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1 3/4 inch (45mm) dia. hole using minimill

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xxxx_guy

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#1
Hi,
I'm using a minimill to try to make an approx. 1 3/4 inch dia. hole in about 3/4" thick steel (low carbon). I'm trying to work out what is a reasonable way to go about this. The mill has a stated drilling capacity of about 3/4" dia. from memory.
Do I drill a 3/4" hole and then use a lot of passes with a boring bar on a boring head to get the hole out to 1 3/4"?
Do I use an end mill to try make a circular pocket just a bit smaller than required and then use a boring bar on a boring head to finish off the rest?
Any other suggestions as to how to achieve this hole?
Cheers,
xxxx_guy
 

Ed ke6bnl

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#2
Might be a good time to have a small rotary table.
 

Smithdoor

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#3
Start with holes saw about 1 5/8" then use a boring head to finish to 1 3/4"

Or just use a 1 3/4"hole saw

Dave

Hi,
I'm using a minimill to try to make an approx. 1 3/4 inch dia. hole in about 3/4" thick steel (low carbon). I'm trying to work out what is a reasonable way to go about this. The mill has a stated drilling capacity of about 3/4" dia. from memory.
Do I drill a 3/4" hole and then use a lot of passes with a boring bar on a boring head to get the hole out to 1 3/4"?
Do I use an end mill to try make a circular pocket just a bit smaller than required and then use a boring bar on a boring head to finish off the rest?
Any other suggestions as to how to achieve this hole?
Cheers,
xxxx_guy
 

Mitch Alsup

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#4
Start with holes saw about 1 5/8" then use a boring head to finish to 1 3/4"

Or just use a 1 3/4"hole saw

Dave
IF there is any tight dimensional requirement, the top paragraph above.
IF there is not, the the second sentence below.
 

gjmontll

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#5
I've not had great luck with a hole saw on thick steel like this in my mini-mill.
I'd drill a circular series of closely-space holes inside your final diameter, maybe use a 3/8 drill.
Then cut or mill to give a roughly-shaped opening.
Finally, bore it to the final diameter.

As Ed suggested above, a rotary table would work too, if your work can be mounted and rotated within the confines of the mini-mill.
Even if it could, I think my way is the quickest. Just ensure you have an accurate way to center the work under the boring head once you've punched out the rough center piece.
 

xxxx_guy

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#6
Thanks for the advice. The tolerances don't need to be very tight. I was worried a bit torque-wise whether a 1 3/4" hole saw would be capable of spinning or if friction would be too great, also I don't currently have a hole saw of the required diameter. Also how good are hole saws depth wise? I've used some for cutting through SS sheet with a drill, however it would appear that the friction will increase as depth increases, and whilst it isn't a problem for these holes are hole saws limited to twice the depth of the internal cavity of the hole saw (if you come from both sides of the material) or are there tricks around this?
For now I've ordered some hole saws, about 2 weeks until they turn up and will see how they go.
 

turnitupper

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#7
I have drilled 60mm holes with a X2 mill. Drill 2 or 3 6mm holes just inside the outer side of the holesaw kerf to allow swarf to drop through. Use a light lubricant such as WD40 or kero to flush. Takes some time but works.
John.
P.S. also a Fourex guy
 

xxxx_guy

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#8
Thanks for your replies. Have attached some photos of my initial attempt (prior to your advice). I drilled out a 1" (25mm) square bar with a 3/8" (10mm) hole. Put a 3/8" (10mm) bolt through the hole and used this for the center point. Used a HSS bit to try to drill the hole. Made a crescent shape by rotating the steel until it hit the column of the mill on either side. Was hoping to then rotate the steel by re-positioning the end mill with the same radius but away from the column. I think the end of the HSS went blunt (not penetrating vertically - I did use lubricant). I may have an attempt of drilling some 3/8" holes (10mm) to try and engage the side rather than the end of the HSS end mill. Sorry about the quality of the photos. Have used 3 pieces of steel to make up the 6/8" thickness (18mm) at about 1/4" each (6mm). Sorry for the metricfication however in Australia we use metric, given this is a USA website I'm obviously using Imperial first out of respect.
 

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xxxx_guy

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#9
Also attached is the machine I'm trying to fix up. One arm broke on first use (new). I'm trying to replace this arm. As long as the new arm fits over the existing bearings it should work (shouldn't need massively tight tolerances). There is probably a few 1/16" adjustments available between centers and the arm and the centers can be out by at least 1/16" to 1/8 " without to many problems. After a bit of a fight (prior to me even having a lathe or mill) I received a refund. I'm trying to make the most of it by hopefully replacing the arm and getting the sheet metal bender working. If the new arm is straight, it shouldn't conflict with the sheet metal going into the machine.
 

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xxxx_guy

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#10
Also in case you are wondering. My vice takes about 2 1/2" and the plates are 3" (murphy's law).
 
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