# Boring a large diameter hole in a lathe



## prasad (Jan 7, 2015)

My equipment: Grizzly G4000 9x20 lathe with 6 inch 4-jaw independent chuck

My work: Most my (recent) work involves boring a large diameter hole - typically to a 5 inch ID in solid metal (aluminum and some brass). I initially did it the hard way. Start off drilling a hole as big as my largest drill bit, and then switch to a boring bar to enlarge the diameter gradually. That took a long time and was very tiring.

I have tried using round nosed turning tool too. It was faster than my first method but I want to know if there is any better way to cut a hole. 

Thanks
Prasad 
Wynnewood PA


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## grandpaswagon (Jan 7, 2015)

How thick is the stock you want to bore the hole in?


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## Terrywerm (Jan 7, 2015)

If this were something that I was going to do often, and the material is not too thick, I would get a hole saw just a bit smaller than the desired ID. Use it first, then finish the hole to size with a boring bar.  If you only need one or two of them, the methods you have used thus far might be your best bet, even though they make the process a bit tedious.


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## 12bolts (Jan 7, 2015)

You can use a "parting style" tool on larger holes like that. It is plunged in to the face of the work. You need to grind a lot of relief on the outer side of the tool to allow for the arc of the cut. But for brass and ally I would go with a holesaw. You can also drill a small hole, (just 1) on the perimeter of the holesaw. Makes it easier going and helps clear the chips

Cheers Phil


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## Hawkeye (Jan 8, 2015)

I use hole saws quite often to cut steel. I've cut 1" holes in 1" steel and larger holes in thinner steel. The key is to slow things down, use cutting oil and clear the chips often. You could use the hole saw in a drill press first, then set the piece up in the lathe to bore to finished size.


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## dracozny (Jan 8, 2015)

prasad said:


> ....... That took a long time and was very tiring.
> ....


I came to a conclusion a couple weeks ago. There is a reason they call it "boring". It is dreadfully so.


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## Karl_T (Jan 8, 2015)

You may not have the equipment, but I'd do a 5" hole in the mill with a rotary table. Or even CNC. Use an endmill to just cut a slot.


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## darkzero (Jan 8, 2015)

If it's not too thick/long you could use a trepan tool. If it is too long for a trepan tool or hole saw, you could drill a number of holes (with a drill press or by hand) in a circle pattern close to the dia you need bored out, then use a coping saw to saw out the slug. 

The coping saw would work fine for alum or brass, remove the blade from the saw, stick the blade through one of drilled holes, put the blade back on the saw & saw away.


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## prasad (Jan 8, 2015)

grandpaswagon said:


> How thick is the stock you want to bore the hole in?




Thickness is 1" typical

-Prasad


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## uncle harry (Jan 8, 2015)

12bolts said:


> You can use a "parting style" tool on larger holes like that. It is plunged in to the face of the work. You need to grind a lot of relief on the outer side of the tool to allow for the arc of the cut. But for brass and ally I would go with a holesaw. You can also drill a small hole, (just 1) on the perimeter of the holesaw. Makes it easier going and helps clear the chips
> 
> Cheers Phil




The tool Phil described is called a trepanning tool. A  good "how to run a lathe" book will probably have a diagram of it.  I "Googled" You Tube Videos & found trepanning by Frank Stearns. It shows an example of what you are trying to do.


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## epanzella (Jan 8, 2015)

I've used a plunge cut on my lathe to cut big holes like you describe but the thickest I ever cut was probably 3/8 steel. I plunged straight in until I ran out of front relief then widened the hole like I was facing and plunged in again. The lack of front relief forces the hole you're cutting to taper in as you go deeper but it was still better than boring all the way. Switching tools can minimize the taper. If you have the capability the hole saw is probably the best idea but if you don't plunging may work for you.
Ed P


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## dracozny (Jan 8, 2015)

trepanning tool. anyone try using one on a mill?


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## Bill Gruby (Jan 8, 2015)

Use a good hole saw to rough it out then finish with a boring bar.

  "Billy G"


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## Karl_T (Jan 8, 2015)

Another option for you:

chain drill holes just smaller than the ID you need. Use a jig saw type unit to finish cutting the core out. Then finish in the lathe.

Trepanning won't work one inch deep.


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## Round in circles (Jan 8, 2015)

Nice one Karl, the aluminium expands due to the heat and nips the blade ..same often happens with a hole saw . 

Doing the chain drill exercise all way round  inside the waste material at a diameter the same as your hole saw & then using a hole saw in the chain holes works well for the hole, not only does it reduce or lose  the heat causing swarf & expansion problem ,there is material to cut because of what you have removed chain drilling. 

The drawback is unless you have set up a four jaw chuck to do it previously you may find it difficult to centeer on th lathe.


This is how I did some 3 inch holes in some 2 & 1/2 thick alu plate . Set the carraige stop up to prevent you hitting things with the cutting tool or the carriage 

Accurately mark & center the hole & dot punch it for repeatable line up's. Now carefully chain drill a concentric ring on the inside of the required final diameter. in 1/4 inch holes  leaving 1/16 thick webs between holes & drill the center pop mark in 1/4 as well so you can get the same reference point when you turn the work piece over . ( If you need to turn it over that is.) .

Put it back on the lathe , line up the centre , using the face plate and some standoffs to lift thebplate off the face plate so you have clearance at breakthrough . Then turn out the webs of the chain drilled holes using the  lathe using center ground point cutter at 90 degrees to the work piece to do most of the work till  your well over half way through  . Now reverse the work piece on the face plate & center it , cut right through ,
 Now  finish the final cuts with a RH  cutter at 90 degrees to the work face .

Do keep things tight at in & on the tool post for  on several occasions I found that the tool posts have rotated a fraction and left me with hollow cones for holes instead of parallel bores 


 On some smaller holes of 1 & 1/2 , 1 & 3/4 " dia in 2 & 1/2 2sqy-uare ally bar  I drilled a 1/4 center hole and whacked a well ground up 1 & 1/4 morse tape drill through  from both sides ,  then used a home made boring bar from one face only to turn out the remainder to the required size .
The cutting angle for the drill  is sharper than that for mild steel , look it up.  ( I've been to sleep since then .)

Because for the size of things & the need for the holes to be at the ends of an 8 inch long billet of ally I had to use the face plate and home made clamping bars & some cut down 6 " bolts , hand cutting the bolts in situ.  right next to the nyloc securing nuts to give me as much clearance as possible , I also used stand off spacers behind the workpiece to put it 1/2 " away from the face plate.  ( Nowt worse than unintentionally hitting your tool end on a bit of hard steel .... apparently :lmao: ) .

I thought it would take ages but once I got the speed right it didn't take long .... done at something like 87 RPM because of the out of balance of things if I recall correctly .


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## Bill C. (Jan 8, 2015)

prasad said:


> Thickness is 1" typical
> 
> -Prasad



Use a hole saw.  Look in the plumbing section of your local home improvement store.  

Good luck


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## uncle harry (Jan 8, 2015)

Karl_T said:


> Another option for you:
> 
> chain drill holes just smaller than the ID you need. Use a jig saw type unit to finish cutting the core out. Then finish in the lathe.
> 
> Trepanning won't work one inch deep.



True, the trepanning approach suggested here will not go deep, but there are trepanning tools that resemble a hole saw that will.  Annular cutters are another type that work that way.


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## Chiptosser (Jan 8, 2015)

If your lathe has the power, you can use a Hougen rotabroach.   You would have to make an adapter to hold the tool. 
 A start cut would have to be made on the piece, with another tool first. They are not cheap, but it is an other option.


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## pdentrem (Jan 8, 2015)

A rotabroach would be a good choice. Limited to fixed sizes. I used a ranger face grooving tool by Manchester. 1" would be it maximum with some modifications. 3/4" is the standard depth of cut I believe.
Pierre

PS in this posting there is a picture showing the tool.
http://www.hobby-machinist.com/show...king-plates-for-my-lathe?highlight=Back+plate


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## batt (Jan 13, 2015)

Sharpen the boring tool like a knife tool and set tool flat against face of job ... should be able to take at least 3 mm cuts ... not too heavy a feed ... should peel the metal of easly.
If need be can send a image of tool set up.
Batt


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## batt (Jan 13, 2015)

Sharpen the boring tool like a knife tool for aluminium, flat top for brass, and set tool flat against face of job ... should be able to take at least 3 mm cuts ... not too heavy a feed ... should peel the metal of easly.
If need be can send a image of tool set up.
Batt


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## cjtoombs (Jan 13, 2015)

Karl_T said:


> You may not have the equipment, but I'd do a 5" hole in the mill with a rotary table. Or even CNC. Use an endmill to just cut a slot.



This is how I would probably do it, since I have the equipment, but if you do, ensure that you either bolt the center section down or leave it attached and do the final cut manually.  When the center waste portion comes loose, it WILL break your end mill.  You will also need to clear the chips some way or another, as gravity won't help you on a mill.  One could also rough cut it with a bandsaw, if you have a way to solder or weld the blade.


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