# Boring flat bottomed holes



## SignalFailure (Nov 11, 2008)

I want to bore a 3/4" (19mm) hole about 1/2" (12.5mm) deep but want the hole to have a flat bottom (i.e. a blind hole). It's for a lightweight 'bucket' type piston.

Normally when boring a through hole I'd drill through with as large a bit as possible then bore out to the required diameter but how to do a flat bottomed blind hole?

Starting seems to be the problem:

If I make a hole with my largest D bit (1/4") then I don't have a tiny boring tool to enlarge it. I suppose I could make one but I think rigidity might be a serious problem) and I find boring on the mini-lathe is very laborious (i.e. I can only seem to take very light cuts or I get non-parallel bores)

I don't fancy shelling out for a 1/2" dia silver steel rod (even if it's made in that sort of size!) to make a 'big' D bit.

Conversely if I drill a 1/2" hole it'll take forever and probably not be very accurate to remove the conical part at the end of the hole left by the drill. It'd be rather tricky to see what's going on too!

The only other thing that occurs to me is to bore through then solder on a flat end piece but I need it to be as accurate as possible and that doesn't sound too good.

Any suggestions anyone?

TIA

Paul
PS I don't have a milling machine or any other fancy equipment


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## Brass_Machine (Nov 11, 2008)

There may be better ways than this, but here is how I have done it. I assume this is being done on a lathe. I was going for a .60 hole to .675 in depth in aluminum.

I drill out the depth I want with a 1/4" drill bit. Replace the bit with a 4 flute 3/8 end mill and take it to depth. Then use a 1/2" end mill and do the same. Swap it out and use the boring bar.

Hope that helps.

Eric


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## Bluechip (Nov 11, 2008)

Hi Signal Failure,

A slot drill will plunge cut. Not really the tool for the job, but it will work. Low speed on steel, but I suppose your piston is aluminium alloy, the bottom will be 'flat-ish'. Put a 1/4" or so twist drill in first to take the load off the slot drill. Inelegant but works.

Dave

Edit: Similar to Eric's post .. Great Minds etc.. ;D


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## Mike N (Nov 11, 2008)

I use a 2 flute endmill as my boring bar to make flat bottom blind holes on my lathe. It works good & you can face off the bottom of the bore, if your tool bit is on the centerline.

Good Luck!


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## SignalFailure (Nov 11, 2008)

It's certainly an idea Eric, many thanks!

I don't have any milling gear but it might be worth investing in a couple of end mills specifically for this purpose as the same problem sometimes arises with cylinders.

Paul


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## SignalFailure (Nov 11, 2008)

Dave & Mike, thanks also.... seems the end-mill/slot-drill idea is popular around here! ;D

I don't tend to use Aluminium, more likely materials would be brass or cast iron. Is it likely to work as well?

Thanks 

Paul


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## Brass_Machine (Nov 11, 2008)

SignalFailure  said:
			
		

> Dave & Mike, thanks also.... seems the end-mill/slot-drill idea is popular around here! ;D
> 
> I don't tend to use Aluminium, more likely materials would be brass or cast iron. Is it likely to work as well?
> 
> ...



Sure... Just adjust your speed and feed rate accordingly.

Eric


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## tel (Nov 11, 2008)

As an aside, to make a weeny boring bar, take a _good quality_ Allen key, cut the short leg even shorter, grind to shape and lash up a holder for it.


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## DickDastardly40 (Nov 11, 2008)

Let me have a nose at work see if we have any 3/4" flat bottom drills (prolly have a 2MT shank, what's the tailstock on your lathe). You still get a small inverted dimple in the centre. If we have you can borrow it for postage and a virtual pint!

Does the hole actually have to have a flat bottom due to design, ie would it matter if it was conical? You wouldn't believe the times I've got away with a conical hole when using allen headed setscrews.

Al


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## Mcgyver (Nov 11, 2008)

Paul, I may be missing something here, but this looks like straightforward boring bar work. The end of the boring bar needs to be less than 3/8 and grind such that it can face. At only 1/2 deep there shouldn't any rigidity issues. To get the tool to repetitively stop at the same,e depth use a carriage stop. here's a sketch i did eons ago.....


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## itowbig (Nov 11, 2008)

tel  said:
			
		

> As an aside, to make a weeny boring bar, take a _good quality_ Allen key, cut the short leg even shorter, grind to shape and lash up a holder for it.



wow thats a great idea thanks (i was just passing by and BAM got some more good info thanks & sorry for russling the post)


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## itowbig (Nov 11, 2008)

nice drowing there mcgyver


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## SignalFailure (Nov 11, 2008)

Thanks again all.

Al, thanks for the offer mate but as this is something that seems to crop up fairly regularly I reckon I better get a permanent solution! (But here, have a virtual pint on me!). The bottom needs to be flat to mount an axial 'bar' to attach the piston rod.

Tel, great idea... as usual ;D

Mcgyver, yes that's how I would have approached the problem (towards chuck then towards centre) to remove the 'cone'. For this application it would probably be good enough (if slow!) but a slight error in winding the cross slide would mess up if using the same technique to create a parallel blind bore for a cylinder.

Thank you for taking the trouble to post the drawings and explanation though 

Maybe I should shut up and go experiment instead?!

Paul


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## deverett (Nov 12, 2008)

SignalFailure  said:
			
		

> Mcgyver, yes that's how I would have approached the problem (towards chuck then towards centre) to remove the 'cone'. For this application it would probably be good enough (if slow!) but a slight error in winding the cross slide would mess up if using the same technique to create a parallel blind bore for a cylinder.
> 
> Thank you for taking the trouble to post the drawings and explanation though




Paul

I would go for mcgyver's method using the boring bar. Take the final cut full depth with the longitudinal feed then move into the centre with the cross slide to ensure parallel sides and flat bottom.

If you do go too deep, then it does not matter what method you use, the result will be the same. You could always face off the outside end afterwards to bring the depth back to what is required.

Dave
The Emerald Isle


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## SignalFailure (Nov 12, 2008)

Thanks Dave 

I've just ordered an end mill so I can play with both options and see what works best


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## deverett (Nov 12, 2008)

Paul

If you are going to use an end mill as a boring bar mounted in the toolpost, or even in the tailstock, make sure that the cutting lip you are using is on centre height. (This assuming that you eventually make the hole bigger than your endmill).

Dave
The Emerald Isle


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## SignalFailure (Nov 12, 2008)

Thanks Dave, will do.


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## SignalFailure (Nov 16, 2008)

Well the endmill followed by boring bar worked a treat! I drilled a 6mm hole then used the 10mm end mill (cut like a hot knife through butter) then switched to a TCT boring bar. Here's the piston with the fork and con-rod fitted (the wrist pin is 1/16").


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## CrewCab (Nov 16, 2008)

Good to hear ...........  8)

CC


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## Dhow Nunda wallah (Nov 19, 2008)

Another source for small boring bars is masonry drills, suitably ground.
I've made some tiny TCT bars this way.


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## greenie (Nov 20, 2008)

Maybe I'm missing a few grey cells upstairs, but, what's with all this lot then ?

Did it even flash through your mind, to go get a bit of toolsteel and just sharpen it to make a very small boring bar ?

Drill a hole and then start boring it out to whatever size you need, no need to worry about anything else, it seems pretty simple to me, flat bottom, no worries.


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## Stan (Nov 20, 2008)

Greenie: I defer to your experience on grinding small boring bars. The problem I have is that to bore a flat bottomed hole, the boring bar has to less than one half the diameter of the drilled hole so that it can be backed up to put the cutting edge on the center line of the hole. This makes a flimsy boring bar compared to one that is almost the full hole diameter that only has to cut on the sides of the hole and not the bottom.

I am amazed when I see the small work done by some of the members here but I am too ham fisted to work with boring bars under .125" so I drill to depth with a .250" drill, change to an mill in the tailstock and run it in to depth and then start boring with a .250" boring bar.


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## shred (Nov 20, 2008)

Likewise I run a small endmill in to flat the bottoms of holes and bore from there. I know endmills aren't actually flat on the bottom, but they are a lot more so than drill bits


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## baldrocker (Nov 20, 2008)

Ahh newbie suggestion.
Would grinding an end mill shank down to one flute work?
BR


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## Tin Falcon (Nov 20, 2008)

baldrocker  said:
			
		

> Ahh newbie suggestion.
> Would grinding an end mill shank down to one flute work?
> BR


It is an old machinist trick to take a worn end mill and  grind it down into a boring bar. 
I have made boring bars from drill rod and ground them fromm sqare HSS blanks sorry do not have the pics handy.
Tin


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## Stan (Nov 20, 2008)

baldrocker: Almost any piece of hard metal or carbide can be ground into a cutting tool. If your question applies to this thread, an endmill ground into a boring bar would still have to be less than one half the diameter of the hole in order to get the cutting point on the center.


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