Drilling Precision Holes

[mfarris]

Thanks jtrout.......that's very helpful advice. One other question somewhat off the original subject. The cylinder will be a blind hole. Any suggestions on how to get the bottom of the hole flat? And, is it critical that it be completely flat?

 

[Marty Feldman]

"Drilling holes to precision" can be more complex than might at first appear. Size, roundness, finish, and especially location are some of the dimensions of the problem. I would suggest that some reading on the subject is in order. An authoritative and complete account of basic procedures for accurate drilling -- and for most other machine shop procedures that you are likely to use -- is the 2-volume set by Moltrecht, "Machine Shop Practice".

 

[jtrout13]

Thanks jtrout.......that's very helpful advice. One other question somewhat off the original subject. The cylinder will be a blind hole. Any suggestions on how to get the bottom of the hole flat? And, is it critical that it be completely flat?

That hole doesn't need to be completely flat, but you can't just drill it, because there isn't enough 'meat' on the top of the cylinder for the tapered tip of the drill bit.

I'd probably drill that hole with a 3/8" drill bit (drill the TIP of the bit 1.625" into the material). Then, I'd run a 3/8 endmill down that hole and flatten the bottom to the correct depth. Then, bore out the hole to to be a good fit on your piston. Make the hole about 1/16" deeper than the plans say just for clearance.

 

[kvom]

It's easier to make a piston to fit a hole than vice-versa. It's unlikely you have a boring bar small enough to bore a 3/8 hole, so reaming 3/8 is a better solution. The bottom may not need to be flat. You should be able to tell how far in the top of the piston goes by adding up some plan dimensions. And you can also make the piston a bit shorter if needed. You can also make the cylinder piece longer.

 

[mfarris]

Super. Thanks for all the info/advice everyone.

 

[lensman57]

It's easier to make a piston to fit a hole than vice-versa. It's unlikely you have a boring bar small enough to bore a 3/8 hole, so reaming 3/8 is a better solution. The bottom may not need to be flat. You should be able to tell how far in the top of the piston goes by adding up some plan dimensions. And you can also make the piston a bit shorter if needed. You can also make the cylinder piece longer.

Hi,
If you want a top quality small boring bar to do small holes with, 1/4" and upwards take a look at these. They should have their own dedicated tool holder but I have used them very successfully by clamping them onto the tool post as normal or making a brass sleeve to hold them in a QCTP style tool holder on my Taig and Sherline lathes. These really are very good boring tools. The one in the link is 6mm diameter but the range starts at 4mm meaning that you should be able to start boring from as small as 4.5mm or 3/16" in the old money, the lenghts differ according to specification.
http://www.ebay.co.uk/itm/New-Sandv...Home_Garden_PowerTools_SM&hash=item1c2a6a54d1

Regards,

A.G