Drilling a hole for a boring bar on a Unimat - How do you go about doing it?

[STEMtheMachining]

So here's the deal, I'm about to do my first machining project on my little Unimat SL.

The job itself is pretty straight forward: turn down a 1 inch rod of acetyl to 25 mm, (removing about .050 if my memory is correct) , make a hole on the centerline of the rod with an ID of 15 mm, and then part them off at 6 mm intervals. Yes, you guessed it, I need to make some new plastic spacers for some chairs that have lost them over the years.

So I've got my acetyl rod, I've got my lathe, I've got my HSS tooling, and now I'm stuck. Here's why.

On a normal lathe, this is an easy deal. Use a spotting drill or a center-drill, then drill it in increasing increments until you get to the final 15 mm ID.

But the Unimat is NOT a normal lathe. The chuck only holds a 1/4 inch shank bit. And a normal 15 mm drill bit is almost as long as my lathe itself.

So I have to go with plan B: boring bar. But that too presents a problem, because the smallest boring bar I can find requires a 5/16" hole, and most drill bits sets are a 3/8 inch shank.

So as I see it: my options are:

a) drill them individually on the drill press after turning and parting them.
b) grinding or turning down a shank on a 5/16" bit to fit and using that for the clearance hole for the boring bar
c) Buying a set of stubby spiral drill bits and still having to turn the shanks down

So Unimat peeps - what do you use to drill a clearance hole for your boring bars?

 

[Liljoebrshooter]

You could try to use a spade bit for wood. It would drill into the plastic easily. Then bore the hole to the correct size.
Joe

 

[STEMtheMachining]

You could try to use a spade bit for wood. It would drill into the plastic easily. Then bore the hole to the correct size.
Joe

Joe, I've enclosed a picture of my lathe with a 6 inch mitutoyo dial caliper for scale.

Also for giggles, I chucked up a 1/4 spade bit just so you can see how small of a space we're dealing with here. I actually had to take the tailstock off to accomplish this.

Sadly, I don't think that is going to work.

 

[Liljoebrshooter]

I would cut the shank off of the drill bit.
Joe

 

[Jim F]

You could grind your own boring bar from a drill bit, or reamer,or endmill.......

 

[Jeff.64]

Here is a source of 1/4" shank drills.

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

Maybe use a 1/4" shank, center cutting router bit or Micro 100 boring bar after using a 1/4" drill.

Amazon.com : micro 100 boring bar

https://www.amazon.com/Micro-100-BB-1801500-Diameter-Projection/dp/B00Q8KO22S/ref=sr_1_3?crid=30KOVQQ7CRAGZ&keywords=micro+100+boring+bar&qid=1673482486&sprefix=micro+100+%2Caps%2C89&sr=8-3

I've found good deals for Micro 100 boring bars on E-Bay.

 

[STEMtheMachining]

I would cut the shank off of the drill bit.
Joe

I thought about this, and you're right, it would solve the problem and allow me to do the work.

But unless I'm given no other choice, I want to do this the proper way and actually machine the parts to within 1-2 thou of their intended dimensions.

 

[markba633csi]

A trick I've used in the past is to use an endmill as a boring bar- works great
-Mark

 

[ErichKeane]

I'd just grind my own out of a piece of HSS. Grab a broken 1/4" drill bit, grind the top half of it off and add a bit of a relief under the cutting edge, and you're all set!