Bridgeport X-axis ways question

[jmiller]

I am currently tearing down my new BP. Just removed the table. Was wondering if someone could explain this feature in the ways:


Right on the center of the ways on both sides it appears to be ground down exactly .003". Judging by the scraping marks i assume its intentional. Never seen anyone mention it or show it in videos I've watched.

 

[benmychree]

I have fwd this page to the expert, I will LYK what I hear back.

 

[markba633csi]

That looks like wear, and possibly that someone tried to cover it up by re-flaking- I hope I'm wrong

 

[jmiller]

It's exactly .003 deep and uniform on both sides. In the right light you can see strirations all down the length which seems like surface grinder marks to me.



There is a similar pattern on the underside of the carriage/saddle.



Both are perpendicular to the travel of the opposing ways. Identical in size and distance from the ends. I can't see how this would be natural wear. This and the 6 other bridgeports in the shop i bought this from had clearly not been touched in probably 20 years or more. The building was collapsing around them. This has to be intentional. Has any other bridgeport owners seen this before?

There are no such patterns on the opposing surfaces either. Maybe a high spot in the casting that had to be ground out, but i would think they'd find it flat along the length. Or an oil pocket?

 

[Uglydog]

Notice that the relieved portion does not have oil grooves.
It does not need lubrication because it was not intended as a weight bearing surface.
The flaking covers some of the tool marks from the planer.

Daryl
MN

 

[Rex Walters]

Rich teaches to relieve the middle like this.

My understanding:

Relieving the center of the top half of longer bearing ways like this ensures it's the ends that determine the alignment.

Relieving the center means there's less to scrape to get things into final alignment, including subsequent touch-ups, reduces friction, and reduces the chances of "tippy" travel (slightly concave is always better than slightly convex when scraping).

I think it's usually easiest to rough scrape the entire surface flat, relieve the middle (how much doesn't matter, but you don't want chips to be able to get in), then do final scrape for points and precise alignment on smaller areas.

Is that the table?! Relieving the middle makes sense for the saddle (Y), because the full length of the saddle is always in contact, but I didn't think you could relieve the middle on the underside of the table (X, because the ends will hang unsupported).

I don't have a bridgeport and have never torn one down. Is the length of the non-relieved sections on the end long enough that there is always a portion bearing?

 

[Uglydog]

Rex,
You are correct in what you have reported.
This is an old method, which many scraping teachers include in their curriculum.

Daryl
MN

 

[jmiller]

Not the table, this is the saddle. Thank you for the explanation Rex.

 

[benmychree]

I post the reply of Rich King in answer to the question;

If they were the old machines that had oil zeros the factory relieved them around .020". The newer chromed machines were approx. .005". A cast on cast I scraped them neg. .0003 to .005". On lathe cross slides .0005"

Be sure to tell the friends at Hobby Machinist I said hello

I'm still teaching classes in MN

My son Alex is helping and will travel to teach. I broke my left arm 3 months ago. Sucks. Rich. I am on Facebook and practicalmachinist.com under my name King-Way Scraping

 

[MyLilMule]

It's purposely relieved in the center. This is normal and is the same way on my Bridgeport.