kyocera 26 and 32mm inserted blades of various widths. Reading both manufacturers data sheets on their websites, they both recommend 0.01mm, 0.004" above centre line (or below if inverted), when parting solid material.

Dr_GMJN04/04/2021 21:46:45
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999 forum posts

Determining centre height to that degree isn't something I could do. I'm currently setting tools initially be eye against a dead centre, then by trial and error to home in on not getting a pip. Pretty tedious.

Macolm04/04/2021 21:53:25
34 forum posts
5 photos

Carbide needs to be kept in compression. Setting the tip slightly high (in a front toolpost) helps ensures this, even if there is a dig in; if the tip is low, there is a good chance the cutting edge will be pulled off.

old mart04/04/2021 21:53:29
2914 forum posts
184 photos

You could get a lathe tool set to cut exactly on centre and then make a gauge that sat on the bed to match the height. A very useful gadget.

Dr_GMJN04/04/2021 22:01:39
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999 forum posts
Posted by old mart on 04/04/2021 21:53:29:

You could get a lathe tool set to cut exactly on centre and then make a gauge that sat on the bed to match the height. A very useful gadget.

What's the difference between that method and using a dead centre? Aren't you still judging it by eye?

JasonB05/04/2021 07:00:11
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You can feel if the tool is the right height as it will either pass under the gauge(too low) or hit it (too high) Why not set a cutting tool to height on the top slide and then undo th erear post, slide it forward and see if the parting tool is above or below.

I would also say that there will be some movement of your blade on those screw shanks as you would have had some clearance for the screws which will be taken up by cutting forces. So keep a lifting force on the cutting end as you do up the screws.

John Baron05/04/2021 07:36:54
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448 forum posts
180 photos

Hi Dr GMJN, Guys,

Here is my lathe tool height gauge, made from bits of scrap !

26-02-2020-01.jpg

26-02-2020-02.jpg

And a drawing !

height gauge.jpg

Its very handy. Just sit it on the cross slide and set the tool edge either under the cap or on the top at the side of the cap.

JasonB05/04/2021 07:43:40
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20474 forum posts
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It is also very handy to know the exact height of your lathes ctr line above the cross slide so that you can accurately set work on it by adding the required packing under it. So once you know the height if you have a height gauge that can also be used for tool setting, scribing ctr lines on the face of the work etc. I have it written behind the lathe but know it is 3.389"

Dr_GMJN05/04/2021 11:07:35
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999 forum posts

Ok thanks guys. So I could make a gauge similar to Johns. To get the height, perhaps put some bar in the chuck, then measure the height by putting slip gauges on the cross slide, then add the radius of the bar? Then machine to that dimension? I don’t have a height gauge, but have got most of a set of second hand slips.

ega05/04/2021 11:56:39
2108 forum posts
175 photos
Posted by Dr_GMJN on 04/04/2021 21:46:45:

Determining centre height to that degree isn't something I could do. I'm currently setting tools initially be eye against a dead centre, then by trial and error to home in on not getting a pip. Pretty tedious.

If you have a height gauge you can scribe a line on a flat piece of metal in the chuck, rotate 90 degrees and repeat until the two coincide.

Any other suggestions?

not done it yet05/04/2021 12:14:33
5870 forum posts
20 photos

I’ve never had any need to set the centre height on my rear tool post - it was fixed when it was made.

Howard Lewis05/04/2021 12:25:44
4744 forum posts
10 photos

Dr

Two comments

1 You do NOT need the extra clamp. It WILL be in the way of the workpiece.

If the rear post does get in the way, it can always be unclamped and rotated so that the parting tool faces away from the workpiece, until needed.

The key in the base prevents the post from rotating. The stud and nut merely clamp it to the Cross Slide so that it does not lift out.of the Tee slot. A stud that size can exert enormous clamp load, probably in excess of 1 ton, but no need to go that far. Just a normal tightening torque, say 30 lb-ft with your calibrated hand!

(As a a guide, a 1/2 UNF bolt in W range steel goes into yield at about 175 lb-ft and exerts a load of 9 tons. )

Take a look at my album.and see my rear toolpost. It only comes off when a collet chuck is used, to allow the Saddle to move closer to the Headstock, or when I take it display at a Show.

2 Make yourself a Centre Height Gauge, to set both tools. . See mine, the first of my albums. (Also look at the other picture showing my four way rear toolpost on my lathe. Obviously, this can be rotated, when needed to keep the tools out of the way.

Make all the components. Note the relief on the base, so that the rim is left for the gauge to sit on.

Having screwed the stud into the base, grip the stud in the chuck and face across the end. This will ensure that the stud is perpendicular to the Base.

Take a piece of bar, preferably steel, and face the end.

Keep shimming the tool until there is no "pip"

The tool is now at Centre Height. Set the Gauge on the Cross Slide, and clamp the upper blade so that it just touches the top of the tool in the FRONT toolpost.

Set the lower blade up against the upper blade. The inverted tool in the REAR Toolpost can now be set to this.

From now on setting tools to centre height will ,be much easier, for both toolposts.

Howard

Martin Connelly05/04/2021 15:44:01
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1713 forum posts
184 photos

Have a look at this Youtube video from Winky's Workshop. Interesting how much difference the chuck overhang causes. Collets always make parting off easier as well.

Martin C
ega05/04/2021 17:21:45
2108 forum posts
175 photos
Posted by Dr_GMJN on 05/04/2021 11:07:35:

Ok thanks guys. So I could make a gauge similar to Johns. To get the height, perhaps put some bar in the chuck, then measure the height by putting slip gauges on the cross slide, then add the radius of the bar? Then machine to that dimension? I don’t have a height gauge, but have got most of a set of second hand slips.

I don't think this post was up when I suggested the use of a height gauge. I believe slip gauges are indeed the pukka way to measure but the bar radius would need to be carefully ascertained.

John Baron05/04/2021 17:24:07
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448 forum posts
180 photos

Posted by Dr_GMJN on 05/04/2021 11:07:35:

Ok thanks guys. So I could make a gauge similar to Johns. To get the height, perhaps put some bar in the chuck, then measure the height by putting slip gauges on the cross slide, then add the radius of the bar? Then machine to that dimension? I don’t have a height gauge, but have got most of a set of second hand slips.

Hi Guys,

The easy way to get your lathe centre height is to put a scriber in the chuck and use a piece of plate on the cross slide and scribe a line ! Measure it and record the figure. If you are not happy with any chuck run out when you do this, just rotate the chuck 90 degrees and scribe again. You will certainly be more accurate than doing it by eye and guessing.

Howard Lewis05/04/2021 18:51:29
4744 forum posts
10 photos

If you decide to use slip gauges, don't forget to use the protective slips.

Building a stack of slips under a bar in the chuck risks damaging the slips, and does not take into account any eccentricity in the chuck. (A good 3 jaw will have less than 0.003" or 0.0075 mm. I have seen a worn one with 0.036" nearly 1 mm!, but still made an effective Centre Height Gauge for that machine. )

Slips are accurate to millionths of an inch, so no need to risk damage by not using the protectives.

If scratched, or worn, they will no longer wring together, and then be difficult to stack for other dimensional checks.

Not sure that using for direct measurement on a machine is really a proper use for them.

Usually, they are used in conjunction with a Height Gauge and DTI on a surface plate. The Height Gauge is adjusted on the machine until the DTI reads Zero. The Height Gauge is being used to compare the dimension against a stack of slips on the surface plate.

(That was how I set up my centre height gauge, taking into account any eccentricity in the chuck. )

That's the way that professioinals, in industry, use slips..

Or, if you have one, you could use the height Gauge, and DTI and a Height Micrometer., instead of slips..

Both methods should allow you to measure to 0.0001", which is probably more than you need for setting a tool to centre Height.

Howard

Howard Lewis05/04/2021 19:04:39
4744 forum posts
10 photos

P S

It is possible to manage without slips,

Make up a bar which is adjustable for length. (Ideally use a fine thread such as ME 40 )

Having zeroed the DTI on the bar (Silver Steel? ) in the chuck (having measured and noted any eccentricity ).

Adjust the length bar until the DTI reads Zero again, when held vertical on a good flat surface. Bedway?.

Measure the length of the bar with a calliper., and note.

Measure the diameter of the Silver Steel

Using the diameter of the bar and the eccentricity, calculate the Centre Height.

Bar length - (Radius of Silver Steel and half the eccentricity )

Adjust the length bar to this new dimension (Measure with the calliper ) and set the Centre Height Gauge to this .

Job done!

Howard

 

Edited By Howard Lewis on 05/04/2021 19:05:59

JasonB05/04/2021 19:09:43
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20474 forum posts
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Easier to turn a bar in the chuck than faff about measuring and compensating for any eccentricity if you want to use that method.

Ron Laden05/04/2021 20:15:49
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2190 forum posts
437 photos

I just set my tools (turning and parting) to centre height by eye against the turned point of my home made scriber set in the chuck. Is it absolutely spot on to a fraction of a thou, probably not but its as close as makes any difference as I dont have any issues in both turning or parting.

DMB05/04/2021 20:26:39
1093 forum posts

I dont possess a tool height gauge. I just set tool by eye, face bar and adjust tool height to remove pip, then lock the adjuster. Jobbie done! Having done each TH, all is set for nxt turning job.

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