Machining- Drilling with high positional tolerance.

[ryandias]

I have a heat sink that i want to insert two cartridge type heaters. Unfortunately the heat sink profiles i have found that fit my assembly do not leave much surrounding material if i drill it out for the cartridge heaters.

I have to drill a 1/8" hole about 1.5" to 2" deep. The surrounding thickness will be about 3/64". My question is, How much should i expect the drill bit to wander?

Is there a rule of thumb that says X diameters deep is the maximum depth with accuracy? My supervisor suggested 3 diameters but that seems very minute.

 

[gfbotha]

Depends on quite a few things, amongst others the material and process used.

Let's assume it is aluminium, your machine is in good condition and set up properly, you don't feed too fast, you first drill a smaller pilot hole, you might then use an end mill to square the hole up for say 3 diameters deep, and then drill the final hole. This done, your center line might be off 0.1-0.2mm which is adequate!

But, it's not to say it's aluminium and may be you you want to use a simpler process...

 

[ryandias]

the material is aluminum. I was weary of trying to drill a pilot hole since the finaly diameter is only 1/8".

 

[TheTick]

Try EDM instead of drilling.

 

[ryandias]

well i'm really not looking for a process that will work. I'm trying to learn what rules to design with so that i don't exceed the capability of the machine shop.

 

[gfbotha]

EDM is slow.

Regarding a 2.4mm pilot hole: in my opinion it does not have to go all the way..., just slightly deeper than the guide distance the end mill has to go.

 

[limct]

The hole you intend to drill is a deep hole (when drilling depth is more than 5 fives of it's drill diameter).
There are a few factors which influence the quality of work.
a) the relative movement of work and drill, for deep drilling, it's easier to produce concentric holes when the work is revolving, with the drill held stationary.
b) starting the hole, use a center drill prior to the deep drilling.
c)quality of the work material. usually material like wrough iron which is not homogeneous and cast iron which has pores in it, will tend to be problematic for deep drilling, drill bits are often devaited when it breaks into porous territory.

Jigging and drill bushing can be considered too.

Good luck to you.

Best regards,
ct

 

[KENAT]

Do you have your own machine shop or ones you regularly use. Why not mark up a print with the maximum deviation you think you can accept and see if they say they can do it for reasonable time & cost?

At the end of the day you probably don't care what process they use just that they can meet the required value for acceptable time & cost.

 

[Heckler]

Some of the factor that affect the drilling process: Drill bit condition, material, feed rate, spindle speed, coolant usage, fixturing material & expected hole tolerances. Now go talk to your shop foreman or forewoman about their capabilities. Then you will have a better understanding.

One other suggestion would be to take some machining classes so you can better understand those processes.

Best Regards,

Heckler
Sr. Mechanical Engineer
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Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[dimjim]

Why don't you simply drill some pieces of aluminum
thru different depths and draw your own conclusions.
You might want to check out tap drilling for threads
and use their general guideline. These are for
steel parts but you will get the drift.
Accurately means something different to all of us.
What precision are you looking for?
They make pull drills too which are putting the
drill in tension as it is pulled thru the material.
You seem to have a lot of options suggested.
If you want blind holes, ie, holes that do not
go all the way thru, your options are different.

 

[dimjim]

I used to put ridiculous true positon tolerances
on my drawings and the shop was very quick to tell
me what they could achieve. They should know your
specific equipment and they are probably the best
guide.

 

[gfbotha]

Ryandias, after all I think your question was a good one to share. I made it a personal rule to consult the shop whenever in doubt, but depending on the shop person's attitude & experience you might sometimes get too negative or too positive an answer.

I like to support the real important point (a) and (b) for that matter, as listed by Limct. If your workpiece has a difficult/non-concentric shape one might consider holding it in a special piece with a round perimeter.

Let us hear of the results if you indeed proceeded with the design as proposed.

Regards