Author Topic: Do you lap your female centres ?  (Read 2159 times)

Offline awemawson

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Do you lap your female centres ?
« on: March 11, 2016, 05:34:41 PM »
I've been doing a bit of between centre turning and cylindrical grinding recently.

Now obviously you need nicely formed centre holes for the work to run nicely - I've always just very carefully drilled my centre holes with a centre drill aiming for the best  finish I can get, but I'm aware that lapping the centres is recommended for precision work.

So do you lap your female centres, and if so how do YOU do it ?
Andrew Mawson
East Sussex

Offline chipswarf

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Re: Do you lap your female centres ?
« Reply #1 on: March 11, 2016, 11:05:42 PM »
In 1969, at the USAF Machinist's Course at Chanute AFB, IL, the grinding shop had a center lapping machine. It was set up like a drill press, with some sort of chuck holding a rod shaped stone in line with a static center on the table below. There was a rickety appendage that could be swung into place to move a diamond tip in a 30 degree angle to the vertical chucked stone. Thus, was put a 60 degree cone point on the working end.

After 'truing' the stone point, you'd settle one end of the workpiece into the lower static center and run the turning stone point into the top center. All this story telling is by way of suggesting how you might set up your own center lapping device.

Offline chipenter

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Re: Do you lap your female centres ?
« Reply #2 on: March 12, 2016, 02:29:14 AM »
Only on stuff that has been hardend and quenched , on softer metal I stop feeding the center and hold it for a few seconds .
Jeff

Offline leg17

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Re: Do you lap your female centres ?
« Reply #3 on: March 29, 2016, 10:10:03 AM »
Center Lapping
As mentioned above, center lapping is for hardened materials, typically for grinding operations.
Abrasive center laps are relatively inexpensive.  They usually are available in a couple of grits.  A medium or fine grit will usually work on properly drilled workpieces.  Chuck the lap in the headstock of your lathe.  Install a conventional center in the tailstock.  Carefully position the tailstock so that the workpiece can be held against the tailstock and not accidentally break off the tip of the fragile center lap in the headstock.  Turn on the headstock, maybe about 500 rpm or so.  Hold the workpiece against the tailstock with the left hand and center it as well as you can.  Advance the tailstock carefully until the lap engages the workpiece.  You will hold the workpiece against the tailstock center to prevent it from turning AND to prevent the weight of it to damage the center lap.  Gently make contact with the lap to clean up the workpiece center.  This should not take much effort if the center hole was carefully machined prior to heat treat.  Be careful backing away so as not to damage the lap.  Reverse the workpiece and repeat.
The lap rotates, not the workpiece.
The lap usually will not be able to bear the weight of the workpiece.
Work carefully.
Let us know how it worked out.

Offline ian99

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Re: Do you lap your female centres ?
« Reply #4 on: March 30, 2016, 10:08:07 AM »
Great title for a thread and I was hoping for some pictures. But I guess I have a dirty mind!

Offline PekkaNF

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Re: Do you lap your female centres ?
« Reply #5 on: October 20, 2016, 08:16:53 AM »
Interesting....I never even knew this. Now I'm considering it.
 
Found this pretty informative "How to lap a Center on a lathe"
https://www.youtube.com/watch?v=Zf5rC7SBevI

Now my questions: Where do you find small carbide "laps" in europe? Can carbide burr used instead? Those can be used only on soft material, but maybe there are harder:
https://www.shop-apt.co.uk/carbide-burrs-60-countersink-type-j.html

I envision carbide being very useful for small centres. 120 grit stone can't be dressed very small and survive.

My next related question would be about how to dress dead centres....but let's wait for it. I don't have this setup:
https://youtu.be/N7Xy_UISuIg

Pekka

Some silly videos too:
https://www.youtube.com/watch?v=MO3pZaVHul4

Turn the voice down...next one has exceptionally irritating muzak
https://www.youtube.com/watch?v=CyrdE9SgKUU

« Last Edit: October 20, 2016, 09:01:38 AM by PekkaNF »

Offline Manxmodder

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Re: Do you lap your female centres ?
« Reply #6 on: October 21, 2016, 07:00:24 AM »
A toolpost grinder does a very good job on centres. It also has many other uses and is relatively simple to build one from scratch if you don't fancy buying one.

Toolpost grinding a lathe centre: https://youtu.be/IHNLSnFSKrM

OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline PekkaNF

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Re: Do you lap your female centres ?
« Reply #7 on: October 22, 2016, 12:05:01 PM »
Thanks.

I wonder how Andrew et.al. finishes centres.

I'll try most simple stuff first and see where it takes.

I did a little reading on 1915 book "Modern toolmaking methods" and there was a little descriotion of conical lampping and lappin on centre holes.

Apparently some sort of nutating or orbital component is needed to prevent rings or distortion of the conical form.

I'm not planning of making any orbital mechanism like on jig grinders that has planetary (orbital?) and reciprocating motion (oscillating) movements. I tried to google those mechanisms for hours, but could not find any illustrations.

Pekka


Offline awemawson

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Re: Do you lap your female centres ?
« Reply #8 on: October 22, 2016, 12:16:51 PM »
Male centres I can do easily on my Jones & Shipman 1300 EUIR cylindrical grinder, but I've still not got any way of lapping female centre holes other than a suitably pointed wooden dowel and lapping paste.
Andrew Mawson
East Sussex

Offline PekkaNF

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Re: Do you lap your female centres ?
« Reply #9 on: October 22, 2016, 01:11:39 PM »
How does it works out? I read somewhere that rings are nuicance and paste has tendency to accumulate at the wider part of the hole (and make taht part bigger). Some texts suggest using brass lap and to make it 0,5 - 1 degree more acute to comesate somewhat that tendency. Other book said that you should "peck" very briefly repeatedly and use slow rpm. Probably the same reason.

Pekka

Offline awemawson

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Re: Do you lap your female centres ?
« Reply #10 on: October 22, 2016, 01:36:25 PM »
I've only done it once as an experiment, and it wasn't very scientific. Certainly improved the centre hole cosmetically but I had no means of any quantitative measurements
Andrew Mawson
East Sussex

Offline PekkaNF

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Re: Do you lap your female centres ?
« Reply #11 on: October 23, 2016, 03:00:03 AM »
I wonder how well small cylindrical rotating diamond file would work, when feed at half angle?

Like those:
http://www.netkonttori.fi/adeona/745949/web?1402058056

Pekka

Offline leg17

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Re: Do you lap your female centres ?
« Reply #12 on: November 04, 2016, 11:16:43 PM »
Male centres I can do easily on my Jones & Shipman 1300 EUIR cylindrical grinder, but I've still not got any way of lapping female centre holes other than a suitably pointed wooden dowel and lapping paste.

Andrew
Buy a center lap and follow my detailed instruction.
Done in the tool and die and machining industry all the time.
You don't have to re-invent the wheel.