Which surface plate?

[gi_984]

Getting set up to start scraping and would like advice on purchasing a surface plate. I'm looking at a A grade 24X36(?) from Enco. Plan on building my own stand from wood with casters to roll around.

 

[turbotadd]

Just a word of caution, you'll probably want to build the frame out of steel as wood will move, and a surface plate's flatness depends on a rigid 3-point mount. I'm sure Richard will elaborate. I just got a 24" x 36" double ledge from a local auction, can't imagine what shipping would cost from enco. They often show up on CL too.

 

[Richard King]

Tadd took one of my classes....Go for it Tadd and teach. I always ask my students to pass on my knowledge and teach. You know the answer. Don't be shy.

 

[DMS]

If you have the space, larger ones show up for pennies on the dollar around here. I have one of the cheap import ones. It works OK. I got it because I wasn't sure how much I would use it. If I did it again, I probably would have gone with a Starrett. I have a 12x18, which has been large enough for what I have needed it for (reference for scraping, and for layout). That 12x36 is gonna be _heavy_, so be prepared

 

[turbotadd]

When you look at a big surface plate you think surely that thing can hold its shape no matter what right? Not so. In the class that Richard taught this summer, we had a guy from a local company stop by and show us how he calibrates surface plates. We had a 4' x 4' plate that checked out pretty good, but we heard stories of plates that weren't mounted properly or abused, and I was amazed with how far from flat they can go. Granite will creep just like any other material, so supporting the weight of the plate evenly is crucial to maintaining flatness. Ever sit on a tippy bar stool you know that it's virtually impossible to get even weight distribution on all four legs. Supporting on three points is critical. If you do design your own frame, plan for three support points where you will put 2" or so diameter rubber disks (1/8" or so thick) that the plate can rest on. This pic illustrates the idea http://www.flickr.com/photos/gomezaddams/2243574164/
I also found this PDF, see page 5 for dimensions.
http://www.evenfallstudios.com/woodworks_library/fed_spec_GGG-P-463c_for_granite_surface_plates.pdf

 

[gi_984]

Thanks for the info. Yes, I've been hunting the craigslist ads without any luck. My (almost) last resort is to order from Enco. I've considered even buying a used plate locally and paying to have it re-ground and calibrated. Problem is nothing decent has been come up so far.

 

[Richard King]

A couple of other points. The spec sheet Tadd gave you is for new plate mfg. If you buy a plate from Enco or who ever look at the bottom and if the rubber pads are glued to the plate or there is X's at 3 locations put you rubber pads there as that is where they had it sitting when they lapped. It may vary a little. I have always been taught to use 30% from the end and sides on the 2 pad end and one at 30% and directly in the middle on the single end. I use to sell Starrett plates and they use 25%, an optical machine building is know uses 28%. Many precision machines are built using 3 points, take for an example a Swiss made Sip Jig bore. It sits on 3 points, but the 2 sides end they have the points under the columns where the heaviest area's are and the other is 30% from the other end directly under the table. So the spec sheet isn't for all uses of the Kinematic mount principal or 3-Points. I also like to see you put adjustable leveling screws around the perimeter of the plate with a light touch, so when you set something heavy on the plate especially on the single point end it won't twist. . Many can debate the spot if your a scientist in a lab looking for billionths, debate but for the average machine shop owner use 30% it's easier to remember and you can hold millionths. If you make a stand, make it so you can adjust it level with the earth too. Thanks Tadd for your help.

 

[gi_984]

Richard and Tadd,
Thanks again for the information. I just ordered the surface plate from ENCO. Got 20% off plus free shipping! I'm going to weld up a stand. What color and brand of marking fluid do you like for scraping and on the surface plates?

 

[Richard King]

I have used all the different brands and I prefer water based brand named Canode 2243. My favorite before that was Dykem High Spot blue, but it would stain your fingers and the surface plate. I was doing some scraping seminars at GM in Indianapolis when I first saw the Canode and at first I did not like it, but I could go to the sink and wash off my hands and no stained fingers was wonderful. You can buy it from DAPRA, ES Dyjak and a company called Volt who advertise on the net. The Japanese and Taiwanese have a great product that looks a lot like shoe-polish and is red, but I have never seen it sold in the USA. The Canode also comes in different colors; Blue, Red, Orange, Yellow. They used to make black and white but have discontinued those colors. I use the Yellow as a red lead substitute and dilute it with window cleaner and I spread it and the blue ink with foam paint roller. The rule of thumb when spreading all the inks you roll on an even coat so you can see the surface thru it. If it is on to thick and you can't see thru it, the ink will smear. Another thing is you always run your hand or fingers thru it so you can feel any dirt that may be in it. You can feel the dirt with your hands, but if you wipe it with a rag you can impart dirt and lint into it. That's why my hands would get dirty and stained using the Dykem. If you use the Dyken use a foam roller or a piece of leather to spread it. Maybe Tadd has some pictures he can add.

 

[turbotadd]

Sounds like you got a sweet deal from Enco! Can't beat free shipping on a plate. Here is a picture of the plate we used in class. I use the Canode water based stuff too, the Dykem is used for pranks like the old anti-seize under the toolbox drawer pull because as Richard says it never comes off! Its a few bucks for a small bottle. I use the 4" wide foam ~1" diameter roller to blue the plate like the one pictured on the left. You will end up with two sections of blue on the plate: first one is where you drizzle the blue and saturate the roller - this will be too thick to use for spotting. Second area is where you apply blue to the plate with the roller, this is where you will blue up the part. As Richard said it is imperative to pull your hand across the blue to wipe the little bits of crud that will inevitably make there way in there. You'll be able to see the littlest bits of crud show up when you blue your part. They show up as little swirls of blue that look like they make by the tip of a needle.