I don't think anyone has mentioned this possibility: Fill in the existing hole
with weld and start over.
Chris
KF7P
On Sep 8, 2015, at 10:07 AM, Grant Saviers wrote:
> 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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>>
>>
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