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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73
Roger (K8RI)
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