If the cutter head is square, the straight edge tests may be applied to all four of its faces, and they may then be tested with a square, and if the head shows no error under these tests, and the bolt holes or slots are of equal diameter and depths, the head will be correct as far as it can be tested without running it.
A cutter head may be roughly tested by placing it between the lathe centres, both centres being oiled and delicately adjusted so as to just prevent end motion of the head without perceptible friction when the head is revolved by hand.
The first thing to test is whether the journals run true, which may be tested by a pointer fastened in the slide seat, and moved up to just touch the journal. The pointer should be soft, and not a cutting tool, unless indeed it be set so high in the slide rest that it cannot cut.
If the journals do not run true, the next thing to test is whether the body of the head runs true to the centres, which may be done by first setting a pointer to just touch the extreme corners of the head at each end and in the middle of its length, and if there is an error in the same direction as the test at the journal shows, then the centres of the head are out of true, and must be corrected before a test of this kind can be proceeded with.
But the body of the head may show true at the corners while the journals do not run true, and if this is the case we may further test the body of the head as follows:
With the lathe slide rest at one end of the head we may set a pointer so that it will just pass on the flat of the cutter seat and make a mark when the slide rest is traversed along the lathe bed. We then move the slide rest so as to bring the pointer to the journal end of the head; give the head a half a revolution on the centres and try the pointer on the flat of the cutter seat, and if it makes a mark of equal strength, then two faces of the head are equidistant from the axis of the head.
The next thing to do is to make the same test at the other end of the head, and in order to do this without moving the pointer, and therefore without altering its adjustment, we must move the slide rest so as to bring the pointer opposite to the lathe centre, and out of the way of the body of the head, and take the cutter head out of the lathe and turn it end for end, and then repeat the test with the pointer, which will show whether both ends of those two flats are alike.
This test we repeat on the other two faces of the head, and if they show true, then the head is true, except the journal, which must be made true with the head.
This testing will clearly show any want of truth in either the head or the journals, and in what direction correction needs to be made.
Now suppose the above tests do not disclose any error, either in the journals or in the head, and we may continue the tests by revolving the head by hand between the dead centres, and apply the pointer to the journals while the head is revolved as quickly as possible; as, however, the head cannot be revolved very fast in this way, we may adjust the lathe centres as before described, and revolve the head as rapidly as possible by hand, and letting it come to rest mark which side is at the bottom, and if on several tests the same side comes to the bottom of the plane of revolution at each test, that side is the heaviest and must be corrected. If it is found to be a flat side or cutter seat that comes to rest at the bottom, the correction can be made by deepening the bolt holes on that side, measuring to see which bolt hole is the shallowest, and making all as nearly as possible equally deep.