[TowerTalk] [Bulk] Re: Grinding hole in thick steel plate?

Tue Sep 8 12:07:59 EDT 2015

Roger makes a number of good points.  I would add a couple.

Annular cutters are basically end mills, so if you have a rigid enough 
setup, one might be carefully be used to off-center enlarge a hole.  A 
Mag-Drill is designed for such cutters to drill into heavy plate but 
they and cutters are not cheap.  The advantage is only a ring of steel 
is being removed, not the entire hole as with a twist drill.  That 
reduces the thrust force needed to a level that can be handled by the 
electro-magnet that holds the drill to the plate. Plenty of cutting 
coolant is needed.

A "hole saw"  would be useless in 3/4" thick steel plate and without a 
pilot drill center just wouldn't work no matter how rigid the setup.  
Don't ask how I've broken off a saw hub.  Metal fatigue won.

I know it is appealing to use an end mill of the hole diameter to move 
the center, but for moving a 1.5" hole 0.25", that is an expensive end 
mill ($150).  So for this job I would use a boring head and carbide 
tipped bar ($15).  it will take a bit longer, but much less downside in 
tooling costs and usually in most hobby machinist tool boxes.  That is 
also a much too large an end mill for a Bridgeport cutting steel.

One other way to enlarge a hole is a "core drill"  which is specifically 
designed for that purpose.  However, they won't move a hole off its 
nominal center.  Usually, they only cut on the outer one half to one 
third of the radius of the drill.  With a tapered lead, they center on 
the existing hole and enlarge it to the drill size.  If used in a mill 
or lathe, near reamer tolerances and finish are possible.  It is 
practical to use one up to about 5/8" dia in a hand drill.  I think the 
source of the name "core" comes from the need to drill "cored" holes in 
castings to the desired size.  They are particularly useful in a lathe 
to quickly increase an inside diameter rather than multiple boring bar 
passes, e.g 1" to 2" in one pass, but that takes all of the 10 
horsepower in my big lathe. Impressive (and dangerous) swarf 'though.

There are core drill sets that are pretty useful to 3/4", particularly 
for pilot holes for a large chassis punch and larger holes in thicker steel.
http://www.ebay.com/itm/NORSEMAN-MAGNUM-SP-8HH-HOLE-HOG-REAMER-DRILL-BIT-4-FLUTE-SET-3-8-3-4-USA-/301607300147?hash=item4639322c33

As Roger points out many "hand held" drills have the torque to hurt.  A 
friend broke his wrist recently with one that grabbed.

Grant KZ1W

On 9/8/2015 3:03 AM, Roger (K8RI) on TT wrote:
>
> Using the original plate as a guide in a drill press, or a hand held 
> with a magnetic base is a good approach.  I don't know if the off 
> center cutting will force the side of the bit to cut into the guide,  
> Use good, substantial, C-Clamps to hold the plate and guide securely 
> to the press base plate, guide.  Don't use small clamps.
>
> The best, safest, and quickest is still a machine shop, or someone 
> with a mill that knows how to use it.
> A high speed grinder with "good bits is slow and tedious work, There 
> is also a learning curve. I know it's tempting and I'm the type who 
> wants to do it himself, but...
>
> I know I'm covering a little more than the original post asked, but I 
> think it's appropriate.
>
> Large drill bits used off center with out a good, solid guide tend to 
> grab and once they grab, they tend to bounce around the hole, grabbing 
> as they go with each grab becoming more dangerous.  They are likely to 
> become a real arm, or leg breaker if you use your leg as a brace 
> because each grab is an impact rather a ramped up resistance easily 
> surpassing several hundred pounds of force.(It's like dropping a 200 
> or 300 rock on your leg, arm, or hand).  IOW, The drill bit can become 
> very dangerous.  Those who are not familiar with what can and likely 
> will happen, believe they can hold it.  I've had a drill grab while 
> enlarging the hole on center. Thankfully it burned up the motor on the 
> largest Dewalt 1/2" drill motor Lowe's sells before it did anything 
> serious to me. I thought I could hold one until then.
>
> Example:  I know an electrician who had the cord get wrapped around 
> his hand. When the drill grabbed, it pulled his thumb out, separating 
> the thumb from his hand completely.  He had to climb down 3 stories on 
> steel ladders after the incident.
>
> I can't emphasize how dangerous Redrilling a large bolt hole, "off 
> center" can be. There are only two safe ways to do it. One is with a 
> milling machine, or a cutting torch (Plasma, or Ox-Acetylene). There 
> is a third way using a heavy duty, high speed grinder using a carbide 
> bit or a heavy duty abrasive grinding bit and not the cheap ones.   
> Colors indicate hardness.   Pink is soft and for finishing. Dark gray 
> is for harder metals and better for hogging out steel metal plates , 
> like 3/4" metal plates.  I use carbide bits and abrasive grinding bits 
> rated for 40,000 RPM.  40,000 is not a misprint.  Typical big box and 
> hardware store bits are rated at 5,000 RPM and are very likely to 
> explode in a real high speed grinder or die grinder.
>
> Always wear a full face shield and safety glasses with side shields 
> with this kind of equipment..  Safety goggles, or safety glasses with 
> side shields by themselves are not considered sufficient.  As I have 
> to rely on experienced volunteers, I'm sometimes reluctant to insist 
> on the full face shield
>
> The strong magnetic brace becomes a necessity, but are not guaranteed 
> to hold a bit that has grabbed when "trying to drill off center.. OTOH 
> they GENERALLY  do tend to only kick to the center of the hole and 
> allow you to let go of the trigger.  As I've said before, it can be 
> almost impossible to let go of the trigger on a hand held drill motor  
> that is grabbing, or has grabbed.  They are dangerous on a drill 
> press, and far worse on a portable setup.
>
> By the far the safest and bet is an end mill of the proper size. 
> Unfortunately, I've never seen one with a smaller shank to fit a drill 
> motor or drill press.  The end mill, fed slowly, with a water based 
> lube spray is almost a necessity and  they give a very clean hole of 
> the proper size.  Never, ever try to use one, hand held.  They are 
> expensive and need a rigid hold as with a milling machine.  The shank, 
> or quill on even an expensive drill press is just too flexible.  There 
> is a reason the larger end mills are gripped with a collet of the same 
> size.
>
> If you can find some one ( or a shop you trust) with a milling machine 
> that will tackle the job, it's a very simple setup and easy to do.  
> It's helpful to have the plate setup of the proper orientation, but as 
> you are using the mill as a drill press, with the plate "tightly 
> clamped", it's not necessary.  However many shops do not like to do 
> this kind of work.  Never suggest "only a drill bit will do the 
> work!".  It tells them you do not know the dangers you are suggesting 
> or want them to do something dangerous.  Depend on them for the proper 
> equipment to use.    The drill bit usually won't and is usually very 
> dangerous to boot.  A complete set of 2-flute and 4-flute end mills, 
> plus ball end mills males a set or top quality drill bits look cheap! 
> Add to the the need for a required set of collets
>
> I have a old, big Bridgeport Mill.  The head needs to be rebuilt as 
> the key has been sheared, probably from just such an operation and why 
> It was so cheap. A mill with a keyless chuck is the best drill press 
> you can find.  A ball end mill makes a very good, but expensive drill 
> bit.
>
> BTW You can purchase a gallon jug of lubricant to mix with water from 
> most any industrial supply house and use it in a little hand held 
> spray bottle.  There are also spray bottles fat last a long time and 
> many small jobs.
> http://www.grizzly.com/products/Accu-Lube-Pump-Spray-Lubricant/H9239
> I'm just a customer, have no interest in the company.
>
> Proper lubricants will make the use easier, while bits, taps, and saw 
> blades last much longer.
>
> 73
>
> Roger (K8RI)
>
>
> On 9/7/2015 2:33 PM, Rich Hallman - N7TR wrote:
>> I had a ¾ inch steel plate made up that will be used to retrofit a 
>> HDX589 base foundation to a HDX572 base.   After giving both bases to 
>> the machine shop and asking them to make sure all of the holes are 
>> exactly the same location as the base, some of the holes are 1/8 to ¼ 
>> inch off.   I had a major issue with this machine shop so I will not 
>> take it back for now.
>>
>> I wanted to see what others have done to grind out holes in thick 
>> steel plate.
>>
>> I was looking at the Carbide Burr bits and getting a ¾ inch burr bit 
>> to see if that would grid enough from the hole so it would then fit 
>> the bases.
>>
>> The holes are pretty close....so I thought grinding them out would be 
>> the best solution for now.
>>
>> Any thoughts?
>>
>> Thanks....Rich
>>
>> Rich N7TR
>> ex KI3V, N3AMK, WB3JOV
>> www.n7tr.com<http://www.n7tr.com>
>> http://www.qrz.com/db/N7TR
>> Telnet: dxc.n7tr.com N7TR DXCluster
>>
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