Enlarging holes

[Bill ThompsonNM]

I need to enlarge 8 holes in a Delta RAS base as part of my renovation of an early 50's saw (sneak pre-gloat). The holes are about 3/8 inch, originally used to mount legs to the base but I'm going to use 1/2 inch bolt supports so I can adjust the whole table to be the same height as my shaper which sits nearby. When I try just drilling them out with a 1/2 inch bit, it chatters and they become irregularly shaped. Suggestions?

[Thomas Hotchkin]

Bill
You could use a step drill. Start at 3/8" drill up to 1/2" . Tom
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[Clint Baxter]

Irwin makes a Unibit that works great for stepping up hole sizes. Should be available wherever you can buy Irwin tools.


I see Thomas beat me to it. Unibit is Irwin's name for their step bit.

Clint

[Rollie Meyers]

I also agree w/ the responses, a step drill will be the best choice but buy a good one, the HF ones are cheap but the larger ones do not perform well.

[ray hampton]

the steel may be hard which will cause a chatter

[Rick Fisher]

A step drill is the right tool for that application..

[Floyd Mah]

Work hardening can occur with repeated attempts to enlarge a hole with a drill bit. I had an interesting experience with this years ago while trying to make a mobile base using bed rails. Unknown to me, those bed rails are extremely hard to drill (or cut). I managed to drill a few holes, but when I got to the last few, the drill bits had become dull and it became increasingly hard to drill. I was drilling progressively larger holes, working my way up to the final size. I thought that I could get by the last step by buying a new sharp drill bit (this was before I had a bench grinder). I quickly discovered that I was wrong. I destroyed the new drill bit trying to enlarge the hole. The metal became red-hot with my drilling attempts. I resolved the problem by fitting a smaller bolt. I learned what work hardening was a few years later when discussing my experience with someone else.

I learned a few lessons. Bed rails can be extremely hard. I learned to cut them using cut-off abrasive discs on a grinder, after destroying a few metal-cutting blades in a Sawsall. As someone once said, do something different if the last thing didn't work. Abrasive techniques can also be used if metal cutting tools don't work. Try a step drill bit, but if that doesn't work, use a grinding tool like a Dremel to finish your work. Hand filing with a chain saw file might be a choice. Fitting an abrasive bit (cylindrical or conical) in a power drill might do the job. If you have already distorted the holes, grinding might be the best (cheapest) choice.

Anyway, a few suggestions in case nothing else works.

[Mike Heidrick]

How thick is the steel? Could you use a knockout punch?

[Bill ThompsonNM]

Too thick for a knockout punch I think, very heavy steel as expected at that time, a bit thicker than 1/8 inch. It is hard, might be work hardened from the original factory holes. I'll try a high end step bit if I can find one today, then may have to resort to drawing some circles and breaking out the dremel and files (groan)

[Michael W. Clark]

Another + for Unibits (not HF) if the steel is not too thick. I have not had good luck with them when the steel is thicker than the steps on the bit. If the steel is thick, drill with 7/16", then the 1/2". Don't take it easy, use high speed and push hard, don't be timid. The quicker you do it, the less heating and work hardening. It is important to have good quality drill bits. I had this experience when drilling some heavy gauge SS. The cheap bits just spin and make it glow, then drilling a hole is almost impossible. If the bit is cutting, the metal doesn't get as hot.

You can also use TapMajic as a coolant if you have thicker steel and this may be advantagous if you are using a DP with slower speeds and more force. The comments above are based on the assumption you are using a hand drill.

Mike

[Bill ThompsonNM]

Thanks to all! I bought an Irwin set of three that included one Cobalt Steel bit for thick metal and it zipped right through all 8 holes. I always figured they were some sort of gimmick that should be advertised on late night TV! So my construction continues, pictures when the rehab is done.

[Gary Stamper]

I would use a drill bit with a flatter angle. A 135 deg bit will cut much better on harder steel and when enlarging holes. It would be better to use a drill press and clamp the work down while drilling. Use a drop or two of oil to lubricate it. Getting the hole lined up as close as possible also helps to eliminate chatter.

Gary

[ray hampton]

I am glad that you succeed in getting out of the hole