Accuracy of Transfer punches & drill bits for Jig/Fixture fabrication

[vonsteimel]

I've got a small question concerning the fabrication of Jig/Fixtures. When I was in school our teacher taught us that the machinist would use transfer punches a lot to transfer holes from one part to the next. For instance, a Drill Jig which requires several steel base/plates to assembled together, located by dowels & secured with SHCS's -- They would make the base-plate and then use transfer punches to locate the holes on the mating plate. Then go back & drill & ream the holes, and this is how the build them; that not every single hole was laid out and drilled.... Recently I got in a debate about the accuracy of this method due to the concentricity issues of standard drill-bits. That the drill-bit would not create a concentric hole with the transfer-punch mark (which is true to an extent) that would have a big effect on accuracy, and that the corret way to do it was to "clamp" the parts together and use a longer drill-bit to drill through them simotaneuously. Now this would probably work given that the parts were "clamped" together in the correct locations but it seems a little unconventional. So what do you all think?

 

[MikeHalloran]

Match drilling is fairly common. It's sort of in the middle of the degree of difficulty in making a part or a jig.

I've worked in shops where a jig borer was necessary, and I've worked in shops where a tape measure, a framing square, and a dull Sharpie were enough.

It all depends on what you're trying to do.


Mike Halloran
Pembroke Pines, FL, USA

 

[mfgenggear]

von

As Mike said before it depends on the application & precision required.
if you match drill & ream it will be precise if done correctly.

however it can not always be matched drill so then transfer with punches is an option but there can be slight mis alignment
due to slight errors.generally it is very close. if it is a one or two job.But it does work, some mechanics can make it perfect.

If I remember right it's been a while, from a master pattern a drill jig was manufactured to coordinate with other fixtures
if the hole have to match exactly, there was the master drill fixture for alignment to each of the other fixtures or parts.

A master pattern was coordinating to a drill jig, then it was coordinated to the other tools.

for most applications it done as noted below.
The Jg Bore, CNC Mill (CNC Mills can now almost replace the Jig Bore) or CNC jig grind that can machine holes to almost exact size & true position

Mfgenggear

 

[Tmoose]

With good workmanship transfer punches would likely be good enough to locate holes to be tapped since the through hole would be a clearance hole oversized 1/32 > 1/16 inch anyhow.

The drilling and reaming for holes to receive snug fitting dowels need to be located and sized accurately. Within 0.001 inch or so, maybe even less. Typically needs to be done with the pieces assembled, or directly and carefully with a fixture fitted with drill bushings, or with CNC equipment and rigid tooling.

More than 2 locating devices or features (or as many as are needed for 6 degree of freedom location) are redundant, and maybe even problematic for manufacturing and assembly.

This Corvette transmission interfaces with the engine block and is located by a couple of snug fitting dowels.

http://allworldautomotive.com/images/userphotos/19204_28266.jpg

The parts are undoubtably made independently, probably using CNC machinery. Assembly is probably a little easier because there is some flexibility of the free part.