grinding a lathe bed

.I have an all angle slideway grinder attachment that fits the frame of my Landis 16x72 Universal.....Question is ,if I grind a bench lathe bed ,how do I hold it down without springing it?.....or maybe I dont hold it down and rely on gravity..Do I chock it with wedges to level it ,and leave it at that?....Or maybe I should make a simple 3 point levelling fixture?

Turn the bed upside down. Shim if neccessary. Lightly chock it onto the table. Skim the feet so that they are all co-planar. turn bed over, clamp, grind.

Obviously you should listen to experienced people, which does not include me. But for a bed which is short enough to grind on your machine, I would have thought that the goal should be a 3-point support which prevents any internal stresses.

Skim the feet as Mark suggested so the feet are flat-bottom and true height to the top to allow holding them tight with hold-downs (or what) with little or no distortion, plus you can unmount the part and re-set it if needed. Good to mark where the feet are in place so the return is very close. If it wobbles from table top error you might shim under a foot.
Wheel selection is very important with an open-wheel the better choice. White aluminum oxide is likely the most common and then red and blue ceramic, brown wheels tend to run hotter. It is not uncommon to use a 24 or 36 wheel to take stock and then finish with a 36 or 46 grit. Full wheeling is common and that can generate enough heat to burn the part and heat-warp it so to make the job troublesome or scrap. The wheel head and the part have to be very solid so wheel grits can penetrate the surface. Tap your grinder head with a brass hammer to see it feels solid. Good to have a slide dresser set to the angle so you might go back and redress. The flats and angles will take a different amount of stock removal so know your finish targets, or blue into the finish sizes. Wet is often a must grinding such a large part. A pause at the ends, off the part might insure not burning the surface as a surface burn can bend a lathe bed and then it is a bugger to make it straight again. Yes, sometimes a cup-type wheel is used.

It is easy to get overconfident as you just touch the ends away from the low place, and when you begin to near or get the full surface you mess up because you are trying to go too fast.

If you are not good at trig you can draw the part at a large scale and measure the drawing to get .001 close to target.
*Good to hand feel the parked wheel to the part with checking your dial so you know where the first contact at both ends and low place (finish) will be on the dial-> write that on a note. Best to not talk to anyone or be bothered when doing a one-chance job.
Most often such as a lathe bed is a solid part but if you were grinding a long part that is not solid then mill jacks are good to just hand feel to the part under side.

I put a piece of tape on my dial to mark my place if I need to leave the job and come back.
Yes, a lot of simple stuff here that likely you already know.

You might find this thread interesting.

Hendey lathe bed grinding

Peter

All good information.....I wont be doing anything larger than the SB9/10 size,and wont be so fussy to get to 10th s.......A friend has a large collection of worn SB s and other small lathes ,and will supply a couple of beds for practice.

if you are going to specialize in one lathe you might make a template of the existing bed..and perhaps one for new specs dimensions

I was going to set up a different way to grind the bed and slide of a 12x24 cylindrical grinder I had......but that one went in the bin ,and oddly enough the ways werent as bad as I had imagined...too late..Not to worry.

We did a couple of lathe beds Well A budy of mine did the grinding to be honest
We always started with the carriage part
Just clean it up First the cross and then the longitudial ways All at the correct angles
When doing the longitudial ways of the carriage the top of the cross slide rests on the magnet So that part is parrallel with the flat back part of the longitudial way (with a convential bed anyhow)
Then we cleaned up the lathe bedways Placed the carriage on top and run a indicator over the cross slide
Run the indicator the same lenght as the distance of the 2 lines that the longitudial ways of the carriage rest on the bedways
That gives you the exact reading how much you have to lower eighter the flat ways or the V-ways I work to it that I can do the flat ways No need to align the carriage then
We had very good results that way grinding the carriage to fit the ways Blued up real nice after a few times scraping

Peter

ballen said:

Obviously you should listen to experienced people, which does not include me. But for a bed which is short enough to grind on your machine, I would have thought that the goal should be a 3-point support which prevents any internal stresses.

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If that would keep you inside the triangle while grinding I would agree
Grinding generates pretty much downforce Even at spark out So I could see the bed twisting where the grinding wheels gets out of the triangle of the 3-point support

Peter

If you look at the specs a lathe bed is not ideal to be flat, but rather slightly high in the middle. Like everything, you want them to wear in before they wear out. Why else when scraping say a lathe carriage, you keep the ends high and the middle low.

Peter from Holland said:

Grinding generates pretty much downforce -- even at spark out.

Click to expand...

I thought that during the final passes there is almost no force -- this is one of the main advantages of grinding. Hence one should creep up on the final depth, so that there is no distortion at the end of the process. Is that wrong?

A very good way to actually visualize grinding pressure is to push a piece of material into a bench grinder and so hand feel the force needed to make x amount of sparks. It is surprising to see the difference between mils and hard steel..or to see the pressure and heat rise for SS.

I will only be doing the beds....I cant see any point going overboard with a hobby lathe.....the saddles will be milled to accomodate plastic pads,if necessary.

RC99 said:

If you look at the specs a lathe bed is not ideal to be flat, but rather slightly high in the middle. Like everything, you want them to wear in before they wear out. Why else when scraping say a lathe carriage, you keep the ends high and the middle low.

Click to expand...

Richard (RC99) has a way grinder and he knows his "Stuff" !!

ballen said:

I thought that during the final passes there is almost no force -- this is one of the main advantages of grinding. Hence one should creep up on the final depth, so that there is no distortion at the end of the process. Is that wrong?

Click to expand...

Put a 0.001mm indicator on anything and everything moves
There is no such thing as no distortion The first couple of mu only need a little force to distort With grinding you are rubbing a rock with practicly uncontrolled sharp edges over a steel surface

Also with 3point mounting you have a unfixed beam While with full surface contact of the underside you have a sort of fixed beam with much bigger loadcapacity And then 3point fixture has little surface to hold it to the magnet and needs a subplate

Peter

South Bend may or may not have hardened ways. Be curious what type of wheel and grit you might use for either hardened or non hardened ways.

texasgunsmith said:

South Bend may or may not have hardened ways. Be curious what type of wheel and grit you might use for either hardened or non hardened ways.

Click to expand...

For both to avoid burning is first priority, then surface finish needs.
I might use a 46 h through L and a very open-wheel running wet with a pause at the ends off the part...and dress so it did not load up.

That wheel may be too fine for a less experienced grinder hand so a 36 or 24 might be the ticket.

Full-wheeling a type 1, or using a cup wheel makes a big difference. the cup runs cooler.

Once you put in a suck-up burn the job becomes a real challenge. Likely the part (bed) will take a warp from the burn and then be worst than when you started.

Three point the bed to make sure it’s on there without coercion
Then with a clock add extra mounts either side of third mount to support bed. Treat it like carefully levelling in a machine
Side clamps to aid clocking in sides true.
Don’t use a magnet

This is how we set up.

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Prior to setting up on the grinder we inspect the base on the granite, if there are open spots on the bottom we will shim when set up on the machine table.