PDA

View Full Version : magnetic table for milling



.RC.
06-20-2010, 04:22 AM
Anyone use a magnetic table on the milling machine for taking very light milling cuts to make two planes parallel? Something that is near impossible to do in a vice unless the sides are perfectly 90 degrees to one another...

macona
06-20-2010, 04:50 AM
Yeah, you can do it.

Also look at other methods like fixturing adhesives.

squirrel
06-20-2010, 08:23 AM
Don't use a standard magnetic chuck, you risk extreme danger from the part launching or tool breaking. Electromagnetic chucks are specifically made for milling but they cost $6000 on up and work extremely well. Fixturing the part like macona says does work well, just get the correct adhesive and nest the part inside a ridgid pocket. Melted fixuring metal also works but I have not personally done that.

Forrest Addy
06-20-2010, 10:38 AM
I've used all kinds of mag chucks in my career. An old weak PM chuck may be a nightmare for milling but newer more powerful PM chucks hold very well.

Mag chucks work great for milling provided you take light cuts as you said. You absolutely positively have to restrain the work from sliding. Most mag chucks have stops along the edges you can raise to keep the part from drifting. To get from the edge stop to the work near the center of the chuck you need parallels of some kind, 16 gage sheet metal power sheared into strips make good stop parallels. Don't go too thick. You don't want the stops to suck up all the magnetism.

Chips are a nightmare around a mag chuck. You have to work very clean. One little crumb stuck on a corner can ruin your day.

Mcgyver
06-20-2010, 11:44 AM
like Forrest says, use the fences......and very light cuts. I've done so when working on some thin pieces needing light milling. But no have delusions; a mag chuck will pull stuff out shape only to have them return to their original curves after....the resultant surfaces are better described as concentric vs parallel :D. A vise holds advantage there

TGTool
06-20-2010, 11:45 AM
Anyone use a magnetic table on the milling machine for taking very light milling cuts to make two planes parallel? Something that is near impossible to do in a vice unless the sides are perfectly 90 degrees to one another...

Hmm, that might also lead to a discourse on the correct sequence for squaring up. That was one of the early lessons in the trade and I've seen machinists who don't know how or the liabilities of machining non-square parts.

This isn't pointed at you, by the way, Ringer. There are plenty of occasions when you legitimately have to machine non-square parts that can't be altered.

The process, IIRC, is to put the stock in the vise with the largest face up, tap it down and machine face no. 1. Then rotate the stock to position the part with this largest face against the fixed jaw of the vise. On the moving side you then put a small rod or piece of scrap so that however un-parallel the opposite side is, the no. 1 side is flat to the jaw. I found some circular segment scrap pieces at a scrap yard once that make nice equalizers for this. So you can now machine the second side square with the first. Take the time to de-burr the corners as you go since they can force the piece out of square and parallel.

Now you can machine the third side parallel with the second, keeping the large flat face against the fixed jaw. Keep your pieces of scrap ready because you may still have an irregular fourth side that may not hold your first surface flat against the fixed jaw. Also snug up the vise and tap the stock down with a soft hammer against the vise bed or parallels before final tightening. If you have two parallels under, as you might when finishing the last large flat parallel to the first, you can try to wiggle them to see that they're both held firmly against the base. If one moves, you can be sure the part is not down flat and the surface you machine won't be parallel.

Also don't forget the thin stock hold downs that apply down pressure to the workpiece when the vise is tightened or the commercial eccentric and toe clamps designed to do that when held to the table.