Newbie- Centre Finder

[Ashish B]

Hi this is Ash.

Till date i am into machining division ( 6 years ), i have never seen anybody using a CENTRE FINDER....i was curious to know about the same....


Generally we use dial indicators for locating a hole, but how does centre finder works...I mean to say it cannot kick off ( i refer to the shiftage of the lower part when the edge finder finds that particular edge )....as a centre finder doesn't has any scope for kick off because centre finder working portion is in the hole....

Than how does it indicates....Is there any another indication for centre finder....What is that ????????/


Ash

[wildwestpat]

Hi Ash

Do NOT fret these devices are for quick and easy centering requiring little or no thought by the operator. A skilled machinist would find little need of these unless they are on a productivity kick!

If you want more details on how it works I will copy the instruction sheet and try and describe the action. It comprises a DTI measuring the gap between the measuring plate held in the spindle being centred and another plate that is being pulled out of line by a measuring probe which is atached. The two plates are spring loaded to the neutra closed position that represents the aglined condition. The DTI is twisted to record the out of line error in the coresponding X or Y (assumed mill axis!) and the work positioned for zero deflection on the DTI in that axis. Repeated for the second axis. Bingo job centered to within 0.001 inch.

Hope this puts these devices in perspective. Worth the cost for repetative work particularly towards the end of a shift!

Regards

Pat

[Ashish B]

Originally Posted by wildwestpat Hi Ash Do NOT fret these devices are for quick and easy centering requiring little or no thought by the operator. A skilled machinist would find little need of these unless they are on a productivity kick! If you want more details on how it works I will copy the instruction sheet and try and describe the action. It comprises a DTI measuring the gap between the measuring plate held in the spindle being centred and another plate that is being pulled out of line by a measuring probe which is atached. The two plates are spring loaded to the neutra closed position that represents the aglined condition. The DTI is twisted to record the out of line error in the coresponding X or Y (assumed mill axis!) and the work positioned for zero deflection on the DTI in that axis. Repeated for the second axis. Bingo job centered to within 0.001 inch. Hope this puts these devices in perspective. Worth the cost for repetative work particularly towards the end of a shift! Regards Pat

Hi thanks for the valuable info........


Can u give a sketch or instruction........that will be really really very thankful....

May be the method which u are refering to like (DTI, Probe )..is not what i am refering to....I have attached a snap of edge finder ( mechanical type ).

& i refer its application to "findd the centre of a pre drilled or reamed hole to enable machining activity"...............

Thanks Pat

[wildwestpat]

Hi Ash

Yes the device I described uses a DTI to indicate the amount off center in the axis in which the steady rod is pointing.

The center finder you have in your pic is much simpler and is more suited to finding the edge of a block or large hole. I have only seen one of these used way back. So here is my recolection - may be missing a vital detail!

The three parts are offset as shown in your picture. The surfaces are lightly lubricated and since they are ground to a good finich they sort of stick to gether. The probe is then rotated at low speed and nudged into the edge to be detected with only the lower part of the probe making contact with the workpiece to be refferenced. (To find the center some simple arithmetic is required hence my preference for the more expensive DTI type of probe as it is all too easy to make a simple mistake particularly if this is a repetative task.)

On approaching the dge being refferenced the wobble firstly became worse - as I recall it the offsets to start with are less than 1/3 the total diameter of the tool. As the axis is advanced towards the refference edge the wobble between the lower part and the middle reduces and is done slowly the three parts suddenly agline themselves. This is the point at which the tool has centered on the edge less half the diameter of the contact part of the probe. Continuing to advance the axis caused the wobble to become worse as the refference point is passed. The accuracy was ok if done carefully and slowly but I wrote it off in favour of a DTI.

I hope others will correct my description as I may have left out some vital step.

Hope this helps and it is my view that as you have the benefit of formal training in the use of machines the device you picture has little to offer in the way of advantage over a good DTIU in expert hands. The device I described just cuts out the mental gymnastics as when the dial pointer is steady that axis is centered - after I had screwed up after a long day - I now use the simplest approach - the repeatability was after all why I love cnc!

Regards

Pat

[wildwestpat]

Hi Ash

Here is the instruction sheet for the DTI probe centering device.

Hope this furthers your curiosity about centering.

Regards

Pat

[Ashish B]

Can u let me know more about the accuracy of centre finder...........

I mean if a hole is dialed by a indicator & also by a centre finder....in what accuracy we can get the reading with respect to the dial indicator....



Ash

[wildwestpat]

Hi Ash

I have been able to center work so that there is no visible movement on the DTI. The bearings in the mill are new and with a reasonable pre load - finger tight plus a little bit more!

The scale on the DTI indicate 0.01mm and the centering is a lot better than this as there is no movement on the DTI. Checking with a conventional DTI bears this out and with an electronic DTI capable of 0.001mm there is no detectable wobble due to re-centering the work piece.

When I get home I will take a photo of the device as it is well worth the £45 I paid for it.

Regards

Pat

[wildwestpat]

Hi Ash

It will appear that I raced home as I did not clik the post button. Any way here is the picture.

The long rod is the one that is hand held in line with the axis being adjusted. The Shaft is held in a collet using the same collet holder I use for the mill cutter to avoid any concentric tool errors.

Hope this helps

Regards

Pat

[kawazuki]

Originally Posted by wildwestpat Hi Ash It will appear that I raced home as I did not clik the post button. Any way here is the picture. The long rod is the one that is hand held in line with the axis being adjusted. The Shaft is held in a collet using the same collet holder I use for the mill cutter to avoid any concentric tool errors. Hope this helps Regards Pat

I've been looking for one of that type since seeing it used on the MIT videos - any info gratefully received - Maker? Where you got it from? I'm UK also!

[wildwestpat]

Hi Kawazuki

They are reasonably freely available. I got mine from Harry at Amerdale in the UK. He has them currently listed on ebay and here is the current link

http://cgi.ebay.co.uk/ws/eBayISAPI.d...fvi%3D1&_rdc=1

Hope you are as pleased with yours.

Regards

Pat

[Ashish B]

Hi Pat.......

Was out of station so couldn't reply....

Thanks for such valueable information on DTI........

Ash