cbn wheel sharpening question

jimbarry · June 21, 2019, 08:36:09 AM

I ordered a 7/39 cbn wheel to use on our WM BMS250MU. What they sent me was this. Note the highlighted red area.

Now refer to this diagram.

My understanding of all this is that 'r' = 7º and 'R' = 39º. Am I right?

If I am correct, then the wheel they send me is 34.5º because if I move the protractor to rest flush on that highlighted area, it measures between 34 and 35º.

When I sharpened a blade using that wheel, this is the result.

Percy · June 21, 2019, 09:08:37 AM

I have had the same problem on occasion with the 13/29 wheel/blades. To remedy this, I set the BMS250 for the lightest face grind possible. After my second pass, the whole tooth is ground. I don't know exactly how the blades are manufactured but perhaps there is a slight variation as some boxes of blades sharpen perfectly on the first grind and occasionally I get the same as you.

terrifictimbersllc · June 21, 2019, 02:39:24 PM

The wheel should have its profile stamped on its side.

I dont know for sure but I would think the back angle is the beginning of the back of the tooth nearest the tip, you would tip your protractor a little bit more to match that. So it would be 7 degrees from vertical and 39 degrees from horizontal, where the back of the blade is horizontal (blade is lying horizontal).

D6c · June 21, 2019, 08:32:39 PM

If the spindle is set to swing at 7deg then the tooth face angle can be undercut and still cut 7 deg. It might be they're putting a relief on the tooth face part of the wheel to avoid burning the cutting tip. I do that often when dressing grinding wheels for form grinding on a surface grinder.

Pabene · June 22, 2019, 04:19:17 AM

I think D6c has the answer. I don't know how the sharpener works in this case but if there is a possibility to adjust the angle for the wheel spindle relative the swing shaft, you would be able to achieve the profile you want with this wheel. If the sharpener has a parallell spindle / swing shaft mounting, it will be hard to hit the profile angles. If the sharpner in case, has a fixed mounted angle (about 4°) between the spindle and swing shaft, it will also be good.
An other thing is that, I can not understand why the bandsaw blades in most cases has the back angle of 30° -35°. In most other saw blades for wood (Circular blades, Blades for proffesionel mills and more) it is reccomended about 12°. I have modified the cam in my sharpener so I can sharpen the back angle on the tooth tip to about 12°. I think the blade keeps sharp much better that way. (I know it can be like to swear in the church to ask this question.)

ladylake · June 22, 2019, 07:50:55 AM

Looking at the pic of the blade you sharpened it looks like you sharpened a 34° blade with a 39° stone. As mentioned above you need to measure the angle right at the tip of the tooth. Steve

nativewolf · July 20, 2019, 10:02:43 AM

bringing this back up because I didn't see a reply to @Pabene question re back angle and I'm trying to learn a bit myself and thought it an interesting question.

Ben Cut-wright · July 21, 2019, 11:16:14 AM

Quote from: jimbarry on June 21, 2019, 08:36:09 AM
I ordered a 7/39 cbn wheel to use on our WM BMS250MU. What they sent me was this. Note the highlighted red area.

Now refer to this diagram.

My understanding of all this is that 'r' = 7º and 'R' = 39º. Am I right?

If I am correct, then the wheel they send me is 34.5º because if I move the protractor to rest flush on that highlighted area, it measures between 34 and 35º.

When I sharpened a blade using that wheel, this is the result.

It appears you have developed/measured angle 'r' and angle 'R' improperly. The angles should be referenced from the perpendicular lines you show as 't' off the band blade body in your diagram.

A measurement of the finished tooth profile after complete sharpening should be withing reasonable tolerances of the wheel profile.