Anyone ever machined the damage from a mill table ?

As the title states..

Not my damage, someone else before.

Its easy enough to strip the table and machine but whats the potential problems, will it bend like a non EU banana?

Thanks

We inherited a Tom Senior light vertical at the museum which had a much abused bed. It is the longer bed model and I determined that removing 1.5mm, about 1/16" would remove 90% of the damage. I bought two sets of 124 blocks on ebay, and was pleased to find that both sets matched exactly in all axes. The center tee slot had 4 evenly spaced holes drilled right through to take SHCS screws holding into tee nuts made to fit our other mill, one of the larger roung column drill mills. The blocks supported the TS bed on the other mill, but it had to be milled in 4 stages, repositioning each time. Then the inverted bed was rubbed by hand on the surface table which was covered with 120 paper held down with double sided tape. Only sheets from one packet were used, as different batched vary in thickness. The rubbing was done over several weeks and was very slow, and I decided to give up when there was just a shadow of milling marks left at the ends of the bed. How accurate is it ? If the head is trammed to 0.0005" or better with the bed in the central position, and then the tramming is checked at either extreme of travel, the ends are both 0.002" higher, this with a tramm about 10" swing. Good enough for our purposes. With an 18" straight edge, our longest, the top of the bed is better than 0.0002".

Would I do it again? I really don't know.

It would be worth checking machine shops and see if any would have a big enough mill to do your bed in one go. Measure the depth of the damage to see if all or part can be removed without compromising the bed.

How bad and what sort of damage? I'd not commit to machining unless it was the only option. Besides the stress relief / bend risk getting it all set up to produce a really flat result after machining all over isn't trivial. The folk who do such things know all the tricks to make the job fairly routine.

For the usual drill and mill cut holes good quality metal loaded filler (devcon) carefully scraped flat works fine with far less risk. I've even used it to build up partially broken edges in cast iron similar to what might happen to a Tee slot with every success despite less than careful handling by customer folk.

My Bridgeport table has some weld filling done when InterCity Machine tools reconditioned it before it was sold to a previous owner.

Clive

Can't see it being a major problem to a competent machinist with the right equipment, you would need to specify how parallel the finished article needs to be but if you thinking about removing loads of material I wouldn't bother.

Tony

The material should be very stable, if there is any stress in it when its produced it might warp as most of the machining is on one side, the casting must be stress relieved by age or heat before machining if its to stay straight.

Could you add a fixture plate on top of the existing bed if the damage is too extensive to live with. Depending on the size, a plate might be cheaper than having the existing bed machined.

Have you a picture to show damage with depth and width etc.

David

I wouldn’t. I agree with Clive on all points. I would fill it rather than removing metal.

How deep? How much metal to the T-slots? A recent machine or an ancient model?

If only needing a surface grind, I would be leaving it to a professional - too risky for me as a non-machinist hobbyist!

Thanks all for the replies

I've not got the mill yet, picking it up shortly.

The damage is not a major problem to me, the only reason I would consider machining the table is I have the access to the machinery and grinders at work so could lightly machine/grind the surface.

I know sometimes these things can bend and twist so wondered if anyone had tried it,

Many thanks everyone.

I am having mine skimmed but it is a 90-degree angle table with stiffening webs. I would worry about skimming a flat table lest it release stresses and bend somewhat. A lot depends on how well it was stress-relieved during production.

When I rebuilt my beaver milling machine, the table had 20 thou of bend on it and three thou of wear. that's on a 48" table. That amount was removed by scraping. The normal machanism for distortion in milling machine tables is peening or stretching of the upper surface by over--tightening T-nuts.

I wouldn't bother removing scars on the table unless I was also going to scrape or grind the ways for alignment purposes.

I think I have seen on this site a description by the late lamented John Stevenson of repairing a table which had a chunk broken out of a tee slot. He used the machine itself to mill out a neat pocket round the break, and bolted in a piece of MS or CI to repair then machined the surface/slots to match the existing. Maybe the Google site search on the home page would turn it up.

If you have access to the machinery to skim the table it would be worth doing, if it is an old English machine it is probably well seasoned and should not move, if it is modern Chinese you may well improve it.

If you have access to that sort of machineshop facilities, talk to the guys who work there. Get their opinion.

I would not bother machining it unless the top of the mill table is damaged beyond usefulness. You can't beat factory finish. (Unless it is some Chinese machine tools.) If it was done properly in the factory , the top of the table will be ground and possibly scraped so the top runs within a few tenths of a thou of the load-bearing ways on the bottom of the table. Not the bottom of the table necessarily, but the load bearing way surfaces themselves. So may take some setting up to regrind properly. If not set up properly, you may end up worse off.

With a grinder you are only going to be skimming a few thou off so chances of it stress bending are lower than if you were milling 100 thou off etc. But if it only needs a light skim, probably not worth messing with. Use it as is.

A long time ago I skimmed the bed on my Warco Major because it was no longer flat. I also cut a groove across the back to support a 50cm length of gauge plate standing proud of the surface for alignment purposes.

Slightly scary moment when I found the hard spot but I regret nothing

For a single mistaken drilled hole, I have heard that tapping, adding a grub screw and stamping OIL is a neat disguise especially if you do another in a symetrical position.

;O)

Martin

The Taiwanese round column drill mill at the museum has a 500mm X axis travel, and when I decided to pair up the two 100mm Bison vises with keys to hold long work, it was found that the rear face of the table slot used by them had a 0.007" run from side to side. I managed to use the mill to skim the high spot out in 3 stages by swinging the head left and right. The error was mostly in the left hand third of the travel, so just touching with the side of a 12mm solid carbide cutter did most of the slot side. Now the error is better than 0.001" over the entire length. I have never bothered to check the other 3 slots.

Good afternoon everyone

Further to my original question,

After collecting the machine i made the decision to grind the table as i could not live with the damage.

An initial measurement on the CMM machine showed no twist or sag and about 0.001” wear to the underside.

After grinding the twist and flatness remained less then 0.0004” so i’m happy

Edited By Brian Abbott on 19/09/2022 19:53:40

It certainly looks good now, that table seems to have led a life of unbelievable carelessness. I have managed to avoid milling the table or vice on my mill or drilling the table of my drill, however i did hit my top slide with a chuck jaw, the bad language didn’t fix it.

Mike

Lovely job, Brian

MichaelG.