One of the great things of Pentax is the possibility to use legacy glass. After the introduction of the crippled K-mount they never changed lens mounts, so there are literally millions of lenses available, often for not more than several tens of bucks. Before the K-mount was introduced, Pentax used the M42-mount, and the lenses with this mount can still be used, even on modern dSLR's, with a cheap adapter.
Many old lenses however share a common problem, a stiff focussing ring, caused by old lubricant. Notorious for this are the old Soviet lenses, and I have a M39 Industar-50 which had precisely this problem. So I decided to take it down in order to clean it. Here are some photos to show the process. I hope this will be helpfull!
In the upper scale ring there are three tiny screws. In the photo you see one just above the 0,65 sign. Unscrew them, but be careful: these screws are really tiny and they tend to jump everywhere, even when you think they're still in the scale ring!
The lens with the removed scale ring
Shown is the lens in nearby focus, with three pals that prevent the lens from focussing beyond infinity and too close. Clearly visible is the old, yellow lubricant. Although it is not really necessary, I chose to remove also the lower scale ring. This is done exactly the same way as you removed the upper scale ring.
The second ring removed
To clean the helicoid completely, you have to unscrew it from the base of the lens. In order to do so, you have to remove the lower pal. You can reach this one when you get the lens in the position as shown below.
In this position you can remove the lower pal
Tip for re-assembling the lens: when the pal is in the centre of the notch in the above ring, the "8" of the lower scale ring is exactly in line with this pal.
When you have unscrewed the pal, you can remove the base of the lens and clean the helicoid and the base. For relubrication use a good lithium lubricant. You can buy it in bike shops etc.
Ready to be cleaned