Lathe holding

[pgk pgkParticipant @pgkpgk17461]

Sorry this is a bit long

Referring back to my question about cutting a groove.. that was a 12mm thick hot rolled plate 115mm x 75mm with the circle centre 30mm from long edge and central with the groove width 8.5mm.

To hold it I used the 4-jaw and reversed two jaws, sat it on the highest jaw flat and ten centred and bumped it until a DTI showed it lying right.

My original idea was to cut the groove then bore a central concentric hole but this wally hadn;t realsied that the one reversed jaw obscured the hole centre from the viewpoint of getting a boring bar right through

I moved it to the mill and swept the groove with a dti to centralise and discovered that the groove is sl elliptical.

Now it turns out that I did have a couple fo digs during the grooving .. so it may have shifted the ite. but it also turns out that whie it ddnlt appear to be a problem at the time – somehow overnight the 4-jaw had loosened on it;s backplate – an obvious vibe i couldn't have missed unless it was way less at the time.

It;s out of round reading 0.05mm less each side of one axis sweep compared to the other.

The questions:

I didn't have any parallels that were a useful 'fit' on the chuck for levelling and i'd have run out of hands trying to centralise the part.. any tricks beyond the setting and bumping I did?

If I put a part like that on a mandrel would the imbalance be too great and if so how to balance it without clamps etc above the working face?

If I used the faceplate should it be skimmed first or just check with a dti?

It using the face plate how best to hold such an item without any holds being above the 12mm thickness?

Now I have two ideas for that.. one would be based on low blocks bolted to the faceplate and then 6 or 8mm grubscrews clamping to the edges or indeed making the plate larger with sacrificial 5/6mm rim that low clamps could hold down onto.

Lastly it occured to me that one could cut the groove on the mill either by making a new tool for the boring tower or a modified flycutter type tool with a vertical cutter specifcially for the radius wanted.to clear the heel of a boring tower tool?

[pgk pgkParticipant @pgkpgk17461]

[Brian WoodParticipant @brianwood45127]

Hello pgk,

I found the description rather hard to follow so instead and for some guidance I might tackle a similar job this way.

Mount a section of decent quality plywood to a faceplate, screwed in from the rear with short screws that don't penetrate the surface to make a dummy faceplate just for the job in hand.

Skim the surface if you wish but unless the depth of this groove is critical I don't think it will be badly wrong.

Now you have a plain surface to screw the job down to, set out however you like from the central position and secured in as many places as you use screws. It would transfer to the mill like that if you elect to use that machine instead of a lathe. Balance on a lathe shouldn't be a problem if the speed is modest, on a mill with orbital milling at a radius it doesn't apply.

If you use a mill, locate it and clamp securely to prevent the endmill trying to drag the job off the table

I hope that gives you some ideas

Brian

[MadMikeParticipant @madmike]

I also struggled with the description, but it seems like you were trying to produce a 15mm round boss in the centre of the 115 x 75 plate. Is that correct?

What exactly do you mean when you refer to the process of bumping? The way to centralise a plate, or whatever, when in a independent 4 jaw is to slacken very slightly the one jaw and adjust the position of the plate by tightening the opposite jaw.

In terms of the roundness tolerance, did you actually measure the spigot with a micrometer, to establish its correct size or are you relying upon the DTI reading?

What lathe are you using and just what condition/state of repair is it in?

Have you ever checked the accuracy of your lathe to establish the roundness tolerance that it produces?

IMHO back plates do not loosen themselves overnight. The laws of physics apply here.

Brians suggestions are very good and should help, but I really do not understand why you couldn't do this job as you started out, on a lathe.

so many questions but if you are to carry out this sort of work then you need to analyse and cure the cause, not simply eradicate the symptom.

[Ian S CParticipant @iansc]

You could get an elliptical shape in the hole if you over tighten the jaws on the 75 mm dimension.

Ian S C

[pgk pgkParticipant @pgkpgk17461]

The confusion I caused is probably because i was combining he job i had done with a second phase to be done.

You're correct that i could ave held the first job to a board on faceplate..or even directly on the faceplate since the central through-hole could have been bored through the faceplate central hole. using the 4-jaw did make aligning my marked out centre-pop to lateh centre easier than on a faceplate.

However if wanting to skim the whole surface of the plate on lathe rather than mill: (and for the second part I plan I'm leaving a raised circle of metal to fit in the groove of the first part).. the need is to attach to faceplate (or 4-jaw) such that I can skim off nearly 6mm of the surface to leave the raised circle. That means hold-downs, screws etc cannot be proud of the job.

I've had more time to consider this and i thik my easiest solution is to rough out with endmill (in the mill vice) down to the depth required and leave the raised area and a safety zone then transfer to lathe and tidy up.

That way hold-downs on the edges can be above the surface.

Or indeed the job could be completed in the mill with the boring tower working both inwards for the outer raised circle and boring it out from the middle to leave the raised part.

[pgk pgkParticipant @pgkpgk17461]

Let me try and answer these new questions:

Lathe i a chester crusader delux and 4 mths old.

For the second job, yes a raised boss bored out centrally but not central on the plate – indeed the boss centered by adjusting the jaws.

Bumping referred to the fat that with 2 jasws reversed and 2 normal on the 4-jaw chuck the workpiece was held proud of any support from behind on 2 jaws than meant the palte didnt naturally sit squre to the chuck and took some tapping (or bumping) to get it absolutey level before really dogging down on the jaws. My take on the eliptical hole is that one of my 'digs' twisted the plate or the loosened 4-jaw on it's backplate caused a 'sag'.

There is no question that the 4-jaw had loosened overnight. the amount of wobble when I starte the lathe up was dramatic (handy E switch) and now way would that have passed me by while machining.

I guess if the chuck had slightly loosened and sagged in one plane then it could lead to an ellipse and then sitting there sagging overnight had loosened further/or sagged off the locating boss.

[Brian WoodParticipant @brianwood45127]

Hello again pgk,

I think now the confusion is complete, I am lost with all sorts of images that conflict.

Mike is right, chuck backplates are either tight or not properly fitted and by that I mean not tightened to the chuck. The chucks are located to the backplate on a close fitting register to stop any misalignment from the lathe central axis and the two components are bolted solidly together

No chuck 'sags' as you say, they are rigid things, it just doesn't happen

Maybe a picture or two will clear the fog a bit

Brian

[pgk pgkParticipant @pgkpgk17461]

It happened. There are 4 bolts from the front of the chuck through to the back-plate (which in turn has the camlocks). In the morning when I found the wobble then one (or more) of those 4 bolts was very loose such that the chuck was tilting from it's backplate.

Obviously i dismounted it, took it apart, cleaned and reassembled and cranked those bolts up tight. When I bought lathe the 4-jaw came in a box to be attached to it's backplate and self-evidently i didn't dog those bolts down enough. Threadlock will be considered if it does it again.

[IanTParticipant @iant]

Hi PGK

Sorry I didn't follow all of the detail in your original post either – but with respect to your question about the faceplate (a much under-used accessory in my view) – you don't need to skim it or anything else if it has been properly set-up in the first place (unless it's been damaged in use).

To avoid cutting into your faceplate, it's a good idea to use a wooden backing plate (which is sacrificial in nature) and you can then simply screw your work piece to it. This can be quite useful for some work. I gathered you were cutting a groove (?) in your work and the 4 jaw would be good for this (as you can make small adjustments to centre the work). The faceplate is a bit trickier to set-up – so I often cheat and use slightly oversize material.

I'll attach a photo of a pair of inside engine frames I needed, which needed an 'arc' cutting in them. I used some slightly longer/deeper material, first 'bored' my arc and then machined my 'edges' based on datum from the arc. I felt this was easier than trying to set an exact arc centre with something so simply screwed in place. Not sure whether this would apply to your situation (or not) but the faceplate is very useful in some situations and is often over-looked when considering work holding options – particularly awkward pieces .

Regards,

IanT

.

[Brian WoodParticipant @brianwood45127]

Hello pgk,

Normally when such bolts are tightened that's it and the combined assembly remains securely assembled until spanners are laid on it again.

This experience is totally alien to me and must have a cause, the most obvious being that the tails of the bolts protrude though the back plate and have been forced in some way with pulling down the cam locks. Without anything moving overnight there were no rotating forces acting and unless you have a new kind of workshop gremlin to undo things behind your back, I can't think of any else that might be remotely logical

Ian's wooden faceplate was just the sort of work holding I had in mind but without a picture to show you.

I hope you can get this mystery resolved

Good luck

Brian

[pgk pgkParticipant @pgkpgk17461]

Ian,

I'm obviously good at confusng folk with my questions <s>. Yes i could have made up the first part using your technique with adequate sacrifical material. As you say the centralising would be tricky (boring the holes while backplate is on lathe etc).

It would also be possible to make the second part in a similar fashion using oversized material with countersunk bolts to allow for facing off some 6mm to leave the raised portion – something i hadn't thought about until your pic triggered the idea. Although my essential mean-ness hates the idea of creating waste.

As it happens i was planning on making some artistic slots to pretty up the part and reduce weight so such could be used to attach it to a faceplate and then modify them up afterwards….hmmm…

If the concept I'm playing with works but isn't neat enough and i remake it then that could well be second approach.

[IanTParticipant @iant]

PGK, there is another option involving "wooden faceplates" which can sometimes be useful when trying to hold 'awkward' work.

This is a plain wooden disk, with a smaller disk screw/glued to the back of it. You mount your 'work' on the front of this assemblage and then grip the whole lot in your four jaw (inside jaws). This will enable some degree of adjustment (for centring your work – although it still needs to be roughly in the right place). There are of course limitations on what size of work you can mount & still 'throw' and where on the work you can set-up to centre & machine. You also need to be gentle – as it's not as rigid as the proper faceplate option. But it's another possibility for you to consider (or for those who don't have a faceplate!) – if not on this piece, then perhaps on something else in the future?

Regards,

IanT

[pgk pgkParticipant @pgkpgk17461]

I've seen videos of that sort of idea.. an ally pallet gripped in 3 or 4 jaw on some bigger machines where swapping chucks is a pain/overhead crane for a quickie job. (or indeed a small check gripped in a large one for tiny work).

[Ian S CParticipant @iansc]

There is one method I used to use for wood turning, a wooden piece screwed to the faceplate, and the block to be turned glued to that with a bit of paper in between the two, after the job is done it just required a chisel placed at the joint, and a bit of a tap separated the two. A 3" diameter area would easily hold a 9" x 3" block of Mahogany to make a salad bowl. Don't know if glue would hold metal.

If the metal you are using could have even only two holes drilled and tapped(doesn't need to be right through), you could bolt it to the faceplate, preferably with thick card/thin MDF between the two.

Ian S C

[pgk pgkParticipant @pgkpgk17461]

My one try at using the glue and paper method on wood turning ended up with the lump coming adrift and needing a fast hop sideways. that was wth PVA.

The brown foamy glue is supposed to hold metal to stone even but I can;t say i fancy trying that on any decent chunk on a metal lathe.

At the moment it's necessary stuff in greenhouses and seed sowing so shop time will be limited to bad weather.

[Ian S CParticipant @iansc]

PVA worked ok for me, I used the pages from a telephone directory, and leave at least overnight to dry. I also cut the corners off the block of wood to make an octagon, more bumps but smaller.

Ian S C

[pgk pgkParticipant @pgkpgk17461]

Posted by Ian S C on 03/04/2015 09:56:48:

PVA worked ok for me, I used the pages from a telephone directory, and leave at least overnight to dry. I also cut the corners off the block of wood to make an octagon, more bumps but smaller.

Ian S C

My only difference was not usng a telephone diectory.. just printer paper. I'd seen and heard of it being done and assume i just got unlucky. At least it lept towards me and not the window. The window wouldn't have ducked.

[Nigel McBurney 1Participant @nigelmcburney1]

workholding using a wooden chuck and glue,is a very old tecnique,mainly for machining very small thin parts or small instrument castings,I would not fancy holding a lump of 12 mm plate by glueing and then trepanning a wide groove, recipe for disaster and dangerous, Just hold the plate in the four jaw and set the front face true ,preferably with a surface gauge or an old DTI ,I would not use a good DTI on a bit of rough black plate,on a piece of plate this size there is no need to set two jaws one way ie external and two internal ,,set them all the same way and just true up by tapping the work with the time honoured way by using a lead mallet or a copper one , grip on about 10 mm of the thickness leaving 2 mm protruding so that a light clean up cut can be taken if required, One method or machining a groove in black plate,is to use a 3mm /4mm wide parting shape tool with a full radius ground on the tip,more clearance is required on one side to clear the rad (similar to a boring tool clearance) plunge in carefully say half a mm then face to nearly the full width of the groove, carry on until full depth is achieved,the use narrow right and left hand tools to clear out the groove to get sharp corners and finish width on the groove,all this carried out at about 150 rpm,go slow and careful, black plate is not the easiest material to machine,

[pgk pgkParticipant @pgkpgk17461]

We went off topic a bit with the glue.. that was wood lathe stuff when its thicker material.

I'd flycut the plate through the rust and scale on the mill before it went on the lathe..

Because the hole was towards one end of the plate it wasn't possible to have all four jaws reversed. To centre the groove position one of the reversed jaws overlapped the centre enough to interfer with the other pair clamping down if reversed (rectangular plate)

[pgk pgkParticipant @pgkpgk17461]

I managed to get into the shed tonight. Part2 is working out better – learning from part one. This part is slightly shorter, same width and double the thickness and to make a raised 'hollow' boss to fit in part one's groove.

One reversed jaw on the 4-jaw with a pair of parallels clamped each side of the opposite jaws for set-up and i was able to get a decent hold proud of all jaw parts so no danger of catching the tool. And no need to DTI the face. I've limited cut depth to 0.75mm since it's facing cuts that start intermittent; to reduce the chances of tilting the part. The lath could take a heavier cut but I'm not in a rush. Power crossfeed is giving a nice finish on this hot-rolled.

All I have to do is avoid stuffing up making the mating circle….

[Gordon WParticipant @gordonw]

I have made a "faceplate" for similar jobs. It is just a short bit of tube with a square of steel welded on one end, 4 holes in the corners of the plate. Screw a piece of good plywood or similar to the plate. Put in the 3 jaw and skim the face of the wood and screw or bolt the job to the face as required. Skim again next time it's used. very useful and quick to set-up.

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