Question about tramming a bridgeport

[pontiac428]

My vise is indicated in (at least well enough). My question is when and how hard to tap the stock down to seat it on parallels or the vice bottom. I was tapping it before fully tightening and I think it was bouncing back up

Tap the part down once while the vise is half snug, then tighten. If your parallels sit tight, you are good, if one or both parallels move, the part isn't seated.

 

[D.sebens]

Tap the part down once while the vise is half snug, then tighten. If your parallels sit tight, you are good, if one or both parallels move, the part isn't seated.

Okay. Now if the bottom hasn’t been machined yet do I just tap down and machine the surface even if one isn’t tight?

 

[pontiac428]

Yes, if the bottom hasn't been machined, your first reference will be the plane of your first cut. You establish square from that point forward. The only reason for splitting hairs on the first step is if you have less stock than you really need for the job and need to keep the dimensions as fat as you can. Otherwise, it's a blank slate until you make that first cut, then square everything to it using your fixed jaw, the mill tool's cutting plane, and when the part touching the parallels is faced, then you need to set the part tight to them.

 

[D.sebens]

Yes, if the bottom hasn't been machined, your first reference will be the plane of your first cut. You establish square from that point forward. The only reason for splitting hairs on the first step is if you have less stock than you really need for the job and need to keep the dimensions as fat as you can. Otherwise, it's a blank slate until you make that first cut, then square everything to it using your fixed jaw, the mill tool's cutting plane, and when the part touching the parallels is faced, then you need to set the part tight to them.

Thank you. That makes sense

 

[Gaffer]

My vise is indicated in (at least well enough). My question is when and how hard to tap the stock down to seat it on parallels or the vice bottom. I was tapping it before fully tightening and I think it was bouncing back up

Yes. Bounce-back is a thing. You want a dead blow hammer, and with some practice, you'll get a feel for how and where to tap. It's easier than you'd think to make things worse when seating a part in the vice. You want the part snug in the vice before whacking the part.

 

[Gaffer]

I like Joe Pie's method for squaring stock.

 

[pontiac428]

The best way to learn about how vise/fixture seating affects parts is to push the flashing green button in a production shop for your (minimum) 40 hour week. You'll see what happens when you are too fast, too tight, too loose, not seated, seated then re-tightened, seated on a fine chip, on and on. Measure all the parts that come out and you'll see where those annoying little .005 tolerances go. The only way to get parts to the level the machine is capable of is to carefully clean and seat them before the op. Deburr between fixtures. Cleanliness being like godliness and all that. If your parallels are loose after the part is tight, it's not right.

 

[Parlo]

Berfore the Kurt style vices we used to use " hold downs ". They were like parallels with a thin edge. They were laid flat along both jaws with the part inbetween them touching the thin edge. The thicker edge against the jaws had a slight taper which drove the thin edges down when tightened. Most times the part did not need tapping down.

Don't be too concerned if the part does not sit on both parallels it may only be a couple of tenths off of the loose parallel.

My preferred method for squaring up is using vee blocks if applicable. They hold two faces vertical at once so a third face can be machined square to both at once.