Press Fit / Reaming Sanity check Please

Boswell

Boswell

Hobby Machinist since 2010
H-M Lifetime Diamond Member
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Feb 27, 2014
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I have two press fits that I need to make and would appreciate it if anyone would check my tool size selections.
  • First Operation is I need to press a .2500 drill rod into aluminum
    • I will use a .2497 diameter reamer
    • I will predrill the hole using a 15/64" drill (.2344)
  • Second Operation is to press in a .5000 drill rod into aluminum
    • I will use a .4995 diameter reamer
    • I will pre-drill a hole using a 31/64" drill (.4844)
Precision reamed holes are a new experience for me so any advice and guidance (or even funny stories) would be a great help.

thanks


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I think your drill sizes are too small. I'd go with a C drill for the 1/4" hole for sure. There isn't a closer imperial drill size to 0.500", so I'd personally start with a more undersize reamer, like a 0.498 and then ream with your final size reamer. ALso depends on what machine you're doing the drilling and reaming in - a mill will be more accurate than a drill press - and how you're pressing in the pins.
 
Boswell said:
I have two press fits that I need to make and would appreciate it if anyone would check my tool size selections.
  • First Operationis I need to press a .2500 drill rod into aluminum
    • I will use a .2497 diameter reamer
    • I will predrill the hole using a 15/64" drill (.2344)
  • Second Operationis to press in a .5000 drill rod into aluminum
    • I will use a .4995 diameter reamer
    • I will pre-drill a hole using a 31/64" drill (.4844)
Precision reamed holes are a new experience for me so any advice and guidance (or even funny stories) would be a great help.

thanks


Note: for your browsing pleasure, possible Subject Drift has been pre-approved for this thread :fat:.
Click to expand...
Those sizes will work. I used a .2495 reamer for .2500 steel pins in aluminum recently.
 
Boswell, it sounds like you already have the reamers so you need to use the right drills in order to hit the fit tolerances you're going for.
  • For the 0.2497" reamer, you need to use a letter D drill. This drill will get you close to the 0.003-0.005 material to be removed with that reamer. You want to spot the hole, then go straight to a C drill, then use the D drill and then the reamer.
  • For the 0.4995 hole, I would use a 12 mm drill to debulk, then a 12.5 drill as the prereamer drill, and then the reamer.
Keep your reaming speeds low (100-200 rpm) so you can keep up with the feed; you should be able to feel a slight resistance to the feed. If possible, do all of this reaming stuff on the mill for best accuracy. Use lots of lubricant (WD-40 works good, Relton A-9 is even gooder). I recommend you ream only on the way in and do not bottom out your reamer, ever. Shut the machine off before retracting the reamer for best accuracy; the reamer will cut on the way out and there goes your 0.003-0.005" tolerance.

Do a few practice runs if the fits are critical.
 
Most drill shafts (of my drills) are a bit thinner than the actual drill size.
 
Thanks for the suggestions so far. To answer a few questions.

I will do this on my PM45M-CNC mill
I would spot drill
then drill, (still working through comments on best drill bits to use) under CNC control (I have 10 of the .25 holes to drill and one .5 hole.

Then not sure about the reaming. I would use the CNC to align with each hole but might feed by hand. then again the CNC feed would give a very repeatable smooth feed, I'll try both methods when I practice

I do plan to practice on some scrap.

I have no problem using 'C' then 'D' to pre-drill. for the .250 hole
I don't have any metric drill bits but drill bits are easy to get so I may try the 12mm then 12.5mm for pre-drill on the .500 hole.

Also, thanks Mikey for the reminder about speed for the reamer. I probably would have been running 3x or 4x that without knowing better
 
The speed I recommended is mostly so you can keep up with the correct feed rate. Reamers don't like to be rushed or they cut oversize.
 
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