Finding Centre Height

[Tony Pratt 1Participant @tonypratt1]

Posted by John Olsen on 28/07/2010 09:46:02:

Hi Guys, I suggest you refrain from trying the face off and check the pip method if you are using carbide inserts. It is liable to wipe the edge off your insert if the height is too low. Don't ask…. actually most of the tips I have done in have been from doing this without remembering, eg the job need facing so you wind the tool into the middle, it turns out to be a smidgeon low, exit one insert.

 

John

I've never had a problem, maybe lucky?

Tony

[Martin ConnellyParticipant @martinconnelly55370]

My centre height is 4" above the ways which have large flats. A steel rule marked in inches and reasonable light (and reading glasses now) are all I need to set or check things out.

Martin C

[John HaineParticipant @johnhaine32865]

Digital height gauge.

[Nick WheelerParticipant @nickwheeler]

Posted by John Haine on 19/12/2022 13:16:24:

Digital height gauge.

That's what I use. I printed a sticker with the centre height from both the bed and the cross-slide

[Douglas JohnstonParticipant @douglasjohnston98463]

This is a device I made some time ago from some bits from the scrap box. The dial gauge is set to the correct height and zeroed and then used to set the correct height for any tool in a quick change toolholder. The plunger on the gauge needs to have a flat foot.

[Martin KyteParticipant @martinkyte99762]

I seem to remember a kind of scribe by the tool method where the tip is passed along the face of a bar in the chuck making a scribed line on a diameter. The workpiece is then rotated half a turn so that the tip and the line coincide at the front. Another line is scribed by cross slide infeed and the result is a shallow V. The next bit is a bit hazy in my mind but something like this. Say the tool is low. If it is raised by half the distance between the ends of the V on the far side of the face then it will be at centre hight.

Of course I could have completely misremembered all this and I cannot for the life of me remember where I read it.

regards Martin

[Clive FosterParticipant @clivefoster55965]

I like the "optical" type of centre height gauge comprising a sheet of decently thick perspex held vertically up right on a suitable base able to sit on the lathe bed and, if the top is flat, cross-slide.

Using a sharp centre lines can be scribed on both sides directly on the machine so they must be accurate. Needs a bit of thought to arrange a stepped down bridge across the centre of the bed to slide the base on for marking but its not intrinsically difficult. Keep the bridge because its useful for getting the gauge into more places.

With lines bot side of the transparent sheet you can reliably eye up the tool centre height from all sorts of distances. So long as you only see one line you must be looking at it level. Out of level wyou will see two. So if the tool tip alings with one visble line it must be on center.

Sitting a mirror behind at 45° allows you to look down rather than across. Or just use it as a plain line gauge by touching the tip of the tool (gently!) to one side.

A desigg was published in the early days of MEW. I modified it to suit my machine. Mine has grown several sets of lines to suit two machines but it works very well. Blade is thinner than ideal at 5mm, 10 would have been better, but I used what I had. If I ever luck into a suitable off cut of 10 mm perspex sheet I shall rebuild it.

Mine with mirror.

Using the mirror.

On the S&B 1024.

I really should do some better pictures but its hard to photograph transparent things.

If I still used a 4 way tollpost with shims to set tool height something like the dial gauge arrangement Douglas uses would be great. Measure the the height with no shims in and select from your carefully sorted set. No reaon you couldn't do the measurements on the bench using a bare tool.

Clive

[Howard LewisParticipant @howardlewis46836]

As many on here know, I am a great fan of making and using a Centre Height Gauge..

My method may sound a bit involved , but it works. And the gauge will be needed every time that you change or grind tools.

I used a piece of Silver Steel, as an Alignment bar but could be a short piece of steel which is known to be round (If in doubt take a light skim. This should remove any eccentricity. )

Making ans setting Centre Height Gauge is a useful learning (and confidence building ) exercise for a newbie, and the tool can be used for many years to come.

SETTING UP A CENTRE HEIGHT GAUGE.

The diameter of the Alignment Bar was measured and noted. The chuck was replaced, and the Alignment Bar gripped in it.

The DTI on the Magnetic base is fixed to the Cross Slide, and brought to bear on the top of the Alignment Bar. The Cross Slide is adjusted to and fro, until the DTI shows a maximum reading, indicating that it is on the centreline. The chuck is rotated until the DTI shows a maximum reading, and is reset to Zero. Rotating the chuck shows what eccentricity there is at that that diameter. The chuck is rotated again until the DTI again shows a maximum reading, before being reset to Zero .

An adjustable pillar had been made, with a ME 40 tpi thread, and this is placed between the bed and the DTI, and adjusted until the DTI again reads Zero, and locked with the knurled locking ring. The dimensions obtained are used to determine the Centre Height.

The adjustable pillar does not have to be 40 tpi, but the finer the thread, the easier it is to set accurately.

So M6 might be as coarse a thread as you would want to use.

The length of the pillar is measured, using a digital calliper, and the dimension noted, as the height of the top of the Alignment bar above the bed.

Half the diameter of the Alignment Bar and half the Eccentricity are subtracted from this figure to arrive at the Centre Height.

The figures obtained, for a particular small lathe were:

Height over Bar at Highest point 3.308”

Alignment Bar diameter 0.706” (Radius 0.353" ) Eccentricity 0.003”. (Not bad for a 3 jaw chuck!)

3.308 – (0.353 + 0.0015) = Centre Height above bed = 2.954” (75.03 mm)

The pillar was then adjusted to this dimension.and locked with the knurled locking ring, when this dimension was reached.

The pillar is then used to set the Centre Height Gauge,

Subsequently, all tools are set to the gauge.

If you look in my Albums, there is a picture of the Centre Height Gauge that I use on my lathe.

It has been used for nearly twenty years, and tools set to it leave no pip when facing!

Tools in the front toolpost are set so that they just touch the underside of the upper blade..

It has two blades because the lower blade is used to set the inverted tools in the back toolpost (Shown a little lower down )

I've added pictures of a simple Centre Height Gauge and setting it to my Centre Height Gauge Album

HTH

Howard

Edited By Howard Lewis on 19/12/2022 15:01:54

Edited By Howard Lewis on 19/12/2022 15:05:22

[old martParticipant @oldmart]

I do it the simplest possible way. Just face off a bit of bar and see if the tool height requires adjusting up or down, and when the tool is on height the "tit" in the end of the work will be very small, or pretty near non existant. That tool is on height and the tip height can be used to adjust a simple gauge which sits on the bed or crosslide. No centres were harmed in the production of this work.

[Chris MateParticipant @chrismate31303]

I have a question about a thought that came up after thinking about all this a few times and that is:…

If your carbide or cutting HSS tool tip is set on centre correctly by any of these methods, you take a light cut, everthing ok, but if you take a heavy cut,1- it may bend under centre for the tip, 2-AND in some cases at deeper cuts with the rest of the cutting edge higher than the tip, how is that effecting the theory of cutting on centre with that part of th insert also cutting but not at the same height as the tip..-?

[Tony Pratt 1Participant @tonypratt1]

Chris Mate, I believe you are over thinking things, go cut some metal.

Tony

[Peter Simpson 3Participant @petersimpson3]

I cannot believe people go the far ends of the earth to find the centre height. I use the pip method every time. No DTI no measuring just a quick rough check against a fixed centre then a final adjustment against a faces off pip check. One of the most simple tasks using a lathe.

[HuubParticipant @huub]

I use a level for setting/checking. That gives a centre height within 0.1 mm. After that, I face a bar and check the pip is turned, not broken off.

My level:

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Peter Simpson 3 on 19/12/2022 20:53:11:

I cannot believe people go the far ends of the earth to find the centre height. […]

.

For a really interesting alternative approach, have a look at the Chronova ‘DTI dial’ video that I linked this morning [and at the later part of the 4-way tool-post video that it references].

MichaelG.

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Ref: __ https://www.model-engineer.co.uk/forums/postings.asp?th=184291&p=1

Edited By Michael Gilligan on 20/12/2022 00:02:50

[SakuraParticipant @sakura]

Posted by Peter Simpson 3 on 19/12/2022 20:53:11:

I cannot believe people go the far ends of the earth to find the centre height. I use the pip method every time. No DTI no measuring just a quick rough check against a fixed centre then a final adjustment against a faces off pip check. One of the most simple tasks using a lathe.

Couldn't agree more.

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