Newb Question About Reverse Spotting with Surface Plates

Hi all,

I'm looking into getting some tools for rebuilding my Maho MH600E. It has two box ways on X and Z (as a vertical mill), and a dovetail way the Y axis. Going by what's written in the Connelly book, I should get a surface plate that covers the length of the longest ways (which is about a 120 by 80 cm surface plate in a standard ISO size), plus a straight edge with a 45 degree angle on it that's about 120 cm.

I don't have any spotting tools, so I thought the easiest thing to do would be to buy a new high-grade granite surface plate to have a known flat plane. I could then get a used straight edge in good length or have one cast at a foundry, since I'd have a known reference plane for scraping. But, unlike cast iron surface plates, granite ones don't come with handles and aren't ribbed for additional stiffness. And I still want to use the surface plate for reverse spotting the ways on the machine casting. If it was just for measuring, I'd get a much smaller one.

This bugged me, so I emailed the local manufacturer of granite plates here, and he replied that they don't drill holes in their products (e.g. to put in threaded inserts and screw on a pair of handles), and that their plates will bend if unsupported correctly on a three-point stand. To be clear, the size I'm after would be 120 mm thick diabase from what he's selling.

In the application I considered for using the surface plate, it would be hung from a winch over the casting for reverse spotting for a lot of the work, which needs some kind of handles for chains or straps, and shouldn't result in ruining the flat plane. So, are grainte surface plates used for reverse spotting at all? Am I looking to buy one as a master surface and then getting a cast-iron surface plate that I'm going to be using for reverse spotting? Or should I rely on a grinding job (for which there is an excellent venue near me) to fix my alignments and only scrape in some oil pockets?

I get that you can spot along the length of the ways with a straight edge and use a small, hand-held surface plate to spot across the ways. But it seems like it increases the chances of error and makes for a lot of additional spotting. For what it's worth, I fully intend to get the spindle inner tapers (vertical and horizontal) ground anyway, and possibly even have some parts reduced in thickness to glue in turcite, so it's not like I want to do everything myself at all costs and pay a maximum price of a bag of peanuts for tools stolen from the scrapyard for the rebuild to be economically viable.

One of the chief reasons I'm leaning towards working on this myself is that most rebuilders I talked to will actually tear down the machine and replace damaged parts, but they still have the ways and sliding members ground by an outside company before rebuilding. That just doesn't seem to be worth the back-and-forth driving and fitting my needs into their schedules. Around these parts, most rebuilders actually specialize in electronics, servos and controls, some basic mechanics, but not real alignments. One of them outright told me it's not worth it and that I should get a different used machine if I'm not happy with the one I got. I agree on principle, but I already replaced the control on my mill (with LinuxCNC, hi how are you), and the only reason I want to do a rebuild is because I'm sitting on designs for two injection molds that have features located to within a tolerance of 8 microns. It seems a bit futile to drive around Europe looking for a bargain bin machine that will make those sorts of parts.

Sorry for the long post, I wanted to be clear about where I'm coming from with this project. I'd be very thankful for any useful input, and if you want to just tear me a new one, I don't really mind either. I got pretty thick skin from hanging around my inlaws on holidays

I personally prefer granite for spotting plates and have verified my prints and alignment with both scraped camel backs, precision levels and opposing corner to corner measurements with a known parallel to check for twist as well. unlike its it conventional orientation the master is supported along the majority of its length during spotting so you can actually go thinner than a conventional surface plate, its the same reason why a prism dovetail straight edge can be used in place of a camelback to spot ways. However if you flipped a prism straight edge on its back and tried to spot a part on it, it would deflect and give a false print. As a general rule of thumb you should always be scraping your Long slide first and using it as master to print its mating short slide. Excuse the dodgy exaggerated Microsoft paint drawing but imagine for the moment that that is the profile Z axis long slide on a knee mill, both sides have been printed with a camelback and a level sitting on three points has been run along its length. It is a true plane along the length of the Z axis but the ways themselves are not technically co planar. As long as the short slide (the knee) is spotted to this it will in all respects act in the same manner as if they were co planar. In saying this if there was a Horizontal spindle you should have to scrape the back of the knee to tip in the alignment otherwise you would have an error that would have chase in the Y axis
(if this had a horizontal spindle the Y axis would technically be the Z but I prefer to treat all knee mills as if they were vertical when calling about axis as it gets a bit confusing)

I personally prefer to use a spotting plate in this case and scrape the reference surface so I can map the flat ways out and scrape them as a true flat plane before tackling the dovetails. Here in aus we have a small group of guys interested in machine rebuilding/scraping and one of the guys does organizes a group buy of precision granite at least once a year. I'm currently waiting on a 200mm x 800mm spotting plate to do the Z reference surface on one of my Toolmaster mills (it was far cheaper than you'd expect), the ways are coplanar within about 0.0002" top and bottom with a central worn area mainly from coolant engress but the reference surface is giving me an error of about 2 thou in one corner(so don't always trust a factory surface). I and you for that matter could scrape a surface like this without a spotting plate but its a lot of additional work to constantly check for twist or convexity. in reality you shouldn't need to but everything adds up if you're looking for a truly accurate machine. its an australian group but if you're interested we have a Private Facebook chat where you can ask for advice on the fly, there's also at least two Polish australian scrapers there could probably give you a hand next time they head home.

Liquid Sun said:

Hi all,
Going by what's written in the Connelly book, I should get a surface plate that covers the length of the longest ways (which is about a 120 by 80 cm surface plate in a standard ISO size), plus a straight edge with a 45 degree angle on it that's about 120 cm.

Click to expand...

Not a bad choice, with an accurate Plate in this demension you can check an scrape all your straight edges you need.

Liquid Sun said:

But, unlike cast iron surface plates, granite ones don't come with handles and aren't ribbed for additional stiffness. And I still want to use the surface plate for reverse spotting the ways on the machine casting. If it was just for measuring, I'd get a much smaller one.
This bugged me, so I emailed the local manufacturer of granite plates here, and he replied that they don't drill holes in their products (e.g. to put in threaded inserts and screw on a pair of handles), and that their plates will bend if unsupported correctly on a three-point stand. To be clear, the size I'm after would be 120 mm thick diabase from what he's selling.

Click to expand...

There you have the wrong manufacter / dealer, ist is no Problem to get Plates with threaded inserts on the sides.
It is also no problem to set an threaded insert in an existing (used) plate.
Im am wondering about 1200mmx800mm and only 120mm thick normaly this size of plates are 160 to 200mm thick. With 160mm thickness the plate has 460kg and that is the reason why i woud not use a plate this size for bluing the column on a MH600. You woud need a real good overhead cran with a real sensitiv control and when bluing, you have to move the 460kg

On Bottom line i woud youse a long enough straigt ede and a precision level (or a Kingway Tool) for the long box Ways.

For the topside of the Dovetail on the column a smal Plate wich you can handle without a overhead cran woud be perfect, but i think with a limited Budget it is better to by one big enough Plate and more (used) straight edges (500mm, 750mm, 1200mm)

Liquid Sun said:

One of the chief reasons I'm leaning towards working on this myself is that most rebuilders I talked to will actually tear down the machine and replace damaged parts, but they still have the ways and sliding members ground by an outside company before rebuilding.

Click to expand...

This has nothing to do with real rebuilding.

If you want to learn scraping in an easy way come and join our KingWay Scraping Class in October in Austria

Flyer EN http://www.maschratur.at/wp-content/uploads/2020/04/BIAX-Schaberschulung-2020-EN.pdf
Flyer DE http://www.maschratur.at/wp-content/uploads/2020/04/BIAX-Schaberschulung-2020-DE.pdf
Video EN KING-WAY Scraping Class - YouTube
Video DE KING-WAY Schaberschulung - YouTube

greatings Franz
from the Lüftinger Brothers
www.maschratur.at

Hey guys, thanks a lot for the informative replies, it's greatly appreciated! Sorry for the late response, but I'm going on vacation on Sunday and getting as much work done as I can before that.

So what type of rigging equipment do you use for handling large machine parts and scraping tools? I just strained my back moving a small (25 kg) vice a bit too carelessly from one table to another and when I think about handling 120 cm cast-iron straight edges it seems kind of crazy to do it by hand. Not to mention just the table on the MH600E is 200 kg, the vertical milling head seems just about over 80 kg, etc. I was thinking that I should maybe build a steel frame around the machine with a winch attachment. I'm working from a garage and only have about 0,6 m of headroom over the machine.

I don't know, however, if a winch can be precise enough to use for spotting. It does look like it in Foundation of Mechanical Accuracy, there are several photos of scraping hands handling masters held by chains, but they don't go into any detail of how they accomplish the fine positioning of these heavy pieces when actually spotting (their surface plates were 1,2 tons, and the picture usually just show one guy working).

From what both of you said, getting a large granite plate is a good investment, though maybe not from that particular source. The plate thickness is a bit small, but it's legit regarding the ISO standard they use. Normally, surface plates here are listed with DIN accuracy grades, this particular company works with ISO. I checked the norm online and their dimensions match up. Maybe I could get a Garant, Mytri or Planolith surface plate, but they are far more expensive and I'm not so sure they're that much better.

This Polish company is a small business that actually manufactures their products and sells them directly which keeps the price low. It's also understandable that they have little to no idea about machine tool rebuilding, they probably started with tombstones or something like that. OTOH, they attach a certificate of inspection with the flatness map of the surface, which is reassuring, and also service their product and lap used plates. If I got a German surface plate, I'd be getting it from a reseller and it would be a pain to have it resurfaced or inspected bu the factory later on. I'd be dealing with sales reps and not the manufacturer (my frame of reference is how the US YouTube machinists are able to interact with service techs from Stanridge in their videos).

The scraping class in October seems like a good idea, we've been to Austria (Vienna) for a few days in 2018 and it was a really fun place.

The good news is that I learned this week I can actually make my injection molds on the Maho even without rebuilding it. When working with the vertcial milling head, I couldn't ever get the work area truly perpendicular to the spindle (there was an overall error of 0,05 mm after facing a large enough plate), and I had no tooling for the horizontal spindle. But I finally got some scrap cast iron for an angle plate to check the horizontal spindle and it sweeps to within 0,01 mm after a facing operation. So the vertical head is out of tram, while the horizontal spindle is able to take out the vertical axis wear error just fine. I'm not worried about locating H7 holes with a closed loop servo system. I also have a spindle speeder that will fit in the horizontal spindle. So I will make the molds first and rebuild the machine later. This is super important to me because it will take away most of the stress associated with the rebuild. I will be able to take my time and do a good job while the parts I'm currently selling are going to come to me ready from the molding shop.

I still need a surface plate first (for inspecting parts), and I want to get a big one to do the rebuild later. Also need some temperature control for fine measurements.

I would also highly recommend attending a Richard king classes while he is still teaching, as to a surface plate it's not a bad idea to go larger than what you currently need if you can afford it/have the room. I bought a 2000mm x 1000mm plate a few years back which was excessively large to my needs at the time but now its been almost perfect for all my projects/all the projects of the local guys in the club and had I gone a 1200mm x 900mm at the time I would have had to upgrade anyway.