How to machine a shim accurately ?

[compact8]

The tram of my Emco FB2 mill is off along the Y axis by 0.05 mm over 150 mm. Not too bad really but as I have got some time now, I am thinking about doing something to correct it. My initial plan is to make a donut-shape shim to go into the tilting joint of the head. The shim will be about 1.5 mm thick, 110 mm in diameter. The critical dimension is the thickness difference across the shim - one side has to be be 0.04 mm thicker than the opposite side. What is the best way to make such a shim on a bench mill ? Will appreciate some ideas.

 

[Flyinfool]

Set it up in the mill like you were going to mill the full surface flat. Slip a 0.04mm shim under one side of it and then mill it flat. You may have to mount it to a thicker piece of something to keep it from flexing, just shim that thicker backing plate.

 

[RJSakowski]

Back around Y2K, I was experimenting with accurately machining very thin epoxy composites for a new medical imaging product. The thinnest we needed was .016" thick. My first attempt was to bond the work to an chilled aluminum back plate (-10ºF) with water. Ice is an excellent adhesive at low temperatures. For production, we ultimately used a vacuum plate to hold the work.

For your application, I would use cyanoacrylate adhesive to bond to a backing plate. I would put the necessary shims to create your desired wedge between the back plate and the bed of the mill so you have intimate contact of your work with the back plate. When finished with your machining. heat the work to break the adhesive bond and clean any adhesive from the part with acetone.

edit: the backing plate should be thick enough to prevent flexing when you clamp it down.

 

[markba633csi]

An easier approach might be to just use strips of tape instead of a single machined piece. That way you can add or subtract strips to get it perfect
-M

 

[jmkasunich]

The tram of my Emco FB2 mill is off along the Y axis by 0.05 mm over 150 mm. Not too bad really but as I have got some time now, I am thinking about doing something to correct it. My initial plan is to make a donut-shape shim to go into the tilting joint of the head. The shim will be about 1.5 mm thick, 110 mm in diameter. The critical dimension is the thickness difference across the shim - one side has to be be 0.04 mm thicker than the opposite side. What is the best way to make such a shim on a bench mill ? Will appreciate some ideas.

0.05mm is 0.0019". Scrape it.

(Edit: I mean, just scrape the correction into the machine castings instead of making a shim.)

 

[jwmay]

Just stopped in to applaud your dedication to accuracy. I turned something today that was within .0005" of being perfect, for the first time, after...7 years of hobby machining. First time.
Also Tom's Techniques, and Joe Pi on Youtube both have very good videos on making shims. You might want to check them out.

 

[compact8]

0.05mm is 0.0019". Scrape it.

(Edit: I mean, just scrape the correction into the machine castings instead of making a shim.)

The first line of your reply made me

seriously, scraping is the first thing came to my mind as the method has been mentioned widely in the internet. The impression I have got is that it requires a lot of skill and if not done right, it can be a never ending process until the whole thing is scraped away. As of now, making a shim is a much saver option. If it doesnt work, the machine will not suffer from any damage.

 

[compact8]

For your application, I would use cyanoacrylate adhesive to bond to a backing plate.

Thanks for the suggestion. Mounting the shim to the backing plate is an issue as there is no places for clamps to be applied. The use of CA glue sounds very promising although I am not sure whether the force needed to break up the heated CA glue ( I think it still sticks to some extent ? ) will deform the shim. I will do some experiment to find that out.

 

[ErichKeane]

So rather than mess with shims, what about making a riser-block of some sort? Make it a 'shimmed plate' that is off by a bit on one side, but ~1/2"/1"/2" thick all the way across (obviously thicker on one side). I know those machines are typically short on Z axis space anyway, so an extra inch or two might be nice!

 

[RJSakowski]

Thanks for the suggestion. Mounting the shim to the backing plate is an issue as there is no places for clamps to be applied. The use of CA glue sounds very promising although I am not sure whether the force needed to break up the heated CA glue ( I think it still sticks to some extent ? ) will deform the shim. I will do some experiment to find that out.

The backing plate should be larger than your workpiece to allow for clamping. Heat should totally break the adhesive bond although it will re-bond as it cools. In machining your shims, you could leave a small tab to enable removal of the shim from the backing plate. There is a product called Goof Off which is much better than acetone at breaking the cyanoacrylate bond.