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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