Drill size for 1/8" reamer in brass

[deere_x475guy]

Hey gang I need some help here. I am reaming the 1/8" holes for pistons on my Siamese Twin. What drill size should I use so that I am not to small or to big of a hole to ream? Did a quick search and didn't really find what I needed. Also these are brass pistons.

 

[compound driver]

HI
H7 reamers are designed to take a few thou at most so id look at somthing in the order of .120 - .122 as a drill size. taking more blocks the flutes and tends towards an over size hole. Cylinder bores dont need reaming rather the bore should be honed after. That said a 1/8th bore would lend its self to reaming.


Cheers kevin

 

[Bogstandard]

Hi Bob,
Machine or hand reamer?
Because I invariably use machine reamers, I would wack the hole thru about .5mm smaller (0.020"), and chunk it out under power, but in your situation I would make it about 0.004" to 0.008" smaller so if you use a 3mm or 3.1mm drill this should leave you about 0.004 to 0.008" to clean up the hole with a reamer.
Some people might say go to 0.002" undersize, but a wobbly drill can easily take you oversize to start with.
The difference between the hand and machine reamers is that the hand held one has a tapered lead in that can be as much as 1/3rd of the length of the reamer whereas a machine reamer usually has just a small lead in, I have some carbide ones with no lead in at all.
I always say that by using a hand reamer, your hole just might be slightly oversized because you can't easily control the side pressure that you put on it by doing it by hand, especially in the smaller sizes.

John

 

[deere_x475guy]

Bogs and compound driver thanks for the followup.

Bogs I am using a chucking reamer and what I am doing is making the hole for the wrist pin. I know about the drills making an oversize hole and thats why I asked about this. I didn't want to go to large and still be oversized. :roll:

 

[compound driver]

HI
Twist drills with a good grind and nicely sharpened should cut bang on but in reality almost always cut a few thou under size. This is the amount your looking to ream out and not turn reaming into a second drilling operation. The best way is do a test piece and make up a go no go for the hole you drilled keep the drill in the chuck so your not changing the setup. Going .5mm under is almost 20 thou and im my opinion much too much to give a true reamed bore.

If a drill makes an over size hole you should be looking for the cutting faces being un even (yup new chinese and cheap drills) or the rake being much too steep.

90% of the work I do goes on to be checked with air gauges before I get my money for it. Hitting a perfect size even with a good setup and a very good reamer isnt easy. Many times I look at a job and for the life of me dont know how I hit the tolerance required by the customer.

We all have different ways of doing jobs so there isnt to be honest a right or wrong way just as long as the results are what we were looking for.
In truth most of the work we do with model steam would work happily for many years if we only aimed for +- 5 thou. But we always try for
+- .0005 I suppose thats a testament to model engineers in general.


Cheers kevin