Rohm ZG 140 3 jaw loses accuracy (but regains it!)
|Oily Rag||12/10/2020 21:57:27|
182 forum posts
Just had a very interesting day in the shop turning some parts requiring a reasonable concentric accuracy. When I first chucked the part I was horrified to see the chucked piece with an almighty wobble. This was a cause for alarm as the chuck - a Rohm SC 3 jaw model ZG 140 had until now been repeating chucking accuracy to less than 0.001" TIR. I had previously swapped back to the 'standard jaws from the set of externals.
The history of the chuck is that I bought it new about 4 years ago when my well worked PB chuck was retired to secondary duties (it was anywhere up to 0.004" TIR out of true and was probably 40+ years old). The Rohm was a revelation as it was so accurate across all diameters.
Believing in the old engineering adage that "cleanliness comes before godliness' I wondered whether there was a bit of swarf impacted into a jaw tooth face or impacted onto the scroll. I therefore set about a grade 7 cleaning operation and gave the jaw teeth, the chuck body jaw slots, and the scroll a good thorough clean. Brake cleaner and a brass brush on the jaws, blue paper in the jaw slots, and a softwood dowel (which was from a pack of epoxy glue mixing sticks proved to be the right size to become a scroll 'tooth pick' was put into the scroll and the scroll wound round its full travel to clean out the gunge - not much but a little swarf particles did come out!
Reassembling with a spray of Rocol Slideway lube got it all back together and working again. Checked with ground parallel mandrels, the chuck now showed a consistent 0.0025" TIR 'wobble' with jaw 2 being the high spot. This was across a range of diameters and was consistently jaw 2 which was 'high' - by inserting a Rizla fag paper between the mandrel(s) and jaw 2 this corrected the TIR to around 0.0006" - running my DTI along the top of the mandrel showed no appreciable movement (less than 0.0003" and mainly due to carriage reversals).
As it appears Jaw 2 is the culprit - a job did spin in the chuck during tapping a hole last week! - does the panel think it is time for the toolpost grinder, a clover leaf plate, and a gentle jaw grind? Or should I count my blessings and live with the Rizla papers??
|Neil A||12/10/2020 22:19:18|
|73 forum posts|
If it were my chuck I think I would be happy to live with the Rizla paper when needed. Perhaps as time went by I might consider grinding, but I would have to weigh up if I could actually improve the accuracy or not.
1268 forum posts
Since the jaws are hardened, it would seem odd that one slip would take that much off a jaw.
|Tony Pratt 1||13/10/2020 08:18:34|
|1277 forum posts|
3 jaw chucks aren't really a precision work holding device.
|Mike Poole||13/10/2020 09:40:52|
2816 forum posts
I have said before that I don’t know how accurate my three jaw chuck is, I am certain is will not be as good as a 4jaw independent or a decent collet. If the concentricity is important and the job needs to be removed from the chuck then the 3 jaw is not the tool for the job. Unless you indicate the job you will not know how accurate the job is, and if you find it is out then there is nothing to be done unless you have a griptru type chuck. If you need to shim then you may as well use the 4 jaw. The accuracy of a 3 jaw seems of great interest to many people but it really doesn’t matter if you just accept it is not a super precision device and work accordingly.
|Chris Evans 6||13/10/2020 09:51:03|
1784 forum posts
A few years ago I found my Pratt-Burnerd 3 jaw to be well worn and suspected the jaws. New jaws where available at £184 but not knowing if they would cure the chuck I bought a cheap chuck from ARC for about£110 (160mm)
The new chuck runs better than any 3 jaw should across all sizes and I now consider it as good enough for 80% of my jobs. The only downside is a poor feeling action when winding the jaws in and out.
|Mick B1||13/10/2020 10:06:10|
|1777 forum posts|
This is a standard Warco chuck on a WM250V, with a bit of ground silver steel. The clock divisions are half-thous (when it comes into focus!):-
Admittedly this is the best result I got, and the reverse jaws and inside gripping configurations tended to get .001 - .002" TIR, but the great majority of standard chuck work uses 'em like this, so I wouldn't *usually* have any worries about removing and replacing the workpiece.
Edited By Mick B1 on 13/10/2020 10:07:31
|Tony Pratt 1||13/10/2020 11:46:17|
|1277 forum posts|
I stand by my statement '3 jaw chucks aren't really a precision work holding device', your chuck is obviously a 'good one' at the moment, but give it a few years of use/abuse & things will change, a 3 jaw is very versatile but its design makeup doesn't lend itself to supreme accuracy, others may disagree?
|Oily Rag||13/10/2020 11:47:10|
182 forum posts
All good points!
I do accept that a 3 jaw is not a high precision instrument but was intrigued by how it suddenly lost its accuracy (maybe the 'swarf elves' are to blame! ). Bill's (Peak 4 ) point about the jaws 1 &3 picking up rather than jaw 2 losing material in the spin is a very valid point. I will investigate further if this is too blame.
However, I'll count my blessings and carry on regardless, using the 4 jaw and collets for accuracy where required and making do with my Rohm as it is. As an aside, the measurements were taken with a Mercer 0.0001" DTI with a +/- 0.010" travel, all test pieces are hardened and ground non taper mandrels which range from 3/8ths to 1 1/4" diameters. The chuck is always tightened and loosened on the referenced key (the one with arrow and zero ), not excessively and then the other two keys are 'seated' with light pressure. Chuck register diameter and internal spindle MT readings are as accurate as one could wish - showing only a tenth deviation on the clock. Off to the newsagents now to by a packet of Rizlas - hope he doesn't think I'm on the whacky stuff!!
Thanks all for the time you have given to respond.
|Neil Wyatt||13/10/2020 12:26:54|
18325 forum posts
|Andy Gray 3||13/10/2020 17:13:07|
|49 forum posts||
Could one of the jaws be 'miss timed' in the scroll when you first fitted them? To be then fitted correctly after dismantling for cleaning?
It would certainly give an 'almighty wobble' and would fit with the sequence of events...
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