Self-stopping drill depth?

[Jabberwocky]

Hey all, I have a question which seems to be best-suited for this particular forum.

I want to drill through a relatively soft material, and once I reach a change in material density/hardness I want the drill to stop itself at that depth. I don't expect a screeching halt, but a general stoppage in depth. Is there any sort of drill tip geometry that would support this maneuver? I'm thinking something like an endmill or a ball-tip which would be able to cut the soft stuff but not penetrate the hard.

Thanks for the help!

 

[CorBlimeyLimey]

What materials are involved?
What are the differences in hardness?
What thicknesses are involved?

 

[MintJulep]

I can imagine some obtusely complicated methods, but nothing cheap, simple and mechanical comes to mind.

 

[Jabberwocky]

Great questions, CBL - I don't have hardness values off the top of my head, but the density would definitely vary in a relatively short distance, on the order of millimeters - unfortunately we're talking anisotropic viscoelastic composite materials (that bleed).

The depth is the variable, the only sure thing is that the far end of the hole will have a harder, denser, stiffer material than the rest of the shaft.

I'm about where you are MintJulep - nothing simple comes to mind.

Thanks for the help!

 

[unclesyd]

They make torque controls for machine drilling that could easily do what you need. One of the main uses is monitoring tool condition, broken, dull, etc.

Here is one that encompasses all aspects of drilling which might be a little much for your application. I will look around for additional types such as the ones used on commercial routers.

http://www.ovationengineering.com/compensatordatasht.pdf

 

[CorBlimeyLimey]

Maybe one of the tapping head types could also be used.

 

[Compositepro]

How about a paper drill? Basically a thin walled tube with a sharp bevel. Could rotate - or not.

 

[IRstuff]

Is there any optical or electrical difference, or something that could be added?

How thick is the base layer?
What is the allowable depth tolerance?
Any dielectric constant difference?
Any optical transmission difference?

Not that this would result in anything simple...

If there was something optical, one could potentially create, effectively, an optical depgth gauge.

Likewise, if there were some dielectric constant difference, one might be able to create an electronic depth gauge.

If the base layer is conductive but the epi is not, then a simple conductivity sensor might work.

TTFN

FAQ731-376

 

[GregLocock]

It depends a lot on the relative hardness of the two layers. For instance when drilling large holes into foam cored with prepreg on each side it was easy to stop at the second skin when using a conventional hole saw, which is not much different to composite pro's suggestion.






Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Jabberwocky]

I appreciate the clever ideas, glad I came to the right forum! I wasn't trying to be coy earlier, but I might as well tell you all that I'm asking about drilling through living bone. There are two types, cortical and cancellous, with cortical being significantly harder, denser, stiffer than cancellous. Think drilling through a textbook - how do you break through the pages without puncturing the cover?

My goal was to geometrically limit the drill shape to prevent puncturing the far cortex of a bone. The tolerance is a few millimeters, but going too long is not an option, only stopping short. (+ 0/- 2mm). Electrical and optical methods may be out just due to cost and complexity (and sterility).

Doesn't a paper drill require a pretty significant amount of force? What would stop it from breaking through something hard on the bottom side?

 

[CorBlimeyLimey]

If it's bone wouldn't you be able to predict the depth from the x-ray/CAT-scan/MRI? If so all you need do set the drill depth.

How is the depth controlled when operatiing done on the skull?

 

[MikeHalloran]

A twist drill ground with zero lip relief angle should cut soft material but not hard material.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

You might investigate a microphone or accelerometer as well.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[unclesyd]

What is your power source for the drill motor?

What is the hole size.

As I understand it you want to drill perpendicular to a somewhat round object with unknown wall thickness to a set distance.

How about a fluoroscope?

My wife has a broken back,no T8 vertebra, and receives monthly injections of pain medication. The doctor uses a fluoroscope to position the point of the injection needle as close to the spine as possible.
I also just had a stent installed and the fluoroscope they used was impressive as to the detail on screen.

 

[pauljohn]

There is a woodworking tool that cuts square holes (mortising attachment). It is really two tools in one, a four sided (square) chisel and a drill bit in the center of the square. The chisel cuts the sides of the wood and the drill starts the hole and removes the waste. If you set the drill bit so it is the same depth or slightly higher in the tool than the chisel blades it will only cut soft surfaces that the chisel can cut and stop at the hard materials. I would think it would easily cut a hole in flesh and stop at the bone.

If you wanted a round hole I'm pretty sure it wouldn't be difficult to make the same tool with a round tube around it that is sharpened appropriately. The blade acts as a safety stop because it will not pass through the bone so stops the drill from piercing it. (OK I'm guessing at this but it sounds feasible to me!!)

 

[IRstuff]

Ok, so you're drilling through near side cortical, and all the way through the spongy bone, but want to stop at the far side spongy/cortical boundary?

Sounds like DEXA might do the job:

http://courses.washington.edu/bonephys/opbmd.html#tz

http://courses.washington.edu/bonephys/opDEXA1.html

TTFN

FAQ731-376

 

[IRstuff]

What type of drill is this? Is it under computer control?

There may be some possibility of running a flatter bit with just a small point and running it high speed to get through the first cortical layer, and then dropping the speed way down going through the spongy bone. As the bit gets to the far side cortical layer, it won't be going fast enough to drill through, and it'll be scraping the inside surface, which might be detectable as a sound or vibration change in the drill.

TTFN

FAQ731-376

 

[ajack1]

I might be well off the mark here but could you not manufacture a drill from a material harder than flesh? but softer than bone?

 

[Jabberwocky]

More neat ideas! I'm very appreciative. You generally would use fluoro- which will show you exactly how far you've drilled, but it doesn't have a physical stop preventing you from going deeper.

I think for cranial screws they use something more like a burr to remove material slowly - although I did find this patent:

http://www.google.com/patents?id=tZ8BAAAAEBAJ&dq=5382250

which seems to only apply if you know the depth ahead of time.

Power source is usually electric. My holes can be 2 to 4mm, depending on the type of screw.

We're not drilling through flesh or muscle, that has all been resected above our bone and we're trying to prevent the drill ever touching the anatomy below the bone. IRstuff has it right for the boundaries - but we're not under any computer control, just surgeon brainpower.

I'll ask around about a flat bit / zero lip relief angle. Unfortunately our drill materials are pretty tightly defined by FDA regs.

Thank you all for the great brainstorm!

 

[Jabberwocky]

http://www.acracut.com/perforators.html

Pretty cool device, seems to do almost what I wanted, although theirs are only used for skull thicknesses, I don't see why you couldn't adapt it for a more variable depth.