Adding A Column Riser To A Solid Column Mini-mill: Lots Of Questions

TIGL

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Pretty much what it says in the subject: I keep running into problems with a lack of of Z-axis height on my SX2 and I thought this would be a good thing to try after seeing that they're fairly common on b-port machines. I found this thread on another forum and am interested in giving it a shot. The two things I'm wondering most about is what material to use and how to finish the top and bottom faces for maximum parallelism. My current plan is to get a 2x3x12 inch piece of gray cast iron from mcmaster and chop in in half and make a pair of 2x3x6 spacers. I can use one or both to get 2 or 4 inches of rise respectively. Is cast iron a good choice for this? Would something like cold rolled steel be better? If the cast iron block is not flat enough to begin with, could I face the top and bottom with a fly cutter and get sufficient accuracy? Also how tough will it be to drill 4x holes for M8 bolts in CI? I'm worked almost entirely in aluminum up to this point. Would this mod completely kill what little rigidity this machine has already? Should I be worried about stripping out the threads in the base casting if I go a little overboard on the tightening?

Thanks!



Relevant picture from the linked thread:

33e43394f97651766b78bff844994343.jpg

33e43394f97651766b78bff844994343.jpg
 
Cast would be a better choice than CRS. Much more stable.
I would decide how much lift you want and do it once. Changing the spacers would be a continual pain in some portion of your anatomy.
As far as stripping the threads in the base, have a look at the recommended torque for the threads in the base material and use a good torque wrench.
 
Grey cast is fairly easy to machine. There are multiple warnings about cast iron chips though. Most machinists protect the surfaces from the machining swarf as it tends to chew up any sliding surfaces.
 
Cover the ways and table with heavy paper or use a strong shopvac. With any casting you need to take at least enough material off to get a clean flat surface. Use a carbide cutter to mill CI. It tends to crumble instead of cutting chips. Sometimes a edge can chip off if the cut is to heavy. Don't use any cutting fluids.
 
Cast would be a better choice than CRS. Much more stable.
I would decide how much lift you want and do it once. Changing the spacers would be a continual pain in some portion of your anatomy.
As far as stripping the threads in the base, have a look at the recommended torque for the threads in the base material and use a good torque wrench.

I can hear my Dad say torque them like head gasket bolts. Start by snugging the front left bolt then the right rear one and so on.
 
Machinable cast iron would be an excellent choice yet messy, mill the non-mating surfaces to size first as these features are simply clearance for the rest of the machine components. Mill the mating surfaces leaving .010-.020" of material on each side for finishing, drill through in 2 places where no other holes will be located then counter bore each face for socket head cap screws.

Clamp a piece of material onto the table leaving enough room to fly cut a spot large enough to hold the part without hitting the clamps. Fly cut this plate in place then drill and tap it for the size holes that you put in the part. Bolt the part down and skim one side, flip it over and skim the other side to dimension, do not remove the fixture between steps. This is about a accurate as you may achieve on a mill much like grinding a magnetic chuck face on a surface grinder in place. You have created a reasonably flat parallel surface relative to both the X and Y axes. Also the hold down screws do need to be very large as this is a finishing operation so the cuts should be very light (-:
Use clamps on 4 sides of the fixture if possible as removing material from one side will often cause CRS and some aluminum flat products to move a good deal.
Like so millinplace_zpsvorrpqen.jpg

millinplace_zpsvorrpqen.jpg
 
Cast would be a better choice than CRS. Much more stable.
I would decide how much lift you want and do it once. Changing the spacers would be a continual pain in some portion of your anatomy.
As far as stripping the threads in the base, have a look at the recommended torque for the threads in the base material and use a good torque wrench.

I'm thinking I'll put both in and leave it like that unless the hit to rigidity is too serious. I'm sure I can borrow a torque wrench from the maintenance shop

Grey cast is fairly easy to machine. There are multiple warnings about cast iron chips though. Most machinists protect the surfaces from the machining swarf as it tends to chew up any sliding surfaces.
Cover the ways and table with heavy paper or use a strong shopvac. With any casting you need to take at least enough material off to get a clean flat surface. Use a carbide cutter to mill CI. It tends to crumble instead of cutting chips. Sometimes a edge can chip off if the cut is to heavy. Don't use any cutting fluids.
They ways have rubber accordion covers, would not be enough? I plan on using the shop vac as well if needed. I'm not sure if the bar stock I'm getting is a raw casting or if its been saw cut to size already. I would guess that its been cut to size already but I'm not sure. If it's the latter it will already have a decent surface finish but I probably would do better to try and improve it. I'll be ordering a left handed turning tool with carbide inserts to put into my fly cutter so that I can use that for surfacing. that way I can crank up the spindle speed and not have to hand crank my table for 2 hours to get a good finish.

I can hear my Dad say torque them like head gasket bolts. Start by snugging the front left bolt then the right rear one and so on.
Thats the plan

Machinable cast iron would be an excellent choice yet messy, mill the non-mating surfaces to size first as these features are simply clearance for the rest of the machine components. Mill the mating surfaces leaving .010-.020" of material on each side for finishing, drill through in 2 places where no other holes will be located then counter bore each face for socket head cap screws.


Clamp a piece of material onto the table leaving enough room to fly cut a spot large enough to hold the part without hitting the clamps. Fly cut this plate in place then drill and tap it for the size holes that you put in the part. Bolt the part down and skim one side, flip it over and skim the other side to dimension, do not remove the fixture between steps. This is about a accurate as you may achieve on a mill much like grinding a magnetic chuck face on a surface grinder in place. You have created a reasonably flat parallel surface relative to both the X and Y axes. Also the hold down screws do need to be very large as this is a finishing operation so the cuts should be very light (-:
Use clamps on 4 sides of the fixture if possible as removing material from one side will often cause CRS and some aluminum flat products to move a good deal.

That a very good idea for maximum accuracy but I'm not sure if my mill table is big enough for that big of a production. I think I will try to to face the blocks either in the vice or bolted to the table normally first and if I can't hold the modest tolerances I'm usually going for I will go back and address it using your method on the bridgeport they have at the local makerspace.
 
Grey cast iron is very easy to machine.

Look up specs and use a torque wrench as stated above. As far as I know, once you get close to the yield point of bolts they actually loosen up.
 
Got an email back from McMaster and apparently it's just a raw casting. Hopefully getting under the hardened skin of the CI won't be too horrible.

Grey cast iron is very easy to machine.

Look up specs and use a torque wrench as stated above. As far as I know, once you get close to the yield point of bolts they actually loosen up.

Looks like the max torque for grade 12.9 M8 bolts is 27 ft-lbs so I'll probably shoot for 25. BTW i'm talking about the threads in the existing mill base which I assume is also cast iron. Thanks for everyone's help so far, any additional input anyone has would be welcome since I'm so new to this material
 
That a very good idea for maximum accuracy but I'm not sure if my mill table is big enough for that big of a production. I think I will try to to face the blocks either in the vice or bolted to the table normally first and if I can't hold the modest tolerances I'm usually going for I will go back and address it using your method on the bridgeport they have at the local makerspace.

One may also clamp a flat piece to the table by drilling, counterboring and clamping it down with socket head cap screws and T-slot nuts, then skim the surface, drill and tap and then screw your part down. It does not need to be any bigger then the part itself, if you have the Z clearance to do it in a vice then there is certainly room to do it screwed to a sub 1" high fixture on the table.
This sort of simple flat fixture may be reused many times, simply bolt it down to the table and skim it again, drill and tap new holes for the new part and have at it.
Good Luck with your project.

If one is overly concerned about accurately "tensioning" threaded fasteners a torque wrench is about the least accurate method (aside from a large handheld non-torque wrench) yet the only method available for general work.
For those hobbyists suffering from crippling OCD about accuracy , Rotabolt as well as several other manufacturers such as Maxbolt™ make exactly what you seek. However you will not be at all happy with the cost.
http://www.rotabolt.co.uk/how-it-works/
Maxbolt https://www.vfbolts.com/product/maxbolt-load-indicating-fastener-system/
 
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