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GeorgeD
27-07-2010, 09:01 PM
many moons ago I went on a 12 months basic engineering course and failed to enrole on the second year.

Anyway the lathes I worked on were self centering chucks,I've purchased a Independant chuck for the CNC project.
Any tips on getting the correct centering of the jaws all the time?

Thank yer very kindly:beer:

Robin Hewitt
28-07-2010, 01:04 AM
First you make a sneaky gadget to hold the DTI face up in the tool post. Doesn't have to be on centre height, but you want it twistable and tiltable so you can point it at the centre line for an external or run it up against the inside of a pre-existing hole. If an alignment hole is small, find something that fits tight inside it. I have always had more luck lining up on a centre pip using the tool tip and turning the workpiece than using the tail centre. You can see better. You end up slackening one side a tadge then tightening the other, watch out if the piece isn't round/square/hexagon it is very easy to compromise the two jaws you didn't loosen. When moving it watch the DTI so you can stop at half the error. If you are using the chuck face for square, use the tail stock to hold it there while you adjust. Is that enough? :beer:

irving2008
28-07-2010, 01:12 AM
'cept he's not doing this on a lathe. Not entirely sure what the spindle and chuck are for in your CNC project George... a 4th axis or the main spindle?

GeorgeD
28-07-2010, 10:20 AM
Maybe,Irving?

Anyway was hoping not to buy a DTI.

Found this youtube vid if anyones interested?

http://www.youtube.com/watch?v=2KMhx4DbyDg

GeorgeD
28-07-2010, 10:48 AM
Have you noticed in the vid that the guy is using a job that has already been cut...erm what if its a raw piece?

Wobblybootie
28-07-2010, 11:28 AM
The same process applies unless the piece is a very weird shape. I know it's a slightly different scenario but with my chunks of wood I try to get the thing to balance in the chuck after marking the approximate centre of the face and bringing up a live centre, then small adjustments. Of course I do not always need the fine tolerances working with metal requires, that said I do produce items that during construction require a fit to within 2 decimal places.

I have no doubt there will be others here who can give you a solution to your problem.

GeorgeD
28-07-2010, 11:46 AM
I'm woking n a jig,Tim

this requires 4 metric 6" rulers affixed to a tin/ally plate in a manner they slide in and out.

The idea is the tin plate has an opening to fitove the jaws and the rulers slide down onto the jaws and using the rulers increments as a guide the jaws can be adjusted for centering on all four.

Sounds complicated and tedious but its not in my mind,at the drawing stage at the mo

irving2008
28-07-2010, 01:33 PM
George, a DTI is a fundamental piece of equipment for aligning things. I dont know how to manage without one. I suppose it will depend on your need for accuracy... a ruler will give you +/- 0.5mm... I'm normally working to +/-0.1mm or better. That youtube vid shows the same technique I use... takes only a couple of minutes to get it lined up...

Here's a pic of something more complex... here I DTI'd on the central bore to get it centred within 0.01mm... it was a tight fit on the minilathe, there was about 5mm clearance to the bed...

GeorgeD
28-07-2010, 01:41 PM
Normally that is the way to go,Irving but as you stated in post #3 I'll not be doing it on a lathe.

Dang,I'll come up with an idea,you mark my words. :smile:

GeorgeD
28-07-2010, 04:53 PM
Might of hit the jackpot here?

Ok this idea might work but only one way ie those who have a lathe or worked on one will know what I'mon about,the chuck jaws are twofold position one way for holding small diameter rod,the other way for holding larger diameter rod or wokpiece.

Using the jaws in the larger format,lets supposing I aquire a lentgh of ally bar the diameter of the lathe spindle bore,cut a 3" piece of the bar,dilled a hole in the center of the bar and then tapped a thread in the drilled hole.


Now then this is the nagging part but can be done as and when one is needed of these disc are needed.

Take a piece of alluminium square 2"x2" plate,scribe a line from one coner to the othe diagonally and the same on the other two corners,in otherwords X marks the spot in this case dead center of the 2b2 square.
Using this center point I use a compass to draw a circle so that it coincides with the edge of the square,cut out the cicle with tin snips? drill a hole the same size as the hole previously drilled in the aluminium bar,screw this disc to the aluminium bar with a countersunk screw,feed bar into chuck spindle and make sure its snug fit.

Take a piece of Aluminium rod thats 2" in diameter that I want to ream/down size?
position it level with the disc we just made up and popped intothe spindle,tighten the jaws up and thats it.

Like I said a disc can be cut and drilled as and when we need it and the same spindle rod can be used all the time.

What do the experts think? :wink:

BillTodd
28-07-2010, 05:25 PM
What do the experts think? :wink:Why is centring a piece causing you so much trouble? (A picture would help)

If you're using an independently jawed chuck for accuracy you'll need to dial it. One can usually do it by eye to about +/- 20 thou", just using the concentric rings. It should only take a few minutes to get the TIR down to 1 thou or so.

If you only need a rough centre ~0.003" then a self-centring chuck with 3,4 or 6 jaws would make your life a whole lot simpler.

I'd guess your plate Idea would work to say 0.01" (if you're lucky ;)) tightening one jaw can throw the reading off by that much.

Tom
28-07-2010, 05:26 PM
popped intothe spindle,tighten the jaws up and thats it.
I'm lost, what does it do? Got a picture? If it's not a lathe what is it? :-~

I'm with Irving - a DTI allows you to centre objects to a precision better than you can see. And they start at a tenner.

Tom
28-07-2010, 05:30 PM
I'm with Irving .....

Edit

I'm with Irving AND Bill

FatFreddie
28-07-2010, 06:09 PM
I'm with Irving, Bill and Tom :-)

GeorgeD
28-07-2010, 06:24 PM
I'm with the missus.:tongue:

irving2008
28-07-2010, 06:30 PM
Might of hit the jackpot here?

Ok this idea might work but only one way ie those who have a lathe or worked on one will know what I'mon about,the chuck jaws are twofold position one way for holding small diameter rod,the other way for holding larger diameter rod or wokpiece.

Using the jaws in the larger format,lets supposing I aquire a lentgh of ally bar the diameter of the lathe spindle bore,cut a 3" piece of the bar,dilled a hole in the center of the bar and then tapped a thread in the drilled hole.


Now then this is the nagging part but can be done as and when one is needed of these disc are needed.

Take a piece of alluminium square 2"x2" plate,scribe a line from one coner to the othe diagonally and the same on the other two corners,in otherwords X marks the spot in this case dead center of the 2b2 square.
Using this center point I use a compass to draw a circle so that it coincides with the edge of the square,cut out the cicle with tin snips? drill a hole the same size as the hole previously drilled in the aluminium bar,screw this disc to the aluminium bar with a countersunk screw,feed bar into chuck spindle and make sure its snug fit.

Take a piece of Aluminium rod thats 2" in diameter that I want to ream/down size?
position it level with the disc we just made up and popped intothe spindle,tighten the jaws up and thats it.

Like I said a disc can be cut and drilled as and when we need it and the same spindle rod can be used all the time.

What do the experts think? :wink:

Firstly I think you need to get your terminology right... reaming is a process for accurately opening out a hole to a known size using a reamer... reducing the diameter of a bar is a process of turning...

Secondly how are you going to drill a hole in the centre of the 3" long bar without centering it? Also how are you going to make it fit the spindle bore without turning it down accurately - each spindle is slightly different and that would have to be made for your specific spindle.

Thirdly, the accuracy of what you have described using compass and tinsnips is pretty useless in engineering terms, and I think would be incredibly dangerous as the workpiece could easily be 0.5mm offcentre.. i wouldnt want to go near that rotating at any speed with a cutter approaching it....

I'm all for innovation, but sometimes theres a right way of doing something devised over generations of trying alternatives... mainly due to the Darwin effect...

Ross77
28-07-2010, 08:35 PM
I'm all for innovation, but sometimes theres a right way of doing something devised over generations of trying alternatives... mainly due to the Darwin effect...


Nicley put, much more conservative than what I was going to say..:whistling:

George
Why dont you just say what it is you are trying to bulid and what you want it to do, it seems people are having to try and guess what the question is... :smile:

GeorgeD
28-07-2010, 09:20 PM
I'm really having a badday here,so I'm not going to answer that.

Really cheesed off I am. :mad: