drilling grade 8.8 metric bolt 2

[scottmaha]

i need to drill a 3.2mm hole through a grade 8.8 metric bolt. the bolt is M8 x 55mm long. the bolt is a production part. i have 1200 psi coolant pressure through the spindle. i would like to drill the bolt from one side all the way through. this has proven difficult. the only thing that is kinda working right now is drilling the bolt half way from one side and then completing the hole from the other side.

anyone have any suggestions? it would be gratly appreciated.

scott

 

[daveaberle]

Hello my name is dave aberle. There are several points to check before deciding what methods to best modify your process. First; is the part to be machined held absolutly solid with no side deflection from drill pressure or pull from drill break thru.(Is there appreciable back lash in the drilling slide that would allow either catching or plunging of drill on break thru.) Second; a hard encirling bushing is needed, with true in line and on center drill bushing holes to minimize drill run out or part deflection (remember to have a flushing coolant system to ensure clear chip free part placement on next part. Also keep the drill as short as possible, yet still allow for chip removal) Third; if your problem is stringy winding chips give the drill several pauses or perferedly pull backs.(Programmed or mechanical) But don't pause long enough to induce work hardening. Once these variables are resolved drill breakage should be reduced to the point where you can truly equilably test drill materials, feeds and speeds and drill geometry. Keep a log of tooling changes or other modifacations to develope the best combination over time, as there is no best solution anyone can tell you except your people running your equiptment with continuing testing and evaluation of tooling life data. Hope this helps and if you know any expierience in severe interupted cut machining of stellite it would be appreciated. Thanks.

daberle@sympatico.ca

 

[Hush]

Drilling a 3.2mm hole in a 8mm bolt doesn't leave you a whole lot of room for error. I'm not too familiar with metric fasteners but I'm guessing that only leaves about .2mm wall below the thread root. If your application allows for a stud and nut instead of a bolt it might be easier to thread a piece of high strength 8mm tube instead.

 

[CoryPad]

The minimum minor diameter for an M8 screw is 6.272 mm. Thus, a 3.2 mm hole would result in a wall thickness of 1.536 mm.

 

[Hush]

Oops, factor of 10 error.

 

[gunsmith]

Hi Scott,

the only way I know (without having a TBT-machine with self-centering canon-drill and drilling sleeve) is to use a center drill (extra short) of 3.2mm first followed by an extra-long 3.2mm drill (there are different lenght in the GÜHRING-Catalog up to 200 mm - take the shortest possible for 55mm drilling lenght). Maximum feed for the drill is 0.01mm per round, otherwise You will fail. Program the cycle to discharge of chips every 3mm (!).

Good luck

Andreas

 

[Viktor]

First of all, I do not see any problem in such drilling. This is a normal application unless the bolt is heat treated to high harness.
Second, you’ve never mentioned what kind of drill you are trying to use, its point geometry and tool material.
Third, what rpm and feed per rev you are trying to use.
You’ve mentioned that you have 1200 psi coolant pressure through the spindle. What for? It is only good if you use a drill with internal coolant supply. What kind of coolant you are using (water soluble or oil based)?
If you use a gundrill (not cannon), please check

http://gundrilling.tripod.com

Viktor

http://viktorastakhov.tripod.com

 

[CoryPad]

The hardness for a property class 8.8 screw is:

250-320 HV/238-304 HB/22-32 HRC

 

[gunsmith]

Hi Viktor,

excuse me for translating wrong from German, won´t happen again to say cannon-drill to the gundrill. But there is questions left I want to ask You. If there isn´t any problem to drill a hole of 20 diameter length with standard equipment - what is the sense of all these hightec special deep hole drilling machines on Your website?

Andreas

 

[scottmaha]

corypad is correct about the bolts hardness. the grade 8.8 metric bolt has a tensile strength of 120,000 psi.

i have used a drill from guhring already. however, i think the design of the drill did not lend itself well to my particular application. it is guhring's GT100 deep hole drill. the drill works well for low carbon steels and other long chipping materials. it is a TiN coated, HSS drill. i ran the drill from 110 sfm down to 60 sfm. i've tried pecking every 3mm down to every 1mm. i am aware that i could be work hardening the material by taking such small peck amounts, but at this point i would try any thing.

also, i tried a through coolant drill from CJT/KOOLCARB on aa a test. the drill was solid carbide, through coolant. it is primarily used to drill cast iron and aluminum. it is a straight flute, G-drill. i was able to put coolant through this tool, but still experienced tool breakage at deeper depths. i ran this tool at a recommended 120sfm at .001ipr.

heat is definitely my enemy here. i have acheived my best results by drilling the hole half way, then flipping the part and completing the other half. i would rather do the part from one side, but the flipping may become a necessary evil. the vendor we were getting them from was drilling them from both sides. they decided that the $.75 a piece they were doing them for was not enough.

i have only tried to come up with quick solutions so that i can provide assembly with these parts. however, i have plans to gun drill these parts on my vertical machining center. i am talking with guhring right now aboout using there gun drills. i plan to use a pilot drill that is .0005 - .001" bigger than the diameter of the gun drill. i will then feed the pilot drill to a depth no greater than one and one half times the diameter of the gun drill diameter. i will then feed the gun drill into the pilot hole until the guide pad of the gun drill is into the pilot hole. i will then start the spindle and the coolant and proceed to drill the hole at 5300 rpm at 2 ipm.

viktor, it sounds as if you could do these parts with little or no problem. i require about 1200 of these little jewels per month. they are yours if you can do them for $.75 each.

regards,
scott

 

[Viktor]

D-drills are good for shorter holes so I do understand why you had problem with them.
I think that gundrilling is the best solution for you. We have implemented gundrills of similar diameter (3.1 mm) for similar application (fasteners) on standard CNC machines with pre-drilled pilots with great results. I would say that the regime you’ve mentioned is reasonable. I would like to tell you that you have to have a coolant system capable of delivering coolant at 1200 – 1400 psi to assure sufficient flow rate for chip removal. Good coolant filtration is also needed for good tool life. I would also recommend you to buy a good drill holder.

About Guhring. It is a good company but I am not sure that they produce gundrills of this small dia in Brookfield, WI. I would recommend you to contact a few other domestic companies to compare delivery and costs.

Specially for Gunsmith

Canon-drill and gundrill are different drill according German terminology although both are used for deep-hole drilling. The former has only one cutting edge located with zero approach angle and is suitable for drilling of large diameter holes. The gundrill has two cutting edges (the inner and the outer) and it is suitable for drilling small diameter holes.

Gundrills are not always require a special machine for their application. More and more often they are used on standard CNC machines. A special machine is required if one needs a special hole in terms of straightness, surface integrity and diametric tolerance. For example gun manufactures do not want to produce guns capable of shooting over a corner. Another application for special machines is high-volume production (for example, in the automotive industry – engine heads, cranks, gas pump, etc).
Viktor

http://viktorastakhov.tripod.com

 

[deeptuna]

Hello Scott,

One other though that seems nobody mentioned here.

Try EDM machining. They could punch small holes in high tensile steel such as the 1 u have. Obviously you have to look at the economics of drill vs EDM.

 

[Viktor]

and it will take from here till doomsday and cost...well, you know. Viktor

http://viktorastakhov.tripod.com

 

[gundriller]

Hey Scott.

Depending on the volumes, this looks like a gundrilling job. The length / diameter ratio would justify this. It should be not too difficult to drill from one side. Your coolant pressure is a bit low, but you can try by drilling a pilot bore that is very close to the 3.3 (Only 5-8 microns over size) Drill for 1.5-2x the diameter. Go in with the gundrill slowly rotating without coolant. When the tool is guided in the puilot hole, put the full coolant pressure to the tool and start rotation. Get yourself a good tool such as a TBT or Nagel gundrill. (I think they also make machines to do this kind of work.)

Let me know if this worked.

 

[trespasser]

Have you thought about getting the bolts in a pre-hardened, or annealed state? Then harden them later, after drilling?