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DICKEYBIRD
05-25-2011, 04:16 PM
Next project for the little CNC lathe is to true up its ER32 chuck taper. I thought originally it was close to perfect but after further review, it looks like it's consistently out about .0015". I've removed the spindle a couple times and repositioned it in the bearings but it stays the same. I must've missed something in my setup between centers when I turned & polished the shank down to fit the bearings.

Anyhoo, I'm going to mount a HF pencil grinder in the tool holder and grind off teeny amounts via CNC control. I have a brand new Dremel AO wheel that's about 5/8" dia by 3/8" thick. I will dress it true before attempting any cuts.

Should the grinder be mounted parallel to the spindle center and the edge of the wheel used to cut the 8 deg taper or should it be mounted at an angle? Any guess on a good lathe spindle speed? The grinder alledgedly turns 35K but my compressor only does 90 psi so I'm thinking it'll do 20-25k.

Any other thoughts on the process? Obviously I'll be making some trial cuts and checking the angle with a collet with a dowel pin slid into it and some blue transfer.

gzig5
05-26-2011, 10:05 AM
I'll give it a shot, and it is worth what you paid for it. Mount the stone parallel to the angle you are facing so you have the full face of the stone working. Otherwise the corner is going to wear off quickly. As for speed, I don't think you'll go too fast with that little stone. It's the air pressure that will determine the speed. The volume capability of your compressor will determine if it can keep the pressure up based on the cfm consumption.

tdmidget
05-26-2011, 10:19 AM
The lathe should rotate about 50-100 rpm. Probably not critical. Your weak point is the bearings in any die grinder, especially HF. They are probably just bushings with a lot of slop. If you encounter chatter, stop before you ruin it. Try to take off about .0002' at a pass and spark out frequently. If it does chatter, fall back and regroup. Can you get a real tool post grinder? Might be your best bet.

DICKEYBIRD
05-26-2011, 11:24 AM
Thanks for the input gents. A "real" T/P grinder isn't an option...no room for it as this is a very small "lathe". 50-100 rpm isn't a problem; I was thinking it should be pretty slow as well, thanks.

I've seen a couple posts here and elsewhere showing similar jobs being accomplished with the little grinder and they seemed to have worked OK. I flushed it thoroughly with oil when I got it and it runs very smooth....at the moment. Here's hoping it stays that way long enough to do the job.

I'm going to make a sturdy mount for it and dress the stone as accurately as possible to help things along. Definitely going to make very small cuts though.

One other question, should I try to dress a small radius into the edge of the stone?

tdmidget
05-26-2011, 01:23 PM
A radius should not be necessary , especially as the total removal is so small. But if you don't have to grind to a shoulder it won't hurt either. The grinding points commonly available will be of unknown hardness and such but for this tiny stock removal light cuts and slow feed should render these things moot.

gzig5
05-26-2011, 04:35 PM
Is it safe to assume that you confirmed the runout is in the spindle by using a tenths indicator in the bore and it's not from a funky collet or shaft you are measuring from? Make sure your compound angle is correct using an indicator on the inner bore and I think it is important that it be on center line because of the taper. When you are ready to grind put a light coat of dykem or sharpie pen on the bore and go until the marker just cleans up. Good luck.

DICKEYBIRD
05-26-2011, 05:00 PM
Is it safe to assume that you confirmed the runout is in the spindle by using a tenths indicator in the bore and it's not from a funky collet or shaft you are measuring from? Make sure your compound angle is correct using an indicator on the inner bore and I think it is important that it be on center line because of the taper. When you are ready to grind put a light coat of dykem or sharpie pen on the bore and go until the marker just cleans up. Good luck.Yup, definitely checked it several times on the taper itself.

Good plan on the other stuff too; thanks!:)

jugs
05-26-2011, 05:53 PM
Use a shop vac, mounted near grinding wheel to get most of grit, cover all ways with oil soaked newspaper to catch the rest.
You dont want to make grinding paste :eek:

john
:)

DICKEYBIRD
05-26-2011, 06:36 PM
Use a shop vac, mounted near grinding wheel to get most of grit, cover all ways with oil soaked newspaper to catch the rest.
You dont want to make grinding paste :eek: Absolutely....Good idea on the newspaper. Fitting use for the news we get these days!