Finding the centre of a hole

[David Brown 9Participant @davidbrown9]

I have bought a boring head and boring bar set to use on my Sieg SX2 mini mill. I need to bore out the bore of a pullet from 19mm to 20mm to fit a 20mm shaft.

I need to find the center of the pulley bore on the milling machine so that the boring bar is centered in the hole, in the milling machine spindle.

I have a coaxial centre locating indicator, but firstly there is not enough room on my mini mill to fit the probes which will reach to the edge of the pulley bore, only the smallest probe will fit and this does not reach the edge of the bore. Secondly this is designed to work with the spindle turning under power and the key slot in the pulley bore is likely to catch and break it.

I I have a dial test indicator which fits in the spindle but the indicator only has 0.8mm of movement and again does not touch the edge of the bore when centered.

I think I need a wiggler with an attachment for a dial test indicator, to which I can fit the indicator at an angle so that it touches the side of the bore. I can then turn the spindle by hand and adjust the x y tables until the bore is centered.

Is this correct? If so where could I buy this?

I do not have DRO.

David

[David Brown 9Participant @davidbrown9]

[JasonBModerator @jasonb]

You can usually move the arm mount on a co-ax indicator, its quite a firm joint but gentle pressure near the clamped end of the arm will move it. Same if its a lever type DTI. Turn by hand so you don't sweep over the keyway

Depending on how good you are at compensating for back lash then just a straight bit of bar or side of a centre drill in a collet and fag paper will do. touch on one side of teh bore, note handwheel reading. Touch on opposite side, note reading and then half the difference between the two. Repeat for the other axis.

Trap a large ball bearing between teh spindle socket and the pully hole is another option.

J

 

Edited By JasonB on 29/11/2015 14:04:32

[Bob RodgersonParticipant @bobrodgerson97362]

David,

Does your centring probe not also work on the outside diameter of the pulley ? if it does, clamp the pulley via the centre hole with something that can be removed later (stud removable from Tee Nut) then fit a probe that will run on the o/d of the pulley. Once centred, clamp the outside and remove the centre hole clamp.

Alternatively you could use the X & Y axis dials to determine the centre. First of all position the spindle roughly in the centre of the hole. start first say in the X direction by locating one side of the hole with a rod of known diameter, zero the dial , then move it across till it reaches the other side and note the reading. the centre point in the X direction will be exactly half this distance from starting edge of the hole. Remember to tak up any backlash, once you have the X direction set repeat the same for the Y direction and you should have the hole centred.

Bob

[John HaineParticipant @johnhaine32865]

A simple way is to make a 19mm mandrel you can fit to the mill spindle in some way (for example an mt2 blank taper with the end turned to fit the pulley hole). Place the pulley on the table, centre on the mandrel and clamp the quill and table while you clamp the pulley to the table. Now you're ready to bore.

[JasonBModerator @jasonb]

David does not have a lathe so some options are out.

[Chris Evans 6Participant @chrisevans6]

In my 50 years of toolmaking nothing beat the piece of bent 1/4" round bar to hold the DTI. Simple and effective and used in a drill chuck or collet. I have several sizes made up to centre work pieces round square of rectangular all use the same bars. So bend a piece of bar to suit the diameter of your attachment for holding the DTI. For most work bend at 90 degrees with say 2" x 3" and job done.

[David Brown 9Participant @davidbrown9]

Thanks for the advice. All I had to do was push the base of the lever on the DTI reasonably hard and it moved further out! It should now touch the outside of the pulley bore.

I guess it should have been obvious, although it would be nice if these things came with instructions! I watched a lot of videos on youtube and none explained this simple thing!

Next weekend I will have my first try at using a boring head and boring bar.

David

[JasonBModerator @jasonb]

Good to know moving the arm was all that was needed.

Have a practice on some scrap first as you don't have much to play with on the pully.

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