# Flat bottom holes.



## KevinM (Dec 28, 2020)

I am building Elmer Vernburg's beam engine and I need to cut a 1/4" diameter, 1 inch deep flat bottom hole in brass for the cylinder.  Am I going to have to bore it?  That seems that it would be quite a challenge for me. Are there any other solutions that will make the hole and have a very smooth walls? 

Elmer's engines are now in the public domain and can be found at http://www.john-tom.com/html/ElmersEngines.html.  A lot of other content is available on the site.


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## Liljoebrshooter (Dec 28, 2020)

2 flute end mill


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## NC Rick (Dec 28, 2020)

The endmill won't be real flat.  Finish with a counterbore with a removable pilot?  I'm not sure how flat is needed.


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## KevinM (Dec 28, 2020)

I am not sure that the 2 fluted endmilI will leave a smooth enough wall finish and probably cut oversize.  I am not sure how flat the bottom needs to be.  I could possibly drill a little deeper and ream to final diameter without a loss of power/performance.


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## Bi11Hudson (Dec 28, 2020)

The end mill will leave a better wall finish (rounder) than a drill. And not as good as a reamer. The bottom will be reasonably flat, especially as compared to a drill. If the mill is long enough for what you are doing. . . Drilling ahead with a slightly undersize drill and using the mill like a reamer would give the best results. Drilling over depth may prove detrimental depending on . . . 

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## benmychree (Dec 28, 2020)

I would drill, flat bottom drill and then ream with a near new machine reamer with little end chamfer.  Remember the need to dub off the cutting edge of the drills to avoid "hogging in".


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## KevinM (Dec 28, 2020)

3/6" end mill and step ream to size?


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## benmychree (Dec 28, 2020)

KevinM said:


> 3/6" end mill and step ream to size?


1/16" stock would be too much for a reamer that size.


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## mikey (Dec 28, 2020)

You don't mention your tolerances. If the hole has tight tolerances, has to be straight all the way down the bore with a good finish and a really flat bottom then boring it would be the best way to do it. Given the depth and closed bottom, you are looking for a positive lead solid carbide boring bar.


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## KevinM (Dec 28, 2020)

Benmychree, I meant 3/16".  I have number and letter sets of reamers.  I just tried a test with the plunging end mill and it left a much better wall finish that I expected.


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## KevinM (Dec 28, 2020)

mikey said:


> You don't mention your tolerances. If the hole has tight tolerances, has to be straight all the way down the bore with a good finish and a really flat bottom then boring it would be the best way to do it. Given the depth and closed bottom, you are looking for a positive lead solid carbide boring bar.


Bore diameter tolerance is quite loose because I can make an appropriate size piston to fit the bore.


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## mikey (Dec 28, 2020)

A simple hole like that would take a few minutes and you're done. Reamers are okay, too, but you have to size the pre-reamer drill so the reamer has to only remove about 0.005" or less if you want accuracy and a good finish. And your drills need to be sharp. Then you need to flatten the bottom. 

For me, I would just buy a Micro 100 boring bar, drill an access hole and bore it. On size, straight walls, fine finish, flat bottom.


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## Shootymacshootface (Dec 28, 2020)

As discussed previously, the problem with using an endmill would be finding one of the correct diameter to use with the reamer.
If it were me, I would use the correct size drill bit to make the hole, then when the leading tip of the drill bit is at the correct depth, grind the end flat, then sharpen the flattened flutes like an end cutting endmill. Go back in and take out the cone at the bottom of the hole. Then ream to final dimension.


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## Mitch Alsup (Dec 28, 2020)

Why does the end have to be "dead" flat ?


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## KevinM (Dec 28, 2020)

Mitch Alsup said:


> Why does the end have to be "dead" flat ?


Not sure that it matters but Elmer always shows drill angles on the bottom of holes.  This one is drawn flat.


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## KevinM (Dec 28, 2020)

I am surprised that my OSG cutter cuts undersize (.243) but cut depth is too short.  I hope a long reach endmill does also.


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## Jimsehr (Dec 28, 2020)

I would drill with drill a little smaller than 1/4 dia then use 1/4 dia flat bottom drill to finish To depth.


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## Illinoyance (Dec 28, 2020)

The hole will have to be bored or reamed to get the finish needed for a cylinder wall.  Boring will get a flat bottomed hole. A conventional reamer will leave a small chamfer at the bottom of the hole even if the hole were flat bottomed before reaming.  A D-bit reamer can give a good finish and a flat bottom.  Ideally you would want a chamfer at the open end of the cylinder to aid starting the D-bit.  Allow a little sacrificial material at the open end of the cylinder for the chamfer.  Machine it away after reaming.

As a practical matter a little extra clearance space at the end of the cylinder will not matter.


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## Bi11Hudson (Dec 30, 2020)

While not familiar with your particular kit, I am aware that most steam engines have the cylinder and the "head" as seperate pieces. A quarter inch bore isn't much, but the small screws are available. Would such be an option?

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## Illinoyance (Dec 30, 2020)

A separate head held by screws seems like a viable idea.
George H Thompson describes D-bits in his book.  I believe Clickspring had one or more videos on the subject.


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## whitmore (Dec 30, 2020)

Illinoyance said:


> The hole will have to be bored or reamed to get the finish needed for a cylinder wall. ,,,
> 
> As a practical matter a little extra clearance space at the end of the cylinder will not matter.


In case of a compressor, clearance space limits the compression in a single stage, and would be
important to minimize.   I'd consider machining a plug with one flat face, and fitting that into the
cylinder (secure with solder or braze, or pin...).

Finishing a cylinder wall can be done with grinding, or a hone, but reaming is more convenient.
I'm unsure that boring generates a good flat bottom, the center might not cut smoothly,
and of course the chip removal is problematic (could score the sidewalls).

Does anyone ever mill/bore with an orbital cut?   Such as setting a boring bar for  0.5 inch diameter,
offcenter by 0.25 inch, and rotate the workpiece to make a 1.00 inch flat bottom?


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## Illinoyance (Dec 30, 2020)

My point was that additional clearance space in the cylinder doesn't really matter in model (toy) engines.  Clearance is detrimental to the performance of real world steam engines and air compressors.
I like the idea of a separate head.  Being able to pass a boring bar, reamer, hone, or lap through the whole length of the bore is the ideal way to finish the cylinder bore.

A D-bit is essentially the same as a half-round bit but with a shorter flute.


			https://www.mscdirect.com/product/details/01150168
		

The end of a half-round drill can be modified to cut a flat bottomed hole.


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## Chewy (Dec 30, 2020)

My 2 cents.  Jump in and point out what I have missed.  It is a number 24 beam engine and once you look at all the cylinder parts, it is not real critical. The cylinder bore is 1/4" x 1-132" (1.0313) long. You have a number 70 hole at 27/32". That means the piston can not come past .8438 (27/32") - .0140 (1/2 # 70 drill) or .8298 from open end of cylinder or .2015 from closed end.   Open end has a #70 hole at 11/64" (.1719) + .0140 means that the piston can't go closer then .1859 from open end. If it goes past this, it will cover the steam inlet holes. Do the math and at closed end the piston is .2015 clearance or cylinder volume.  At open end the piston is .1859 clearance or cylinder volume. About .016 difference.  The flat bottom is so the volume differences don't get too great.
Next, two piston rings are equally spaced on a 3/16" piston or 1/16" spacing. Subtract .0625 from .8438 at closed end and the wall only need to be smooth to .7813 from open end.  Or .25 from bottom hole.  As long as the reamer taper stops before .25, the bore is good. 

I would drill 15/64" so tip touches final depth.  Grind the point off and flat bottom drill.  Run a 1/4" chucking reamer until it exactly touches and that should do the job.  Store brand reamers are around $9, nothing stops you from grinding a little of the taper if you want.  I would try this on scrap first.

People, I have a whole page of numbers, arrows and directions to get this to make sense, and add up to what I would do. Go to the web site and look at the whole page and see if I got it right, or totally blew it. Something fun to do on a cold windy day.
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## Chewy (Dec 30, 2020)

Follow up.  I had to know.  The taper on my .250. .251 and .312 reamer is about .040.  That means you don't have to bottom out. just go past the steam hole and let it fly.


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## KevinM (Dec 30, 2020)

I have been making and testing D bits.  I think that I am pretty close.  I have 2 new bits I need to try when my wife lets me.  Another tactic that I may try is to make it a through hole and press fit a plug to give me a nice "square bottom".  At this point I will most likely end up using an appropriate D bit to get the bottom square and ream to size.  I am waiting for material.


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