Finish Thickness

[VirgilJ]

My dead center arrived today so I'm ready to start turning between centers. My plan is to use the bushings as a guide and to turn to size using calipers.

My question to those of you that use calipers for sizing is how much do you allow for the finish? I realize that it can vary quite a bit depending on how many caots are put on.

Currently I use two or three coats of thin (depending on grain) followed by four coats of medium.I then level with 400 grit Abranet and micro mesh, then polish.

I'm guessing I need to finish sand the wood to about 8-10 thousands under size.

I'm not sure this is worth the effort because there's quite a bit of variablity in the size of the parts. How close do I need to be to part size to achieve an excellent fit?

 

[its_virgil]

Turn a scrap piece of wood round then sand and apply your finish as you normally do. Measure before applying your finish and then again after the finish is complete. Subtract and divide by 2 and you will have the thickness of your finish..data that will work for you. Even if you and I use the same number of coats individual layers may not be the same thickness. I think your question is best answered with data from your shop.
Do a good turn daily!
Don

My question to those of you that use calipers for sizing is how much do you allow for the finish? I realize that it can quite vary a bit depending on how many caots are put on.

 

[Ironwood]

What works for me is I take the blank down to 0.3mm ( 12 thou. ) undersize, then just keep adding coats of CA until I am about 0.2mm oversize, then sand back down to size.
Before doing it this way, I wasnt so particular with the measurements and I was regularly sanding through in spots, or my finish wasnt thick enough to sand out all the low spots.

 

[plano_harry]

Don's recommendation is the best, but if you are lazy, .008 is probably a good guess. Your thickness will probably vary a bit between pens. Best practice is to measure the CA after your 400 grit and compare with the mating part. If you are low add more finish and resand. If you are high, take a little more off the ends with the 400 until you get it right. MM won't change it much after that.

 

[toddlajoie]

For me (and this is just me, if it works for you that is fantastic!!), the .008 to .012 (!!!!) is overkill for turning under size to build up. I typically aim for .003 to .005 under size for CA finishes, then I build up CA and dry sand with 400 grit between coatsto the point that I have a smooth and uniform finish at about .005 oversize (I apply a bit heavier in the middle of the blank, as you usually only measure at the ends...) After that, I wet sand with 800, 1200, and 2000 grit paper, then use the 2 wheel acrylic buffing wheels. If you are going to run a full course of Micromesh instead of the buffing wheels, I would suggest going to .008 or so oversize, as you are going to remove a bit more material with the MM... My ideal final size is .002 or so oversize, as if you aim for being dead on, it may end up slightly undersize and reveal the sharp edge of the hardware, while the .002 over is not noticeable to the touch.

 

[VirgilJ]

Thank you all for the advice. I tried Don's suggestion. The finish increased the blank size by .017. That makes Harry's guess .0005 off. Pretty darn close for a guess.

I also turned an acrylic blank using the calipers to match the size to the measured parts. That took the finish thickness out of the equation. I left the blanks .010 over size to comjpensate for the sanding. I had to do good bit more sanding than I normally would to get the blanks to size.

 

[plano_harry]

Close enough for gov'ment work :biggrin:

Yes, it takes a fair amount of sanding to remove .01. My standard practice on plastic is to mic the part loosely and when I can barely get the calipers on the end, I am done. I turn pretty smooth in those final passes, so a little wet sanding with a worn 400 abranet and I am ready to fit the first 5 MMs and on to Texatdurango's plastic buffing process

Thank you all for the advice. I tried Don's suggestion. The finish increased the blank size by .017. That makes Harry's guess .0005 off. Pretty darn close for a guess.

I also turned an acrylic blank using the calipers to match the size to the measured parts. That took the finish thickness out of the equation. I left the blanks .010 over size to comjpensate for the sanding. I had to do good bit more sanding than I normally would to get the blanks to size.