Boring small holes

Good evening all,

Just playing around with my beginner project- a unit steam engine and would like some advice on boring for the cylinder.

The instructions say that it should be drilled 19/64 and bored/flat bottom drilled before finishing 5/16 with a reamer- is there a small enough boring tool available to save me having to modify a drill with my awfull grinding skills?!

im hoping the answer is completely obvious as my searches for small boring tools has come up empty!-piston size is .3125

any advice is massively appreciated and hello to you all!

KR,

Matty

I presume your boring in the lathe, in which case use a 6mm or similar slot drill with the flat face very slightly angled to give you side clearance to bore the hole, that way you'll achieve the flat bottomed bore. Take the bore as close as you can to finished size before reaming as the leading edges of the reamer will not produce a size diameter at the bottom of the bore.

Yes it will be done in the lathe,So i will be bolting the slot drill into the tool holder and setting at say 5 degrees?

Thank you for the help so far!

Welcome to the forum Matty

Reaming a blind hole isn't a very good idea for a cylinder bore, even a machine reamer won't go right to the bottom.

A better choice would be a d-bit made from sliver steel with the end cut across square at the end and only the tiniest radius on the corner.

For a small boring bar, get a piece of 1/4" round HSS and grind away just over half its thickness, then grind across the end at about 10-15 degrees. Make a holder by putting a metal block in your toolpost and making a good drilled or ideally reamed 1/4" hole holding the drill/reamer in the 3-jaw chuck. Either add a pair of locking screws, or split the block so that tightening down the toolholder locks the bar in place. use the bar angled towards you by just a few degrees to provide front clearance, and with the bar rotated to put the cutting corner at centre height.

Neil

It appears that this engine has a blind cylinder so reaming will not work as reamers have a leade on the end to aid insertion .

If i were making this engine i would use the 19/64 drill to remove the bulk of the material and only drill deep enough so the point of the drill reaches to full depth then use a 5/16 endmill in the tailstock chuck to clean up to size and square the end of the cylinder up to full depth.

Then make the piston to fit .

A much more rigid set up for the lathe is to mount the cylinder up in the toolpost or to an angle plate fixed to cross slide and put the drill / milling cutter in the chuck but may be more work than necessary for this engine.

You could of course do the same operations on a mill or even a drill press .

I would not recommend that a beginner tries to bore a blind hole let alone one so small , much better to learn on through holes first and preferably a larger diameter .

The problem with really small boring bars is flex and they will flex causing a tapered bore , not hard to fix but you have to account for it .

Ian.

Thank you for the ideas,there seems to be a few ways to complete this one.

I may have a go on some scrap with the D bit and see what results i acheive-i assume if it tapered the bore it would be larger near the bottom?

The end mill sounds like a good option too,Would it produce a clean cut/bore for the complete 34MM depth?I am not sure how long the working end of the bit is.

KR,

Matty

I have said many times before a reamer is a sizing tool that will not necessarily produce a round hole, neither will it produce a good finish. The 'end mill' or better still slot drill is the route I would take. A single point tool will give you a hole that is round and with a little perseverance can give you a hole with an excellent finish. I have used this method many times and in fact have a boring bar made for the purpose. This is in effect a length of 5/8" silver steel with a 1/4" hole drilled in the end then cross drilled and tapped for a grub screw. A taper was then turned on the end to reduce the dia to give a better view of the tool in action. You will probably need a long series end mill/ slot drill for that length.

Edited By Dusty on 04/10/2016 09:11:38

Hi Matty,

Neils suggestion of using a 'D' bit is the best way to go and it will not produce a tapered hole. There is plenty of advice available on the internet on how to both make and use them.

Regards,

IanT

Thank you all for the advice,I will give it a try!

KR,

Matty

Matty, I am not familiar with the Unit engine, but another possibility could be to drill the cylinder right through and then use a reamer to finish which you could also go right through with, then make a cylinder cover to close the top. This all depends on what material you have around the bore of course. Most larger engines have through bores and cylinder covers so they can be machined more easily than a blind hole.

Chris Gunn