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  1. #1
    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

  2. #2
    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?

  3. '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?

  4. #4

    Anyway was hoping not to buy a DTI.

    Found this youtube vid if anyones interested?

  5. #5
    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?

  6. #6
    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.
    Tim G-C

    I disapprove of what you say, but I will defend to the death your right to say it.

    (attrib. Voltaire but written by Evelyn Beatrice Hall "The Friends of Voltaire" 1906)

  7. #7
    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

  8. 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...
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  9. #9
    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.

  10. #10
    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?

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