good basic tutorial on machining

All of the older units that I know of have the arc gap HF generator. The newer ones are solid state. Miller has their "Blue Lightening" module. I think they came out with it in 2008. You have to adjust the arc gap over time where there is nothing to do with the solid state units. Mine has arc gap HF start and a water cooled torch. I also have the foot pedal that controls the power and does the start. This lets you continuously control the heat as you are putting down the bead. It's great for aluminum. You can set the heat high, floor the pedal to get a puddle started and then back off. My pedal is wired. Miller has a wireless pedal and I suspect Lincoln does too.

Miller has a touch start. It seems they run a very small current in the electrode and detect when the tip touches the work piece. You touch and lift and you get the arc. It is supposed to be a cleaner start for high precision (x-rayed) welds. I still like the HF start.

Hi, John:-

I must be missing something- I see the A-A section line with arrows just above and to the right of the 0.875 dimension.

Best regards, Spehro Pefhany

Not sure I am parsing that sentence correctly, but that article was aimed at a fairly rudimentary level of design expertise. GD&T, although very useful for some kinds of parts, might be a bit much to take in when you have not learned not to just spec every part to

Yup, which is why the automotive guys use it. I think it works the other way too.. particularly things like perpendicularity and parallism which are often not acceptable if as bad as the 90°+/-1° typically shown on a title block.

I've used _Fundamentals of Geometric Dimensioning and Tolerancing_ by Krulikowski (my edition is based on Y14.5M-1994, and I think there's a later version of the standard). Pretty straightforward, if tedious, slogging.

Yes. My limited experience (anecdotal, weasel, weasel) is that putting GD&T symbols on a drawing may well increase the quoted price of a one-off machined part. Maybe the shops have had bad experiences with fussy customers correlated with GD&T use.. dunno why.

Best regards, Spehro Pefhany

Luckily, n/c machining is so deadly accurate that complex tolerencing is seldom necessary. The stuff that we buy from sheet metal shops is phenomenal; they even get the bends dead-on. Everything fits. And n/c milling is accurate to a thou.

Our fab path is generally dxf directly to n/c programming, with no human interpretation in the process, so the tolerances are determined by the quality of the n/c machine. That's plenty good enough for electronics packaging.

Some of these GDT drawings look like a history of the Egyptian kings.

Perhaps, if you can afford milling. ALmost everything we do is punched or extruded (or both). Fortunately, I just give the ME's the big picture and they make it work. ;-)

;-)

Yeah, we mostly do sheet metal, but we do machine extruded enclosures, and machine things like VME front panels and end plates and heat sinks. Nowadays with CAD and n/c machining (which includes punching and bending) practically everything works first time.

If we were making optics or jet engines or something, things would be trickier.

I think the big advance, here, is in 3-D modeling, rather than in NC machining. NC makes it cheaper, I guess.

I think 3-D modeling is even more important in jet engines. Optical, maybe not. There still is a lot of plumbing and tin-knocking in RF, even. ;-)

Sorry, I let a bit of sarcasm leak out without a smiley.

GD&T is usually worse than useless to the machinist; it makes the drawing harder to read with little if any benefit. For example a function based circular tolerance zone needs interpretation to use with rectilinear machine axis coordinates; the machinist needs to calculate what size square tolerance box fits inside the specified tolerance circle, as opposed to seeing rectilinear tolerances which map directly to the machine coordinates.

On the other hand GD&T can be extremely valuable to an inspection department which does not want to turn perfectly serviceable parts into scrap because rectilinear tolerances are unable to accurately reflect functional requirements. This becomes more worth the trouble as tolerances get tighter and the value of the total production run goes up. It makes sense for a one-off only if it is a very expensive part with tight tolerances. In my direct but limited experience less than half of all mechanical engineers who use GD&T really have a solid understanding of how and when to use it to minimize product cost, and the small machine shop is likely to have seen some misapplication.

I would classify any use of GD&T for design of parts which do not need the to be inspected by an inspection department familiar with GD&T as likely misapplication.

These days, when I hand sketch parts in preparation for walking the drawing out to my shop for me to fabricate, I never use GD&T :-).

Regards, Glen