Small Holes.. really small holes 4

[Mb17]

Hi,

I need some advice on how to enlarge some current holes in a 41340 steel part.

The part is a plate with a series of small hole in, varying from 0.030" through to 0.060". The person who designed it, who is no longer working for us, had the machinist drill the holes using a regular drill bit. But unfortunately they are the wrong size holes and we believe some of the holes aren't concentric.

We'd like to open up these holes by on average 0.015", as the size is very important for its application.

Could anyone suggest a method of opening these holes up?

I have had limited experience with EDM, but do not know if it is capable of opening up an existing hole to this particular accuracy. We'd like the holes to within 0.0005" of their designed spec. if possible? IS EDM suitable? Are there any other more efficient methods?

Cheers,

Michael

 

[unclesyd]

Your hole size is large when it come to small sizes. I’ve been trying to get my company to fabricate or make tooling for microdrilling and hole sizing to no avail so I’ll have to give you the next best.
There are several ways to approach you problem, redrilling, redrilling and broaching, broaching and reaming, reaming. and as you mention micro EDM plus broaching.
You should give these people a call as they are well suited to give you direction and tools for several approaches.

http://www.najet.com/

 

[ajack1]

You would have no problem putting these right with EDM provided the holes are not oversize, unlike broaching or drilling it does not matter where the original hole is as it will not “wander”

Limits of 0.0002” are easily achievable and this can be increased if desired, however it is not the cheapest way of manufacturing, only you can decide if the cost meets the needs.

 

[diamondjim]

With that kind of accuracy on hole size,
what kind of true position is required?

 

[Mb17]

diamondjim,

The holes diameters need to be ± 0.0005", and the true position should be to the same precision. What do you suggest?

Michael

PS it is 4140 steel, not the fictional 41340....

 

[diamondjim]

With that kind of accuracy,
I would recommend the three
holes in one plate be to that
size and have the matching
plate have only one hole
with that accuracy and the
other two be slightly oversize
by the amount of true position
accuracy that you can maintain.

 

[vzeos]

Mb17,
Check out the attached link. This EDM contractor could probably help.

http://www.echoedm.com/wire_edm.htm?x=116&y=16

 

[unclesyd]

Your hole size tolerance is achievable but with added cost as not every shop can generate this tolerance. The location tolerance is another matter, this work would have to fall under a shop that real close control of the conditioned air. Achieving these tolerances are for new holes, reworking an existing hole is another matter.

Just remember that all these dimensions will change with changes in operating temperatures. I would recommend that you take a real good look and see if you could loosen your tolerance. The little zeros will very quickly get you in trouble with your pocket book.

Personally I would select, if possible, a more stable and predictable alloy that 4140 for your hole and positional tolerance.

 

[diamondjim]

Another thought would be to align one
hole with a pin and taper ream the
other two holes for two taper pins.
The trouble with this is that you cannot
replace one of the plates as they would
always be a matched assembly.
How thick is the 4140. We drill it all
of the time and it is 250 BHN min.

 

[btrueblood]

Agree with unclesyd, call national jet. They are experts at small holes. Have them send you their "business card" (10+ years ago it held a piece of human hair with a hole drilled thru it and a piece of wire threaded thru the hole. You had to view it under a 'scope to verify.