CT Bore Size

[bergerud]

I was making an ER11 adapter for my friend's C machine. I assumed that the bore was going to be a fairly accurate 1/2". I found instead that it was 0.5025". That seems a little large to me. This explains why I could see the bit wobble. I thought it was a poor adapter, but it turns out the adapter is 0.4995" which does seem reasonable. With the side clamping, there is a guaranteed run out of around 0.003". My question is this: Are all of the CT bores this large or did my friend just get a bad one? This is an important question if 3rd party ER adapters were to be made. With the side clamping, such adapters have to be a snug fit. Could some of you measure your bore sizes and post?

[bergerud]

Can someone please measure their CT bore and post? It would only take a few minutes.

[CNC Carver]

CT does not seem to be round. Measures 0.4995" to 05305". Mostly .4995" to .5100"

[bergerud]

Really? That seems too excessive. (I think you have to stay away from the paw hole.) How do the bit adapters feel? Do they fit snugly or do they wobble quite a bit before you tighten the paw on them? If you put a little piece of Scotch tape on a side of the adapter, does it still fit in?

Sorry to ask all these questions but I have spent quite a bit of time playing with a sample size of only 1. I was all set up to do a run out comparison test with my new dial indicator which reads to 0.0001". I thought that if I had a non typical one, I should not post the results. MT reported that his adapters fit very snug and so I considered that this one was a bad example. It sounds better than yours!

[mtylerfl]

Really? That seems too excessive. (I think you have to stay away from the paw hole.) How do the bit adapters feel? Do they fit snugly or do they wobble quite a bit before you tighten the paw on them? If you put a little piece of Scotch tape on a side of the adapter, does it still fit in?

Sorry to ask all these questions but I have spent quite a bit of time playing with a sample size of only 1. I was all set up to do a run out comparison test with my new dial indicator which reads to 0.0001". I thought that if I had a non typical one, I should not post the results. MT reported that his adapters fit very snug and so I considered that this one was a bad example. It sounds better than yours!

I just measured my CT bore with Wixley Digital Calipers. It reads .498

I also have another CT still in the box! I just remembered as I was typing...going to measure that one now...be right back.

- OK, that one measured .495

I have no idea how accurate the Wixley calipers are...I would have expected the measurements to be at least .5

[Jeff_Birt]

Most calipers are only accurate to +-0.001" so you alwyas have to take the reading with a grain of salt. Taking an internal ID measurement is also a bit tricky as the calipers's tips are slightly offset so you can't get them set true on the diameter. There are special micrometers made for taking ID measurements that are more accurate: http://www.grizzly.com/products/Digi...r-2-1-2-/H2691

[TerryT]

Mine was .499 inside and .4985 on the insert. Using a harbor freight digital caliper so don't know how close to accurate.

[bergerud]

Mine was .499 inside and .4985 on the insert. Using a harbor freight digital caliper so don't know how close to accurate.

Those also seem a little small. The difference of 0.0005, however, sounds perfect. (If the bore was 0.499, I do not think you would be able to fit in a regular 1/2 inch shank router bit.) The difference on my sample is more like 0.0020 - 0.0025 which results in some run out.

Jeff, I have to agree (it hurts), these calipers can be a little flaky. I never really trust them for inside measurements for the reasons you stated as well as the fact that the zero is determined by the grinds and is really unknown.

[badbert]

For inside measurements I use my calipers in motion, I try to keep them slightly moving in different directions and take the high reading.