Problems with rotating around a Primary Part?

local runService = game:GetService("RunService");

local model = script.Parent; local defaultCF = model:GetPrimaryPartCFrame();

runService.Heartbeat:Connect(function() model:SetPrimaryPartCFrame(defaultCFCFrame.Angles(0,(tick()/4)%(math.pi4),0)); end);

Something wrong with the way I do this? For some reason it very slowly moves outwards from the Primary Part for reasons I can't understand.