View Full Version : drilling holes

11-17-2005, 05:52 PM
i need to make a very smooth .250 hole for my compressed air eng cyl bore,how do i get the smooth and at .250 inch. ural

11-17-2005, 06:13 PM
Drill it undersized by about 5 or 10 thou and then ream it.

charlie coghill
11-17-2005, 06:15 PM
Than take an expandable lap and lap it.

11-17-2005, 06:16 PM
If you want smooth, suggest you burnish the hole.

The simplest burnish is push a ball thru it after you carefully ream it undersize.

The ball (and lubricant) is chosen from appropriate material, and what it does is mash the micro peaks and valles into a near smooth situation.


11-18-2005, 07:06 AM
A reamer ought to give you a good enough finish. If it doesn't, follow up with charlie's suggestion of lapping. Places like MSC www.mscdirect.com (http://www.mscdirect.com) and Travers www.travers.com (http://www.travers.com) sell expandable laps and lapping compound. #320 ought to be about right. The only trouble is, the smallest quantity you may be able to buy from them is half a pound, which will last several lifetimes.

Another source of lapping compound may be Brownell's www.brownells.com (http://www.brownells.com) . Years ago they sold a set of 6 or 8 two-ounce cans of various grits, which is just about ideal for the amount of lapping a home shop guy is going to do. Dunno if they sitll sell that set though.

Forrest Addy
11-18-2005, 07:29 AM
You're gonna get dozens of favorite methods.

I'd advise agaist lapping the bore except as the last possible refinement of surface conditioning.

I suggest you drill indersized and ream the hole 0.001" under the target size then drive a hardended dowel pin through it. Use an extreme pressure lube like hypoid oil or animal fat for a drawing lubricant.

The hardened pin will burnish the shole smearing the peaks into the tool marksleaving a very good durable suface. If you need to size the hole lap out tha final few tenths, no more.

For lapping you need either a series of cast iron laps graduated graduated by 0.0001" increments or an expandable lap. Start with 1200 grit or finer compound finishing with 10,000 for a mirror bright finish. Work from the quill in a lathe tailstock or a turret mill or drill press. Hand operate laps tend to produce hour-glassed bores and rounded-off corners.

11-18-2005, 09:55 AM
The only problem with lapping with an expandable lap or a reamer for that matter, is that it is very easy for inexperienced hands to bell mouth or barrel a hole.
Ball sizing as was mentioned(or dowel pin) is a simple and easy way to assure a straight hole providing you have a press.
You could also lap the reamer to get an acceptable straight finish in the hole. My .02.

11-18-2005, 05:19 PM
I dont know if a Sunnen wrist pin hone (automotive machine shop) will go that small, but it's a good way to do it if they will.