07 November 2009
Hi Big Daddie,
You can't be a rookie if you can build rifles ! First see my reply to Mogwar (bullet mold diameter) which covers some of the ground you mention. Nominal groove dia of the 7 x 57 is 284. Most die makers make standard and one thou over, ie 284 and 285 for the 7mm. That's why you can't get 286 off the shelf. Some rifles shoot as well or better with a groove dia bullet, so before you do anything else test some sized 285 and see what results you get.
If no good, then consider a 286 die. KenK is tight, they are not difficult to polish out if you don't want to pay for a custom die. But you can't use the punch as a mandrel. It is too tight and will not run into the die with abrasive paper wrapped round it.
I have polished a few dies to bigger dia and made a few from scratch. The grade of abrasive you need depends on how much metal you want to remove. This is a whole topic of its own but briefly, 320 or 400 grit will leave too rough a surface and will need finishing with finer paper up to at least 1000 and preferably 1500 grit. That's OK if you need to take off two or three thousandths, because polishing that much with 1000 grit will take forever, so you use coarser grit for faster stock removal in the early stages.
But you need to polish only one thousandth. That's half a thou all round. Your big risk is polishing too much. Start with 1000 and finish with 1500.
But before you start polishing you need a means of measuring the bore. Best to make a plug gauge with two diameters, 2855 and 286. The 2855 is to tell you when you are getting close.
Clamp your electric drill in the vise. Cut a three inch length of quarter inch wood dowel and chuck in the drill. Cut a one inch wide strip of 1000 paper, attach to the dowel with masking tape, turn the paper round the dowel and cut of bit by bit until it is an easy push fit into the dies bore. Or to be more precise, till the die can be pushed on to the mandrel.
Keeping a firm grip on the die, start the drill and work the die back and forth along the rotating mandrel. This takes some judgement because of the risk of getting taper or ovality or both. You can only judge that by feel and by ensuring that no part of the die gets heavier treatment than the rest.
Don't do this longer than one minute at a time. After each rotational treatment, switch off the drill and run the die back and forth on the mandrel longitudinally without rotation. That is to cut in the opposite direction. It also provides a good feel as to even cutting. That's why the mandrel must be a tightish fit, then a loose spot in the die will be felt. After each rotation plus straight polish clean out the die and test the dia. Continue until the plug gauge feels as if it will enter but not quite. Then finish with a single application of the 1500 grit.
It would be ideal if the bottom punch could be made first and serve as the plug gauge, but I found that it is near impossible to polish the die so close as to achieve that. I found it much easier to make the bottom punch afterwards. That's because it is much easier to make a male fit a previously made female than the other way round.
Like I said, I have enlarged factory dies and made new dies this way but you have to be very careful because of the accuracy required. But absolute precision is not necessary. Whether the resulting die is exactly 286 or 2858 or 2862 will make no practical difference.