Headstock raising block, How to make? (accurately)

Following my (successful, to me) centre height increasing experiment I am now thinking about making a more engineered solution.

What I don't know is a reliable way of machining opposite sides of a block whilst ensuring both faces and features are exactly parallel to each other.

The lathe in question is a Boxford, and the headstock locates on one prismatic and one flat shear. The height of the flat face has to be just right relative to the triangular groove if it is to sit properly. A method of machining male and female versions using only/mainly a vertical mill has defeated me so far.

On the lash-up I showed on this thread,

**LINK**  (its quite safe, its only a forum topic)

I used a pair of standard Vee blocks and a length of ground bar. After machining the face of a 10.5" diameter disk I could not see daylight at any point across the face with ground straight edge so as crude as the rasing method was there was no sacrifice of accuracy.

I was thinking of using aluminium for the headstock block, it will never see any wear so I cannot forsee it being a problem. Raising the tailstock can be done with a spacer plate interposed between the upper and lower castings.

Any thoughts anyone?

Edited By Ian Phillips on 04/08/2013 21:42:15

When I made a set of risers for my dividing head and tailstock I started with a chunk of hot rolled steel; less likely to distort after machining. The risers were made as a pair, and only separated as the last operation. They were marked with two 'V' stamps so they can always be placed in the same orientation as they were machined.

I machined the surfaces using a 1" side and face cutter in a horizontal mill, with the work clamped directly to the table. The sides and a T-slot for tenons were machined at one setting with a narrower side and face cutter. Having fitted tenons I then used those tenons in the T-slot of the mill table to align the block to machine a slot on the other side for the tenons on the dividing head to sit in.

Of course this assumes that the mill table T-slots are parallel to the movement of the table, but on any decent mill they should be.

Regards,

Andrew

The saddle has a surface that is exactly parallel crossways relative to the V and flat on its underside and in relation to the bed. Thus if you place it (on parallels) upside down on a surface plate with a bar in the V you can measure the height of the bar relative to the flat. This enables you to calculate the depth of the V and and after machining to check it. You would probably want to scrape it to the bed before moving to machine the top.

For the top, after machining the inverted V leaving the flat too high you can place the saddle on it and from its deviation from parallel to the plate calculate how much to take off the flat.

Once close it might be a viable trick to place the block on the bed and use a tool on the saddle to rack it back and forth to act like a shaper to make a fine cut off the flat that is parallel to the bed.

You could just cheat by slitting the block horizontally along the flat side, parallel to the bed, and put in forcing screws

Actually I think the most critical part is not the heights but getting the two Vs, male and female, dead parallel without introducing a twist on the head about a vertical axis. Reference flats on both sides of the block dead true to one another and reference parallel clamped on the mill table should take care of that.

Height and lateral shift errors can be taken out in making and setting the tailstock but twist is a killer to fix.

Much of this kind of work used to be done on shaping/planing machines, then finished by hand

More recently it's milled, then finished with a surface grinder

Ian,

Have you thought of by-passing the tricky machining ?

John McNamara seems to be having great success using Epoxy Concrete.

MichaelG.

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Edit: Added hyperlink

Edited By Michael Gilligan on 05/08/2013 07:22:09

How about mounting the block so that where you want the vee is horizontal put your cutter in and cut a vee groove on one side then move the table across and put another vee groove on the opposite side.

So now you have your two grooves opposite one another ( both female ) mount a round bar of the appropriate dia, in one groove to become the male ( held in by socket head screws )

Maybe not what you were after but it would work

Paul

Hi Ian

To Michaels comment Re Epoxy concrete

It would be quite easy to make a base made from epoxy concrete. 15% epoxy and 85% Sand and 5mm aggregate both by volume.

Prepare timber or MDF formwork making the inside dimensions the size of the base you require. the formwork can be clamped directly to the bed. the bearing rails on your machine bed are to be left exposed within the formwork. other bed areas can be masked off, use two or three layers of masking tape to create clearance. the edge of the base plate should be kept at least 20mm away from the v-way to avoid a fragile sharp half v being cast. it may chip.

for v-ways with a flat top as most have these days normally around 8 to 10mm wide, cover the small flat with a couple of layers of masking tape, this is to create the clearance that is standard best practise on machine slides. the bas should bear on the sides of the v-way and the bearing flat only.

When I did this on my lathe (Epoxy bedding the cast steel saddle after a bed regrind, it worked perfectly. The saddle is ) it is very heavy I had to crane it into position it has been in constant use for several years now. In this case I used pre set jack screws set in blocks of steel attached to the saddle using the way wiper mounting holes to mount them, to pre position the lathe saddle then applied the epoxy to the saddle and carefully placed it on the lathe bed sitting it back on the jack screws to set. Positioning the saddle in three planes took hours! The "Pour" only a few minutes

Once the formwork is in position all small cracks and holes should be filled with plasticine and masked off with tape, otherwise the epoxy will weep out.

Before the epoxy is placed (15% epoxy aggregate has the consistency of peanut butter) the entire inside of the mould and the exposed ways need to be coated with mould release available from fibreglass suppliers who also supply Epoxy. Use plenty I used a couple of coats ..... Epoxy sticks very well. Be careful not to scratch the mould release. I used a small stick to gently tamp the epoxy the top will self level within a couple of mm if you give it a hand.

Once the epoxy has set the formwork can be stripped and the base lifted off the bed, it may need a tap around the edges. to finish the casting sharp edges can be rounded with a small angle grinder. Any small holes and blemishes can be filled with body filler and sanded. The same way you would prepare a cast iron casting ready for painting.

The height of the base mould should be the height you want less the thickness of a mounting plate made from steel or aluminium less about 5mm for epoxy sand grouting I will explain grouting later. the base mould should also have several cast in position threaded steel inserts. These will be used to attach the steel top plate onto which you will attach whatever device you are building.

If you plan to machine the top plate after it is grouted in position you may wish to drill and tap any holes before to avoid drilling into the epoxy concrete later (Epoxy concrete is easy to drill with a masonry bit but it is not very kind to taps, or use a tap nearing its use by date)

After the base is cast and the top smoothed when wet it will not be dead level and planar to the base probably within a couple of mm, clearly not what we want.

Aligning the top plate? I deliberately made this a second operation. It would be possible to pour the whole base in one hit with a precision made mould, however you would spend a great deal of time preparing it. Instead I propose using the steel threaded inserts that will ultimately hold the top plate in position to level the top which can be precision set up in position on your machine using 3 or four small jack screws inserted in tapped holes in the base plate

The jack screws bearing against the epoxy base can be set, opposing and together with the mounting screws, to position the plate in line with the your machine bed, you should be able to align it very accurately indeed. A dial indicator on a sliding block running on the machine bed could be used. maybe a precision level would help if your machine is truly level. The reason for the 5mm gap is to allow the accurately positioned top plate to be epoxy grouted to align with the base plate.

Once the base plate with aligned top is set up the assembly could be turned on its side and the gap filled with an epoxy grout maybe 20% epoxy with sand only on aggregate (it has to be runny in order for it to run within the 5mm gap) If the gap is too small the epoxy may refuse to fill the slot.
The bottom and two sides having been masked off. The jack screws can be cut of flush if they refuse to come out. Grouting machine bases to the floor and assembling components aligned on jack screws is often done today.

Unlike Water based concretes epoxy has minimal shrinkage most papers state it to be "negligible". This does not apply to solvent based polyester resins as used with fibreglass boats, they have significant shrinkage.

Interestingly there is almost no machining apart from the threaded inserts and drilling a few tapped holes with this method. Oh and a bit of woodwork. Allow a little more epoxy for testing your mix before doing the real pour. You will find it an easy process once you have done a couple of small test runs.

Regards
John

Edited By John McNamara on 05/08/2013 15:55:42

Edited By John McNamara on 05/08/2013 16:02:02

Edit above should have said all measurments by volume not "both"

Brilliant write-up, John

On the subject of threaded inserts; have you tried bigHead fasteners ?

I used them years ago, on F-Board [honeycomb cored panels, with GRP skins] and found them excellent.

MichaelG.

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Paul,

Yes, that arrangement works very well ... although matching the diameter of the bar to the size of the vee is tricky.

The camera mount on my Zeiss macro stand uses exactly that arrangement; but they use a slightly oversize bar, to make sure it clamps properly.

Strictly speaking; Ian's job would need perfect registration of the bar with the plane surface; and this might prove to be a tricky fitting job.

MichaelG.

I've now got quite a lot of food for thought! so thanks to all.

From the replies I now realise that maintaining the headstock spindle axis parallel to the bed (in both planes) is not as hard as I thought as the 'half' machined block can be inverted and then aligned as suggested by Andrew.

Epoxy as described bt John McN (in a detailed and very lucid explanation) all makes sense but I'm not sure its the way to go for this raising block. Basically it is only half an inch thick and sits between the lathe bed and the underside of the headstock with the critical fact being the exact parallelism and close fitting to all the mating geometry. Once made and fitted its a fixed part and there is no sliding movement, its purely a spacer.

I used a round bar and vee blocks for my lash-up but to preserve as much rigidity as possible I aim for maximum surface contact area, that must be better than line contact only.

I think the hardest part is getting the height of the flat face correct relative to the vee.

Gang milling a block of cast iron followed by a bit of hand fitting/scraping is what it needs really, there are a lot of 4.5" Boxfords out there that are waiting to be converted to 5" (and 5 to 5.5).

Ian P

both the upper and lower faces

Must admit I had not thought of that, it certainly reduces the precision require to make the blocks, Now I'm thinking, no gib strips, but still have the screws to push the faces into contact, then inject an epoxy of some type into the gap to make the whole thing into a solid entity.

Overall I have high regard for Boxfords engineering and design, but one thing that does not impress me is the really marginal headstock to bed fixings. There are just two 3/8" Whit bolts with seriously restricted head access that are screwed upwards into the head casting I have never needed to get to the ones on my lathe but I have been told that its only possible to loosen/tighten them with the aid of specially cranked spanners.

With the alignment provided by the V the head hardly needs more fixing. I made a special spanner decades ago - it only needed to be 1/8 mild steel but I think those flat ratchet spanners now available would do.

Don't some modern import lathes have jacking and adjusting screws now so rather small areas actually taking the strain. Not brilliant long term but at least you can correct them.

The disadvantage of the 5in is no greater width on the ways so less solid than 4 1/2 in. Not sure how often a mere 1 in dia is going to help.

Bazyle

You are right, its a valid point about the bed not being any wider so in theory there is a loss of overall rigidity.

Boxford decided the bed was substantial enough for a 5" centre height so converting a 4.5 to 5" should be completely acceptable providing its done properly.

Raising a 5" to 5.5 is a more dubious procedure and I have not tried it yet. What I did was raise a 4.5" headstock by more than 1" using vee blocks and odds and ends, and surprise, surprise it works well,

Apart from turning 10.5" diameter aluminium parts I have today been taking heavy cuts off large 6" diameter lumps of EN24T. I say heavy but the depth of cut is about 0,5mm and the feed rate as fast as I can push it using the top slide. There is some chatter but only at the most overhanging point (the 6" blank is 4" long) which when I look at it, seems quite a long way from the front bearing (backplate+chuck+4".

Ian P