I have to drill the 4 holes in a growser pad from 18mm to 20mm, not the optimum situation but have no choice as little to no options available for this machine after dealing with 4 undercarriage sales men, 3 of which had no chains available for this machine at all, so chains and drill pads or scrap machine, did I mention 82 pads.
No good local machining job shop available? I have shop nearby specializing in custom jobs like that which could turn them out in no time after setup.
Yeah, CNC mill would be the felines behind.... Lacking that: Mag drill and annular bits, drill from the backside. Make two jig plates, each with two 20mm ID drill bushings for two holes, one with 18mm holes for the other two spots, the second with 20mm holes, so you can bolt the jig plates securely, have half a chance of getting the bolt pattern to stay right. You could try it with just a good quality drill bit, might wander or knock the corners off though. Just a thought.
If your just opening the holes up 2MM id go with a good bridge reamer.
What are the tracks for ? Just curious
A masonry drill bit will sometimes drill through metal other bits wont faze. Ive used them to drill holes in leaf springs.
I dropped a pad off at just such a machine shop yesterday, he does a great job, I think he is looking at using the bridge port to machine them, he happens to be super busy right now, and I am probably going to be involved somehow just on a speed up production basis. The pads are from a Fiat Allis FD 14C Dozer.
Just opening up (leaving on location) ? I would put them on my small radial drill (lots of cutting oil and nice slow speed, as well as power feed) That's if they are drill-able..... Having to use carbide would probably preclude the use of an endmill (and maybe why the o.p.'s shop is going with a bridgeport) Moving the location would need a drill jig as stated above by Lantraxco. BTW 82 pads and 4 holes each is not really that bad of a job, figure 10 minutes each pad (hitting all 4 holes) floor-to-floor time, thats 820 minutes/60 minutes = 13.7 hours Hardest part is the handling.
CNC plasma tables are pretty common these days. I would think it wouldn't be too big a deal to make a jig to hold the pad on a table. Don't know what kind of tolerance they could hold for the hole diameter, but moving location of the holes would be no problem and the hardness of the pad would be a non issue.
I don't think the pattern is changing, just bigger bolts? I wouldn't really consider plasma, you're talking about removing forty thousandths of metal all the way around the hole, plasma kerf is a lot more than that usually.
What he said.... And plasma aint straight (top to bottom of a hole), the pad surface might not be flat, the plasma torch head may not fit in the area, Heat affected zone might cause cracking problems. I could maybe attempt oxy/propane on my little C.N.C. machine, with water up to top of plate surface, to keep the heat affected zone down. But with the pre-heat times for each hole, drilling would be faster.