Is stability of cast iron table affected by drilling holes?

richard newman · Jul 13, 2010

Wondering if drilling holes in an old cast iron saw table will upset the "balance of stresses"?

I have an old Hammond Glider that I have tuned and modified to make a super smooth and accurate little sliding table saw. I'd like to drill and tap a grid of 1/4-20 holes (maybe 2" c-c) in the table so I can easily fixture it, but not if there's any chance of the table moving on me. It's dead flat now, and I'd hate to screw it up.

Thanks,
Richard

moonlight machine · Jul 13, 2010

I put a grid of 1/2 13 holes in an iron plate with no problems.......

jim rozen · Jul 13, 2010

Just out of curiosity, how do you know it is dead flat right now?

MwTech Inc · Jul 13, 2010

Remember Jim...........

dead flat to a woodworker is 1/16" in two feet

stephen thomas · Jul 13, 2010

You guys oughta see the precision Richard uses in his work. Even besides the banjos. & some of the machinery he makes.

A few years ago I bought one of his surplus gages, an 8' Challenge straight edge he acquired to scrape a large jointer flat. Though as I recall, he did not proceed with that project and may have had it ground instead?

Richard: You might get away with it, you might not. If there is not enough thickness or the geometry would suffer from re-flattening after drilling/tapping, I wouldn't do it. Can you use a sub-plate?

smt, woodworker, who scrapes, grinds and planes all kinds of metal stuff pretty flat

richard newman · Jul 13, 2010

Hey guys, I work tp pretty tight tolerances.... I even have a red head girl friend for metrication standards!

Seriously tho, I've checked it with a precision straight edge and its flat within a few thou over a 12" x 18" surface, which is fine with me. Just don't want to lose that.

For those not familiar with these saws, they were made for the printing industry to trim lead plates to size. They are very well made and accurate, the table slides on ball bearings in V ways, and there is an adjustment to take up slop. Makes a great mini sliding table saw for precision work.

Thanks for the kind words Stephen, I like the idea of a subplate. Maybe make it out of aluminum and trim it right on the saw for a tight throat. Probably easier than taking the saw apart and fixturing it on the mill table.

Oh, and I never did get around to dealing with that jointer table. For the volume of wood I'm using these days its easier to just kiss the edges with a hand plane. Good to keep my chops up. Wish it was one of your planes tho...

R

jim rozen · Jul 13, 2010

"flat within a few thou over a 12" x 18" surface"

I think that's what a hand-scraper calls "roughed out and ready to start
making it flat."

The way to check, is to blue it up and compare to a surface plate.
It's doubtful that drilling a regular pattern of holes in this will make it worse
than that - but for a surface that small, you could trivially use a scraper,
surface plate, and some blue, to regain the few thou runout.

Basically unless you are comparing with a surface plate, you don't know
what's really going on. It could well be much better than a few thou right
now, and maybe you really need that degree of flatness.

Jim S. · Jul 14, 2010

Richard,
I also use a Hammond Glider as my primary woodworking saw. I have modified several of the castings by drilling, milling, sawing, etc, in order to accomodate dado blades, molding heads and a standard 10" blade. I have experienced no warping of the table top, notably on the hinged part of the table to the right of the blade where I removed quite a bit of metal to accommodate a dado blade.

Some pictures and description of my modifications are here: MachineJunkie :: View topic - New Member with Hammond G100

Although I have not drilled much into the sliding table, I doubt that 1/4" holes will have an appreciable effect.

I really like this saw and have attempted to make modifications consistent with Hammond's quality and workmanship.

BTW, a number of years ago, Hammond sent me an original factory brochure and parts lists for the model G4 and G100 saws. Nicely printed on glossy paper. If you want a copy, I would be happy to scan and send.

Regards,
Jim

Clive603 · Jul 14, 2010

Richard

If you decide to go the sub-table route check out Thor Labs solid aluminium breadboards Thorlabs.com - Solid Aluminum Breadboards, standard series would probably do for you Thorlabs.com - Solid Aluminum Breadboards. Prolly around $200 for 12" x 18" which isn't silly in the modify / make / buy equation.
Hopefully the US web site will come up for you which will have $ prices, the link gives £ for Brits like me.

Clive

richard newman · Jul 14, 2010

Jim R

"flat within a few thou over a 12" x 18" surface"

I think that's what a hand-scraper calls "roughed out and ready to start
making it flat."

That's what woodworkers call flat enough! Jim, I have done some scraping, including flattening the sole of hand planes, which really makes a difference and is the only sane way to do it. But for a saw table it just isn't necessary, and I've learned to pick and choose my battles carefully these days, it takes too much effort to just stay afloat in this economy.

Jim S - Very impressive work on your Glider! I have several bigger saws, I just use mine for really precise, intimate work. I shrinked (shrank?) a piece of 4140 on to the spindle and turned a flange and 5/8" arbor. Used a standard left hand arbor nut also. I cut out the base and beveled the underside of the slider so I could use an 8" blade with a good size outer flange. With a good Forrest blade the cut is beautiful. I'll try to take some pix and post them if I have a spare moment.

Clive - Thanks for the link, those breadboards are a great idea!

Thanks for the responses guys,
Richard - who knows and appreciates the difference between flat and FLAT.

alex71 · Jul 14, 2010

worked out OK for this one (1/2-13 holes with helicoils)

Before:

After:

jim rozen · Jul 14, 2010

+1 on the thorlabs breadboards. Any optical table company like newport, oriel, etc
makes a similar version. But the thorlabs company is good stuff.

Just be sure you purchase the one you want, they come both 1/4-20 and also M6.

Mcgyver · Jul 14, 2010

richard newman said:
I have done some scraping, including flattening the sole of hand planes, which really makes a difference and is the only sane way to do it. .

so i'm not the only one. I feel slightly less compulsive -it really does a nice job doesn't it.

Jim, curious why you' think a table saw needs to flater than .001? i bet they'd move that much from leaning on them

imo the answer to you question is probably not enough to matter. There are stresses in CI too, just a less than in other materials or fabrications. Altering them will change the equilibrium and shape...but so will a fly landing on the tailstock of my lathe. Bottom line i don't think it will matter ...if does move you know how to scrape it back

richard newman · Jul 15, 2010

Mcgyver, I think scraping is the only way to go for flattening planes. Everyone I knew used to try to lap them flat, on abrasive paper or plate glass with loose grit. It took forever and most wound up slight convex. Or they'd take it to someone with a surface grinder, and then complain when they got it back that it wasn't flat. Probably they were distorted in fixturing, or warped from the heat.

With surface plates so cheap these days, especially big used one, every woodworker should have one.

The jointer Stephen refers to is a 24" Yates, and the outfeed table is crowned a bit (maybe .010" - .015". It's just enuf to be annoying. I was going to scrape it in, I figured 30 min after lunch every day eventually would do it. But I love the look of the planed top, and I know it would bother me having a planed infeed and scraped outfeed. Talk about obsessive, eh!

R

mark thomas · Jul 15, 2010

Doesn't stephen have a planer that could fix it up for you?

You can also get woodworking-level flat pretty easily with lapping, leaving a surface finish that blends nicely with a ground finish. I love scraped surfaces, but I don't think it's better (aesthetically) in every context.

To lap a table like that, I use a 8" round cast iron lapping plate upside down. I just push it around in a figure-eight'ish pattern, focusing on the high spots, and rotating the lap. No downward pressure is required; the weight of the lap does the work.

To gauge flatness, I use one or both of two tools, depending on circumstances. I spot with a camel-back style straight edge to highlight the high spots, though very roughly using a pretty thick coating of spot fluid. I also use a narrow rule-type straightedge and feeler gauge.

It's easy get flat within a couple thou, and I think it's a lot easier than scraping and surface plate (no lifting and turning your table over repeatedly, and in fact, you don't even have to take your table off the machine)

Btw, this style of lapping is also how granite surface plates are made flat to within millionths, though the measuring technique is obviously more precise.

stephen thomas · Jul 15, 2010

I've drilled grids in relatively thin steel, and seen it go hollow or potato chip shaped. If the top on the saw is "thick" (say over 5/16" between the webs), and if you adjust the grid so as not to drill through any webs, it would probably be ok. I don't think a few holes here and there would matter. But there is something about a uniform grid of a "lot" of holes within a tight border that seems to stretch the metal a bit.

But yeah, an obsessive recognizes a superior obsessive

, and I have a sense if the top did move a bit it would bother you. If it twisted, it would be a major pain

You might want to clarify for some of the metal workers that the jointer is 24" -wide-. (I can't recall if that one is over 6' long)

Here is the 8' Challenge straight edge Richard sold me. Being used to align the tables on a lumber jointer (automatic edge jointer). I had to scrape all 4 tables on this machine due to the fact they were too long for my grinders or mill. One of the final jobs that convinced me it was time to get a planer. If the track/ways had been worn out (center section with the Vee grooves) it would have been a problem since a new one is $10,500. So fearing the need for a planer to re-machine that ~5' long piece was the clincher.

smt

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