Originally Posted by
Chris D
I agree with the comments that something changed between when it was working and now. However, the original process, based on the small amount of details provided, is fundamentally wrong. Drilling a hole for a "slip fit" is not the proper process. Twist drills are not even made to an accurate enough diameter to do this consistently.
The videos showing the indicating process really show us nothing. You have run-out, apparently more as you get further away from the spindle. Does the tool holder fit tight in the spindle? Have you put a shaft in the chuck that you know is straight and round and measured that to see if there is run out? Who sharpened the drill, factory or by hand on a bench grinder? It is very strange that a 6mm reamer is also cutting "Still too oversized ".
I believe you are making a lot of assumptions and you are attempting to do precision machining operation yet you have no way to measure what you have with any level of accuracy. Using terms like "Way looser" doesn't mean much in the world of precision machining.
I would suggest you do the following:
Find a straight 6 mm shaft that you can chuck up with your various tool holders. Put an indicator on it like you have in the video and rotate the tool by hand so you can actually measure to a value as opposed to watching the indicator's needle wobble back and forth.
Check to make sure that you can't deflect the tool holder by pushing/pulling on it sideways while you have the indicator on it.
Assuming everything runs true and your spindle and tools holders are not messed up, review your speeds and feeds, have they been changed since it last worked?
You mentioned that you have tried a new drill and the old drill both with bad results. You also stated that drilling undersized and reaming resulted in poor results. Did you check the size of the hole before the reamer performed its operation? The drill could have drilled oversize and you would believe that the reamer is the problem.
Finally, getting back to the proper process.
In order for a drill to cut on size, it cannot wander when starting the hole. Without a spot drill, the probability of the drill wandering is increased substantially. I would recommend spot drilling to help prevent wandering and the ill effects that will cause.
The drill must be sharpened properly, if it is not, it will NOT cut on size or close to size. If the drill measures 6.1mm in diameter, do NOT expect it to drill a 6 mm hole. In all likelihood, a drill will cut bigger than it is. MEASURE THE DIAMETER OF YOUR TOOLS WITH A MICROMETER so you know what you are starting with. Common twist drills are NOT designed for making holes so close to size you can count on them to give you a "slip fit" every time. They are a roughing tool which is why Reamers are used!
Reamers are ground to very tight tolerances and can be purchased for under/over size applications. They are designed to cut: straight, round, and accurately with regard to diameter.
In my very humble opinion (based on a few decades of doing this) you should:
Spot drill
Drill the hole under size by about .3 mm
Ream the hole with a 6.025 reamer
Be sure to use tool holders that run true
Be sure to use proper speeds and feeds for the materials you are cutting
Be sure to use proper lubricant when cutting
Don't expect the wrong tool or wrong process to perform magically all the time, sometimes luck happens though.
Chris D