Hole pattern positional error

[Corn]

Hi,

I faced this issue in hole positions when drilling two hole pattern to bronze. I got two parts which slide against each other by two 6mm axles. This far both have been made from Aluminium and I haven’t had any issues drilling exact positions to that material. However now I tried to do the other part from Bronze (SAE 660 / C93200) but for some reason the distance between the two holes became 0.1mm (0.0039”) short. Needless to say that is way too much for them to match in playless installation. Drill cycle goes like this.
1. Spot drill dia 10mm 90deg 1.5mm depth
2. Peck drill hole dia 5.8mm
3. Ream by 6mm reamer

Can’t find anything wrong with the G-code. It’s 24mm between the two hole and not 23.9mm.. Same tools used to do this to Aluminium without issues.

What do you think is causing this? Is this material more challenging? Is it the 90 deg spot drill and 118 deg drill bit which cause the drill to wander and miss the spot, or something else?

Here’s the G code:

(DRILL1 10mm carbide spot drill 90deg)
T1 M06
S5000 M03
G54
G00 X5. Y29.
G43 Z15. H01
G00 Z5.
G98 G81 X5. Y29. Z-1.5 R5. F300.
Y5.
G80
Z15.
M05
G53 G00 Z0.
G49

(DRILL2 5.8mm HSSCo drill)
T4 M06
S2210 M03
G54
M08
G00 X5. Y29.
G43 Z12. H04
G00 Z5.
G83 X5. Y29. Z-26.742 R5. Q10. F122.
Y5.
G80
Z12.
M05
G53 G00 Z0.
G49

(DRILL3 6 flute HSS reamer)
M09
T11 M06
S1523 M03
G54
M08
G00 X5. Y29.
G43 Z15. H11
G00 Z5.
G85 X5. Y29. Z-23. R5. F150.
Y5.
G80
Z15.

Thank you in advance.

 

[13engines]

A short length 120 degree spot drill is a 118 degree drills best friend, and will follow it anywhere.

 

[Mfg_Engineer]

Definitely use a greater spot drill angle then you drill, like a 120 degree. Also, you can position the hole with and undersized endmill first after drilling, then ream. We do this to ensure hole location on critical parts.

 

[CITIZEN F16]

I haven't ran any bronze in a while so I cannot remember the difference in types, some were definitely worse than others. There isn't any bronze on the planet that can be machined as fast as 6061 aluminum, not even close. Many types of bronze are very abrasive, check your drill and spot for wear and slow your SFM down. Your code other than feeds and speeds is not the issue, your drill is walking because of a dull spot/drill and going too fast.

 

[Corn]

Those feeds and speeds were from HSM advisor (50%) and not the same I'm using for Aluminium.

So would the ultimate accuracy be achieved by following? By smaller endmill bore the hole same size or hair smaller than the drill and depth so that the drill land is supported by the milled hole. Then drill by twist drill to specified depth.

 

[DDoug]

Those feeds and speeds were from HSM advisor (50%) and not the same I'm using for Aluminium.

So would the ultimate accuracy be achieved by following? By smaller endmill bore the hole same size or hair smaller than the drill and depth so that the drill land is supported by the milled hole. Then drill by twist drill to specified depth.

Why doo you want to keep using the &^%$#@! drills after endmill ?

Drill, Endmill, and then ream, as you were advised up at #3

Period, end of sentence.