[How-To] Pecking. For drilling, yes; how about for reaming?

JustMe3

JustMe3

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I am drilling a 0.3750" diameter hole 1.250" completely through a piece of T6 Aluminum. The notes I have for this state that the dimensions on this hole are critical.

So I am planning on under drilling the hole at 23/64th (0.359375") diameter and then use a precision reamer to get it to 0.3750". My drill code uses a standard "pecking" cycle. Taking 0.020" depth each peck, doing a full withdrawal, then repeat all the way through the block.

My question is what methods exist for reaming? Do I peck with the reamer, or just slowly move through the piece in one motion?

Any suggestions, pointers, comments or ideas are appreciated.

Thanks is advance to all responding.
 
My understanding is that pecking is to help clear chips so that you're not recutting or packing chips that create problems. I don't think you have that issue with reamers as you're taking a fairly thin cut with plenty of fluting in the reamer to handle chip clearance. Note - I'm strictly an amateur!! so take what I say with a grain of salt.
 
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Offhand, a 0.020 peck with a 0.359 drill seems way too small. Otherwise, I agree with rabler.
 
The gcode was generated by the software the model was held in. Here is a sample with just 6 "pecks" with the different depths and feed rates. The Z zero is the surface of the piece.

Z.200 F20. Z-.150 Z-.180 F2. Z.200 F20. Z-.170 Z-.200 F2. Z.200 F20. Z-.190 Z-.220 F2. Z.200 F20. Z-.210 Z-.240 F2. Z.200 F20. Z-.230 Z-.260 F2. Z.200 F20. Z-.250 Z-.280 F2.

I too am an amature learning from others.

Any suggestions on a feed rate to use on the reamer? My thoughts are about 1.0 in/min.

Clearing chips is not a problem. I have set up a system that delivers compressed air from one feed and lubricating fluid (WD-40) from another. So I can mix them at the jet with any ratio from straight WD-40 to straight compressed air and any mix in between. My best results have been from a good compressed airflow with enough WD-40 to keep the part and tool coated with mist. Then the WD-40 flows down to the reservoir for separation and reuse.
 
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Don't peck with a reamer, and use a fairly fast feed, if you get too fine with the feed, they have a tendency to cut a bit oversize. Try one test hole and see how the chips behave, hopefully they will be forced out ahead of the reamer and not pack tight in the flutes.
 
I did a CNC job many years ago that required a very deep reamed (blind) hole. To deal with the chips packing and heat, pecking (with dwells at full retract) had to be used. I can't think of many circumstances this method would ever be needed, and I would not do it for a through-hole.

The secret to our success in this instance was to make sure the reamer was dead straight. We indicated at the collet to be sure it was centered at full depth and we bent it as straight as we could to keep from smashing the tip on peck re-entry. It worked, but it was tedious.
 
DavidR8 - thanks for the link. Turns out my first intuition was exactly what is on the table, 23/64th.

benmychree - Being very new to CNC, what is your suggestion for a "fairly fast feed" in inches per minute?

I am cutting with the drill at 2.0 in/min and a spindle seed as fast as the machine can provide. This yields a good finish in the hole.

I have several scrap blocks of aluminum I can start testing on.
 
I really can't say, because I am strictly a manual machinist, and just go by feel when hand feeding, and am fairly aggressive with feed pressure.
 
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