Centec 2B Mill Table—Surface Grind or Scrape?

[Graham Horne 2Participant @grahamhorne2]

I am learning to ask specific questions in threads as answers have a tendency of exploding, so to be specific, I have a Centec 2B mill, and my concern in this thread is the mill table. It’s not terrible, but not great either—there are no major dings or mill marks, which is a plus, but it has a ±0.25mm concave bend and noticeable wear in the middle of the ways.

My first thought was to get the table surface ground, but I’ve struggled to find anyone willing to take on the job near me in the midlands around Rugby. While I’d still prefer surface grinding for the table, I’m now considering scraping it flat instead. It won’t look as nice as a ground finish (I’m sure options will vary) but scraping the ways also gives the benefit of oil retention.

For those with experience, would you go for scraping in this case? Are there any pitfalls I should watch out for, or alternative approaches worth considering?

[Andrew TinsleyParticipant @andrewtinsley63637]

Not an expert at scraping, but I have never tried to scrape such a large amount of material. It would take me forever to do such a task!

How are you measuring the plus or minus 0.25mm? would it be by putting a DTI in the quill and then traversing the table underneath it?. If so, then there are other reasons you could be observing the plus or minus 0.25mm.

Other more experienced people will be able to comment.

Andrew.

[duncan webster 1Participant @duncanwebster1]

Unislide grinding in Coventry advertise doing Bridgeports. Whether they can be bothered with smaller machines requires a phone call

Tell them you’re not in a hurry and they can use it as a fill in job

[peak4Participant @peak4]

I’m absolutely no expert, having only attempted a job like this once, and without any training whatsoever.

During the first lockdown, I scraped in the table on a Herbert Junior surface grinder.
It was similarly bowed; the top surface was flat, so I scraped away any high points, so that it showed a uniform blue transfer all over from a surface plate.

The underside was bowed, with about 14 thou wear in the middle, so I effectively need to take of about 15 thou from each end. The dovetail was well worn too, so I made an angles straight edge from a length of cast iron for that job.
It was all done by hand, but did take rather a long time; a better scraper would have helped.

Essentially I flattened the top, and made the underside parallel to it within a couple of tenths.
Scraped the front edge square to the top and bottom, so I could then use that as a reference for scraping the fixed dovetail parallel to it.

The flat table then had a handle attached to either end, so I could use it as a portable surface plate to scape in the column.

Lots of photos in a Flickr album, many showing the various lashups for measuring wear.

The photos seem to show an excess of blue on the surfaces; in reality, it wasn’t as thick as it looks at the later stages.

Bill

[DC31kParticipant @dc31k]

I read the initial post three times and I think it is the underside (the ways) of the table that need attention.

As they are a dovetail shape, the inclined sides might be tricky on a surface grinder. It might need a slightly more dedicated way grinder. Searching for ‘slideway grinder’ might assist.

https://www.ebay.co.uk/itm/325718395803

TE-BAR Slideway Grinding

As the Centec is not very big, a larger-sized tool and cutter grinder (something with 12″ or more stroke on the X-axis e.g. KO Lee https://www.lathes.co.uk/kolee/) might have the capacity.

As a prelude to final scraping yourself, the ways could be trued up on a shaper or planer. A Centec is not large, so a shaper with a little over a 12″ stoke might do it.

The table is small enough to go onto the table of a milling machine and a dovetail cutter be used to remove the worst wear, again to be followed by hand scraping.

[Andrew Tinsley]

On Andrew Tinsley Said:

How are you measuring the plus or minus 0.25mm?

The table is off and I first cleaned up any burrs etc and lay it flat, face down on the surface plate. The table is the same length as my granite surface plate and use feeler gauges. The exact value is not important for now but it has a decent bow.

G

[duncan webster 1]

On duncan webster 1 Said:

Unislide grinding in Coventry advertise doing Bridgeports. Whether they can be bothered with smaller machines requires a phone.

I have spoken with them. Their machine has a 3m bed and the setup costs are way too high before you start.

[peak4Participant @peak4]

The table on mine is 5″ x 25″
For those unfamiliar with the machine, there is a male dovetail sticking downwards on  the base of the table, rather than a female dovetail groove machined into the bottom.

Hence a machine shop who handles cylinder heads could deal with the T slotted top, but the underside would need a slideway grinder.
I guess one possibility would be to have the top ground, and hand scrape the ways and dovetails underneath.

It really depends on how bent the actual table is overall.
No mention thus far of the wear level on the slides, just the bow on the top.
I would think there is a need to take lots more measurements to decide a plan of action.

You could try James Garside in Halifax who regularly grinds Myford beds.

Bill

[peak4]

On peak4 Said:

It was similarly bowed; the top surface was flat, so I scraped away any high points, so that it showed a uniform blue transfer all over from a surface plate.

Essentially I flattened the top, and made the underside parallel to it within a couple of tenths.

 

So my thinking is pretty much the same as yours. Get the table flat and on the surface plate (without rocking) and the get the ways flat and parallel. I like the idea of scraped ways to provide good oil lubrication.

I checked out your photos. You are one patient man to provide such photographic detail. I have most things I need now to perform these same steps. It took me time to gather and acquire the equipment and tools necessary.

Hence the core of my question. Yes I can scrape the table flat and get a good number of points blued. I know I can do that but what would the preference be, to have a scraped finish or a ground finish for the table top and facing and back edges?

 

[Graham Horne 2]

On Graham Horne 2 Said:

On duncan webster 1 Said:

Unislide grinding in Coventry advertise doing Bridgeports. Whether they can be bothered with smaller machines requires a phone.

I have spoken with them. Their machine has a 3m bed and the setup costs are way too high before you start.

Maybe, but could well be cheaper than scraping as much metal off the bed as your measurements suggest is required.

Scraping is a slow painstaking process used to get an already flat surface tip-top, rather than a quick way of fixing significant wear.  Takes even longer if the bed is hardened, so test yours with a file to see.   Could take months to fix by scraping!

Though scraping is simple enough in principle, it’s a skill, so don’t expect instant gratification.  Learning will eat more hours.

Scrapers are cheaply made from old files, and blue is cheap, but a flat reference is needed, such as a camel-back surface plate: they’re not cheap!

Add it all up and grinding may be cheapest.

Buying ex-industrial machines is always risky.  They were well-made when new, but that’s irrelevant if they were hammered during their working lives.  Some appear on the market because an accountant assessed the cost of a regrind and decided it was cheaper in a commercial setting to replace it.  And he knows a worn bed means other parts are worn too and spares being full-price makes repair uneconomic.  Condition is all when buying second-hand.

Model Engineers often bring worn machines back from the grave by working for love.  Lots of workshop effort, searching the web for affordable spares, and calling in favours etc. Not my cup of tea!  Could be your time is too valuable to waste on a major scraping project too, how many hours can you spend on this?  Scraping is certainly achievable, but it may take too long…

Dave

 

 

[DC31k]

On DC31k Said:

https://www.ebay.co.uk/itm/325718395803

TE-BAR Slideway Grinding

I contacted Joe from this eBay link. They are in Burton-on-Trent which is not far way. He is able to do the grinding at what seems a fair price to me. I think it’s a good option. I just have to decide, grind or scrape. Or perhaps grind and scrape.

[Graham Horne 2Participant @grahamhorne2]

Thanks so much for all the contributions here. I think I have made a decision which is to have the table, front and rear edges and the sideways surface ground. After surface grinding the ways I will probable do some light scraping of the ways to improve oil carrying and of course match the table ways to the cross slide ways which I will scrape.

Last question:

if I am going to this trouble with the table would you get the cross slide ways ground as well or as they are relatively small would you just say just scrape them?

[peak4]

On peak4 Said:

I guess one possibility would be to have the top ground, and hand scrape the ways and dovetails underneath.

No mention thus far of the wear level on the slides, just the bow on the top.

Thanks for your input, Bill. I’ve found someone experienced with smaller machines to do the way grinding not too far from me. I think the best approach is to have the table surface ground along with the front and rear edges, plus the underside and ways. As I understand it, the mating surfaces are best with some light scraping to improve oil retention and prevent galling.

Pete Rimmer generously gave me some hands-on scraping instruction, lent me one of his camelback straight edges, and even checked out my surface plate to get me ready for the next steps. It’s great to see that kind people are still willing to help others learn and improve.

The hardest tool to source was a scraper, but I’ve now got a Sandvik Coromant 25mm carbide scraper, a diamond grinder setup, and I’m ready to get to work!

Graham

[peak4Participant @peak4]

Graham, in case you find the Sandvik scraper a bit too big for the dovetails, I found these carbide blades seemed to hold an edge OK.
I cut a length off and brazed it onto a length of stainless bar, topped with a file handle.
This is an Amazon link, just to show the style of the ones I bought, but obviously I can’t be sure that this is the same grade of carbide.
I wasted a lot of time trying to source a suitable lump of carbide when I should have just gone out and bought the same blade as you, though it was all a bit more difficult during lockdown.
I had various carbide pieces in stock, but found that many, whilst tough, would not take, or hold a fine enough edge.
The various eBay links at the moment all seem more expensive and in China

Bill

[Mark EasingwoodParticipant @markeasingwood33578]

Those TCT Knives are used in wood machining, rebate blocks etc, and are widely available from woodworking machinery suppliers and sharpening services. Many other sizes are available.

One of the sharpening services I used to use, used to collect the blunt ones, re-grind them, and sell them on for decorators paint scrapers.

If you are near a Joinery Manufacturer/wood machinist you could probably get used ones for free, they will need re-sharpening though.

Mark.

 

 

[ryan.carter848Participant @ryan-carter848]

Sandvik inserts are H10 / K10 grade carbide

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