Author Topic: Centec 2A Rebuild  (Read 1713 times)

Offline mm289

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Re: Centec 2A Rebuild
« Reply #25 on: December 31, 2017, 03:06:12 PM »
So with a decent reference point I was able to start work on scraping - a week later than planned  :bugeye:

Connely talks about getting the face of the column flat and perpendicular as the first step. On the Centec though the column face isn't a load bearing surface as the knee runs purely on the ways.

So next step in the process is to get the "guiding way" (opposite side to the gib way) flat and square. To do this I used a combination of a dial indicator mounted in the arbour and a longs straight(ish) edge. The straight edge allows you to measure deviation as you swing the DTI in the arbour. Pic 1 shows the setup if you were using it to check the face, but I just used it in the way. Also had the arbour (Jacobs huck :bugeye:) clocked with a seperate DTI to keep an eye on errors there.

Rather strangely to my thinking the bottom of the face is further out from the chuck than the top, this is true on both side and is about a .12 - .14mm difference. I could understand the bottom being deliberately "proud" to combat sag when built up but 5-6 thou seems a lot. Also I expected the bottom to be most work but it is actually the middle that was showing more wear.

After a rather abortive effort (see post in "scraping lessons learnt") I eventually got the way pretty flat. I am using a combination of a dovetail straight edge kindly lent to me by Julian (but this only covers 3/4 of the knee) and a prism kindly lent by Pete (which covers the whole knee but is very heavy and difficult to balance on the way). As a result my work method was to mainly use the small edge to get the bottom 2/3rds of the way flat up and down and outside to inside edge and then check the overall flatness with the prism. the top 1/3 of the way isn't actually used on this mill as the knee never goes that far so I wasn't too concerned with this part.

After the first attempt it looked like pic 3 and the gradually worked it in via pic 4 till we ended up with pic 5. It is about 15 - 20 PPI across 60% of the main bearing surface and that is good enough I think. The "incline" on the column is now .06mm (3 thou) top to bottom i.e. the base of the way is .06mm proud of the top measured relative to the spindle.

So that's way 1 done only another 15 surfaces to go :doh: - might wait till next year to do those :)

Cheers,

Paul.

Online PekkaNF

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Re: Centec 2A Rebuild
« Reply #26 on: December 31, 2017, 06:43:58 PM »
Thank you for this thread. Very interesting and good read. I'm following this with great intest for next 15 or so surfaces to scrape down and straighten. I think you are making good progress.

Pekka

Offline mm289

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Re: Centec 2A Rebuild
« Reply #27 on: January 02, 2018, 01:55:23 PM »
Cheers Pekka  :beer:

So yesterday, once I was awake after New Years eve (Happy New Year everyone  :thumbup:) I started on the gib way. Bit of a challenge as I needed to take about 8 thou off. Also the wide blade I had for the Biax was on a long fairly flexible shaft and I was struggling with chatter and the cast iron seems rock hard compared to the lovely block of cheese we used at the scraping training week :D

Eventually got it down albeit with a low spot down the middle as I was over focussed on not getting it low on the edge  :scratch: Ended up with a decent print although this time I didn't spend a lot of time getting the top level where the saddle never reaches, just scraped it to smarten it up.

Next job is to work on getting the dovetails parallel then we can look at offering up the knee.

Cheers,

Paul.

Offline Meldonmech

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Re: Centec 2A Rebuild
« Reply #28 on: January 03, 2018, 03:28:29 AM »
 
  Hi Paul

            Nice spotting on the column, well scraped. Sympathise with you scraping hard cast iron, probably chilled during the pour.
                                      The projects coming along well

                                                                                         Cheers David

Offline mm289

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Re: Centec 2A Rebuild
« Reply #29 on: January 21, 2018, 02:28:40 PM »
Bit of an update.

Have completed work on the column dovetails now. Started out by checking for parallel between the dovetails at different points along the column using a couple of milling cutters as pins to measure with. Used a pair that pressed against the middle of the dovetails.

Pic 1 shows me measuring up, was pretty good actually with only about .02mm variation across most of the column widening out to .06mm at the top where it isn't worn.

Then ran the straight edge across the dovetails to see where the high spots were. The Gib dovetail (pics 2 & 3) actually looked flatter than the non gib (pics 4 & 5) so I decided to use that as my "master" and scrape this flat first.

It actually didn't need much scraping to get it flat, but it had a couple of nasty scratches in it so I took it down a bit more to get rid of these until I had a nice finish - pic 6.

Once this was sorted I did the guiding way dovetail (pic 7) and re-measured. Dovetails are now within .02mm all the way along so I am happy with that.

Just for laughs I then blued up the column and took my first print of the knee  :bugeye: (pic 8). Not a lot of bearing surfaces here then, just resting on the four corners.

Next job is to attack this.

Cheers,

Paul.

Offline vtsteam

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Re: Centec 2A Rebuild
« Reply #30 on: January 21, 2018, 05:41:41 PM »
First eleven photos in thread edited to reduce size.
I love it when a Plan B comes together!
Steve
www.sredmond.com