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Backplate debacle

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Martin of Wick23/08/2019 18:58:36
196 forum posts
4 photos

In cutting the register on a steel back plate for a new 5" chuck, the 'just shave off some dust' resulted in the inevitable - an undersize register. teethDiscouraging words have been edited to avoid offence.

I was aiming for a firm push fit. What I have now is between about 1/4 to 1/2 thou undersize as measured with a clock while the chuck on the plate (but not bolted down).

Question is are there any way to remedy the back plate that may work, like dolloping some blobs of weld or cutting the register further and pressing or gluing on a thick shim? The register is very shallow at 3mm, but if I turn it all off and start again, the mandrel nose may foul the back of the chuck.

If the plate is now scrap, could I get away with slathering the register with Loctite 648, then tap up the chuck to give the minimum run out, before bolting down and hoping for the best...?

Apart from paying attention and being more careful, are there any tips that others may have to achieve a stress free perfect fit?


Mikelkie23/08/2019 19:18:55
106 forum posts
13 photos

I never bothered with a perfect fit of back plate to chuck. Years ago i also wanted a perfect fit but about 15 thou under size, so i bolted the chuck and got it running perfectly true by tapping with a lead hammer. The advantage of this i can true up again as the jaws wear.

old mart23/08/2019 19:19:12
1921 forum posts
151 photos

I have deliberately reduced the registers of several of my chucks to give them at least 0.010" clearance, for fine adjustment of any size work that will fit. If you are worried about the chuck moving when in use, there is plenty of room on the back of a chuck to drill and tap more fixing holes. Of course, the bolt holes in the backplate have to be slightly oversize, or they would impede the adjustment.

A tight register means there is no way that variations in manufacturing tolerances can be overcome, some sizes of work may run well, but not all.

Ian Johnson 123/08/2019 19:20:07
281 forum posts
82 photos

Try centre popping around the register, it should raise up the surface a couple of thou, then bolt up the chuck on to the back plate. Try to get all the centre pops equal depth to get minimal run out. Its a similar technique to knurling a sloppy shaft, bit rough and ready but it works, and If it looks like the back plate is scrap what is there to lose?


Baz23/08/2019 19:21:57
453 forum posts

If it’s less than a thou undersize just get and use it, don’t bother with loctite etc.

Clive Foster23/08/2019 19:23:58
2323 forum posts
76 photos

I've done the glue'n screw a ring on process before for a register type fitting not vastly dissimilar in size to yours. Min wasn't a chuck but I see no reason why it wouldn't be fine for you.

As I recall it the ring was something over 1/2" wide and started out rather deeper than was needed so I could get the size right on waste material first before cutting down to length. I imagine something like 1/4" or 3/8" thick would give you ample to work with.

Probably used one of the Loctite structural adhesives as I was working my way through a largish bottle bought in for a special job in those days. Eight 3 mm countersunk head screws in a wavy circle pattern to hold it whilst the glue goes off and help carry cutting loads. Remember to set them deep enough so most, if not all of the head is left when you trim the ring down.


PS Wavy circle pattern is 4 screws alternately on each of two suitable PCD diameters.

Andrew Tinsley23/08/2019 19:50:50
1171 forum posts


I always make my back plate register undersize by around 5 thou. If you make the holding screw holes oversize, then you have a cheap Griptru chuck that you can adjust to get zero run out for any work diameter you want.


Roderick Jenkins23/08/2019 20:31:56
1925 forum posts
503 photos

As Andrew says. I've been using my 3 jaw like that for the last 30 years.


old mart23/08/2019 20:48:42
1921 forum posts
151 photos

If you are thinking about a glue approach, remember that chucks need to be dismantled and cleaned occasionally.

David Standing 123/08/2019 20:52:35
1289 forum posts
48 photos
Posted by Andrew Tinsley on 23/08/2019 19:50:50:


I always make my back plate register undersize by around 5 thou. If you make the holding screw holes oversize, then you have a cheap Griptru chuck that you can adjust to get zero run out for any work diameter you want.


Yep, exactly what I was going to say! yes

old mart23/08/2019 21:03:47
1921 forum posts
151 photos

This debate comes up on all the forums on a regular basis, those who must have a tight register have obviously never tried any other approach. Then they tell everyone that a self centring chuck can never be relied on for accuracy.

Lathejack23/08/2019 21:17:58
281 forum posts
328 photos

Yes I agree, I would leave your backplate as it is Martin, there is nothing wrong with it at all. I have fitted several new chucks of different types over the years, and a few of them were new TOS three jaw chucks that always needed an undersized register machining on the new backplate to get rid of the 1 1/2 to 2 thou runout in the chuck when fitted to a firm fitting register.

Martin of Wick23/08/2019 23:47:55
196 forum posts
4 photos

Hmm, thanks gents, it seems that I have been labouring under a doctrinal delusion and have now suddenly become enlightened.

I was dimly aware that one dodge for an old or bad chuck was to shave a bit off the register to be able to offset it, but apparently the power of dogma has meant I never considered extending the concept as suitable method for the fitting of new chucks.

No reason in principle why it shouldn't be OK, so as suggested will leave the back plate as is and use slightly larger mounting holes with some sort of low strength adhesive just to assist the hold down bolts.

I guess this is what they mean by lifelong learning, so thanks to all of you for your help, not least because I can stop kicking myself now!

not done it yet24/08/2019 02:23:16
4900 forum posts
20 photos

Guessing here, but make sure the next one you machine is cold (same temperature as the chuck) before taking that last skim. I look at it this way - the bolts hold it in its place, but the spigot regulates the position precisely and fixes the direction of the forces applied to the chuck bolts. Chuck removal for cleaning means it can go back exactly from where it came off.

Self centring chucks should not be expected to be that precise, for re-mounting a part machined item. I would go as far as to say that if you expect any old, well-used scroll chuck to be precisely concentric at all diameters, you will be lucky. I expect the same goes for any well-used griptru chuck as well - it may well require adjustment between closed and fully open jaws. If you want it right, use a 4 jaw independent (and they employ a spigot as well - I wonder why...).

Howard Lewis24/08/2019 08:17:09
3544 forum posts
2 photos

If you have the register within a quarter to half a thou, that should be good enough for a precise location (short of size/size fit )

The precision with which a 3 jaw chuck holds work probably varies with the size of work being held, anyway. Doubt if many chucks will hold work to a thou. 3 -5 would be a good one, in my book.

If absolute concentricity is essential, either use a 4 jaw and clock it, or turn two or more diameters without disturbing the work in the 3 jaw.


fat fingers again!

Edited By Howard Lewis on 24/08/2019 08:18:18

thaiguzzi24/08/2019 09:32:23
698 forum posts
131 photos

Half a thou U/S?

You done good son, you done good.

Martin of Wick24/08/2019 10:43:47
196 forum posts
4 photos

When I started this game years ago, one of the first jobs I had to do was prepare a back plate for a new chuck. So I went to consult the professionals in the blacksmiths shop and someone recommended cutting the register to a 'grip fit' in the chuck body.

This I did using a cast iron backplate and hardly daring to breath during the operation. As the cast iron was quite soft and easy than expected to machine, I was able to shave dust off at the final stage and get the desired grip fit. Needless to say, the 3J chuck when so fitted still had about 1.5 TIR.

Until yesterday I was still clinging religiously to that original advice on register size. Until yesterday I hadn't had to machine a steel back plate... It was very hard steel too, in the end I had to resort to carbide to get a decent finish at a reasonable speed.

So two lessons learned,

The back plate register doesn't need to be a precise fit and it may be beneficial as 5 thou smaller (so why have a register at all I wonder).

And don't attempt to get a precise finish with carbide tools - switch to HSS for fine cuts because I think what happened is the tool just rubbed at a very light cut and as I increased the pressure to start the cut, it eventually bit and chewed off a thou rather than the dust I had intended. An yes, the plate was bloody hot too... piles of beautiful blue swarf, so will certainly bear that in mind if I need a close tolerance in future

So not only was the back plate prep not a debacle as I assumed, but thanks to the experience of all those that helped, I have been re-educated too!

Thanks to all.

Tim Stevens24/08/2019 15:48:17
1259 forum posts

A back plate has usually got plenty of thickness (when new, at least). So, if you take too much off it should be an easy matter to face right across, and then start again to cut the recess.

As the upright toy ferret says - simples.

Cheers, Tim

Martin Johnson 124/08/2019 16:33:58
129 forum posts
1 photos

Several points worth flagging up on this saga:

1) "1/4 to 1/2 thou undersize" for what is probably about a 4" register, that is pretty fine tolerances anyway.

2) Another piece of received wisdom is that lathe tools have to be at centre height. If you are trying to take wee shaving cuts and the tool is actually a shade OVER center height, then the tool won't cut. So you put on a bit more and try again. Then, the tool finally gets "in" and takes off a load more than you bargained for. Moral - keep tools a shade BELOW centre height - especially on fine finishing work.

3) Funny stuff is cast iron. The finish straight off the tool (HSS at any rate) will be "whiskery", which gives a false over size reading on a mic or calipers. Before measureing take the whiskers off with a bit of wet & dry paper.

4) Another way to rescue such a situation is to let in and loctite 4 plugs, say about 20 mm diameter for your example, equispaced at 90 degrees and straddling the register. Then machine the plugs back to (hopefully) finish at size. Assemble it and hope you are long gone by the time someone else finds out your rescue bodge. Seen it done in my working days to rescue a very urgent part.

5) I agree with the others - no need to resort to bodges in this case. Crack on and well done.

Hope that helps,


Ian Johnson 124/08/2019 19:06:43
281 forum posts
82 photos

Have a look on YouTube for MrCrispin he's just released a new video on fitting a collet chuck to a back plate on a Myford. He explains his methodology very well including some of the issues brought up in this thread. Doesn't mention centre popping the register though!


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