View Full Version : keeping DTIs perpendicular for runouot checks
Maybe a daft question, but I'll ask anyway.
Is there an easy way to keep a DTI stem perpendicular to the end of the shaft on which run out is measured? This is not on a lathe and access is a bit problematic, also the chassis is out of ali so I can't easy mag base the DTI in place. Attachment shows how the runout is measured and what errors I'm expecting.
11-03-2014, 02:01 PM
Do you have a good heavy vice or somesuch that would grip the DTI stand, or a lump of steel that the mag base would stick to? I have the same problem with my MDF bed!
Indeed Neale, I do have a bigish angle plate or a big long V-block with a detachable arm for a DTI. So that side is covered, it's only getting the DTI perpendicular to the end of shaft that is problematic.
11-03-2014, 03:23 PM
One other point is that that it depends on whether you are trying to measure runout accurately or just centre a cutter or piece of work. For example, if I'm setting up something in a 4-jaw chuck, I'm only looking for zero needle movement and actual value doesn't matter too much, so so bit of misalignment is ok. But if you are measuring actual runout...
Is the problem that you can't see the point of contact, so you can't see if the plunger axis is aligned with the shaft you are measuring?
11-03-2014, 03:28 PM
Set up DTI with base and support block, etc, so plunger is horizontal. That's easy enough clear of the spindle. Put DTI on bed and move carefully up to spindle so plunger starts to move. Rotate whole block/DTI "lump" slowly and carefully about the centre of the supporting block so the plunger moves across the shaft you are measuring, and the max plunger movement corresponds to perfect alignment. That might work...
Set up DTI with base and support block, etc, so plunger is horizontal.
That's the issue Neale, how do I know if plunger is horizontal? is it good enough to just set it by eye and assume it's horizontal? or is there some way to have it 100% horizontal, parallel to the surface on which the mag base is?
The other issue is that I have to points off which I can measure, one is a surface which is perpendicular to the shaft, the other surface is an unknown, it's definitely not perpendicular to the shaft as it's slightly off. If I measure off the first surface, all I need is the plunger to be parallel in relation to the surface, then it's perpendicular to the shaft. If I go with the second surface (much easier, more room), I need to set the plunger in relation to the shaft as I have no other point of reference.
11-03-2014, 08:33 PM
You're the man to decide what's "good enough", but as a guide, a 5deg error in alignment will give just under 0.5% error in measurement. 10deg error is about 1.5% error. Personally, I just set the thing by eye, and as long as you can align it against a known horizontal line on the machine somewhere, I doubt if you'd be far out. I'm not quite sure what you mean when you talk about setting the DTI in relation to the shaft, though - can you provide a little sketch or something?
I simply thought there's a method / tooling to allow to set the DTI perpendicular to the shaft, I can go by eye it's not a biggie, I was just checking to avoid a duh moment later.
As for the shaft related question, ignore it, I made it too complicated than it should be:)
If you need it very accurately centered, then the only real way is a suitable jig.
However as Neale says, by eye is usually good enough.
I've decided to go with a lever DTI, which makes things a bit easier and more accurate. I can always clamp a small V-block to the end of the shaft, align with that and remove the block to allow for measurements. Will give this a go this weekend.
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