recommended strategy to recondition an 18" x 4" knee mill table?

I have an antique Elgin knee mill with an 18" x 4" table. The top of the knee has the original scraping in an X pattern, presumably for oil; the bottom of the table does not have any scraping. As the table moves to the far right and far left it tightens up so much that using the table at the outer limits of travel is not feasible. I have read about and watched videos about scraping, planing, and surface grinding. Can someone please explain to me why scraping, either with a surface plate or a camelback, is a preferable technique to flatten the bottom of the table as opposed to running it through a grinder or planer.

If anyone knows a shop or person near to Washington, D.C. that could do it at a reasonable price, please let me know.

Scraping can be more accurate than planing or low quality grinding and it is easier for difficult to fixture parts. The oil retention may also be better than grinding but for home/light use that isn't going to honestly matter all that much. Scraping also doesn't introduce fixturing distortion or heat like would be found with conventional machining or grinding but it is slow so it is best to mill or plane before scraping if there is much wear. It is possible to only grind parts to high levels of accuracy but without very special equipment and careful setup but it is easier/cheaper to scrape them to the same accuracy for restoration or one-offs.

Depending on what you need someone with a planer or large mill could recondition your machine ways to be much improved. I have no idea of the potential accuracy of your machine, are we talking super refined or garage grade?

Luke

Luke Rickert said:

Scraping can be more accurate than planing or low quality grinding and it is easier for difficult to fixture parts. The oil retention may also be better than grinding but for home/light use that isn't going to honestly matter all that much. Scraping also doesn't introduce fixturing distortion or heat like would be found with conventional machining or grinding but it is slow so it is best to mill or plane before scraping if there is much wear. It is possible to only grind parts to high levels of accuracy but without very special equipment and careful setup but it is easier/cheaper to scrape them to the same accuracy for restoration or one-offs.

Depending on what you need someone with a planer or large mill could recondition your machine ways to be much improved. I have no idea of the potential accuracy of your machine, are we talking super refined or garage grade?

Luke

Click to expand...

It's an early mill, an Elgin VM2, made in Chicago. It was advertised as a precision mill and I imagine accuracies were probably .001" or better. .001" would be fine for me, but use is limited due to the table wear.

A careful scraping of the table will provide a better fit than grinding since you "mate" the two parts during scraping. This will give you a better feel and better precision through the entire distance of travel if done correctly, and often as fast or faster than proper grinding.

Scraping, not grinding, is most likely the way to go for your mill as outlined above. I would suggest thinking about more than just the table. Really you want to start at the base of the machine and work up aligning the machine as you go. Getting someone who knows what they are doing isn’t cheap. If you’re like me and others on PM it’s easier to learn how, get some tools, and do it yourself. Your mill is small and doesn’t need lifting gear, or a big surface plate. It sounds like an interesting project!

For an 18x4” table I would just throw it up on the table of my Deckel or one of the VMCs at work and mill the dovetails with a dovetail cutter. No reason to get too fancy. After milling you can scrape for fit, tuning the accuracy and wear characteristics. Or just use it as-milled.

Anyone with a bridge style VMC (like an Okuma M560) should be able to rework this without trouble. C-frame mills experience a bit of table rock in X, so if I did it on one of those I would setup the table parallel to the y-axis. But at 18” long you could still fit it on a standard 4020 VMC, assuming a ~1.5” dovetail cutter can get the whole dovetail.

Either way the first step is to take things apart and actually inspect and measure the wear etc. Remember even if the table is perfectly scraped or ground, you still have to fit the mating dovetails and gib to it, and this is very much more work than the initial machining of the table. If you aren’t equipped to properly inspect/measure the table wear before sending it out to be machined, fitting the old saddle dovetails to the freshly-machined table is going to be even worse.

Good luck! Machinery rebuilding can be a hobby unto itself, but is also kindof satisfying if you’re into that sort of thing.

In my classes I say if it is worn more then .005" get it machined. That could be by milling or grinding. One thing you will need to remember if you mill the table it will probably warp as the way even though it's cast iron has a hard face and I suspect when you machine it it will change the whole table. You will be releasing the structure and it will bend or warp up on the ends. Then you will have to machine or grind the top too. If it is under .005" then you can scrape it. As Lucky said on a small project like this it will be a good learning curve. That table would be a great project for you if you attended aa scraping class (plug, lol) I have all the tools and straight-edges that you could use for free, just need to pay for the class..lol. I have one here in MN in April.

In the old days we didn't have a lot of dovetail cutters, we used a fly-cutter.

Thanks for your input. It may not be a standard practice but do you know if a camelback can be used with feeler gauges to determine the extent of wear? Any idea where I would source a 30" or 36" long camelback that is ALREADY scraped?

I have worked with these straight-edges. They are good and "cheap" He sells them retail less then I can have mine made. https://www.martinmodel.com/collect...amelback-straight-edge?variant=32412638019667

Thanks