How can I get good finish inside a small (5mm) hole ?

[picclock]

The title says it all. The metal is mild steel. I plan to drill to 4.5mm then get the final size with an endmill, as the finish seems superior this way. I need a good finish because a 3x1mm oring will slide in here (3mm id, on a 3.6mm id groove giving 20% squash or so. I can adjust the groove ID if the hole is slightly oversize, but I'm more concerned about the finish and its abrasive effect on the oring.

Many different ways to do this like using a d bit drill for the final size - seems good on guns, or turning up a piece of aluminium just undersize and using fine valve grinding paste, or wrapping with cloth and metal polish.

Any help or advice much appreciated.

Best Regards

picclock

 

[tel]

... or a combination of all of the above. Actually, a wooden dowel, turned to size and loaded with Brasso does a good job.

 

[rcfreak177]

Hello Picclock,

5mm reamer would be the way to go, here is a link to a supplier in China.

http://www.ebay.com.au/itm/5mm-Stra...ultDomain_15&hash=item3f11ee8615#ht_500wt_715

Around $10 it is a bargain, says hand reamer but I am sure you could run it in a machine providing you use plenty of lubricant. And the size should be spot on.

Cheers Baz.

Ps, As Tel says, a Wooden dowell an brasso does work well

 

[Omnimill]

A 5mm reamer from ArcEuro will cost you £3.76 plus post:

http://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/HSS-Reamers

I always keep a look out for cheap reamers at model engineers shows as you never know when they'll come in handy!

Vic.

 

[lensman57]

Hi,
If you are using an o-ring then the hole needs to be lapped to get it as smooth as possible so the ring doesn't get sheredded in the hole. the easiest way is to use a very sharp reamer with a lot of cutting fluid. Allow the reamer to remove at least 0.1 mm so the hole prior to reaming is drilled under size. You can use a machine reamer on the lathe but make sure your tail stock alignment is spot on and the hole is running true on the spindle as a machine reamer has a very shallow taper compared to a hand reamer, if however the hole is blind a machine reamer is the only choice and good ones are quite expensive. i think the tolerances are quoted in increasing H nos but you need to check this.
You can also use a softer material such as aluminium or brass as a lap with some very fine lapping compound and trail and error is the order of the day here.

Good luck.

Regards,
A.G

 

[Mainer]

If you want to lap the hole, get a piece of brass or aluminum rod just smaller than the hole size. Make a longitudinal slot in the rod just back from one end. (The slot should not extend completely to the end of the rod; both ends should be closed.) Use a screwdriver or similar to expand the slot into a barrel shape such that the rod has a slight drag when pushed through the hole.

Or buy some of these: http://www.acrolaps.com/index_007.htm

Chuck the lap in your mill (I assume) with the part with the hole on the table with space below it. Apply a little lapping compound to the hole, start the mill, and run the expanded rod up and down through the hole at about 200 rpm. If you've reamed the hole, #600 lapping compound may be okay, or you may want to start with #320. Go easy on the amount of lapping compound. An excess contributes to the chances of getting a bell-shaped hole. If needed, expand the lap more if it gets too loose and the hole finish isn't satisfactory yet.

 

[mu38&Bg#]

To the OP, I'd go with a reamer unless the bore is not deep then, I'd bore it with a hand made bar.

Mainer, how does one make sure you don't barrel the bore with that type of lap? I'm thinking about a small pump (think diesel injection), with maybe a 3mm bore. Any leakage would be detrimental. The bore will be deep, maybe 10-12x.

Greg

 

[picclock]

OK I think I'm sold on a reamer. RDG do a set for £16 inc vat and carriage which looks like it could be useful. If I drill with a 3/16 (4.76mm) drill I would think that should get me very close. Its a through hole with a depth of 12mm so not too bad.

I notice the RDG set have far more twists than the ones they sell separately. How does this affect things? is it better or worse ?

I plan to set it up on my mill and do all the operations with the table locked to ensure good alignment.

I like the idea of lapping it to get a really good finish. Have to get some lapping compound as well.

Many thanks

picclock

 

[lensman57]

Hi,
I just had a look at them and they seem to be spiral flute ones which are ok but the helix is quite tight, it should be ok for what you have in mind but do make sure to use good cutting fluid and remove the reamer while still rotating in the forward direction. I have dulled reamers forgetting to do this.

A.G

 

[Ed T]

If the above suggestions don't work out and w/o benefit of knowing what you are making; maybe you could sleeve a rough bore with some DOM seamless tubing like the Brass K&S hobby stuff . Not sure it's available where you are, but I imagine it is.

 

[Mainer]

Mainer, how does one make sure you don't barrel the bore with that type of lap?

Greg

I'm no lapping expert.... The little lapping I've done, a barrel-shaped hole has not been a problem; it's been bell-mouthing.
What I do is put a small amount of lapping compound in the hole, start the part (or the lap) rotating at ~250 rpm, then run the lap all the way through the hole, then back all the way through, froward, back, etc. Don't dwell in one spot and go all the way through so the expanded section of the lap is free of the hole, both forward and back. GO easy on the amount of lapping compound or it will pile up at the ends of the hole, increase the cutting action of the lap, and make the hole bell-mouthed. At least that is what I think happens.

You will probably be able to feel, or at least hear, the lap cutting. As it stops cutting, expand it out a little more and continue, if need be. Periodically you'll want to wash out the hole and see how you're doing. When you're satisfied, give it a thorough scrub so residual lapping compound doesn't remain and keep cutting.