Centering Indicator

I bought a centring indicator (the cheap version) which arrived this morning so I thought I would offer an opinion on it.

This is the Chinese version of the 'Blake' instrument, doubtless not as well made - but is it good enough to be useful? A quick test showed it was capable of centering the bore of a workpiece to within about 0.002", maybe better on some workpieces than others (this was a very quick test!). That may not be as accurate placement as using a standard DTI but it is easier to set up and use with all the probes that are included. If there is considerable backlash in your handwheels I can imagine it would be more difficult to use - my mill isn't too bad but the adjustment method demands you continually reverse the feeds as you try to find the position of minimum movement of the dial pointer.

Some of the small blacked parts in my example came pre-rusted (!) but they cleaned up OK. The 'rotation-proof rod' actually fell off the little screwed stub it had on the end due to the fact it had been glued in place but without the rod being seated properly in the 4mm hole. I used a 4mm drill to clean the hole out and stuck it back with a drop of Loctite 638. So some (inevitable) niggles with QC as can only be expected I suppose within this price bracket. With those issues fixed it seems to perform adequately, though for how long I can't say. It does fulfill a need I had.

Chris

Edited By Chris Heapy on 29/05/2013 16:27:31

Chris

I bought one of those a couple of years ago, you might want to take it apart and clean the bits and then lubricate it. Mine worked a lot sweeter after I had done that.

You can also use it in the lathe chuck for re-setting your tailstock if you have ever moved it for taper turning. Or for setting up jobs on the cross slide etc.

Phil

Thanks for the tip Phil, I'll do that. I don't use the lathe cross slide much these days but good idea about the tailstock, I will try that too.

I'll tell you what the Chinese are brilliant at making - little wooden boxes. I must have half a dozen empty ones right now and they're far too nice to throw away

I'd love one of those but I don't think there's enough headroom on my little mill.

Roy.

It does demand a fair amount of headroom, I guess you could minimise that by using a simple endmill holder (plain bore, screw in the side - fairly inexpensive as there isn't much to them). Failing that, make one from a blank arbour, again pretty cheap and you could cut it as short as possible.Either would get it as close to the spindle as possible, the DIY blank arbour being the best option (but more work)..

Edited By Chris Heapy on 29/05/2013 22:02:40

Chris

I just looked in your photo album of workshop photo's. It looks like you have the same problem as me. ie way too much stuff in too small a space.

I am intrigued by your power saw stood vertically on the corner of the bench, do you use it that way up, or does it fold out somehow ?

Phil

I don't have too much stuff - just too little space

Yes, the bandsaw hinges upwards when I need to use it horizontally, but I can also use it in the vertical position using springs to tension blade.

I have one of these too, bought from DRO (usual disclaimer); as stated it is only an indicator, the scale is not defined so you could in effect call ea. div' what you want, imperial or metric; I chose to call mine 'units' & always followed up with a DTI as confirmation when checking things out, so in effect it gets you in the very close ball park, virtually next to 1st base as our cousins might say, but the DTI gets you ON 1st base, decent piece of kit to set up with although not what you might class as 'Cheap'

ps. I used mine to check out tailstock to head alignment & tram my mill head in to zero.

Cheers

George

Edited By mechman48 on 04/06/2013 01:44:54

They are cheap compared to those from Blake or Haimer, for instance. I bought a Haimer one last year. Initial experiments were a little disappointing, but now that I'm getting used to it I reckon I can consistently get centred to within half a thou. I suspect that the limitations are now the finish on the bore one is centring on, and the issue of moving the mill table by a few tenths here and there.

Regards,

Andrew

I suppose I should check mine against a DTI to see what sort of accuracy I can expect from using it. That last thou or so movement is tricky and I have yet to see zero movement on the dial so I don't know if that is me, the bore, the probe, or mechanical linkages within the instrument.

I cleaned and lubed mine (as suggested above) and it does seem to be less sticky in operation.

Chris,

I've never managed to get the needle exactly stationary either, slightly less than one division is the best I've achieved. Rather than measure the four cardinal points by trapping a fag paper and reading the DRO I suppose I should measure using a more accurate means.

The ball on my device is a very good mirror finish, so I suspect it's not the problem. My device seems to be spring loaded, or similiar, so I'm also discounting internal backlash. That leaves the surface finish of the reference cylinder. I think this may be the problem. My reasoning is as follows. A 1µm Ra finish is quite reasonable for a turned finish. However, Ra is an average value, I believe that Rz, which is a peak to peak measurement, is now preferred. There is no exact conversion between Ra and Rz, but rules of thumb seem to be that Rz is generally 10 to 20 times Ra. So for a 1µm Ra finish that gives an Rz of 10µm to 20µm, ie, 0.01mm to 0.02mm peak to peak. In imperial half to one thou. I'm seeing an error of about half a thou, and slightly less than one division of movement on the dial. While the divisions are arbitrary I'm assuming that one division is on the order of half a thou, or 0.01mm. The manual for my device claims an accuracy of ±0.003mm, which would be difficult to detect if each division spanned a bigger value.

Regards,

Andrew

I have been very pleased with my SHARS unit at £65 or so, it doesn't get constant use and is a lot easier to use than the swing round DTI method; for critical work of course that method can't be bettered.

It was really handy when I was centreing a batch of 4 foot long bars for end drilling and threading on the lathe. The business end was supported in a vice on a vertical slide bolted to the cross slide, the remote end was held in the fixed steady at the very end of the lathe bed with the tailstock removed.

The instructions that came with my unit point out very clearly that the marking on the dial are relative and cannot be related to any scale or measurement.

One of the best views of a unit in use is on the U Tube video made by the Glacern Machine Tool company, the crash course in milling #5 [Work holding and location] in particular. Google Glacern to find them

Brian

Edited By Brian Wood on 05/06/2013 09:36:10

Edited By Brian Wood on 05/06/2013 09:37:44

I think it was John Stevenson who very aptly described the markings "as a scale of no dimension", the last one I bought (I bought the cheapest I could find) from Amadeal for around £50 works very well for the price.

Regards

Paul

In deference to the forum we should mention there was a design in MEW a few years ago. Obviously a need for a follow up article modifying it for low headroom.

John: Do you mean just turn the input shaft with the unit not attached to the milling machine? If so then I can't detect any movement of the needle whatsoever on my unit.

I've just noticed that there's a scale printed on the dial of my unit; it appears that one division equals 0.01mm, which is about what I deduced. It says nothing in my manual about the divisions being arbitrary units.

I bought a range of probes with my unit, but so far I haven't any problems with lack of working depth.

Regards,

Andrew

sorry pictures did not wortk

Edited By Stephen Benson on 06/06/2013 09:28:02

I tried to make a video of this center finder in operation (although I really needed 3 hands to do it so it's a bit wobbly!). The video starts with the workpiece already aligned as best I could - less than one division movement, the workpiece is just a scrap piece of aluminium with a counterbore machined into it.

Then I move the piece 0.001" on the handwheel dial to observe the difference on the indicator dial, and attempt to move it back again (difficult with the backlash in the handwheel - and whilst trying to film the whole thing at the same time). Anyway it was obvious that 0.001" movement causes a significant change in the deflection of the needle. I think. Sorry about the crap commentary but my voice always sounds terrible on vids like this.

Edited By Chris Heapy on 12/06/2013 14:12:50