Best angle grinder disc for grinding steel from slightly bumpy to flat

[hotponyshoes]

Big flap disc is normally good enough for an anvil top.
7" is a good balance between being easy enough to handle but big enough to cover the face.

If you can reach both edges of the face with 1/3 of the disc it's pretty easy to get a good finish.
Go from one end to the other once. Then turn around and go the other way and the grind marks will show any flat spots.
I go down to 120g with a 7" flap disc.
From that I use a belt (floor) sander if I want a polished finish or you can draw file with a farriers rasp easily/quickly enough then polish (if you want) with any type of sander at 240g.

120g is perfectly useable. You could just go down to that then scotchbrite (or one of the 100 other products) over that to make it look shiny.

 

[not done it yet]

The proper job, in my opinion, is doing it on a shaper. Flat, straight and level. But don’t secure it like the muppet on utoob that tied it down with ratchet straps!

 

[Mark 2]

He might also have a belt sander but just not thought of using it - don’t see a problem in recommending other alternatives

Absolutely, I don't get why the title is a loaded question.

 

[BrokenBiker]

if your only tool is a grinder, firstly, does the size of your grinder allow you to contact the rail over its full width in 1 pass? i.e if your rail is 4" wide, you will need atleast a 9" grinder to do it in 1 pass. if you have a smaller grinder then multiple passes in alternating directions would work just means more work and having to pay bit more attention to the grinding marks to make sure you arent creating a divot in the centre.

if you can do it in 1 pass i would make a jig that sat either side of the rail, to allow to grinder to be run across consistently. this does however rely on a few things like having a very flat surface to work on, the base of the rail being parallel to the top face, the jig being accurate etc...

if you could meet that criteria you can make the jig to a sniff under your finshed dimension for the rail and then space it up with thin stock or even playing cards, take a pass, pull a card from each corner, take a pass etc until you are content with the flatness and the finished dimension. that extruded ally frame stuff is pretty good for these tasks, theres a guy on youtube made a lathe and used this method to grind the bed ways in with pretty good results.

that said, if you have got a very flat surface to work from such as a surface plate or even a sheet of glass i would just stick some sand paper down and figure 8 it until it was good enough. marking blue will help show the low spots.

personally for 0.5mm id use a bastard cut file to get it down to 0.1ish then a second cut todraw file down to finish size and polish with a wheel and compund to remove any file marks. may take a bit of time, but the accuracy you can acheive with a file is mind blowing compared to an angle grinder that is 1 twitch away from scrapping the part

 

[ELL]

Thanks for all your replies. I got a shaper in the end.

 

[brightspark]

but its an option to get it right, how long is your track? draw filing that would not take long with "a good going file" as a guy I used to work with used to say.

90 percent of folk wouldnt know what draw filing is probably think its putting the paperwork away in the cabinet. it gives a good flat smooth surface ideal for finishing after roughing out

 

[Shedendman]

@ELL you'll be surprised how well you can remove material with a bastard cut file,not only that its good exercise,but if you want the grinder method i use 150mm velcro discs in the polisher/sander start off with 40grit and work right the way down to 1200 grit that should polish it up well
Thing is its not so much the high spots its the lows,there nothing so bloody annoying that after getting it shiny smooth you find a low or little hole/scratch then its start all over again